From the Cover: Production of ammonia makes Venusian clouds habitable and explains observed cloud-level chemical anomalies

The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include the unexpected presence of ∼10 ppm O₂ in the cloud layers, an unknown composition of large particles in the lower cloud layers, and hard to explain measured vertical abundance profiles of SO₂ and H₂O. We propose a hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH₃), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH₃ dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO₂ as ammonium sulfite salts. This trapping of SO₂ in the clouds, together with the release of SO₂ below the clouds as the droplets settle out to higher temperatures, explains the vertical SO₂ abundance anomaly. A consequence of the presence of NH₃ is that some Venus cloud droplets must be semisolid ammonium salt slurries, with a pH of ∼1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH₃ is unknown but could involve biological production; if so, then the most energy-efficient NH₃-producing reaction also creates O_2, explaining the detection of O₂ in the cloud layers. Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements.

Venus is often called Earth’s sister planet because of its similar mass and size to Earth. Yet, owing, in part, to the greenhouse effect from its massive CO₂ atmosphere, Venus’s surface temperature is higher than 700 K—too hot for life of any kind. The Venusian surface is therefore a complete contrast to Earth’s temperate surface and rich surface biosphere. Nonetheless, scientists have been speculating on Venus as a habitable world for over half a century (1–7). Such speculations are based on the Earth-like temperature and pressure at the altitudes of 48 km to 60 km above the surface (8, 9).Venus is perpetually shrouded in an ∼20-km-deep layer of clouds, including the temperate atmosphere layers at 48 km to 60 km. The prevailing consensus is that the clouds of Venus are made from droplets of concentrated sulfuric acid. This conclusion is inferred from the presence of small amounts of sulfuric acid vapor in the atmosphere (10, 11) and the refractive index of cloud droplets (12, 13). While the clouds are often described as “temperate” or “clement,” such a statement is misleading when it comes to habitability. If the cloud particles are actually made of concentrated sulfuric acid, then it is difficult to imagine how life chemically similar to life on Earth could survive (7, 14). Specifically, the aggressive chemical properties of sulfuric acid and the extremely low atmospheric water content (14, 15) are orders of magnitude more acidic and 50 to 100 times drier than any inhabited extreme environment on Earth.     

From the Cover: Growth of Carnobacterium spp. from permafrost under low pressure,temperature, and anoxic atmosphere has implications for Earth microbes on Mars

The ability of terrestrial microorganisms to grow in the near-surface environment of Mars is of importance to the search for life and protection of that planet from forward contamination by human and robotic exploration. Because most water on present-day Mars is frozen in the regolith, permafrosts are considered to be terrestrial analogs of the martian subsurface environment. Six bacterial isolates were obtained from a permafrost borehole in northeastern Siberia capable of growth under conditions of low temperature (0 °C), low pressure (7 mbar), and a CO₂-enriched anoxic atmosphere. By 16S ribosomal DNA analysis, all six permafrost isolates were identified as species of the genus Carnobacterium, most closely related to C. inhibens (five isolates) and C. viridans (one isolate). Quantitative growth assays demonstrated that the six permafrost isolates, as well as nine type species of Carnobacterium (C. alterfunditum, C. divergens, C. funditum, C. gallinarum, C. inhibens, C. maltaromaticum, C. mobile, C. pleistocenium, and C. viridans) were all capable of growth under cold, low-pressure, anoxic conditions, thus extending the low-pressure extreme at which life can function.     

From the Cover: PNAS Plus: Thermodynamic origin of surface melting on ice crystals

Since the pioneering prediction of surface melting by Michael Faraday, it has been widely accepted that thin water layers, called quasi-liquid layers (QLLs), homogeneously and completely wet ice surfaces. Contrary to this conventional wisdom, here we both theoretically and experimentally demonstrate that QLLs have more than two wetting states and that there is a first-order wetting transition between them. Furthermore, we find that QLLs are born not only under supersaturated conditions, as recently reported, but also at undersaturation, but QLLs are absent at equilibrium. This means that QLLs are a metastable transient state formed through vapor growth and sublimation of ice, casting a serious doubt on the conventional understanding presupposing the spontaneous formation of QLLs in ice–vapor equilibrium. We propose a simple but general physical model that consistently explains these aspects of surface melting and QLLs. Our model shows that a unique interfacial potential solely controls both the wetting and thermodynamic behavior of QLLs.In general, surfaces and interfaces yield unique phase transitions absent in the bulk (1–5). Surface melting (or premelting) of ice (3, 4) is one typical and classical example that has been known since the first prediction by Michael Faraday in 1842 (6). He hypothesized that thin water layers, now called quasi-liquid layers (QLLs), wet ice crystal surfaces even at a temperature below the melting point. Since then, this phenomenon has attracted considerable attention not only because of its importance in the fundamental understanding of melting (a solid-to-liquid transition) itself but also as a link to a diverse set of natural phenomena in subzero environments: making snowballs, slippage on ice surfaces, frost heave, recrystallization and coarsening of ice grains, morphological change of snow crystals, electrification of thunderclouds, and ozone-depleting reactions (3, 4, 7). Furthermore, it is now recognized that surface melting is not specific to ice but rather is universally seen in a wide range of crystalline surfaces such as metals, semiconductors, ceramics, rare gases, and organic and colloidal systems (8–12). Its underlying physics is therefore also inseparable from material science and technology.Although the origin of surface melting, including the nature of QLLs themselves, is still far from completely understood and a matter of active debate (13–18), it is at least phenomenologically believed that surface melting is driven by the reduction of the surface free energy by the presence of intervening liquid between the solid and gas phases (3, 4, 13, 19). More sophisticated approaches have also been proposed in terms of surface phase transitions (1, 3, 4, 20). In contrast to such theoretical speculations, however, the direct observation and the accurate characterization of QLLs by experiments are still highly challenging because of their thinness, assumed to be less than tens of nanometers (21). Experimental efforts in the past have often been bedeviled by large uncertainties depending on the experimental methods and researchers (see table S1 in ref. 22 for details). Even the first convincing evidence for the existence of surface melting of ice was not provided until 1987 (13, 14), more than one century after Faraday’s suggestion. Thus, the conventional theories, although rigorous themselves, have suffered from the lack of reliably experimental support.Recently, we succeeded in making in situ observations of QLLs on ice surfaces using an advanced optical microscope (laser confocal microscopy combined with differential interference contrast microscopy: LCM-DIM), whose resolution in the height direction reaches the order of an angstrom (22, 23). Surprisingly, this work revealed that, contrary to the common belief that QLLs completely and homogeneously wet ice surfaces, they are spatiotemporally heterogeneous and are absent in the equilibrium conditions (22, 24–26). Furthermore, we have observed that QLLs exhibit more than one wetting morphology: droplet type, thin-layer type, and their coexistence (sunny-side-up type) at supersaturation (22, 24–26). This finding fundamentally requires us to recast the conventional understanding based on spatiotemporally averaged equilibrium theories and experiments (e.g., scattering, spectroscopy, and ellipsometry), because of ignorance of the counterintuitive nature of QLLs.In this paper, we present a simple physical model bridging the gap between the conventional interpretation and the above aspects of surface melting based on in situ observations with our advanced optical microscopy combined with a two-beam interferometer. Here we revisit the thermodynamics of wetting (27). The general nature of surface melting suggests the relevance of the phenomenological approach. Starting from the phenomenological interfacial free energy, we robustly determine a full interfacial potential between ice and vapor in the medium of a QLL, governing both the selection and stability of the wetting states, and the thermodynamic condition for the existence of QLLs. As a theoretical consequence, we extend the concept of surface melting into nonequilibrium regimes, more specifically, supersaturation and undersaturation, which has a significant implication for exploring the possible existence of this phenomenon in a wider range of crystalline surfaces. Our model provides not only a clear-cut answer to the long-standing question of the origin of surface melting of ice but also offers a general insight into the origin of surface melting of other solid–gas interfaces.     

From the Cover: Glacier loss on Kilimanjaro continues unabated

The dramatic loss of Kilimanjaro''s ice cover has attracted global attention. The three remaining ice fields on the plateau and the slopes are both shrinking laterally and rapidly thinning. Summit ice cover (areal extent) decreased ≈1% per year from 1912 to 1953 and ≈2.5% per year from 1989 to 2007. Of the ice cover present in 1912, 85% has disappeared and 26% of that present in 2000 is now gone. From 2000 to 2007 thinning (surface lowering) at the summits of the Northern and Southern Ice Fields was ≈1.9 and ≈5.1 m, respectively, which based on ice thicknesses at the summit drill sites in 2000 represents a thinning of ≈3.6% and ≈24%, respectively. Furtwängler Glacier thinned ≈50% at the drill site between 2000 and 2009. Ice volume changes (2000–2007) calculated for two ice fields reveal that nearly equivalent ice volumes are now being lost to thinning and lateral shrinking. The relative importance of different climatological drivers remains an area of active inquiry, yet several points bear consideration. Kilimanjaro''s ice loss is contemporaneous with widespread glacier retreat in mid to low latitudes. The Northern Ice Field has persisted at least 11,700 years and survived a widespread drought ≈4,200 years ago that lasted ≈300 years. We present additional evidence that the combination of processes driving the current shrinking and thinning of Kilimanjaro''s ice fields is unique within an 11,700-year perspective. If current climatological conditions are sustained, the ice fields atop Kilimanjaro and on its flanks will likely disappear within several decades.     

From the Cover: Control of the multimillennial wildfire size in boreal North America by spring climatic conditions

Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire–climate–vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.     

From the Cover: Warming enabled upslope advance in western US forest fires

Increases in burned area and large fire occurrence are widely documented over the western United States over the past half century. Here, we focus on the elevational distribution of forest fires in mountainous ecoregions of the western United States and show the largest increase rates in burned area above 2,500 m during 1984 to 2017. Furthermore, we show that high-elevation fires advanced upslope with a median cumulative change of 252 m (−107 to 656 m; 95% CI) in 34 y across studied ecoregions. We also document a strong interannual relationship between high-elevation fires and warm season vapor pressure deficit (VPD). The upslope advance of fires is consistent with observed warming reflected by a median upslope drift of VPD isolines of 295 m (59 to 704 m; 95% CI) during 1984 to 2017. These findings allow us to estimate that recent climate trends reduced the high-elevation flammability barrier and enabled fires in an additional 11% of western forests. Limited influences of fire management practices and longer fire-return intervals in these montane mesic systems suggest these changes are largely a byproduct of climate warming. Further weakening in the high-elevation flammability barrier with continued warming has the potential to transform montane fire regimes with numerous implications for ecosystems and watersheds.

Fire is an integral component of most forested lands and provides significant ecological services (1). However, burned area, fire size, the number of large fires, and the length of fire season have increased in the western United States in recent decades (2, 3). Increasing fire activity and the expansion of wildland urban interface (4) collectively amplified direct and indirect fire-related loss of life and property (5, 6) and contributed to escalating fire suppression costs (7). While increased biomass due to a century of fire exclusion efforts is hypothesized to have partially contributed to this trend (8), climate change is also implicated in the rise of fire activity in the western United States (9–11).Although increases in forest fire activity are evident in all major forested lands in the western United States (2, 12, 13), an abundance of moisture—due to snowpack persistence, cooler temperatures, and delayed summer soil and fuel drying—provides a strong buffer of fire activity (13) and longer fire-return intervals (14) at high elevations. Recent studies, however, point to changing fire characteristics across many ecoregions of the western United States (15), including high-elevation areas of the Sierra Nevada (16), Pacific Northwest, and Northern Rockies (12, 17). These studies complement documented changes in montane environments including amplified warming with elevation (18), widespread upward elevational shift in species (19), and increased productivity in energy-limited high-elevation regions that enhance fuel growth and connectivity (20). These changes have been accompanied by longer snow-free periods (21), increased evaporative demand (9), and regional declines in fire season precipitation frequency (11) across the western United States promoting increased fuel ignitability and flammability that have well-founded links to forest burned area. A warmer climate is also conducive to a higher number of convective storms and more frequent lightning strikes (22).In this study, we explore changes in the elevational distribution of burned forest across the western United States and how changes in climate have affected the mesic barrier for high-elevation fire activity. We focus on changes in high-elevation forests that have endured fewer direct anthropogenic modifications compared to drier low-elevation forests that had frequent low-severity fires prior to European colonization and have been more subject to changes in settlement patterns as well as fire suppression and harvest (23, 24); we also pose the following questions: 1) Has the elevational distribution of fire in the western US forests systematically changed? and 2) What changes in biophysical factors have enabled such changes in high-elevation fire activity? We explore these questions across 15 mountainous ecoregions of the western United States using records from large fires (>405 ha) between 1984 and 2017 [Monitoring Trends in Burn Severity (MTBS) (25)], a 10-m–resolution digital elevation model, and daily high-spatial–resolution surface meteorological data [gridMET (26)].We focus on the trends in Z₉₀—defined as the 90th percentile of normalized annual elevational distribution of burned forest in each ecoregion. Here, the term “normalized” essentially refers to the fraction of forest area burned by elevation. We complement this analysis by examining trends in burned area by elevational bands and using quantile regression of normalized annual forest fire elevation. We then assess the interannual relationships between Z₉₀ and vapor pressure deficit (VPD) and compare the upslope advance in montane fire to elevational climate velocity of VPD during 1984 to 2017. Specifically, we use VPD trends and VPD–high-elevation fire regression to estimate VPD-driven changes in Z₉₀ and BA₉₀— defined as annual burned area above the 90th percentile of forest elevational distribution in each ecoregion—during 1984 to 2017.     

From the Cover: An adaptability limit to climate change due to heat stress

Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wet-bulb temperature T_W, is surprisingly similar across diverse climates today. T_W never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.     

From the Cover: A 4,565-My-old andesite from an extinct chondritic protoplanet

The age of iron meteorites implies that accretion of protoplanets began during the first millions of years of the solar system. Due to the heat generated by ²⁶Al decay, many early protoplanets were fully differentiated with an igneous crust produced during the cooling of a magma ocean and the segregation at depth of a metallic core. The formation and nature of the primordial crust generated during the early stages of melting is poorly understood, due in part to the scarcity of available samples. The newly discovered meteorite Erg Chech 002 (EC 002) originates from one such primitive igneous crust and has an andesite bulk composition. It derives from the partial melting of a noncarbonaceous chondritic reservoir, with no depletion in alkalis relative to the Sun’s photosphere and at a high degree of melting of around 25%. Moreover, EC 002 is, to date, the oldest known piece of an igneous crust with a ²⁶Al-²⁶Mg crystallization age of 4,565.0 million years (My). Partial melting took place at 1,220 °C up to several hundred kyr before, implying an accretion of the EC 002 parent body ca. 4,566 My ago. Protoplanets covered by andesitic crusts were probably frequent. However, no asteroid shares the spectral features of EC 002, indicating that almost all of these bodies have disappeared, either because they went on to form the building blocks of larger bodies or planets or were simply destroyed.

Despite the large number of samples in the meteorite record that originate from the crust or mantle of rocky bodies (about 3,100 are known today), these rocks provide an incomplete picture of the diversity of the differentiated bodies that formed in the early solar system (1). Indeed, about 95% of these meteorites originate from only two bodies, with 75% coming from the crust of a single asteroid (possibly 4 Vesta) and the other 20% from the mantle of a presumably larger object, the now-destroyed ureilite parent body (2, 3). Thus, until recently, known achondritic lavas were essentially basalts (eucrites) from 4-Vesta and a handful of other basaltic rocks from unknown parent bodies [the angrites and some ungrouped achondrites such as Northwest Africa (NWA) 011 (4) or Ibitira (5)]. Although certainly not representative of the magmatic activity of all the planetesimals, these achondritic lavas strengthened the general view that their crusts were essentially basaltic in composition. However, the discovery of some rare achondrites of andesitic or trachyandesitic composition [e.g., Graves Nunataks 06128 and 016129 (6, 7), ALM-A (8), NWA 11119 (9)] demonstrated that the diversity of the lavas formed on protoplanets may have been more important than previously thought. Experimental studies motivated by these new meteorites have shown that the generation of silica-rich liquids is possible from the melting of chondrites (10–12). Thus, the formation of andesitic crust was possibly common on protoplanets, especially for those that were not Na and K depleted (12), contrary to what the meteorite record suggests. However, the processes that built such a crust, and the genesis of protoplanetary andesites are not well known due to the rarity of the samples. Here, we report on Erg Chech 002 (EC 002), a unique andesite achondrite found in the spring of 2020 in the Sahara. This meteorite is the oldest magmatic rock analyzed to date and sheds light on the formation of the primordial crusts that covered the oldest protoplanets.     

From the Cover: Perceptions of water use

In a national online survey, 1,020 participants reported their perceptions of water use for household activities. When asked for the most effective strategy they could implement to conserve water in their lives, or what other Americans could do, most participants mentioned curtailment (e.g., taking shorter showers, turning off the water while brushing teeth) rather than efficiency improvements (e.g., replacing toilets, retrofitting washers). This contrasts with expert recommendations. Additionally, some participants are more likely to list curtailment actions for themselves, but list efficiency actions for other Americans. For a sample of 17 activities, participants underestimated water use by a factor of 2 on average, with large underestimates for high water-use activities. An additional ranking task showed poor discrimination of low vs. high embodied water content in food products. High numeracy scores, older age, and male sex were associated with more accurate perceptions of water use. Overall, perception of water use is more accurate than the perception of energy consumption and savings previously reported. Well-designed efforts to improve public understanding of household water use could pay large dividends for behavioral adaptation to temporary or long-term decreases in availability of fresh water.Fresh water is used increasingly beyond sustainable levels (1). Do people know how much water is used by a variety of daily activities? If people were asked to conserve water, would they know which behaviors are more effective than others? Gleick (2) estimated that 13.2 gallons of clean water are required per person per day for human needs (drinking, sanitation, hygiene, and food preparation). In 2005, the average American used about 98 gallons of water per day (3), of which ∼70% was used indoors (4). Thus, the average American uses more than seven times the water estimated by Gleick as needed. To understand how water use is distributed among daily activities in American households, Mayer et al. (5) surveyed 12 study sites during 1996 through 1998 to disaggregate residential end-use water consumption. Fig. 1 shows the average distribution for six categories. They also found that indoor water use was fairly homogenous across the 12 sites, except for the category “leaks”; whereas outdoor water use varies substantially depending on local climate (5).Open in a separate windowFig. 1.Disaggregated residential indoor water use based on 12 study sites in the United States published in 1999, adapted from Mayer et al. (5).Most Americans assume that water supply is both reliable and plentiful. However, research has shown that with climate change, water supply will become more variable due to salinization of ground water and increased variability in precipitation (6, 7). Some have argued that rather than focusing on increasing freshwater supply alone, we need also to reduce water demand (8). Demand-side policy responses to future freshwater variability will benefit from a deeper understanding of public perceptions of water use, which is the focus of this study.Similar to Attari et al. (9), a study that explored public perceptions of energy use, here actual water use is compared with perceived water use for a variety of indoor and outdoor activities. Perceived energy consumption is a fairly flat function of actual consumption. Such a compression bias (9, 10) could result from participants’ lack of knowledge about energy in its different manifestations. The flatness is also partly due to the judgment heuristic of anchoring and insufficient adjustment (11, 12), which arises when a person generates a numerical estimate by first adopting a salient reference as a starting point and then adjusts this estimate in the desired direction, but insufficiently. Attari et al. (9) also showed that participants overestimate energy consumption for activities that use small amounts of energy, and underestimate consumption for activities that use large amounts.Do similar over- and underestimations exist for judgments of water use? Given the consistent tangible physical quality that exists for water but is somewhat obscure for energy as well as the familiarity of the unit of measurement, one could expect more accurate estimates for water. Additionally, Attari et al. (9) found that both numeracy and proenvironmental attitudes are associated with more accurate perceptions of energy use. Similar predictions for individual difference variables are tested here for judgments of water use.     

From the Cover: Pluvials,droughts, the Mongol Empire,and modern Mongolia

Although many studies have associated the demise of complex societies with deteriorating climate, few have investigated the connection between an ameliorating environment, surplus resources, energy, and the rise of empires. The 13th-century Mongol Empire was the largest contiguous land empire in world history. Although drought has been proposed as one factor that spurred these conquests, no high-resolution moisture data are available during the rapid development of the Mongol Empire. Here we present a 1,112-y tree-ring reconstruction of warm-season water balance derived from Siberian pine (Pinus sibirica) trees in central Mongolia. Our reconstruction accounts for 56% of the variability in the regional water balance and is significantly correlated with steppe productivity across central Mongolia. In combination with a gridded temperature reconstruction, our results indicate that the regional climate during the conquests of Chinggis Khan’s (Genghis Khan’s) 13th-century Mongol Empire was warm and persistently wet. This period, characterized by 15 consecutive years of above-average moisture in central Mongolia and coinciding with the rise of Chinggis Khan, is unprecedented over the last 1,112 y. We propose that these climate conditions promoted high grassland productivity and favored the formation of Mongol political and military power. Tree-ring and meteorological data also suggest that the early 21st-century drought in central Mongolia was the hottest drought in the last 1,112 y, consistent with projections of warming over Inner Asia. Future warming may overwhelm increases in precipitation leading to similar heat droughts, with potentially severe consequences for modern Mongolia.Abrupt climate changes have immediate and long-lasting consequences for ecosystems and societies. Although studies have linked the demise of complex societies with deteriorating climate conditions (1–4), few, if any, have investigated the connection between climate, surplus resources, energy, and the rise of empires. The rapid expansion of the Mongols under Chinggis Khan (also known as Genghis Khan) from 1206 to 1227 CE resulted in the largest contiguous land empire in world history (Fig. 1, Inset). The Mongol conquests affected the history of civilizations from China to Russia, Persia to India, and even left a genetic fingerprint on the people of Eurasia (5). Although historians have proposed climate as a possible factor in Mongol history (6), few paleoenvironmental data of the necessary temporal resolution are available to evaluate the role of climate, grassland productivity, and energy in the rise of the 13th-century Mongol Empire.Open in a separate windowFig. 1.Tree-ring drought reconstruction site (green cross) and inferred temperature site (8) (white cross) are 50 km apart. Map of the Mongol Empire near its zenith (aqua) in 1260 CE (Inset). The ancient capital city of Karakorum (black triangle) and current capital of Mongolia, Ulaanbaatar (black star).Lake sediment data from central Mongolia suggest that the climate of the Mongol Empire may have been unusually wet (7), but the temporal resolution of these records is too coarse to capture conditions during the 2 decades of rapid growth of the Mongol Empire. Annual tree-ring records of past temperature from central Mongolia extending back to 558 CE document warm conditions during the 11th century, consistent with other Northern Hemisphere records, but also indicate a subsequent warm period during the 12th and 13th centuries (8, 9). Millennium-long reconstructions of past precipitation in western China, mostly located on the Tibetan Plateau and north central China (10–15), document drought during the early 1200s. However, periods of drought in central Mongolia are generally out of phase with drought on the Tibetan Plateau (2), and there is little reason to believe that moisture conditions on the Tibetan Plateau and north central China would be consistent with that of central Mongolia. Here we present the first, to our knowledge, annually resolved record of moisture balance covering the last millennium for the Asian steppe. This new record allows us to evaluate the hypothesis that drought drove the 13th-century Mongol expansion into Eurasia (6, 16).     

From the Cover: Noachian and more recent phyllosilicates in impact craters on Mars

Hundreds of impact craters on Mars contain diverse phyllosilicates, interpreted as excavation products of preexisting subsurface deposits following impact and crater formation. This has been used to argue that the conditions conducive to phyllosilicate synthesis, which require the presence of abundant and long-lasting liquid water, were only met early in the history of the planet, during the Noachian period (> 3.6 Gy ago), and that aqueous environments were widespread then. Here we test this hypothesis by examining the excavation process of hydrated minerals by impact events on Mars and analyzing the stability of phyllosilicates against the impact-induced thermal shock. To do so, we first compare the infrared spectra of thermally altered phyllosilicates with those of hydrated minerals known to occur in craters on Mars and then analyze the postshock temperatures reached during impact crater excavation. Our results show that phyllosilicates can resist the postshock temperatures almost everywhere in the crater, except under particular conditions in a central area in and near the point of impact. We conclude that most phyllosilicates detected inside impact craters on Mars are consistent with excavated preexisting sediments, supporting the hypothesis of a primeval and long-lasting global aqueous environment. When our analyses are applied to specific impact craters on Mars, we are able to identify both pre- and postimpact phyllosilicates, therefore extending the time of local phyllosilicate synthesis to post-Noachian times.     

From the Cover: Antarctic climate signature in the Greenland ice core record

A numerical algorithm is applied to the Greenland Ice Sheet Project 2 (GISP2) dust record from Greenland to remove the abrupt changes in dust flux associated with the Dansgaard-Oeschger (D-O) oscillations of the last glacial period. The procedure is based on the assumption that the rapid changes in dust are associated with large-scale changes in atmospheric transport and implies that D-O oscillations (in terms of their atmospheric imprint) are more symmetric in form than can be inferred from Greenland temperature records. After removal of the abrupt shifts the residual, dejumped dust record is found to match Antarctic climate variability with a temporal lag of several hundred years. It is argued that such variability may reflect changes in the source region of Greenland dust (thought to be the deserts of eastern Asia). Other records from this region and more globally also reveal Antarctic-style variability and suggest that this signal is globally pervasive. This provides the potential basis for suggesting a more important role for gradual changes in triggering more abrupt transitions in the climate system.     

From the Cover: Proton transfer through the water gossamer

The diffusion of protons through water is understood within the framework of the Grotthuss mechanism, which requires that they undergo structural diffusion in a stepwise manner throughout the water network. Despite long study, this picture oversimplifies and neglects the complexity of the supramolecular structure of water. We use first-principles simulations and demonstrate that the currently accepted picture of proton diffusion is in need of revision. We show that proton and hydroxide diffusion occurs through periods of intense activity involving concerted proton hopping followed by periods of rest. The picture that emerges is that proton transfer is a multiscale and multidynamical process involving a broader distribution of pathways and timescales than currently assumed. To rationalize these phenomena, we look at the 3D water network as a distribution of closed directed rings, which reveals the presence of medium-range directional correlations in the liquid. One of the natural consequences of this feature is that both the hydronium and hydroxide ion are decorated with proton wires. These wires serve as conduits for long proton jumps over several hydrogen bonds.The mechanism by which protons move through water is at the heart of acid–base chemistry reactions. Understanding the reaction coordinates of this process has been one of the most challenging problems in physical chemistry due to the sheer complexity of water’s hydrogen bond network (1–4). Developing a molecular basis for these phenomena is of great relevance in energy conversion applications such as in the design of efficient fuel cells (5). Over 200 y ago, von Grotthuss proposed a mechanism by which water would undergo electrolytic decomposition (6). He imagined that proton conduction involved the collective shuttling of hydrogen atoms along water wires. The early 20th century found many of the great scientists of the time developing conceptual models to understand the properties of water and its constituent ions (7, 8). Detailed insights into the mechanisms of proton transfer (PT) came much later from a combination of both ab initio molecular dynamics (AIMD) simulations (3, 9–13) and force-field approaches based on the empirical valence bond formalism (14–16). The current textbook picture of the Grotthuss mechanism that has resulted from these studies involves a stepwise hopping of the proton from one water molecule to the next (1, 17, 18). This process occurs on a timescale of 1–2 ps. For a successful transfer, the model requires solvent reorganization around the proton-receiving species to develop a coordination pattern like that of the species it will convert to, a process known as presolvation. In all of these characterizations of the Grotthuss mechanism, the role of the connectivity of the water network was not brought to the forefront (3, 19).Sometimes PT has also been thought to take on coherent character involving jumps of several protons simultaneously. In this spirit, Eigen (20) suggested that the proton could delocalize over extended hydrogen-bonded wires. There is evidence that this behavior can occur when water molecules form isolated chains, in confined environments like proteins and nanotubes (21–23). In addition, several spectroscopic experiments examining acid–base reactions in ice and water have suggested that fast PT occurs through the formation of transient water wires (24, 25). We have recently shown that, when the hydronium and hydroxide ions approach each other at ∼6 Å, a water wire that always bridges the ions undergoes a collective compression during their recombination (26). This event results in a concerted motion of three protons on the timescale of tens of femtoseconds. It appears as though the formation of polarized water wires is a necessary precursor for correlated PT events. The question then arises whether wire-like structures exist in liquid water and around its constituent ions, and if they do, whether they serve as conduits for different PT mechanisms. Currently, the prevailing view is that concerted PT through proton wires does not occur in liquid water (1, 3, 4, 19, 25).In this work, we revisit the currently accepted view of the Grotthuss mechanism. Using AIMD simulations of large periodic systems, we find that PT in water occurs over a broader distribution of pathways and timescales than normally assumed. The migration of charge involves bursts of activity along proton wires in the network characterized by the concerted motion of several protons, followed by resting periods that are longer than expected, similar to a jump-like diffusion mechanism. This striking dynamical activity is driven partly by the ability of the proton wires to undergo collective compressions. Understanding the structural origins of this behavior, requires a refined picture of the 3D hydrogen bond network. Our inspiration comes from the results of previous studies where it was observed that liquid water is characterized by a broad distribution of closed rings (27–29). In all of these studies, the directionality of the hydrogen bonds in the network has been ignored. By including this feature within our analysis of the rings, we reveal striking medium-range directional correlations in the network. One of the important consequences of this feature is that proton wires naturally decorate the atmosphere of the ions and subsequently influence the mechanisms by which they diffuse through water.     

From the Cover: Warm spring reduced carbon cycle impact of the 2012 US summer drought

The global terrestrial carbon sink offsets one-third of the world’s fossil fuel emissions, but the strength of this sink is highly sensitive to large-scale extreme events. In 2012, the contiguous United States experienced exceptionally warm temperatures and the most severe drought since the Dust Bowl era of the 1930s, resulting in substantial economic damage. It is crucial to understand the dynamics of such events because warmer temperatures and a higher prevalence of drought are projected in a changing climate. Here, we combine an extensive network of direct ecosystem flux measurements with satellite remote sensing and atmospheric inverse modeling to quantify the impact of the warmer spring and summer drought on biosphere-atmosphere carbon and water exchange in 2012. We consistently find that earlier vegetation activity increased spring carbon uptake and compensated for the reduced uptake during the summer drought, which mitigated the impact on net annual carbon uptake. The early phenological development in the Eastern Temperate Forests played a major role for the continental-scale carbon balance in 2012. The warm spring also depleted soil water resources earlier, and thus exacerbated water limitations during summer. Our results show that the detrimental effects of severe summer drought on ecosystem carbon storage can be mitigated by warming-induced increases in spring carbon uptake. However, the results also suggest that the positive carbon cycle effect of warm spring enhances water limitations and can increase summer heating through biosphere–atmosphere feedbacks.An increase in the intensity and duration of drought (1, 2), along with warmer temperatures, is projected for the 21st century (3). Warmer and drier summers can substantially reduce photosynthetic activity and net carbon uptake (4). In contrast, warmer temperatures during spring and autumn prolong the period of vegetation activity and increase net carbon uptake in temperate ecosystems (5), sometimes even during spring drought (6). Atmospheric CO₂ concentrations suggest that warm-spring–induced increases in carbon uptake could be cancelled out by the effects of warmer and drier summers (7). However, the extent and variability of potential compensation on net annual uptake using direct observations of ecosystem carbon exchange have not yet been examined for specific climate anomalies.In addition to perturbations of the carbon cycle, warmer spring temperatures can have an impact on the water cycle by increasing evaporation from the soil and plant transpiration (8–10), which reduces soil moisture. Satellite observations suggest that warmer spring and longer nonfrozen periods enhance summer drying via hydrological shifts in soil moisture status (11). Climate model simulations also indicate a soil moisture–temperature feedback between early vegetation green-up in spring and extreme temperatures in summer (12, 13). Soil water deficits during drought impose a reduction in stomatal conductance, thereby reducing evaporative cooling and thus increasing near-surface temperatures (14). Stomatal closure also has a positive (enhancing) feedback with atmospheric water demand by increasing the vapor pressure deficit (VPD) of the atmosphere (15). The vegetation response thus plays a crucial role for temperature feedbacks during drought (16).Given the opposing effects of concurrent warmer spring and summer drought, and an increased frequency of these anomalies projected until the end of this century (SI Appendix, Fig. S1), it is imperative to understand (i) the response of the terrestrial carbon balance and (ii) the interaction of carbon uptake with water and energy fluxes that are associated with these seasonal climate anomalies.The year 2012 was among the warmest on record for the contiguous United States (CONUS), which experienced one of the most severe droughts since the Dust Bowl era of the 1930s (17, 18). The drought caused substantial economic damage, particularly for agricultural production (SI Appendix). Annual mean temperatures were 1.8 °C above average, with the warmest spring (+2.9 °C) and second warmest summer (+1.4 °C) in the period of 1895–2012 (19). Precipitation deficits started to evolve in May across the Great Plains and the Midwest (17), but eventually affected more than half of the United States (20). By July, 62% of the United States experienced moderate to exceptional drought, which was the largest spatial extent of drought for the United States since the Dust Bowl era (19). Severe drought conditions with depleted soil moisture persisted throughout summer, and unprecedented precipitation deficits of 47% below normal for May through August were observed in the central Great Plains (17).Here, we analyze the response of land-atmosphere carbon and water exchange for major ecosystems in the United States during the concurrent warmer spring and summer drought of 2012 at the ecosystem, regional, and continental scales. We combine direct measurements of land-atmosphere CO₂, water vapor, and energy fluxes from 22 eddy-covariance (EC) towers across the United States (SI Appendix, Fig. S2 and Table S1) with large-scale satellite remote-sensing observations of gross primary production (GPP), evapotranspiration (ET), and enhanced vegetation index (EVI) derived from the space-borne Moderate Resolution Imaging Spectroradiometer (MODIS), and estimates of net ecosystem production (NEP; i.e., net carbon uptake) from an atmospheric CO₂ inversion (CarbonTracker, CTE2014). This comprehensive suite of standardized analyses across sites and data streams was crucial to constrain the impact of such a large-scale drought event with bottom-up and top-down approaches (21), and something only a few synthesis studies have achieved so far (4, 22).We test the hypothesis that increased carbon uptake due to warm spring offset the negative impacts of severe summer drought during 2012, and examine the relationship between early-spring–induced soil water depletion and increased summer temperatures. When using the term “drought,” we refer to precipitation deficits that resulted in soil moisture deficiencies (9).     

From the Cover: Contribution of a mutational hot spot to hemoglobin adaptation in high-altitude Andean house wrens

A key question in evolutionary genetics is why certain mutations or certain types of mutation make disproportionate contributions to adaptive phenotypic evolution. In principle, the preferential fixation of particular mutations could stem directly from variation in the underlying rate of mutation to function-altering alleles. However, the influence of mutation bias on the genetic architecture of phenotypic evolution is difficult to evaluate because data on rates of mutation to function-altering alleles are seldom available. Here, we report the discovery that a single point mutation at a highly mutable site in the β^A-globin gene has contributed to an evolutionary change in hemoglobin (Hb) function in high-altitude Andean house wrens (Troglodytes aedon). Results of experiments on native Hb variants and engineered, recombinant Hb mutants demonstrate that a nonsynonymous mutation at a CpG dinucleotide in the β^A-globin gene is responsible for an evolved difference in Hb–O₂ affinity between high- and low-altitude house wren populations. Moreover, patterns of genomic differentiation between high- and low-altitude populations suggest that altitudinal differentiation in allele frequencies at the causal amino acid polymorphism reflects a history of spatially varying selection. The experimental results highlight the influence of mutation rate on the genetic basis of phenotypic evolution by demonstrating that a large-effect allele at a highly mutable CpG site has promoted physiological differentiation in blood O₂ transport capacity between house wren populations that are native to different elevations.An important question in evolutionary genetics is whether certain mutations or certain types of mutation make disproportionate contributions to phenotypic evolution (1–6). Within a given gene, the mutations that contribute to evolutionary changes in phenotype may represent a biased, nonrandom subset of all possible mutations that are capable of producing the same functional effect. The preferential fixation of particular mutations (substitution bias) could have several causes. Most theoretical and empirical attention has focused on causes of fixation bias, i.e., mutations have different probabilities of being fixed once they arise, due to differences in dominance coefficients or the magnitude of deleterious pleiotropy (1, 2, 4, 7–9). In principle, substitution bias can also stem directly from mutation bias (some sites have higher rates of mutation to alleles that produce the change in phenotype) (4, 9–11). However, empirical evidence for the importance of mutation bias is scarce for an obvious reason: even in rare cases where it is possible to document the contributions of individual point mutations to evolutionary changes in phenotype, data on rates of mutation to function-altering alleles are typically lacking. Rare exceptions include cases where loss-of-function deletion mutations can be traced to hot spots of chromosomal instability or highly mutable changes in the copy number of repetitive elements (12). Documenting cases where genetic changes at highly mutable loci contribute to phenotypic divergence is therefore important for elucidating the evolutionary significance of mutation bias. This is especially true for cases where mutations cause fine-tuned modifications of protein activity rather than simple losses of function.Here, we report the discovery that a single amino acid replacement at a mutational hot spot in the avian β^A-globin gene has contributed to an evolutionary change in hemoglobin (Hb) function that has likely adaptive significance. By conducting experiments on native Hb variants and engineered recombinant Hb mutants, we demonstrate that a nonsynonymous mutation at a CpG dinucleotide in the β^A-globin gene of Andean house wrens (Troglodytes aedon) has contributed to an evolved difference in Hb–O₂ affinity between high- and low-altitude populations. In mammalian genomes, point mutations at CpG sites occur at a rate that is over an order of magnitude higher than the average for all other nucleotide sites (13, 14), and available data suggest a similar discrepancy in avian genomes (15, 16).Andean house wrens are compelling subjects for studies of Hb function because this passerine bird species has an exceptionally broad and continuous elevational distribution, ranging from sea level to elevations >4,500 m (17). At 4,500-m elevation, the standard barometric pressure is ∼450 torr, so O₂ partial pressure (PO₂) is <60% that at sea level (∼96 torr compared to ∼160 torr). Under such conditions, enhancements of pulmonary O₂ uptake and blood O₂ transport capacity are required to sustain O₂ flux to the tissue mitochondria in support of aerobic ATP synthesis (18). To complement changes in the cardiorespiratory system and microcirculation, changes in the O₂-binding affinity and cooperativity of Hb can enhance the O₂ capacitance of the blood (the total amount of O₂ unloaded for a given arteriovenous difference in O₂ tension). Because the optimal Hb–O₂ affinity is expected to vary according to the ambient PO₂, genetic variation in oxygenation properties of Hb may be subject to spatially varying selection between populations that inhabit different elevations. House wrens colonized South America in the late Pliocene or early Pleistocene via the newly formed Panamanian land bridge (19, 20), so the species may have been resident in the Andean highlands for up to ∼3 million years.The Hb tetramer is composed of two semirigid α₁β₁ and α₂β₂ dimers that undergo a mutual rotation during the oxygenation-linked transition in quaternary structure between the deoxy (low-affinity “T”) conformation and the oxy (high-affinity “R”) conformation (21). This oxygenation-linked structural transition between the T and R states is the basis for cooperative O₂ binding, and is central to the allosteric function of Hb as an O₂ transport molecule. Our analysis of house wren Hb highlights the influence of mutation rate on the genetic basis of phenotypic divergence by demonstrating that mutation at a CpG dinucleotide produced a large-effect amino acid replacement at an α₁β₁ intradimer contact (β55Val→Ile)—a replacement that produced a significant increase in Hb–O₂ affinity.     

From the Cover: Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation

An understanding of the mechanisms that control CO₂ change during glacial–interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO₂ (δ¹³C-CO₂) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO₂ from 17.6 to 15.5 ka, these data demarcate a decrease in δ¹³C-CO₂, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO₂ rise of 40 ppm is associated with small changes in δ¹³C-CO₂, consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in δ¹³C-CO₂ that suggest rapid oxidation of organic land carbon or enhanced air–sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO₂ coincident with increases in atmospheric CH₄ and Northern Hemisphere temperature at the onset of the Bølling (14.6–14.3 ka) and Holocene (11.6–11.4 ka) intervals are associated with small changes in δ¹³C-CO₂, suggesting a combination of sources that included rising surface ocean temperature.Over thirty years ago ice cores provided the first clear evidence that atmospheric CO₂ increased by about 75 ppm as Earth transitioned from a glacial to an interglacial state (1, 2). After decades of research, the underlying mechanisms that drive glacial–interglacial CO₂ cycles are still unclear. A tentative consensus has formed that the deglaciation is characterized by a net transfer of carbon from the ocean to the atmosphere and terrestrial biosphere, through a combination of changes in ocean temperature, nutrient utilization, circulation, and alkalinity. Partitioning these changes in terms of magnitude and timing is challenging. Estimates of the glacial–interglacial carbon cycle budget are highly uncertain, ranging from 20–30 ppm for the effect of rising ocean temperature, 5–55 ppm for ocean circulation changes, and 5–30 ppm for decreasing iron fertilization (3, 4), with feedbacks from CaCO₃ compensation accounting for up to 30 ppm (5, 6).A precise history of the stable isotopic composition of atmospheric carbon dioxide (δ¹³C-CO₂) can constrain key processes controlling atmospheric CO₂ (7, 8). A low-resolution record from the Taylor Dome ice core (9) identified a decrease in δ¹³C-CO₂ at the onset of the deglacial CO₂ rise that was followed by increases in both CO₂ and δ¹³C-CO₂ (Fig. 1). A higher-resolution record from the European Project for Ice Coring in Antarctica Dome C (EDC) ice core (10) provided additional support for the rapid δ¹³C-CO₂ decrease associated with the initial CO₂ rise, and box modeling indicated that this decrease was consistent with changes in marine productivity. The record also included other rapid changes in δ¹³C-CO₂, albeit at low precision, supporting large variations of organic carbon fluxes, notably a sharp increase in δ¹³C-CO₂ during the Bølling–Allerød (BA) interval attributed to carbon uptake by the terrestrial biosphere. A combined record including higher-precision EDC and Talos Dome data (11) documented a δ¹³C-CO₂ decrease beginning near 17.5 ka. This shift in δ¹³C-CO₂ was interpreted to indicate that some process in the Southern Ocean (SO), possibly changes in upwelling, drove the initial CO₂ rise. This previous work did not resolve high-frequency variability in the δ¹³C-CO₂ records that may be essential for discerning mechanisms of change.Open in a separate windowFig. 1.Carbon isotope records during the last deglaciation. Taylor Glacier δ¹³C-CO₂ data from this study (red). Previous work from Taylor Dome (gray open circles) (9), Grenoble EDC data (open green squares) (10), Bern EDC data (orange circles) (11, 45), sublimation measurements from EDC (blue triangles), and Talos Dome (purple squares) with an estimate of the 1-sigma uncertainty from a compilation of previous ice core δ¹³C-CO₂ data (11).Here we use an analytical method (12) that employs dual-inlet isotope ratio mass-spectrometry to obtain precision approaching that of modern atmospheric measurements [∼0.02‰ 1-sigma pooled SD based on replicate analysis compared with ∼0.05–0.11‰ for previous studies (9–11)]. We extracted atmospheric gases from large (400–500 g) samples taken from surface outcrops of ancient ice at Taylor Glacier, Antarctica, at an average temporal resolution of 165 y between 20 and 10 ka, and subcentury resolution during rapid change events. This resolution allows us to delineate isotopic fingerprints of rapid shifts in CO₂ that were previously impossible to resolve. Our study complements recent precise observations of CO₂ concentration variations during the last deglaciation, which revealed abrupt centennial-scale changes (13) (Fig. 2).Open in a separate windowFig. 2.Carbon cycle changes of the last deglaciation. WAIS Divide continuous CH₄ (green) (14) and discrete CO₂ (blue) (13) concentration data plotted with Taylor Glacier CO₂ and δ¹³C-CO₂ data (this study) (red markers, black line is a smoothing spline), the five-point running Keeling intercept with shading indicating the R² for each time interval. Blue bars indicate intervals of rapid CO₂ rise identified in the WAIS Divide ice core (13).During the initial 35-ppm CO₂ rise from 17.6 to 15.5 ka, we find a 0.3‰ decrease in δ¹³C-CO₂ that is interrupted by a sharp minimum coincident with rapid increases in CO₂ and CH₄ around 16.3 ka (13, 14) (Fig. 2). The 16.3-ka feature in the CO₂ and CH₄ concentration records, which corresponds to a 0.1‰ negative excursion in δ¹³C-CO₂, has been plausibly tied to the timing of Heinrich event 1 (13, 14) and signals a mode switch in the deglacial CO₂ rise. The subsequent slower rise in CO₂ from 15.5 to 14.8 ka is not accompanied by large changes in δ¹³C-CO₂. Across the Oldest Dryas to Bølling transition (14.6–14.3 ka) and coincident with a 10-ppm CO₂ increase and large CH₄ increase, we resolve a 0.08‰ increase in δ¹³C-CO₂ (Fig. 2). Rapid increases in CO₂ and CH₄ at the Younger Dryas (YD) to Preboreal transition (11.6–11.4 ka) are associated with minor variability δ¹³C-CO₂. On the other hand, the onset of the YD (12.8–12.5 ka) is characterized by a small rise in CO₂ associated with a 0.15‰ decrease in δ¹³C-CO₂ that appears tightly coupled to the timing of the large CH₄ decrease. The recovery from this excursion is characterized by increasing CO₂ and δ¹³C-CO₂. Broadly, our data confirm the results of Schmitt et al. (11) (Fig. 1). However, some of the large swings in δ¹³C-CO₂ indicated by the earlier EDC record (10), may be inaccurate and require reexamination.     

From the Cover: Light-driven dynamic surface wrinkles for adaptive visible camouflage

Camouflage is widespread in nature, engineering, and the military. Dynamic surface wrinkles enable a material the on-demand control of the reflected optical signal and may provide an alternative to achieve adaptive camouflage. Here, we demonstrate a feasible strategy for adaptive visible camouflage based on light-driven dynamic surface wrinkles using a bilayer system comprising an anthracene-containing copolymer (PAN) and pigment-containing poly (dimethylsiloxane) (pigment-PDMS). In this system, the photothermal effect–induced thermal expansion of pigment-PDMS could eliminate the wrinkles. The multiwavelength light–driven dynamic surface wrinkles could tune the scattering of light and the visibility of the PAN film interference color. Consequently, the color captured by the observer could switch between the exposure state that is distinguished from the background and the camouflage state that is similar to the surroundings. The bilayer wrinkling system toward adaptive visible camouflage is simple to configure, easy to operate, versatile, and exhibits in situ dynamic characteristics without any external sensors and extra stimuli.

Camouflage is the behavior of deceiving an observer or a detection device by adaptively changing the appearance to match the surroundings; this behavior has broad applications in nature, engineering, and the military. Some cephalopods (such as octopus, squid, and cuttlefish) and reptiles (such as chameleons) exhibit the exceptional ability of adaptively changing color in response to their surroundings, which is enabled by their chromatophores and iridocytes driven by muscles beneath the skin, to hide from predators (1–4). Inspired by these animals, camouflage enables a device or a robot to seamlessly blend into its environment for effective environment and species monitoring (5). Reconnaissance and anti-reconnaissance play an important role in target survivability on the battlefield. Camouflage helps the military objects to avoid detection by the enemy, thus resulting in fewer casualties (6). Contrary to monitoring theories, various camouflage technologies such as visible and infrared camouflage have been developed to hide the signal of light, electromagnetism, sound, or heat. Owing to the rapid development of modern technology, the need for simple-to-configure, user-friendly, stable, low-cost, and energy-efficient camouflage methods is increasing.To date, many adaptive camouflage materials and systems have been reported. For instance, soft machines with microfluidic networks can alter their visible or thermal appearance through injection of liquids with different colors or temperatures into the channels (7). Adaptive optoelectronic camouflage systems with multilayer and multiplexed arrays of unit cells can produce black and white patterns that match the surroundings (8). The infrared thermochromic properties of vanadium dioxide endow variable emissivity upon temperature changes owing to its unique phase transitions (9–11). In addition, there are various camouflage materials or systems based on metamaterials (12–14), transparent materials (15), hydrochromics (16), thermochromic liquid crystal (17), electrochromic skins (18, 19), electroluminescent skins (20), MXenen-based actuators (21), electro-mechano-chemically responsive elastomers (22), dynamic plasmonic tuning (23), programmable liquid crystal materials with tunable structural colors (24–26), cloaks (27–32), dynamic reflective systems (33–35), or transmission systems (36–38). However, most dynamic camouflage systems work in active form and require extra mechanical or electric stimuli and even external sensors. These requirements increase the design complexity and mass, leading to clumsiness and an awkward appearance. Moreover, high supply voltages increase the energy consumption. Therefore, the development of a camouflage strategy with chromatic diversity, in situ dynamic characteristics, and high energy efficiency is necessary (39, 40). Surface wrinkles with micro/nanostructures, widely found on the skin of creatures, have the ability to manipulate light propagation such as reflection, absorption, and scattering (34, 38, 41–45), thus tuning the optical signal arriving at the observer or a detection device. This characteristic endows such wrinkles their potential applications in visible camouflage. According to the linear buckling theory (46–48), dynamic wrinkles are obtained by regulating the modulus of the film or the strain in the bilayer system (41–44, 49–60), leading to in situ tuning of the optical signal in response to environmental stimuli such as multiwavelength light; such tuning may find application in adaptive camouflage that uses passive control without an extra sensing method.Here, we demonstrate a feasible strategy for adaptive visible camouflage using light-driven dynamic surface wrinkles (Fig. 1). Inspired by cephalopods, the soft substrate of the bilayer system is composed of poly (dimethylsiloxane) (PDMS) containing pigments or dyes, which absorb and reflect visible light of specific wavelength and displays a specific color analogous to the chromatophores. A polymer film with thin film interference serves as the rigid skin layer of the bilayer system similar to the iridocytes. Using the wrinkled structures that are formed on the surface of the system, an observer can easily see the interference color, owing to the strong light scattering; a flat surface limits the visibility of the interference color. Light-driven dynamic surface wrinkles via the photothermal effect of pigments enable the on-demand switching between two states. Moreover, this light-driven bilayer wrinkling system may enable in situ adaptive camouflage under multiwavelength light and even sunlight.Open in a separate windowFig. 1.Strategy for the production of light-driven dynamic surface wrinkles for adaptive camouflage. (A) Schematic illustration of the dynamic wrinkles in response to light and the regulation of reflected optical signal arriving at the eyes by wrinkled pattern. (B) Photographs of a carpenter worm–shaped cPhG-PDMS with (Left) or without (Right) surface wrinkles illustrating the adaptive camouflage under multiwavelength light irradiation and without light irradiation. (Inset) Image of a carpenter worm.     

From the Cover: Condensing water vapor to droplets generates hydrogen peroxide

It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A., 116, 19294–19298 (2019)] that hydrogen peroxide (H₂O₂) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H₂O₂ is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water–air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H₂O₂ production in condensate microdroplets showed that H₂O₂ was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H₂O₂ was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H₂O₂ generation. We also found that the H₂O₂ production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H₂O₂ occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H₂O₂ from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.

Water molecules in liquid water are considered stable and inert. We and other investigators have reported that water molecules become electrochemically active and catalytic for various reactions when bulk water is formed into micrometer-sized droplets (microdroplets). Reaction rates for various chemical reactions are accelerated in microdroplets by factors of 10² or more compared to bulk solution (1). The microdroplet environment provides conditions for a lowered entropic barrier, which allows thermodynamically unfavorable reactions to proceed in microdroplets at room temperature (2, 3). We also have shown that water microdroplets induce spontaneous charge exchanges between solutes and water molecules to induce the spontaneous reduction of organic molecules and metal ions as well as the formation of nanostructures without any added reducing agent or template (4, 5). Moreover, we have reported that water molecules undergo spontaneous oxidation to form reactive oxygen species, including hydroxyl radicals (OH) and hydrogen peroxide (H₂O₂) (6–8). Recent investigations attributed the origin of these unique physicochemical properties observed in microdroplets to the enrichment of reactants at the interface (9–11), restricted molecular rotations (12), partial solvation at the water surface (1, 13), and a strong interfacial electric field at the surface of the water microdroplet (14).Microdroplets can be formed either by atomizing bulk water (top down) with various methods such as high-pressure gas nebulization (15), ultrasonic nebulization (16), vibrating micromesh nebulization (17), and piezoelectric nebulization (18), or by condensing vapor-phase molecules (bottom up) (19). A question may be asked whether those unique properties of microdroplets arise only in microdroplets formed by atomization of bulk water. In addition, it may be wondered whether the spontaneous oxidation of water to form H₂O₂ in microdroplets (6) was caused by the atomizing process involving friction or vibration. These questions motivated us to investigate whether H₂O₂ becomes spontaneously generated in water microdroplets formed by the condensation of water vapor in air on cold surfaces, and how universal might this process be. We have paid special attention to the influence of different surface properties, including hydrophilicity and surface roughness, as well as environmental factors, including relative humidity and surface temperature.