Energetic cost of nonequilibrium

Minimum energetic cost to maintain a target nonequilibrium state

ABSTRACT: In the absence of external driving, a system exposed to thermal fluctuations will relax to equilibrium. However, the constant input of work makes it possible to counteract this relaxation, and maintain the system in a nonequilibrium steady state. In this Article, we use the stochastic thermodynamics of Markov jump processes to compute the minimum rate at which energy must be supplied and dissipated to maintain an arbitrary nonequilibrium distribution in a given energy landscape. This lower bound depends on two factors: the undriven probability current in the equilibrium state, and the distance from thermal equilibrium of the target distribution. By showing the consequences of this result in a few simple examples, we suggest general implications for the required energetic costs of macromolecular repair and cytosolic protein localization.

 

Power of deeper networks

The power of deeper networks for expressing natural functions

ABSTRACT: It is well-known that neural networks are universal approximators, but that deeper networks tend to be much more efficient than shallow ones. We shed light on this by proving that the total number of neurons m required to approximate natural classes of multivariate polynomials of n variables grows only linearly with n for deep neural networks, but grows exponentially when merely a single hidden layer is allowed. We also provide evidence that when the number of hidden layers is increased from 1 to k, the neuron requirement grows exponentially not with n but with n1/k, suggesting that the minimum number of layers required for computational tractability grows only logarithmically with n.

From Geo- to Biogeo-chemistry

The Transition from Geochemistry to Biogeochemistry

Abstract

Paradigm-changing discoveries about stellar and planetary evolution, the survival of organic molecules and microorganisms under extreme conditions, and geochemical environments on early Earth and other planets are sparking a synergistic dialogue between geoscientists, chemists, and biologists to understand how life originated. To achieve this goal, we must (i) explain the non enzymatic synthesis of biologically relevant organic molecules under geologically plausible conditions; (ii) overcome the rigid conceptual dichotomy of the “RNA world” versus the “metabolism-first” hypotheses; and (iii) develop high-throughput analytical systems to sample the myriad possible combinations of environmental conditions to find those that could initiate life. This issue of Elements highlight the roles of minerals and geochemical environments in the emergence of protocells, the cell-like entities that might have preceded the Last Universal Common Ancestor.

 

A prototype Sprite nanosatellite

Reaching for the Stars, Breakthrough Sends Smallest-Ever Satellites into Orbit

A prototype Sprite nanosatellite packs power sources, microprocessors, sensors and transmitters into a single tiny circuit board. Credit: Zac Manchester.

World’s smallest satellites launched in quest to reach nearby stars

Breakthrough Starshot, a $100 million program to launch robotic interstellar missions, has sent into orbit the smallest-ever satellitesScientific American reports. The six prototypes, dubbed Sprites, weigh only 4 grams and contain solar panels, computers, sensors, and radios on a surface equal to that of a U.S. postage stamp. Developed by researchers at Cornell University and transported into space as secondary payloads on a rocket built by the Europe-based company OHB System AG, the nanosatellites are being tested for electronics and communication performance in orbit. The Breakthrough Starshot mission aims to show that Sprites could one day become light-propelled spacecraft that would fly at 20% of light speed and reach Alpha Centauri, our nearest star system, in just over 20 years.

 

 

Water in lunar deposits

Remote detection of widespread indigenous water in lunar pyroclastic deposits

Abstract

Laboratory analyses of lunar samples provide a direct means to identify indigenous volatiles and have been used to argue for the presence of Earth-like water content in the lunar interior. Some volatile elements, however, have been interpreted as evidence for a bulk lunar mantle that is dry. Here we demonstrate that, for a number of lunar pyroclastic deposits, near-infrared reflectance spectra acquired by the Moon Mineralogy Mapper instrument onboard the Chandrayaan-1 orbiter exhibit absorptions consistent with enhanced OH- and/or H2O-bearing materials. These enhancements suggest a widespread occurrence of water in pyroclastic materials sourced from the deep lunar interior, and thus an indigenous origin. Water abundances of up to 150 ppm are estimated for large pyroclastic deposits, with localized values of about 300 to 400 ppm at potential vent areas. Enhanced water content associated with lunar pyroclastic deposits and the large areal extent, widespread distribution and variable chemistry of these deposits on the lunar surface are consistent with significant water in the bulk lunar mantle. We therefore suggest that water-bearing volcanic glasses from Apollo landing sites are not anomalous, and volatile loss during pyroclastic eruptions may represent a significant pathway for the transport of water to the lunar surface.

 

Reduced Diversity of Life

Reduced Diversity of Life Around Proxima Centauri and TRAPPIST-1

ABSTRACT: The recent discovery of habitable exoplanets around Proxima Centauri and TRAPPIST-1 has attracted much attention due to their potential for hosting life. We delineate a simple model that accurately describes the evolution of biological diversity on Earth. Combining this model with constraints on atmospheric erosion and the maximal evolutionary timescale arising from the star’s lifetime, we arrive at two striking conclusions: (i) Earth-analogs orbiting low-mass M-dwarfs are unlikely to be inhabited, and (ii) K-dwarfs and some G-type stars are potentially capable of hosting more complex biospheres than the Earth. Hence, future searches for biosignatures should prioritize planets around K-dwarf stars.

 

Elements of Eoarchean life

Elements of Eoarchean life trapped in mineral inclusions

Metasedimentary rocks from Isua, West Greenland (over 3,700 million years old) contain 13C-depleted carbonaceous compounds, with isotopic ratios that are compatible with a biogenic origin. Metamorphic garnet crystals in these rocks contain trails of carbonaceous inclusions that are contiguous with carbon-rich sedimentary beds in the host rock, where carbon is fully graphitized. Previous studies have not been able to document other elements of life (mainly hydrogen, oxygen, nitrogen and phosphorus) structurally bound to this carbonaceous material. Here we study carbonaceous inclusions armoured within garnet porphyroblasts, by in situ infrared absorption on approximately 10−21 m3 domains within these inclusions. We show that the absorption spectra are consistent with carbon bonded to nitrogen and oxygen, and probably also to phosphate. The levels of C–H or O–H bonds were found to be low. These results are consistent with biogenic organic material isolated for billions of years and thermally matured at temperatures of around 500 °C. They therefore provide spatial characterization for potentially the oldest biogenic carbon relics in Earth’s geological record. The preservation of Eoarchean organic residues within sedimentary material corroborates earlier claims for the biogenic origins of carbon in Isua metasediments.

“Biological entities in the Eoarchean would most likely be formed from the same elements as extant life, which are mainly H, C, O, N and P. The ratio between the elements varies but usually follows the order given above with hydrogen beeing the most abundant. For example, in guanine (DNA nucleobase) the ratio is 38% H, 27% C, 19% O, 14% N and 3%P. We observe 4 out of 5 of these elements in our inclusions. Hydrogen was below the detection limit of the instrument, but this is in accordance with the sample history.”

Danskere finder nye beviser for tidligt liv på Jorden

Danske forskere har med nye metoder fundet spor efter 3,7 milliarder år gammelt liv, som sidder indkapslet i ædelsten fra Grønland. Det er et af de ældste tegn på liv i Jordens historie.

 

Life as We Don’t Know It

Life as We Don’t Know It

Günter Wächtershäuser

Theories of the origin of life on Earth fall into two general categories. Prebiotic broth theories postulate a protracted origin by the self-assembly of high-molecular weight structures, such as RNA, proteins, and vesicles, in a cold prebiotic broth of preaccumulated modules. More recently, theories based on a hydrothermal origin have gained ground. For example, the theory of a pressurized iron-sulfur world suggests a fast origin by an autotrophic metabolism of low-molecular weight constituents, in an environment of iron sulfide and hot magmatic exhalations. Cody et al.‘s results on page 1337 of this issue provide key support for the latter theory and greatly strengthen the hope that it may one day be possible to understand and reconstruct the beginnings of life on Earth.

Pyruvic acid, CH3-CO-COOH, is one of the most crucial constituents of extant intermediary metabolism. It occurs in numerous metabolic pathways, notably the reductive citric acid cycle and the pathways that produce amino acids and sugars. It has been suggested that pyruvic acid or its anion pyruvate formed primordially by double carbonylation. Cody et al. provide experimental support for this suggestion. They show that pyruvic acid forms from formic acid in the presence of nonylmercaptane and iron sulfide at 250°C and 200 MPa. Water is initially absent and forms only by the dehydration of the formic acid. This result poses fascinating thermodynamic and kinetic questions. Pyruvic acid is an extremely heat-sensitive compound that decomposes at its boiling point of 165°C. It appears paradoxical that at the very high temperature required for dehydration of formic acid, the relatively unstable pyruvic acid can form and exist at detectable concentrations. Moreover, it is astonishing that acetic acid is formed at a lower yield than pyruvic acid. The explanation may well lie in the very high pressure.

The work is particularly exciting because experience with organic synthesis in the high-pressure/high-temperature regime is very limited. The experiments require a combination of 200 MPa (corresponding to a rock depth of about 7 km or a 20-km water column) and 250°C, in addition to high CO pressure in the absence of water. It remains to be established whether such conditions are geophysically possible.

The new finding, if it holds, fills a critical gap in the experimental picture of the iron-sulfur world (see the figure below). All individual reaction steps for a conversion of carbon monoxide 1 to peptides 8 have now been demonstrated: formation of methyl thioacetate 4, of pyruvate 6, of alanine 9 by reductive amination of pyruvate 6, and of peptides 8 by activation of amino acids with CO/H2S. The challenge will now be to overcome the discrepancies in the reaction conditions and to establish the right conditions for autocatalysis (reproduction) and evolution. This may involve a primitive version of the citrate cycle in which (methyl) thioacetate and pyruvate participate and/or ligand (notably peptide) feed back to the catalytic metal center.

The reaction scheme in the figure is in substantial agreement with extant metabolism in terms of overall metabolic patterns, reaction pathways, and catalysts. The newly demonstrated formation of pyruvic acid by double carbonylation, however, has no analog in extant metabolism. It may have disappeared because of metabolic takeover, first by a reverse pyruvate-formate-lyase reaction and later, after the advent of thiamine pyrophosphate, by carboxylation with pyruvate oxidoreductase.

Cody et al.‘s results support the view that the primordial organisms were autotrophs feeding on carbon monoxide. But more importantly, the reactions shown in the figure can still occur today because the required conditions are in general still available on Earth, albeit at a lesser frequency. They may thus be a source for geoorganics today; these geoorganics may serve as food for extant heterotrophs, and primitive microbes feeding on CO might still be tracked down in hot pressurized spaces previously inaccessible to exploration.

The reaction conditions chosen by Cody et al. are a compromise between the requirements of geochemical modeling and the requirements of the experimental technique. CO gas cannot be used at these very high pressures without extreme danger. Decomposition of formic acid was therefore used as a source for CO. This requires a temperature of 250°C and the absence of water. But in the real world, the temperature may well have been lower, as may have been the pressure. On early Earth, outgassing (the release of gases by volcanic activity) must have been massive and omnipresent, with a wide spectrum of physical conditions, which only later became restricted to vents and volcanoes because of a thickening crust.

It is occasionally suggested that experiments within the iron-sulfur world theory demonstrate merely yet another source of organics for the prebiotic broth. This is a misconception. The new finding drives this point home. Pyruvate is too unstable to ever be considered as a slowly accumulating component in a prebiotic broth. The prebiotic broth theory and the iron-sulfur world theory are incompatible. The prebiotic broth experiments are parallel experiments that are producing a greater and greater medley of potential broth ingredients. Therefore, the maxim of the prebiotic broth theory is “order out of chaos.” In contrast, the iron-sulfur world experiments are serial, aimed at long reaction cascades and catalytic feedback (metabolism) from the start. The maxim of the iron-sulfur world theory should therefore be “order out of order out of order.”

 

3D Global Climate Model: TRAPPIST-1

Climate diversity on cool planets around cool stars with a versatile 3-D Global Climate Model: the case of TRAPPIST-1 planets

ABSTRACT: TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our Solar System and possibly habitability. First, we derive from N-body simulations possible planetary evolution scenarios, and show that each of the planets are likely to be in synchronous rotation. We then use a 3-D Global Climate Model to explore the possible climates of cool planets of the TRAPPIST-1 system. In particular, we look at the conditions required for cool planets to prevent possible volatile species to be lost by permanent condensation, irreversible burying or photochemical destruction. We also explore the resilience of the same volatiles (when in condensed phase) to a runaway greenhouse process. We find that background atmospheres made of N2, CO or O2 are resistant to atmospheric collapse. However, it should be difficult for TRAPPIST-1 planets to accumulate significant greenhouse gases like CO2, CH4, or NH3. CO2 can easily condense on the nightside, forming glaciers that would flow toward the substellar region. A complete CO2 ice cover is possible on TRAPPIST-1g and h only, although CO2 ice deposits could be gravitationally unstable and get buried beneath the water ice shell in geologically short timescales. Given TRAPPIST-1 planets large EUV irradiation (at least 1000x Titan’s flux), CH4 and NH3 should be photodissociated rapidly and thus be hard to accumulate in the atmosphere. Photochemical hazes could then sedimentate and form a surface layer of tholins. Regarding habitability, we confirm that few bars of CO2 would suffice to warm the surface of TRAPPIST-1f and g above the melting point of water. We also show that TRAPPIST-1e is a remarkable candidate for surface habitability. If the planet is today synchronous and abundant in water, then it should always sustain surface liquid water at least in the substellar region, whatever the atmosphere considered.

 

Negative CO2 emissions

Young people’s burden: requirement of negative CO2 emissions

Abstract: Global temperature is a fundamental climate metric highly correlated with sea level, which implies that keeping shorelines near their present location requires keeping global temperature within or close to its preindustrial Holocene range. However, global temperature excluding short-term variability now exceeds +1 °C relative to the 1880–1920 mean and annual 2016 global temperature was almost +1.3 °C. We show that global temperature has risen well out of the Holocene range and Earth is now as warm as it was during the prior (Eemian) interglacial period, when sea level reached 6–9 m higher than today. Further, Earth is out of energy balance with present atmospheric composition, implying that more warming is in the pipeline, and we show that the growth rate of greenhouse gas climate forcing has accelerated markedly in the past decade. The rapidity of ice sheet and sea level response to global temperature is difficult to predict, but is dependent on the magnitude of warming. Targets for limiting global warming thus, at minimum, should aim to avoid leaving global temperature at Eemian or higher levels for centuries. Such targets now require negative emissions, i.e., extraction of CO2 from the air. If phasedown of fossil fuel emissions begins soon, improved agricultural and forestry practices, including reforestation and steps to improve soil fertility and increase its carbon content, may provide much of the necessary CO2 extraction. In that case, the magnitude and duration of global temperature excursion above the natural range of the current interglacial (Holocene) could be limited and irreversible climate impacts could be minimized. In contrast, continued high fossil fuel emissions today place a burden on young people to undertake massive technological CO2 extraction if they are to limit climate change and its consequences. Proposed methods of extraction such as bioenergy with carbon capture and storage (BECCS) or air capture of CO2 have minimal estimated costs of USD 89–535 trillion this century and also have large risks and uncertain feasibility. Continued high fossil fuel emissions unarguably sentences young people to either a massive, implausible cleanup or growing deleterious climate impacts or both.

Citation: Hansen, J., Sato, M., Kharecha, P., von Schuckmann, K., Beerling, D. J., Cao, J., Marcott, S., Masson-Delmotte, V., Prather, M. J., Rohling, E. J., Shakun, J., Smith, P., Lacis, A., Russell, G., and Ruedy, R.: Young people’s burden: requirement of negative CO2 emissions, Earth Syst. Dynam., 8, 577-616, https://doi.org/10.5194/esd-8-577-2017, 2017.