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In many planetary atmospheres, including that of Earth, the base of the stratosphere—the tropopause—occurs at an atmospheric pressure of 0.1 bar. A physically based model demonstrates that the pressure-dependence of transparency to infrared radiation leads to a common tropopause pressure that is probably applicable to many planetary bodies with thick atmospheres. A minimum atmospheric temperature, or tropopause, occurs at a pressure of around 0.1 bar in the atmospheres of Earth 1 , Titan 2 , Jupiter 3 , Saturn 4 , Uranus and Neptune 4 , despite great differences in atmospheric composition, gravity, internal heat and sunlight. In all of these bodies, the tropopause separates a stratosphere with a temperature profile that is controlled by the absorption of short-wave solar radiation, from a region below characterized by convection, weather and clouds 5 , 6 . However, it is not obvious why the tropopause occurs at the specific pressure near 0.1 bar. Here we use a simple, physically based model 7 to demonstrate that, at atmospheric pressures lower than 0.1 bar, transparency to thermal radiation allows short-wave heating to dominate, creating a stratosphere. At higher pressures, atmospheres become opaque to thermal radiation, causing temperatures to increase with depth and convection to ensue. A common dependence of infrared opacity on pressure, arising from the shared physics of molecular absorption, sets the 0.1 bar tropopause. We reason that a tropopause at a pressure of approximately 0.1 bar is characteristic of many thick atmospheres, including exoplanets and exomoons in our galaxy and beyond. Judicious use of this rule could help constrain the atmospheric structure, and thus the surface environments and habitability, of exoplanets.

The Wet Chemistry Laboratory on the Phoenix Mars Lander performed aqueous chemical analyses of martian soil from the polygon-patterned northern plains of the Vastitas Borealis. The solutions contained approximately 10 mM of dissolved salts with 0.4 to 0.6% perchlorate (ClO₄) by mass leached from each sample. The remaining anions included small concentrations of chloride, bicarbonate, and possibly sulfate. Cations were dominated by Mg²⁺ and Na⁺, with small contributions from K⁺ and Ca²⁺. A moderately alkaline pH of 7.7 ± 0.5 was measured, consistent with a carbonate-buffered solution. Samples analyzed from the surface and the excavated boundary of the approximately 5-centimeter-deep ice table showed no significant difference in soluble chemistry.

Isotopic studies indicate that natural perchlorate is produced on Earth in arid environments by the oxidation of chlorine species through pathways involving ozone or its photochemical products. With this analogy, we propose that the arid environment on Mars may have given rise to perchlorate through the action of atmospheric oxidants. A variety of hypothetical pathways can be proposed including photochemical reactions, electrostatic discharge, and gas‐solid reactions. Because perchlorate‐rich deposits in the Atacama desert are closest in abundance to perchlorate measured at NASA's Phoenix Lander site, we made a preliminary study of the means to produce Atacama perchlorate to help shed light on the origin of Martian perchlorate. We investigated gas phase pathways using a 1‐D photochemical model. We found that perchlorate can be produced in sufficient quantities to explain the abundance of perchlorate in the Atacama from a proposed gas phase oxidation of chlorine volatiles to perchloric acid. The feasibility of gas phase production for the Atacama provides justification for future investigations of gas phase photochemistry as a possible source for Martian perchlorate.

Carbonates are generally products of aqueous processes and may hold important clues about the history of liquid water on the surface of Mars. Calcium carbonate (approximately 3 to 5 weight percent) has been identified in the soils around the Phoenix landing site by scanning calorimetry showing an endothermic transition beginning around 725°C accompanied by evolution of carbon dioxide and by the ability of the soil to buffer pH against acid addition. Based on empirical kinetics, the amount of calcium carbonate is most consistent with formation in the past by the interaction of atmospheric carbon dioxide with liquid water films on particle surfaces.

Phosphorus is an essential component for life, and in-situ identification of phosphate minerals that formed in aqueous conditions directly contributes toward one of the main goals of the Mars 2020 Perseverance rover: to seek signs of ancient habitable environments. In Jezero crater, proximity science analyses within a conglomerate outcrop, “ Onahu ” demonstrate the presence of rare Fe 3+ -bearing phosphate minerals (likely metavivianite, ferrolaueite, (ferro)beraunite, and/or santabarbaraite) embedded in a carbonate-rich matrix. While Fe-phosphates have been inferred previously on Mars, this work presents the most definitive in-situ identification of martian Fe-phosphate minerals to date, using textural, chemical, spectral, and diffraction analyses of discrete green-blue grains. The Fe-phosphate minerals’ textural context along with comparisons to Earth analogs suggest they likely formed after oxidation of Fe 2+ -phosphate vivianite, the most common Fe-phosphate in sedimentary environments on Earth, often associated with microbial activity and organics. While there is no obvious evidence of biological inputs in Onahu , if the Fe-phosphates’ formation environment was similar to vivianite-rich sedimentary environments on Earth, these minerals likely originally precipitated in conditions favorable to potential martian life — in a low temperature, reducing aqueous medium with high concentrations of bio-limiting elements, and Fe-redox gradients that could provide an energy source. If the sample collected from Onahu ( Otis_Peak ) is returned to Earth, analysis of the Fe-phosphates may provide new insights into ancient habitable environments on Mars. The Perseverance rover has made the most definitive identification of Fe-phosphate minerals on Mars to date. High-resolution chemical and textural PIXL analyses suggest they originally formed after vivianite in a potentially habitable environment.

Chemical analyses of three Martian soil samples were performed using the Wet Chemistry Laboratories on the 2007 Phoenix Mars Scout Lander. One soil sample was obtained from the top ∼2 cm (Rosy Red) and two were obtained at ∼5 cm depth from the ice table interface (Sorceress 1 and Sorceress 2). When mixed with water in a ∼1:25 soil to solution ratio (by volume), a portion of the soil components solvated. Ion concentrations were measured using an array of ion selective electrodes and solution conductivity using a conductivity cell. The measured concentrations represent the minimum leachable ions in the soil and do not take into account species remaining in the soil. Described is the data processing and analysis for determining concentrations of seven ionic species directly measured in the soil/solution mixture. There were no significant differences in concentrations, pH, or conductivity, between the three samples. Using laboratory experiments, refinement of the surface calibrations, and modeling, we have determined a pH for the soil solution of 7.7(±0.3), under prevalent conditions, carbonate buffering, and PCO2 in the cell headspace. Perchlorate was the dominant anion in solution with a concentration for Rosy Red of 2.7(±1) mM. Equilibrium modeling indicates that measured [Ca2+] at 0.56(±0.5) mM and [Mg2+] at 2.9(±1.5) mM, are consistent with carbonate equilibrium for a saturated solution. The [Na+] and [K+] were 1.4(±0.6), and 0.36(±0.3) mM, respectively. Results indicate that the leached portion of soils at the Phoenix landing site are slightly alkaline and dominated by carbonate and perchlorate. However, it should be noted that there is a 5–15 mM discrepancy between measured ions and conductivity and another species may be present.

We report on the atmospheric structure derived from atmospheric entry of NASA's Phoenix Mars probe using Phoenix Atmospheric Structure Experiment (ASE) data complemented by Mars Climate Sounder (MCS) temperature‐pressure profiles. Oscillations in temperature, caused by thermal tides, have vertical wavelengths of tens of kilometres. Their amplitudes are much larger in individual profiles than in dayside and nightside zonal mean MCS profiles which is inconsistent with sole control by the migrating diurnal tide. In the fixed local time reference frame of dayside MCS observations, temperature varies by >15 K with longitudinal wavenumber 3, which could arise from non‐migrating tides produced by the interaction of surface topography with the migrating diurnal tide.

The low O2content of the Archean atmosphere implies that methane should have been present at levels ∼102to 103parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH4is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH4implies that hydrogen escapes to space ( ↑ space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O2and H, and methanogenesis transfers the H into CH4. Hydrogen escape after CH4photolysis, therefore, causes a net gain of oxygen [CO2+ 2H2O→ CH4+ 2O2→ CO2+ O2+ 4H( ↑ space)]. Expected irreversible oxidation (∼1012to 1013moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized.

Tian et al. (Reports, 13 May 2005, p. 1014) proposed a hydrogen-rich early atmosphere with slow hydrogen escape from a cold thermosphere. However, their model neglects the ultraviolet absorption of all gases other than H 2 . The model also neglects Earth's magnetic field, which affects the temperature and density of ions and promotes nonthermal escape of neutral hydrogen.

The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between martian crustal fluids and the parent igneous rocks. The resultant secondary minerals -- which comprise up to 1 vol.% of the meteorites -- provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 - 150 C) brines. This is consistent with the simple mineralogy of these assemblages -- Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays -- and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 \"rosettes\". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event post-dating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last 1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO sub(2) level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50-250 mbar of CO sub(2) being trapped in the uppermost 200-1000 m of the martian crust. Large fractionations in d super(18)O between igneous silicate in the meteorites and the secondary minerals (,30ppt) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (400C) and have been taken to suggest low temperatures (e.g. ,150C) of precipitation from a hydrous fluid.