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Chlorine is ubiquitous on Mars, some of it in the form of oxy-chlorine salts. Chlorine-containing salts have been found at several landing sites, including that of Phoenix and Curiosity, in the form of perchlorates and chlorides. Several intermediate states also exist, of which chlorate is the most stable. While perchlorates have received much attention in the past few years, chlorate salts are much less studied. The ratio of perchlorate to chlorate on Mars is not well-defined but may be approximately 1:1. Chlorate salts have similar properties to perchlorates: high solubility, low eutectic temperatures, and likely low deliquescence relative humidities. Laboratory studies were performed to determine the ability of sodium and magnesium chlorate salts to take up water vapor at low temperatures (296 K to 237 K). These studies were performed using a Raman microscope equipped with an environmental chamber and a single particle optical levitator equipped with a Raman spectrometer. The deliquescence of sodium chlorate (NaClO3) was found to be temperature-dependent with the average relative humidity (RH) values ranging from 68% RH at 296 K to 80% RH at 237 K. Additionally, there was a slight deviation between experimental deliquescence values for this salt and those predicted by equilibrium thermodynamics. The observed efflorescence (recrystallization) of NaClO3 occurred at lower RH values ranging from 18% RH at 264 K to 24% RH at 249 K, demonstrating the hysteresis common to salt recrystallization. Several experiments were performed below the reported eutectic temperature of NaClO3 which resulted in supercooling of the brine and depositional ice nucleation. Based on the supercooling effects observed during our experiments, a revised metastable eutectic temperature of 237 K is suggested for NaClO3 compared to the previously reported value of 252 K. Two phases of magnesium chlorate (Mg(ClO3)2) were observed and exhibited different water uptake behavior. The most common form of Mg(ClO3)2 appeared to be a hydrated, amorphous phase, Mg(ClO3)2 • X H2O(a) that continuously took up water when the RH was increased. This water uptake behavior was even observed at very low humidity values, 5.0 (±1.9)% RH, with little temperature dependence. This detectable water persisted down to RH values close to 0%, averaging 0.5 (±0.6)% RH with no visible temperature dependence. The deliquescence relative humidity (DRH) of the hexahydrate, Mg(ClO3)2 • 6 H2O, was found to range from 50.9 (± 7.5)% at 227 K to 55.8 (± 6.6)% at 224 K and was consistent with thermodynamic calculations. Under conditions measured by the Remote Environmental Monitoring Station (REMS) instrument at Gale Crater and conditions modeled in the shallow subsurface, magnesium chlorate, if present, likely interacts with water vapor during some diurnal cycles.

We analyze Phoenix Thermal and Electric Conductivity Probe (TECP) instrument data and previous experimental data to present constraints on the parameters for regolith-driven adsorption at the Phoenix landing site. Modeling Brunauer–Emmett–Teller (BET) adsorption of the data across Mars-relevant materials yielded fairly constant surface coverage and enthalpy values, θ = 0.336, corresponding to 2.96 × 10 −7 kg of H 2 O/m 2 and ΔH = 52.783 +/−1.206 kJ/mol, respectively. With our modeled BET adsorption coefficient, C = 89.4, and ideal specific surface area, SSA = 1.7 × 10 4 m 2 /kg, we conclude that the regolith at the Phoenix landing site is most likely a mixture, which we bracket with a range of possible adsorption conditions. Ultimately, we explain adsorbed water content in the regolith at the Phoenix landing site and thus, adsorption being driven by localized, diurnal variations in the relative humidity.

We describe and interpret the geometry and surficial characteristics of an oblong 98 km depression named Hekla Cavus in the (informally named) Cthulhu Macula, Pluto. The feature is located near the equator west of Sputnik Planitia at the New Horizons Close Encounter Hemisphere. Images from the Long Range Reconnaissance Imager along with digital elevation maps reveal that the feature is located within a larger ridge-trough system, bounded by a raised rim with surficial fissures radiating from the rim. At the wall-floor contact, a mass wasting feature is also observed. The floor of the feature is asymmetric of near-elliptical shape with its long-axis oriented southwestward. This study also comparatively measures the dimensions of impact craters and sublimation pits of Pluto to define Hekla Cavus to be a rare feature. This feature is estimated to have a volume of 5617 km3 of removed ice with possible mechanisms of formation like that of a collapse from subsurface deflation, possibly from cryovolcanic processes. Plain language summary Hekla Cavus is a large, irregularly shaped depression that is part of a larger ridge-trough system through eastern Cthulhu Macula, Pluto. Recently acquired high-resolution images and topography derived from the New Horizons mission were used to describe the geometry and geologic setting of this possible subsurface collapse pit. Estimations of the removed ice volume at Hekla Cavus also provided insight to its possible collapse history. This study also comparatively measures the dimensions of impact craters and sublimation pits of Pluto to define Hekla Cavus to be a rare feature. Thus, the origins of Hekla Cavus may have been a subsurface cryovolcanic collapsing process.

International audience; Sulfates and Fe-oxides identified on the Martian surface by orbital and in situ missions indicate that oxidizing conditions have existed on early Mars, at least locally and/or episodically. In the context of rock alteration and weathering, redox conditions are especially critical for the behavior of iron, which is soluble in its divalent state but insoluble in its trivalent state. Here, we combine results from a series of laboratory experiments conducted under Mars-like conditions to address the influence of highly-oxidizing compounds such as hydrogen peroxide (H2O2) on the alteration pathways of primary materials. We show that, if early Mars had a dense CO2 atmosphere allowing for relatively “warm and wet” conditions and surface weathering, highly-oxidizing conditions would have strongly inhibited the formation of Fe/Mg-smectite clays from alteration of igneous ferromagnesian minerals, and possibly enhanced the formation of carbonates. But a decade of mineral mapping of the Martian surface show abundant, widespread Fe/Mg-clays and rare carbonates, which we interpret here as a mineralogical record of poorly-oxidizing (or even reducing) conditions during most of the Noachian era. Oxidizing conditions would have occurred later in Martian history as a consequence of a higher rate of H2 escape or of a lower rate of volcanic outgassing, or both.

The source of the unusually high radar reflectivity signal found on the highlands of Venus is hypothesized to be caused by a mineral with a high dielectric constant. We propose that this source could be a combination of tellurium, sulfur, and bismuth. All three elements are commonly outgassed in terrestrial eruption plumes and thus are likely to be found on Venus. To test our hypothesis, we used a Venus simulation chamber and studied the stability of various tellurium, bismuth, and sulfur mixtures at Venus temperatures and pressures and in atmospheres of CO2, 100 ppm SO2 in CO2, or 100 ppm COS in CO2. When mixed together bismuth, tellurium, and sulfur phases preferentially formed tetradymite (Bi2Te2S). The remaining minerals that formed in each experiment strongly depended on the initial mixture. For instance the Bi2S3/Bi2Te3 mixture experiments resulted in the original minerals as well as BiTe at hotter temperatures, meanwhile the Bi/Te/S mixture produced Bi2S3 and occasionally Bi2Te3, Bi(S,Te), and Bi4(S,Te)3 depending on the temperature/pressure. There is evidence that COS, but not SO2, affected the stability of some minerals. Due to the presence of these elements in volcanic gases we propose that they can be present in the highlands and can, at least in part, account for the high radar reflectivity signal on the highlands.

Calcium perchlorate (Ca(ClO4)2) is a highly deliquescent salt that may exist on the surface of present-day Mars; however, its water uptake properties have not been well characterized at temperatures and relative humidity conditions relevant to Mars. Here, we quantify the deliquescent relative humidity (DRH) and efflorescent relative humidity (ERH) of Ca(ClO4)2 as a function of temperature (223–273 K) to elucidate its behavior on the surface of Mars. A Raman microscope equipped with an environmental cell was used to simulate Mars relevant temperature and relative humidity conditions and monitor deliquescence (solid to aqueous) and efflorescence (aqueous to solid) phase transitions of Ca(ClO4)2. Deliquescence and efflorescence were monitored visually using optical images and spectroscopically using Raman microscopy. We find that there is a wide range of deliquescence RH values between 5% and 55% RH. This range is due to the formation of hydrates in different temperatures regimes, with the higher DRH values occurring at the lowest temperatures. Experimental deliquescence results were compared to a thermodynamic model for three hydration states of Ca(ClO4)2. The model predicts that the higher hydration states deliquesce at a higher RH than the lower hydration states. Calcium perchlorate was found to supersaturate, with lower ERH values than DRH values. The ERH results were less dependent on temperature with an average 15 ± 4%, but values as low as 3 ± 2% were measured at 273 K. Levitation experiments were performed on single particles of Ca(ClO4)2 and Mg(ClO4)2 at 298 K. While efflorescence was observed around 15% RH for Mg(ClO4)2, the efflorescence of Ca(ClO4)2 was not observed, even when exposed to 1% RH at 298 K. Additionally, a 17-h experiment was conducted to simulate a martian subsurface diurnal cycle. This demonstrated Ca(ClO4)2 aqueous solutions can persist without efflorescing for the majority of a martian sol, up to 17 h under Mars temperature heating rates and RH conditions. We find that Ca(ClO4)2 aqueous solutions could persist for most of the martian sol under present-day conditions. The aqueous phase stability and metastability quantified for Ca(ClO4)2 under Mars relevant temperature and relative humidity conditions has important implications for the water cycle and the stability of liquid water on present day Mars.

We analyze the recalibrated in situ humidity measurements by the Phoenix lander, which landed at 68.2°N, 234.3°E and operated from Ls 78° through Ls 148°, ∼152 sols. Vapor pressures demonstrate significant day–night variations with values ranging from 0.005 Pa to 0.37 Pa, an order of magnitude lower than previously reported, and evening derived enthalpies less than that for a purely ice deposition-driven process, suggesting other water vapor sinks may contribute to the near-surface humidity at the martian polar regions.

To test the effects of meteorite impacts on martian phyllosilicate deposits, we heated two smectites (nontronite and montmorillonite) to temperatures ranging from 350 °C to 1150 °C for durations of 4–24 h in two different atmospheres, under air and a steady flow of CO 2 . Samples were analyzed using X-ray diffraction (XRD) and near-infrared (NIR) and mid-infrared (MIR) reflectance spectroscopy. Interlayer water was lost after heating to temperatures of ∼400 °C. Between 400 °C and 700 °C, nontronite converted to an intermediary phase which conserved the XRD pattern of untreated nontronite with the exception of the 0 0 1 peak. Nanocrystalline high-temperature phases formed for both smectites at temperatures between 700 °C and 1000 °C. Finally, after being heated to temperatures above ∼1100 °C, the samples melted and recrystallized into secondary phases. Secondary high-temperature phases included sillimanite and cristobalite for both smectites plus hematite for nontronite. NIR and MIR reflectance spectra significantly evolved with increasing temperature. NIR spectra of smectites showed that 1.4 and 1.9 μm bands decrease in intensity and disappear above 700 °C. Similarly, the 2.2–2.3 μm metal–OH band showed a decrease in intensity as well as an overall shift towards lower wavelengths (for nontronite) due to the thermal resistance of the Al–OH bond compared to the Fe–OH bond. NIR spectra of montmorillonite showed a gradual increase in band depth up to temperatures between 500 °C and 600 °C, then decreased with increasing temperature. In the MIR spectra of samples heated to temperatures above ∼1100 °C, newly formed bands confirmed the secondary phases identified by XRD. Similarities between the NIR spectra of our heated samples and the phyllosilicates in some martian craters imply that these phyllosilicates were altered by the impact-generated heat and thus were not formed post-impact. In addition, NIR reflectance spectra provide a proxy for shock temperatures of smectites up to 700 °C while MIR is optimum for identification of high-temperature phases of smectites above 700 °C.

We have studied the sublimation of ice and water vapor transport through various thicknesses of clay (

We have studied the effect of wind velocity on the sublimation rate of pure water ice under martian conditions. Measurements were made for wind velocities ranging from 0.7 to 11.4 m s −1 , a typical range observed by the meteorological instruments on the surface of Mars, and at − 15 ° C a value typical of the daily high temperature for most of the year at the Pathfinder landing site. At this temperature, and for a low-humidity environment (relative humidity around 1%) sublimation rates increase following a linear trend of equation E S = 0.68 + 0.025 V ( E S is the sublimation rate in mm h −1 and V is the wind speed in m s −1 ). In high relative humidity (30–35%) atmospheres, the effect of wind velocity is negligible, and the sublimation rate remains nearly constant at 0.33 ± 0.04 mm h −1 . Pure forced convection theory did not provide a satisfying description of the data in terms of the range of values and their wind speed dependency. Therefore, a new semi-empirical expression for the sublimation rate that combines free and forced convection was developed using analogy with heat transfer models. Using this expression, sublimation rates of ice as a function of wind velocity for any temperature can be calculated. In general, temperature is more important that wind speed and atmospheric humidity in determining the rate of sublimation of ice on Mars.

In order to advance our understanding of the long-term stability of subsurface ice, the diurnal martian water cycle, and implications for liquid water, we determined diffusion coefficients and adsorption kinetics for the water vapor produced by the sublimation of ice buried beneath various layers of fine-grained ( 1.56 ± 0.53 × 10 −4 , 2.05 ± 0.82 × 10 −4 , and 3.42 ± 1.36 × 10 −4 m 2 s −1 for the 52.6 ± 8.3 at 270 K for 39.0 ± 6.4 at 267 K for 63–125 μm, and 54.3 ± 9.3 at 266 K for 125–250 μm, resulting in surface areas of 2.6 ± 0.1 × 10 4 , 1.7 ± 0.3 × 10 4 , 1.5 ± 0.3 × 10 4 m 2 kg −1 , respectively. These results suggest that while diffusion is too rapid to explain the purported diurnal cycle in water content of the atmosphere, adsorption is efficient and rapid, and does provide an effective mechanism to explain such a cycle. The present diffusion data suggest that very thin, 10 h at ∼224 K, just above the freezing point of saturated CaCl2. Temperatures can remain above ∼224 K over most of the planet, which means that water, even as saturated brine, will sublimate before the freezing point is reached and liquid could be formed. On the other hand, 1 m ice layers below 1 m of fine-grained basaltic regolith at 235 K and 10 Pa of atmospheric water could last 600 to 1300 years. At deeper depths and lower temperatures, ice could last since the last major obliquity change 400,000 years ago.