Relative Humidity on Mars: New Results From the Phoenix TECP Sensor

Present-day thermal and water activity environment of the Mars Sample Return collection

The Mars Sample Return mission intends to retrieve a sealed collection of rocks, regolith, and atmosphere sampled from Jezero Crater, Mars, by the NASA Perseverance rover mission. For all life-related research, it is necessary to evaluate water availability in the samples and on Mars. Within the first Martian year, Perseverance has acquired an estimated total mass of 355 g of rocks and regolith, and 38 μmoles of Martian atmospheric gas. Using in-situ observations acquired by the Perseverance rover, we show that the present-day environmental conditions at Jezero allow for the hydration of sulfates, chlorides, and perchlorates and the occasional formation of frost as well as a diurnal atmospheric-surface water exchange of 0.5-10 g water per m2 (assuming a well-mixed atmosphere). At night, when the temperature drops below 190 K, the surface water activity can exceed 0.5, the lowest limit for cell reproduction. During the day, when the temperature is above the cell replication limit of 245 K, water activity is less than 0.02. The environmental conditions at the surface of Jezero Crater, where these samples were acquired, are incompatible with the cell replication limits currently known on Earth.

How does humidity data impact land surface modeling of hydrothermal regimes at a permafrost site in Utqiaġvik, Alaska?

Humidity is a basic and crucial meteorological indicator commonly measured in several forms, including specific humidity, relative humidity, and absolute humidity. These different forms can be inter-derived based on the saturation vapor pressure (SVP). In past decades, dozens of formulae have been developed to calculate the SVP with respect to, and in equilibrium with, liquid water and solid ice surfaces, but many prior studies use a single function for all temperature ranges, without considering the distinction between over the liquid water and ice surfaces. These different approaches can result in humidity estimates that may impact our understanding of surface-subsurface thermal-hydrological dynamics in cold regions. In this study, we compared the relative humidity (RH) downloaded and calculated from four data sources in Alaska based on five commonly used SVP formulas. These RHs, along with other meteorological indicators, were then used to drive physics-rich land surface models at a permafrost-affected site. We found that higher values of RH (up to 40 %) were obtained if the SVP was calculated with the over-ice formulation when air temperatures were below freezing, which could lead to a 30 % maximum difference in snow depths. The choice of whether to separately calculate the SVP over an ice surface in winter also produced a significant range (up to 0.2 m) in simulated annual maximum thaw depths. The sensitivity of seasonal thaw depth to the formulation of SVP increases with the rainfall rate and the height of above-ground ponded water, while it diminishes with warmer air temperatures. These results show that RH variations based on the calculation of SVP with or without over-ice calculation meaningfully impact physically-based predictions of snow depth, sublimation, soil temperature, and active layer thickness. Under particular conditions, when severe flooding (inundation) and cool air temperatures are present, care should be taken to evaluate how humidity data is estimated for land surface and earth system modeling.

Water Vapor Adsorption Provides Daily, Sustainable Water to Soils of the Hyperarid Atacama Desert

Water is necessary for all life on Earth. Water is so critical that organisms have developed strategies to survive in hyperarid environments. These regions with extremely low water availability are also unique analogs in which to study the physico-chemical conditions of extraterrestrial environments such as Mars. We have identified a daily, sustainable cycle of water vapor adsorption (WVA) and desorption that measurably affects soil water content (SWC) in the hyperarid region of the Atacama Desert in southern Perú. We pair field-based soil temperature and relative humidity soil profiles with laboratory simulations to provide evidence for a daily WVA cycle. Using our WVA model, we estimate that one adsorptive period-one night-increases SWC by 0.2-0.3 mg/g of soil (∼30 μm rainfall). We can plausibly rule out other water inputs during our field campaign that could account for this water input, and we provide evidence that this WVA cycle is driven by solar heating and maintained by atmospheric water vapor. The WVA may also serve to retain water from infrequent rain events in these soils. If the water provided by WVA in these soils is bioavailable, it could have significant implications for the microorganisms that are endemic to hyperarid environments.

Surface solvation of Martian salt analogues at low relative humidities

Salt aerosols play important roles in many processes related to atmospheric chemistry and the climate systems on both Earth and Mars. Complicated and still poorly understood processes occur on the salt surfaces when interacting with water vapor. In this study, ambient pressure X-ray photoelectron spectroscopy (APXPS) is used to characterize the surface chemical environment of Martian salt analogues originating from saline lakes and playas, as well as their responses to varying relative humidities. Generally, APXPS shows similar ionic compositions to those observed by ion chromatography (IC). However, XPS is a surface-sensitive method while IC is bulk-sensitive and differences are observed for species that preferentially partition to the surface or the bulk. Element-selective surface enhancement of Cl⁻ is observed, likely caused by the presence of SO₄²⁻. In addition, Mg²⁺ is concentrated on the surface while Na⁺ is relatively depleted in the surface layer. Hence, the cations (Na⁺ and Mg²⁺) and the anions (Cl⁻ and SO₄²⁻) show competitive correlations. At elevated relative humidity (RH), no major spectral changes were observed in the XPS results, except for the growth of an oxygen component originating from condensed H₂O. Near-edge X-ray absorption fine structure (NEXAFS) measurements show that the magnesium and sodium spectra are sensitive to the presence of water, and the results imply that the surface is fully solvated already at RH = 5%. The surface solvation is also fully reversible as the RH is reduced. No major differences are observed between sample types and sample locations, indicating that the salts originated from saline lakes commonly have solvated surfaces under the environmental conditions on Earth.

Salt aerosols play important roles in many processes related to atmospheric chemistry and the climate systems on both Earth and Mars.

Complex Brines and Their Implications for Habitability

There is evidence that life on Earth originated in cold saline waters around scorching hydrothermal vents, and that similar conditions might exist or have existed on Mars, Europa, Ganymede, Enceladus, and other worlds. Could potentially habitable complex brines with extremely low freezing temperatures exist in the shallow subsurface of these frigid worlds? Earth, Mars, and carbonaceous chondrites have similar bulk elemental abundances, but while the Earth is depleted in the most volatile elements, the Icy Worlds of the outer solar system are expected to be rich in them. The cooling of ionic solutions containing substances that likely exist in the Icy Worlds could form complex brines with the lowest eutectic temperature possible for the compounds available in them. Indeed, here, we show observational and theoretical evidence that even elements present in trace amounts in nature are concentrated by freeze–thaw cycles, and therefore contribute significantly to the formation of brine reservoirs that remain liquid throughout the year in some of the coldest places on Earth. This is interesting because the eutectic temperature of water–ammonia solutions can be as low as ~160 K, and significant fractions of the mass of the Icy Worlds are estimated to be water substance and ammonia. Thus, briny solutions with eutectic temperature of at least ~160 K could have formed where, historically, temperature have oscillated above and below ~160 K. We conclude that complex brines must exist in the shallow subsurface of Mars and the Icy Worlds, and that liquid saline water should be present where ice has existed, the temperature is above ~160 K, and evaporation and sublimation have been inhibited.

The Mars Environmental Dynamics Analyzer, MEDA. A Suite of Environmental Sensors for the Mars 2020 Mission

NASA's Mars 2020 (M2020) rover mission includes a suite of sensors to monitor current environmental conditions near the surface of Mars and to constrain bulk aerosol properties from changes in atmospheric radiation at the surface. The Mars Environmental Dynamics Analyzer (MEDA) consists of a set of meteorological sensors including wind sensor, a barometer, a relative humidity sensor, a set of 5 thermocouples to measure atmospheric temperature at ∼1.5 m and ∼0.5 m above the surface, a set of thermopiles to characterize the thermal IR brightness temperatures of the surface and the lower atmosphere. MEDA adds a radiation and dust sensor to monitor the optical atmospheric properties that can be used to infer bulk aerosol physical properties such as particle size distribution, non-sphericity, and concentration. The MEDA package and its scientific purpose are described in this document as well as how it responded to the calibration tests and how it helps prepare for the human exploration of Mars. A comparison is also presented to previous environmental monitoring payloads landed on Mars on the Viking, Pathfinder, Phoenix, MSL, and InSight spacecraft.

Meteorological Predictions for Mars 2020 Perseverance Rover Landing Site at Jezero Crater

The Mars Regional Atmospheric Modeling System (MRAMS) and a nested simulation of the Mars Weather Research and Forecasting model (MarsWRF) are used to predict the local meteorological conditions at the Mars 2020 Perseverance rover landing site inside Jezero crater (Mars). These predictions are complemented with the COmplutense and MIchigan MArs Radiative Transfer model (COMIMART) and with the local Single Column Model (SCM) to further refine predictions of radiative forcing and the water cycle respectively. The primary objective is to facilitate interpretation of the meteorological measurements to be obtained by the Mars Environmental Dynamics Analyzer (MEDA) aboard the rover, but also to provide predictions of the meteorological phenomena and seasonal changes that might impact operations, from both a risk perspective and from the perspective of being better prepared to make certain measurements. A full diurnal cycle at four different seasons (L_s 0°, 90°, 180°, and 270°) is investigated. Air and ground temperatures, pressure, wind speed and direction, surface radiative fluxes and moisture data are modeled. The good agreement between observations and modeling in prior works [Pla-Garcia et al. in Icarus 280:103-113, 2016; Newman et al. in Icarus 291:203-231, 2017; Vicente-Retortillo et al. in Sci. Rep. 8(1):1-8, 2018; Savijarvi et al. in Icarus, 2020] provides confidence in utilizing these models results to predict the meteorological environment at Mars 2020 Perseverance rover landing site inside Jezero crater. The data returned by MEDA will determine the extent to which this confidence was justified.

Global Temporal and Geographic Stability of Brines on Present-day Mars

We combine experimentally verified constraints on brine thermodynamics along with a global circulation model to develop a new extensive framework of brine stability on the surface and subsurface of Mars. Our work considers all major phase changes (i.e., evaporation, freezing, and boiling) and is consistent, regardless of brine composition, so it is applicable to any brine relevant to Mars. We find that equatorial regions typically have temperatures too high for stable brines, while high latitudes are susceptible to permanent freezing. In the subsurface, this trend is reversed, and equatorial regions are more favorable to brine stability, but only for the lowest water activities (and lowest eutectic temperatures). At locations where brines may be stable, we find that their lifetimes can be characterized by two regimes. Above a water activity of ~0.6, brine duration is dominated by evaporation, lasting at most a few minutes per sol. Below a water activity of 0.6, brine duration is bound by freezing or boiling; such brines are potentially stable for up to several consecutive hours per sol. Our work suggests that brines should not be expected near or on the Martian surface, except for low eutectic water activity salts such as calcium or magnesium perchlorate or chlorate, and their (meta)stability on the surface would require contact with atmospheric water vapor or local ice deposits.

Mimicking the Martian Hydrological Cycle: A Set-Up to Introduce Liquid Water in Vacuum

Liquid water is well known as the life ingredient as a solvent. However, so far, it has only been found in liquid state on this planetary surface. The aim of this experiment and technological development was to test if a moss sample is capable of surviving in Martian conditions. We built a system that simulates the environmental conditions of the red planet including its hydrological cycle. This laboratory facility enables us to control the water cycle in its three phases through temperature, relative humidity, hydration, and pressure with a system that injects water droplets into a vacuum chamber. We successfully simulated the daytime and nighttime of Mars by recreating water condensation and created a layer of superficial ice that protects the sample against external radiation and minimizes the loss of humidity due to evaporation to maintain a moss sample in survival conditions in this extreme environment. We performed the simulations with the design and development of different tools that recreate Martian weather in the MARTE simulation chamber.

DISTRIBUTION AND HABITABILITY OF (META)STABLE BRINES ON PRESENT-DAY MARS

Special Regions on Mars are defined as environments able to host liquid water that meets certain temperature and water activity requirements that allow known terrestrial organisms to replicate^1,2, and therefore could be habitable. Such regions would be a concern for planetary protection policies owing to the potential for forward contamination (biological contamination from Earth). Pure liquid water is unstable on the Martian surface^3,4, but brines may be present^3,5. Experimental work has shown that brines persist beyond their predicted stability region, leading to metastable liquids^8-10. Here we show that (meta)stable brines can form and persist from the equator to high latitudes on the surface of Mars for a few percent of the year for up to six consecutive hours, a broader range than previously thought^11,12. However, only the lowest eutectic solutions can form, leading to brines with temperatures of less than 225 K. Our results indicate that (meta)stable brines on the Martian surface and shallow subsurface (a few centimeters deep) are not habitable because their water activities and temperatures fall outside the known tolerances for terrestrial life. Furthermore, (meta)stable brines do not meet the Special Regions requirements, reducing the risk for forward contamination and easing threats related to the exploration of the Martian surface.