Water is a requirement for life as we know it 1 . Indirect evidence of transient liquid water has been observed from orbiter on equatorial Mars 2 , in contrast with expectations from large-scale climate models. The presence of perchlorate salts, which have been detected at Gale crater on equatorial Mars by the Curiosity rover 3,4 , lowers the freezing temperature of water 5 . Moreover, perchlorates can form stable hydrated compounds and liquid solutions by absorbing atmospheric water vapour through deliquescence 6,7 . Here we analyse relative humidity, air temperature and ground temperature data from the Curiosity rover at Gale crater and find that the observations support the formation of night-time transient liquid brines in the uppermost 5 cm of the subsurface that then evaporate after sunrise. We also find that changes in the hydration state of salts within the uppermost 15 cm of the subsurface, as measured by Curiosity, are consistent with an active exchange of water at the atmosphere-soil interface. However, the water activity and temperature are probably too low to support terrestrial organisms 8 . Perchlorates are widespread on the surface of Mars 9 and we expect that liquid brines are abundant beyond equatorial regions where atmospheric humidity is higher and temperatures are lower.At the appropriate range of relative humidity and temperature, perchlorates (ClO 4 − ) deliquesce into the aqueous phase creating brines (that is, solutions of salt in water) that are stable in the liquid state. Deliquescence 6,7 occurs when, simultaneously, the ambient relative humidity (RH) is above the deliquescent relative humidity (DRH) of the deliquescent salt and the ambient temperature (T ) is above the eutectic temperature (T e ) of the resulting solution. The stability of transient aqueous salt solutions on Mars was first postulated in the 1960s 10 , and has been inferred from indirect observations at polar and near-polar regions 7,11 . The present-day activity of equatorial recurring slope lineae has been attributed to seasonal flow of brines 2 ( Supplementary Fig. 1). Nevertheless, on the basis of the large-scale predictions of global circulation models (GCMs) and the remote-sensing large-scale observation of RH and T , it is believed that deliquescence conditions could be theoretically reached on the surface of Mars only poleward of ±60 • and only during the northern spring 11,12 (when the water vapour content of the martian atmosphere peaks 13 ). Transiently stable liquid water in the form of brines was unexpected at an equatorial region 11 such as Gale (4.6 • S, 137.4 • E, at 4.5 km below the datum), where the Mars Science Laboratory (MSL) landed and has been operating since 6 August 2012.However, here we show that the RH, air temperature (T a ) and ground temperature (T g ) observations at Gale by the Rover Environmental Monitoring Station 14 (REMS) on the Curiosity rover at the MSL mission 15 are compatible with the presence of liquid brines during night time due to the increased RH associated with night-time ...

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We develop a shape model of asteroid 16 Psyche using observations acquired in a wide range of wavelengths: Arecibo S-band delay-Doppler imaging, Atacama Large Millimeter Array (ALMA) plane-of-sky imaging, adaptive optics (AO) images from Keck and the Very Large Telescope (VLT), and a recent stellar occultation. Our shape model has dimensions 278 (−4/+8 km) × 238(−4/+6 km) × 171 km (−1/+5 km), an effective spherical diameter D eff = 222-1/+4 km, and a spin axis (ecliptic lon, lat) of (36°, −8°) ± 2°. We survey all the features previously reported to exist, tentatively identify several new features, and produce a global map of Psyche. Using 30 calibrated radar echoes, we find Psyche’s overall radar albedo to be 0.34 ± 0.08 suggesting that the upper meter of regolith has a significant metal (i.e., Fe–Ni) content. We find four regions of enhanced or complex radar albedo, one of which correlates well with a previously identified feature on Psyche, and all of which appear to correlate with patches of relatively high optical albedo. Based on these findings, we cannot rule out a model of Psyche as a remnant core, but our preferred interpretation is that Psyche is a differentiated world with a regolith composition analogous to enstatite or CH/CB chondrites and peppered with localized regions of high metal concentrations. The most credible formation mechanism for these regions is ferrovolcanism as proposed by Johnson et al. (2020).

[1] Recurring Slope Lineae (RSL) are small scale seasonal flow features identified by Mars Reconnaissance Orbiter that present several interesting characteristics such as an albedo contrast, seasonal dependence, and a strong preference for equator-facing slopes. All of these characteristics strongly suggest a thermally driven mechanism such as a liquid triggered or dominated flow. Here we investigate the possibility that these features are formed by melting of brines of various compositions via a combination of thermodynamic and kinetic numerical models. Results suggest that a solution with a freezing temperature of $223 K can best reproduce the observed seasonality. Relatively high surface evaporation rates at the RSL locations make the flows quickly disappear over a single season. Our model reproduces well the seasonality of RSL and can explain the preference for equatorfacing slopes suggesting that brine flows, and therefore liquids, are possible on a small time and space scale today on Mars. However, if the RSL are indeed formed by brines, it may indicate that a recharge mechanism is active in order to maintain a source of brine over even short geological timescales, which would have important implications for the Martian water cycle. Citation: Chevrier, V. F., and E. G. Rivera-Valentin (2012), Formation of recurring slope lineae by liquid brines on present-day Mars, Geophys.