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 ...

Two hundred fifty‐two transient drops in atmospheric pressure, likely caused by passing convective vortices, were detected by the Rover Environmental Monitoring Station instrument during the first Martian year of the Mars Science Laboratory (MSL) landed mission. These events resembled the vortex signatures detected by the previous Mars landers Pathfinder and Phoenix; however, the MSL observations contained fewer pressure drops greater than 1.5 Pa and none greater than 3.0 Pa. Apparently, these vortices were generally not lifting dust as only one probable dust devil has been observed visually by MSL. The obvious explanation for this is the smaller number of strong vortices with large central pressure drops since according to Arvidson et al. [] ample dust seems to be present on the surface. The annual variation in the number of detected convective vortices followed approximately the variation in Dust Devil Activity (DDA) predicted by the MarsWRF numerical climate model. This result does not prove, however, that the amount of dust lifted by dust devils would depend linearly on DDA, as is assumed in several numerical models of the Martian atmosphere, since dust devils are only the most intense fraction of all convective vortices on Mars, and the amount of dust that can be lifted by a dust devil depends on its central pressure drop. Sol‐to‐sol variations in the number of vortices were usually small. However, on 1 Martian solar day a sudden increase in vortex activity, related to a dust storm front, was detected.

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