Spectral Evidence for Hydrated Salts in Recurring Slope Lineae on Mars Lujendra Ojha et al. Presented by John Hossain 1

Recurring Slope Lineae: Narrow, low reflectance features possibly due to the transient flow of liquid water. 2 Nature NASA

Hydrated Salts: Crystalline salt molecules loosely attached to water molecules. 3 The International Scientific Association of Grocers

Salts such as sulphates, chlorides, and perchlorates have been found on Mars: Can lower the freezing point of water by up to 80 K Lowers the evaporation rate of water Can absorb atmospheric moisture The Implication is that the presence of salts indicates liquid water. But is the water flowing on the surface? 4

Recurring slope lineae (RSLs) extend downslope on warm slopes (>250 – 273 K), fade when inactive, and reappear annually. Disappear and reappear, possibly because of flowing water??? Suggests the presence of salts lowering the freezing point of water. 5 Mars Reconnaissance Orbiter (MRO) houses the: High Resolution Imaging Science Experiment (HiRISE) camera for pictures of RSLs Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) for mineralogic compositions.

Both liquid water and hydrated salts have absorption bands at ~1.4, ~1.9, and ~3.0 micrometers. The idea is that using CRISM to match the presence of hydrated salts and/or liquid water to the occurrence of RSLs seen through HiRISE will support the hypothesis that RSLs on Mars form due to transient surface water activity. 6

Palikir Crater: RSLs are longest and widest towards the end of the southern hemisphere summer 7 a)RSLs in the crater b)Spectroscopic observations in the crater c) Spectra from colored regions d) Laboratory spectra of salts and water

Palikir Crater Absorptions observed by CRISM are too narrow for liquid water, but are consistent with hydrated salts. Implies rapid change of hydrologic activity, but less rapid change in hydration state of minerals. Or deposition of hydrated minerals by hydrologic activity. Spectra match mix of Martian soil with magnesium perchlorate, chlorate, and chloride. Similar observations done in mid-summer, when RSLs are shorter and narrower, found no absorption at ~1.4 and ~1.9 micrometers. 8

Horowitz Crater 9 a) RSLs at central peak of craterb) RSLs at a different peak c) CRISM spectra

Hale Crater: spectra consistent with a linear spectral mixture of magnesium perchlorate and Martian soil 10 a) HiRISE showing RSLs on central peakb) Spectra

Coprates Chasma Many RSLs, dark fans with high absorption. Only ~1.9 micrometer absorption found; not possible to assign a particular salt mineralogy. Change in albedo may be due to water modifying grain sizes or precipitating salts. 11 a)RSLs and dark fans b)Concurrent CRISM observations c)Spectra

In Situ Measurements Perchlorates on the surface of Mars confirmed at Gale Crater by the Mars Science Laboratory The northern plains by the Phoenix Mission Possibly the Viking landing sites Perchlorates and chlorates found in Martian meteorites. Consistent with remote sensing detections 12

Origin of water forming RSLs? Possibly surface melting of ice Unlikely in the middle-latitudes Through the absorption of moisture from the atmosphere Unclear if Martian atmosphere can provide enough moisture Seasonal discharge from a local aquifer Likely that different RSLs form in different parts of Mars through different mechanisms. 13

Conclusion There is a genetic connection between hydrated salts and seasonally- appearing Recurring Slope Lineae. Salts lower the freezing point of water and allow running water, which forms the RSLs. The water is liquid and flowing, but briny instead of fresh. Might support near-surface microbial life. 14