Recurring Slope Lineae: Evidence of liquid water on Mars today? The High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter (MRO) has monitored surface feature changes over several Mars years. One of the more intriguing observations has been of the seasonal occurrence of numerous (multi-thousand) dark streak features advancing down equator-facing slopes in several mid-latitude and equatorial regions of the southern hemisphere. Termed Recurring Slope Lineae (RSL), the streak behavior resembles that of a fluid mobilized by increasing surface temperatures – initiating in spring, incrementally extending downslope through summer, and then fading in the fall and winter seasons. The phenomenon is then seen to repeat the following spring. RSLs are narrow in width (meter-scale), requiring our best resolution imaging to see them, but may extend down-slope for hundreds of meters before terminating. Science teams involved in the discovery of RSL have been cautious in concluding a water-based origin since most of the RSL locations are found in environments well outside the pressure-temperature stability field for liquid water. However, the presence of salts such as perchlorate can depress freezing points by tens of degrees. MRO Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observations of multiple RSL locations show hydrated mineral absorption signals most consistent with magnesium perchlorate, magnesium chlorate and sodium perchlorate. That raises the possibility of a Mg-, Na-, Cl-rich brine that would also leave these minerals as a kind of evaporite deposit. However, the amount of water, if present, has been constrained at one location by independent Thermal Emission Imaging System (THEMIS; Mars Odyssey) data analysis and a numerical heat transfer model to 0 wt.%, and up to 3 wt.% given observational uncertainties, unlikely to support a sustained flow. This limit on the volume of water in the uppermost surface layer and observations that the run-out of the RSL is close to the angle of repose (expected for dry avalanches) suggest that non-water-based hypotheses cannot be ruled out. The mysterious nature of the RSL remains to be determined. Figure 1. RSL on the central structure of Horowitz Crater (32°S, 140.8°E), MRO Primary Science Phase (PSP) image PSP_005787_1475 (LS = 334: late summer). Altimetry map (A) locates the full 5.1-km-wide HiRISE image (B), with the white box indicating the color enlargement (C). Yellow arrows in (B) show some concentrations of RSL within the central peaks and pits. Colors in (C) have been strongly enhanced to show the subtle differences, including light orange streaks (black arrows) in the upper right that may mark faded RSL. North is up on all images in this paper except fig. S4. Figure and caption from McEwen et al., (2011); see below. Relevant references: Edwards, C. S., and S. Piqueux (2016), The water content of recurring slope lineae on Mars, Geophysical Research Letters, 43(17), 8912-8919, doi:10.1002/2016gl070179. Ojha, L., A. McEwen, C. Dundas, S. Byrne, S. Mattson, J. Wray, M. Masse, and E. Schaefer (2014), HiRISE observations of Recurring Slope Lineae (RSL) during southern summer on Mars, Icarus, 231, 365-376, doi:10.1016/j.icarus.2013.12.021. Ojha, L., M. B. Wilhelm, S. L. Murchie, A. S. McEwen, J. J. Wray, J. Hanley, M. Masse, and M. Chojnacki (2015), Spectral evidence for hydrated salts in recurring slope lineae on Mars, Nature Geoscience, 8(11), 829-+, doi:10.1038/ngeo2546. McEwen, A. S., L. Ojha, C. M. Dundas, S. S. Mattson, S. Byrne1, J. J. Wray, S. C. Cull, S. L. Murchie, N. Thomas, V. C. Gulick (2011), Seasonal flows on warm martian slopes, Science, 333, Issue 6043, pp. 740-743, doi:10.1126/science.1204816. Stillman D. E., Michaels T. I., Grimm R. E.; Hanley, J., (2016) Observations and modeling of northern mid-latitude recurring slope lineae (RSL) suggest recharge by a present-day martian briny aquifer. Icarus 265, 125-128. Heinz J., Schulze-Makuch D., Kounaves S. P. (2016) Deliquescence-induced wetting and RSL-like darkening of a Mars analogue soil containing various perchlorate and chloride salts. Geophysical Research Letters, 43, 10: 4880-4884. .