The recent announcement by NASA that there is evidence of salty, liquid water seeping out of the ground on Mars is both exciting and scientifically puzzling at the same time. As a member of the science team for the High Resolution Imaging Science Experiment (HiRISE) camera on board the Mars Reconnaissance Orbiter (MRO), I’ve been hearing about these possible seeps, or Recurring Slope Lineae (RSL), for several years now. The RSL are relatively darker, small (5 meters or less across), streaks that appear during the warm season on Mars, expanding downslope for a time, before fading as colder conditions set in later in the year. The atmosphere on Mars is very low density compared to Earth. As a result, surface temperatures and pressures are also relatively low, thereby making conditions enabling the occurrence of liquid water difficult to envision. So while the possibility that the RSL are the result of water seeping out of the ground appears consistent with their occurrence and form, how this might happen has been much harder to explain.
Dark, narrow streaks on Martian slopes such as these at Hale Crater are inferred to have been formed by seasonal flow of water on contemporary Mars. Detection of hydrated salts at the streaks supports that interpretation. The features are called Recurring Slope Lineae
Dark narrow streaks, Recurring Slope Lineae, emanate from the walls of Garni Crater on Mars, in this view constructed from observations by the HiRISE camera on NASA's Mars Reconnaissance Orbiter.
Nevertheless, a paper published in the journal Nature Geoscience on September 28 of this year (Ojha et al.) reports that data from HiRISE and the Compact Reconnaissance Imaging Spectrometer for Mars instrument, also onboard MRO, from four different RSL locations reveals hydrated salts are present when the features are most extensive. As reported in the paper, these salts, probably occurring as perchlorate and chlorate, are interpreted to strongly support the interpretation that the RSL are the result of liquid water at the surface. The discovery is exciting because it means the liquid water survives at and just beneath the surface of Mars today. Could this also mean that conditions that might be habitable also occur? Moreover, access to liquid water for astronauts traveling to Mars in the future means that vital resource may not have to be carried with them from the Earth. That would help reduce and/or free up some of the payload required to get to Mars.
The presence of these salts suggests the water is a brine and helps explain how it could remain liquid under present surface conditions. However, it does not solve the mystery of why the water is where it is or how it got there. If the RSL are related to groundwater seepage, it is not certain why they occur where they do versus where they do not. For example, some occur on the sides of the central peak of an impact crater named Horowitz which is a mountain of sorts that is isolated from rocks outside the crater. So how does the water get there? Why do they occur on some slopes, but not on others relatively nearby? So while the evidence points strongly to the RSL being due to seeps of briny, liquid water, a number of mysteries about their formation and occurrence remain to be solved. The RSL continue to be the target of study and new data from MRO will undoubtedly lead to additional hypotheses and interpretations regarding their origin so stay tuned!