On Mars, salts consistent with briny flows are present in enigmatic streaks that seasonally appear and fade away on slopes, according to a study published online in Nature Geoscience. These surface features - called recurring slope lineae - have been hypothesized to form by the activity of brines, but direct evidence has been lacking.

High-resolution images have shown that recurring slope lineae, narrow features typically less than 5 metres in width, appear on slopes during warm seasons, lengthen and then fade during cooler seasons. The range of surface temperatures over which the recurring slope lineae are active suggests that salty liquid water may be involved in their formation. However, spectral spacecraft data are at a coarser resolution than the lineae widths and compositional analyses that are normally averaged over several pixels of data have detected neither salts nor water.

Lujendra Ojha and colleagues analysed spectral data from the CRISM instrument onboard NASA’s Mars Reconnaissance Orbiter for locations where the recurring slope lineae are present. They devised a method to extract spectral information from individual pixels of CRISM data, and the spectra they obtain in all locations examined are consistent with the presence of hydrated salt minerals that precipitate from water. In contrast, the spectral signature for salts is not present in the background spectra of the surrounding terrain.

The findings strongly suggest a link between the transient streaks on Martian slopes and the flow of liquid brines.