There is possibly water flowing during the warmest months on Mars, according to strong evidence from NASA's Mars Reconnaissance Orbiter, or MRO.
Scientist have been searching Mars for decades hoping to find signs of the precious liquid that's said to be vital for life.
Scientists announced on Thursday that they've observed dark, finger-like features appearing and extending down some Martian slopes during late spring through summer, which fade in the winter and return during next spring. These recurring features were located on several steep slopes in Mars' southern hemisphere, according to NASA.
"The best explanation for these observations so far is the flow of briny water," said Alfred McEwen of the University of Arizona, Tucson, in a statement.
McEwen is the principal investigator for the orbiter's High Resolution Imaging Science Experiment, or HiRISE, and the lead author of a report about the recurring flows published in Thursday's edition of the journal Science.
"NASA's Mars Exploration Program keeps bringing us closer to determining whether the Red Planet could harbor life in some form," NASA Administrator Charles Bolden said in a statement, "and it reaffirms Mars as an important future destination for human exploration."
NASA said there are some aspects of the observations that still puzzle researchers, but flows of liquid brine fit the features' characteristics better than alternate hypotheses.
Saltiness lowers the freezing temperature of water, according to NASA.
"These dark lineations are different from other types of features on Martian slopes," said MRO project scientist Richard Zurek of NASA's Jet Propulsion Laboratory in Pasadena, Calif., in a statement. "Repeated observations show they extend ever farther downhill with time during the warm season."
The brine streaks are about 0.5 to 5 yards or meters wide, with lengths up to hundreds of yards down these slopes. Additionally, the width is much narrower than previously reported gullies on Martian slopes, according to NASA.
Some of those locations display more than 1,000 individual flows. Also, while gullies are abundant on cold, pole-facing slopes, these dark flows are on warmer, equator-facing slopes, the space agency said.
The images of the findings show flows lengthen and darken on rocky equator-facing slopes from late spring to early fall, and the seasonality, latitude distribution and brightness changes suggest a volatile material is involved, but there is no direct detection of one, NASA said.
The agency also said the settings are too warm for carbon-dioxide frost and, at some sites, too cold for pure water, which suggests that it's possibly brines, which have lower freezing points.
These salt deposits over much of Mars show that brines were plentiful in Mars' past, and these recent observations suggest brines still may form near the surface today in limited times and places, NASA said, adding that these features may quickly dry on the surface or could be shallow subsurface flows.
"The flows are not dark because of being wet," McEwen said in a statement. "They are dark for some other reason."
NASA said a flow originated by briny water could reorganize grains or change surface roughness that could darken the appearance. But the scientists found it hard to explain how the features brighten again when temperatures drop.
"It's a mystery now, but I think it's a solvable mystery with further observations and laboratory experiments," McEwen said.