Many of the surface formations observed on Mars—once thought to be signs of flowing liquid water—are now believed to be dry sand features, according to a new study examining the nature of seasonal streaks on the planet.
The question of whether liquid water currently exists, or once existed, on the Martian surface has long been a central topic in planetary science. Particular attention has focused on dark streaks that periodically appear along steep slopes. These features, known as Recurring Slope Lineae (RSL), form seasonally on Martian hillsides and crater walls.
Earlier research had strengthened the case for water. High-resolution imagery and instrument data from the Mars Reconnaissance Orbiter, operated by NASA, detected hydrated salts within the streaks—salts chemically bound with water molecules. This was widely interpreted as evidence that briny liquid water might be flowing intermittently on Mars.
However, a new analysis led by scientists from the United States Geological Survey (USGS), the University of Arizona, Durham University, and the Planetary Science Institute challenges that interpretation.
Their findings suggest the streaks are more consistent with dry granular flows triggered by dust movement and sandstorms rather than liquid water activity.
“We’ve thought of RSL as possible liquid water flows, but the slopes are more like what we expect for dry sand,”
“This new understanding of RSL supports other evidence that shows that Mars today is very dry.”
— USGS scientist and lead author Colin Dundas.
To reach this conclusion, the researchers analyzed the morphology of 151 individual RSL sites. They found that every formation occurred on slopes steeper than 27 degrees. Moreover, the streaks consistently halted when the terrain became less steep—a behavior typical of dry sand avalanches but inconsistent with flowing liquid, which would be expected to continue downslope.
Despite these findings, the debate over present-day liquid water on Mars is far from settled. Direct confirmation may ultimately require close-range investigation—either through robotic landers sent to RSL sites or future crewed missions capable of in-situ analysis.
Reference:
Dundas et al., Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water, published in Nature Geoscience, November 20, 2017. DOI: 10.1038/s41561-017-0012-5.