A new study casts doubt on the widespread interpretation that seasonal dark streaks on Martian dunes are caused by water.
By Jay Bennett
At some point, there was water on Mars. We have known this to a high level of certainty since at least 2004, when the Opportunity rover discovered Martian spherules, also known as “blueberries” for their round shape and blue hue in false-color images. These small mineral deposits of hematite could only have formed in liquid water.
In 2015, planetary scientists surmised that significant deposits of liquid water exist on Mars today, lurking just below the red surface. The primary evidence for this theory are seasonal dark streaks that periodically appear on Martian dunes, thought to be briny water that seeps close to the surface during the warmer seasons.
Unfortunately, that interpretation of these dark streaks, technically called recurring slope lineae (RSL), might be incorrect. A study published in Nature Geoscience argues that the dark streaks appear not because of water seepage, but rather due to natural avalanches of dry sand and dust.
“We’ve thought of RSL as possible liquid water flows, but they seem to act more like dry sand,” said lead author Colin Dundas of the U.S. Geological Survey in a press release. “This suggests that the surface of Mars is quite dry today.”
The researchers point out that RSL only appear on dunes that are steep enough for dry sand to descend the slope. Using images from the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter (MRO), the research team analyzed 151 RSL features at 10 sites. (More than 50 RSL sites have been discovered from the Martian equator to the tropics.) The team found that RSL only occur on slopes steeper than 27 degrees, and the flows abruptly end when the slopes reach an “angle of repose” where dry sand accumulates and forms a lump, observed on dunes on both Mars and Earth.
“The RSL don’t flow onto shallower slopes, and the lengths of these are so closely correlated with the dynamic angle of repose, it can’t be a coincidence,” said HiRISE Principal Investigator Alfred McEwen at the University of Arizona, a co-author of the new study.
If the new study is accurate, it could be a huge blow to the search for microbial life on Mars. Substantial liquid water near the surface of the planet has been the primary argument for possible Martian life. However, the new study can only speculate as to where the darker material of the streaks comes from if it is not water.
“The RSL on Mars behave in a similar way to laboratory experiments on Earth,” said Jim McElwaine of the Planetary Science Institute who contributed granular flow and fluid dynamics expertise to the study. “What is still not understood is where the supply of fresh material comes from, though we do have some speculative ideas.”
One idea is that minute amounts of water do exist in the sand, and another is that the dunes wick some moisture from the thin atmosphere, triggering the sand flows. And there are still more unanswered questions. For example, hydrated salts exist at some of the RSL sites, which suggests the presence of trace amounts of water. The seasonal reappearance, gradual growth, and rapid disappearance of RSL features also remain a mystery, particularly if significant water is not the cause.
“RSL probably form by some mechanism that is unique to the environment of Mars, so they represent an opportunity to learn about how Mars behaves, which is important for future surface exploration,” said McEwen.
The search for Martian life is not over, and the debate about RSL on Mars is likely only beginning. The new USGS study, however, does not bode well. From the abstract:
“The preference for warm seasons and the detection of hydrated salts are consistent with some role for water in their initiation. However, liquid water volumes may be small or zero, alleviating planetary protection concerns about habitable environments.”