Gullies on modern Mars are most likely not the result of flowing liquid water, data from NASA’s Mars Reconnaissance Orbiter show.
The finding could help researchers narrow their theories on how the gullies are carved out and reveal more details about the red planet’s recent evolution.
“What we’ve found helps us get a better picture of how these interesting features on the surface of Mars can form and change,” says Jorge Núñez of Johns Hopkins University, lead author of a paper in Geophysical Research Letters. “That in turn gives us better insights into the recent geologic history of the planet.”
100 sites with gullies
Scientists define Mars gullies as features that share three characteristics: an alcove at the top, a channel, and an apron of deposited material at the bottom.
Gullies are different from another feature on Mars, streaks called “recurring slope lineae” that darken and appear to flow down steep slopes during warm seasons and fade in cooler seasons. Scientists have evidence that flowing water is involved in the formation of RSLs.
For the study, Núñez and others from the Johns Hopkins Applied Physics Laboratory collected high-resolution data on minerals at more than 100 gully sites around Mars. These data, from the CRISM instrument on board MRO, were then correlated with images from spacecraft cameras.
The team found no mineralogical evidence for abundant liquid water or its by-products, thus pointing to mechanisms other than the flow of water as being the major driver of recent gully evolution. Freezing and thawing of carbon dioxide frost is a possible alternative explanation.
How did they get there?
Gullies are a widespread and common feature on the Martian surface, mostly occurring between 30 and 50 degrees latitude in both the northern and southern hemispheres, generally on slopes that face toward the poles. On Earth, similar gullies are formed by flowing liquid water. But, at least today, liquid water is not common on the surface of Mars, and may only appear as small amounts of brine even at RSL streaks.
The lack of sufficient water to carve gullies has resulted in a variety of theories about how they got there.
MRO scientists “had shown there was seasonal activity in gullies—primarily in the southern hemisphere—over the past couple of years, and carbon dioxide frost is the main mechanism they suspected of causing it,” Núñez says. “However, other researchers favored liquid water as the main mechanism.”
The MRO cameras couldn’t decide the issue on their own, because images alone can’t tell scientists what material was in the gullies.
“We used CRISM, an imaging spectrometer, to look at what kinds of minerals were present in the gullies and see if they could shed light on the main mechanism responsible,” Núñez says.
There was no evidence of clays or other hydrated minerals in most of the gullies studied.
“When we did see them,” Núñez says, “they were erosional debris from ancient rocks, exposed and transported downslope, rather than altered in more recent flowing water.” In other words, he says these modern gullies are—without water—carving into ancient terrain and exposing clays that likely formed billions of years ago when liquid water was more prevalent on the Martian surface.
APL built and operates CRISM, one of six instruments on MRO. The Jet Propulsion Laboratory at CalTech manages the MRO mission for NASA.
Source: Johns Hopkins University
