While the Mars gully looks a lot like river channels here on Earth, it likely was not carved out by flowing water, NASA officials said.
"The dates of the images are more than a full Martian year apart, so the observations did not pin down the Martian season of the activity at this site," officials wrote in a description of the gully image on Wednesday.
But, they added, "before-and-after HiRISE pairs of similar activity at other sites demonstrate that this type of activity generally occurs in winter, at temperatures so cold that carbon dioxide, rather than water, is likely to play the key role.
However, MRO has observed other Martian features that do seem associated with liquid water — dark streaks known as recurring slope lineae.