Ice deposits waxed and waned on Mars: study

Scientists have found new evidence that glacier-like ice deposits advanced and retreated multiple times in regions of Mars in recent past.
For the study, researchers from the Brown University looked at hundreds of gully-like features found on the walls of impact craters throughout the Martian midlatitudes.
They conclude that many of those gullies were formed by meltwater from icy deposits, which are known to have covered the Martian midlatitudes within the last 2 million years.
The study also turned up evidence of multiple gully-forming events, suggesting that these ice deposits waxed and waned several times over the last several million years - relatively recently in Mars' 4.5-billion year history.

"These recent climate cycles have been predicted by computer models, but have not been documented with widespread geological evidence until now," said Jay Dickson, a researcher at Brown and the study's lead author.
"This research shows that gullies have been episodic across the entire southern hemisphere, a distribution that is required for this to be a signal of global climate change," said Dickson.

At present, most of the water ice on Mars is concentrated at its poles, but there's a wealth of evidence that it wasn't always that way.
The researchers looked at detailed images taken by NASA's High Resolution Imaging Science Experiment (HiRISE) of 479 gullies in the midlatitudes of Mars' southern hemisphere.

The gully systems, which form on steep crater walls, consist of an alcove at the top from which sediment is excavated, a channel through which material is carried, and a delta-like fan at the bottom where material is deposited.
The survey showed gully systems in various states of erosion and degradation. In some places, older gully fans, eroded over many years by the elements, had been crosscut by new gully fan systems.
That suggests at least two gully-carving events. In other examples, gully fans were clearly visible, but the alcoves and channels that supplied them had disappeared, covered by a new layer of ice-rich soil. That too suggests multiple periods of glacial deposition, researchers said.

"We show solid evidence of at least two periods of emplacement of the latitude-dependent mantle," said Brown geologists James Head, an author on the new paper.
"That's consistent with the idea of cyclical ice ages on Mars related to its obliquity," said Head.
The work also bolsters the idea the many of gullies were carved by flows of liquid water, researchers said.
The research was published in the journal Icarus.