MARTIAN GULLIES
Sharp-featured, relatively recent gullies (blue arrows) and degraded older gullies (gold) in the same location on the surface of Mars suggest multiple episodes of liquid water flow, consistent with cyclical climate change on the Red Planet.NASA HiRISE

Glaciers advanced and retreated many times in the mid-latitude regions of Mars in the "recent" past, say Brown University geologists, indicating that the region is the place to look for signs of past life on the Red Planet.

The hundreds of gully-like features found on the walls of impact craters throughout the Martian mid-latitudes led the team to conclude they were formed by melt water from ice deposits in the last million years.

Multiple gully-forming events suggest that the ice deposits waxed and waned several times over the last several million years — recent in Mars' 4.5-billion year history.

The study also indicates that climate change is an ongoing process on the planet, aided by its wobbly rotation axis.

"I think people have this idea of Mars as an inactive place, that it is now as it has been for billions of years," James Head, one of the authors, says. "But it seems likely that climate cycles and global climate change are still occurring."

He points to similar features in Antarctica, where despite low temperatures, the sun melts enough ice for gully activity to occur.

"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."

Wobbly axis
The cycles of ice age on Mars have been linked to its wobbly rotation axis sans a satellite like the moon that locks the earth's axis to a large extent.

Computer models predict that when the obliquity of Mars exceeds 30 degrees, increased sunlight at the poles melts the ice caps and the water is transported closer to the equator in the form of glacial snow and ice.

Mars is known to have crossed the 30-degree threshold several times during the last 20 million years.

The scientists studied images taken by Nasa's High Resolution Imaging Science Experiment (HiRISE) of 479 gullies in the mid-latitudes of Mars' southern hemisphere.

The gully systems on steep crater walls consist of an alcove at the top from which sediment is excavated, a channel to carry this material out and a delta-like fan at the bottom where material is deposited.

Various stages of gully channels and erosion suggest multiple gullies carved at different times of glacial deposition. At least two such periods were noticed that point to a time when ice-rich deposits carved the gullies.

Earlier evidence
A 2003 research led by Brown geologists showed proof of layers of ice-rich soil and dust in the mid-latitude regions of Mars.

Curiosity's findings in the Gale Crater on Mars suggest that the ancient climate on the planet could have sustained many lakes in the past.

Mt Sharp was built with sediments deposited in a large lake bed a million years ago. The rock layers on Mt Sharp – alternating between lake, river and wind deposits – are proof of the constant filling and evaporation of a large and long-lasting Martian lake, said Nasa.

Previous evidence of rocks smoothed by water as also sedimentary rocks in the valley suggests it once was a freshwater lake.

All this however contrasts with a recent ETH Zurich study where lead author Giovanni Leone categorically ruled out oceans on the Red Planet at any time.

"Before becoming the cold and dry desert of today, this planet was characterised by intense heat and volcanic activity, which would have evaporated any possible water and made the emergence of life highly unlikely," he said in his study which gave evidence of a massive moon-sized object slamming into Mars, triggering volcanic activity and resulting in the dual nature of its two hemispheres.