Domed polygons found throughout the Athabasca Vallis head-regions area.
The weather on Mars was much balmier in the recent past than scientists have previously thought, according to a new interpretation of the formation of certain landforms on the surface.
The finding could have implications for the possibility of finding signs of life on Mars.
Matthew Balme, of The Open University in the United Kingdom, studied detailed images of equatorial landforms taken by the HiRISE (High Resolution Imaging Science Experiment) instrument onboard NASA's Mars Reconnaissance Orbiter (MRO), currently circling the red planet.
Ample evidence has been found to show that liquid water once covered parts of the Martian surface in the early history of the planet several billion years ago, but whether water flowed more recently has been less certain. Several studies in recent years have started to point to possible evidence of more recent water flow though.
For example, a system of gullies suggests the most recent period of water flow was only 1.25 million years ago. Another recent study found that rivers might have flowed on the Martian surface within the last billion years.
Balme's analysis indicates that the landforms he examined formed by the melting of ice-rich soils during "freeze-thaw" cycles that continued until as recently as 2 million years ago. The pictures show polygonally patterned surfaces, branched channels, blocky debris and mound/cone structures. These features are similar to landforms on Earth in areas where permafrost is melting.
Permafrost is ground that remains frozen for years or even millennia, such as in the Arctic, but which can melt if the climate changes.
"The features of this terrain [on Mars] were previously interpreted to be the result of volcanic processes. The amazingly detailed images from HiRISE show that the features are instead caused by the expansion and contraction of ice, and by thawing of ice-rich ground," Balme said. "This all suggests a very different climate to what we see today."
All of the landforms observed are in an outflow channel, thought to have been active as recently as 2 million to 8 million years ago. Since the landforms exist within, and cut across, the pre-existing features of the channel, this suggests that they too were created within this timeframe.
"These observations demonstrate not only that there was ice near the Martian equator in the last few million years, but also that the ice melted to form liquid water and then refroze. And this probably happened for many cycles," Balme said.
Balme's study, funded by a UK Science and Technology Facilities Council grant, will be detailed in an upcoming issue of the journal Earth and Planetary Science Letters.
Signs of more recent water activity could aide the search for past or present life on Mars as such life, or signs of it, would be more likely to have survived than any that might have existed earlier in the planet's history.
"Given that liquid water seems to be essential for life, these kinds of environments could be a great place to look for evidence of past life on Mars," Balme said.