Mars’ watery past

Researchers identify site within Martian crater which likely held water in the not-too-distant past.

The question of the existence of life beyond our planet has intrigued humankind since time immemorial. For many years, Mars has been the focus of the search for extraterrestrial life. The Red Planet is the most similar to Earth and is considered the most habitable planet in the Solar System after our own.

Water, or more specifically liquid water, is vital to life on Earth, and in their quest to discover evidence of biological life on Mars, scientists have adopted a “follow the water” approach, seeking sites which show signs of a watery past or present.

A collaboration between researchers from Trinity College Dublin and Oxford University recently pinpointed an ancient valley within a Martian crater – which shows characteristics of having held water in the not-too-distant past.

Trinity’s Dr Mary Bourke and Professor Heather Viles, Head of Oxford University’s School of Geography and the Environment, were struck by the similarities between patterns on the Martian surface with those associated with desert flooding on Earth.

“On Earth, desert dunefields are periodically flooded by water in areas of fluctuating groundwater, and where lakes, rivers and coasts are found in proximity. These periodic floods leave tell-tale patterns behind them,” explained Dr Bourke. “You can imagine our excitement when we scanned satellite images of an area on Mars and saw this same patterned calling card, suggesting that water had been present in the relatively recent past.”

These patterns – knowns as ‘arcuate striations’ – had been noted on the surface of migrating sand dunes in the Namib desert in remote sensing images taken from an earlier study. Subsequent fieldwork showed that these striations were the result of geochemically cemented dune salts left by evaporating groundwater. These sediments later become relatively immobile, and are left behind as the dunes migrate downwind.

Example of geochemically cemented crossbed strata in the interdune near Walvis Bay, Namibia. a) Google Earth image of crossbeds upwind of the windward slope of a barchan in Namibia. White arrows highlight particularly prominent examples b) Ground image of crossbeds showing general relief of interdune (<10cm) c) Wall of pit excavated in interdune (location seen in b) shows dipping sediment layers in subsurface are contiguous with protruding layers on the surface. Alternating high and low albedo layers have different salt composition and grainsize. The darker layer forms the more pronounced micro-ridges.

The researchers hypothesize that a similar process occurred on Mars, and that these arcuate striations exposed on the surface between dunes are indicative of fluctuating levels of salty groundwater, during a time when dunes were actively migrating down the valley.

Striations exposed on the surface between dunes indicate fluctuating levels of salty groundwater. a) Exposure of putative crossbeds on windward slope of dunes on Mars b) Interdune strata exposed in planform showing contrasting albedo and crosscutting relationships similar to that exposed in the dune. Subset of false-colour HiRISE image ESP_013319_1685. The Infrared, red and blue bands are displayed as red, green and blue.

The findings, recently published in Geophysical Research Letters, are significant. Not only do they shed light on Mars’ past hydrology, but also offer a geological target for detecting past life forms on the Red Planet, which is vital for selecting sites for future missions to Mars.