Vast lakes of melted ice existed on Mars more recently than previously thought during a warm, wet spell on the red planet, new images suggest.
The lakes might have been habitats for life, if there ever was life on Mars. So far, however, there is no firm evidence of any Martian biology, past or present.
The photographs, taken by NASA's Mars Reconnaissance Orbiter, reveal a network of winding channels linking together several depressions in the Martian surface. Researchers say those channels could only have been caused by Martian lake water running between the depressions about 3 billion years ago — which is 1 billion years more recent than earlier estimates.
"Most of the research on Mars has focused on its early history and the recent past," said researcher Nicholas Warner, who led the study at the Imperial College of London. "Excitingly, our study now shows that this middle period in Mars' history was much more dynamic than we previously thought."
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The new images suggest lakes as large as 12 miles (20 km) wide once dotted the equatorial regions of Mars, researchers said.
Scientists already know that water ice exists today beneath the Martian surface based on data from landers, rovers and Mars images taken from orbit. But past studies have hinted that Mars was warm and wet enough to support liquid lakes around 4 billion years ago.
Using the images captured by the Mars Reconnaissance Orbiter, Warner and his colleagues concluded that Mars could have sustained lakes even later, in a period known as the Hesperian Epoch.
"Scientists had largely overlooked the Hesperian Epoch as it was thought that Mars was then a frozen wasteland," Warner said.
But Warner and his team found that during that epoch 3 billion years ago, Mars could have been warmed by volcanic activity, meteorite impacts or even orbital shifts. The result would be a temporary increase in planetary temperature as the gases created in those events thickened the Martian atmosphere.
The research is detailed in the Jan. 4 issue of the science journal Geology. Scientists at the University College London also participated in the study.
The channels revealed by the Mars Reconnaissance Orbiter are the key, researchers said. They run between huge depressions in the Martian surface located near a 1,242-mile (2,000 km) gorge called Ares Vallis across the planet's equator.
Scientists previously thought the depressions were formed by a process called sublimation, when ice transitions directly into gas. That process would create gaps as ice sublimates from the Martian surface, leading to a collapse that would form the depression.
But sublimation alone would not explain the channels, researchers said. Similar channels can be seen on Earth in Alaska and Siberia, where permafrost melts to carve drainage channels that connect different lakes, they added.
Warner and his team are unsure how long the more recent warm, wet period on Mars may have lasted, or how long liquid water could have flowed between the lakes. But the ancient lakebeds from the current study could provide attractive targets for future probes to seek out regions on Mars that may have once been habitable for microbial life, they added.
The scientists plan to seek other potential lakebeds in other regions along Mars' equator using more images from the Mars Reconnaissance Orbiter. Launched in 2005, the orbiter is NASA's most powerful spacecraft circling Mars today and has collected more images of the Martian surface than all other missions to the red planet combined.
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