A dried-up riverbed has been discovered on Mars which is the first direct evidence of an ancient network of fast-running streams on the Red Planet, scientists said today.
Although there has been strong indirect evidence that large amounts of water once existed on our neighbouring planet, the latest observations from Nasa's Curiosity rover confirm that Mars was once a watery world in which microbial life may once have thrived.
The rover, which is just a few weeks in to a two-year mission, has captured images showing gravel beds in the Martian rocks that have clearly been eroded away over many years to form smooth pebble-like stones just like those found in terrestrial riverbeds.
Scientists from the Jet Propulsion Laboratory (JPL) in Pasadena, California, estimated from the size and shape of the eroded rocks that fast-moving stream was up to several feet deep and could have existed for thousands and possibly millions of years for a period between 3.5 and 4 billion years ago.
“From the size of the gravels it carried, we can interpret the water was moving about three feet per second, with a depth somewhere between ankle and hip deep,” said William Dietrich of the University of California, Berkeley, the science co-investigator on the project.
“Plenty of [scientific] papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars,” Dr Dietrich said.
“This is a transition from speculation about the size of streambed material to direct observation of it,” he said.
Curiosity, a six-wheeled mobile laboratory the size of a family car, has just begun its exploration of the north rim of the huge Gale Crate where it landed in August. It was examining two outcrops of rock, called Hottah and Link, when its mast-mounted cameras detected the distinctive gravel beds of a fast-moving stream.
“Hottah looks like someone jack-hammered up a slab of city sidewalk, but it's really a tilted block of an ancient streambed,” said John Grotzinger of the California Institute of Technology in Pasadena.
Both of the outcrops contain rounded pebbles embedded in stone to form what is known as rock conglomerates. The gravels in the conglomerates range in size from grains of sand to rocks the size of golf balls, and although some have sharp edges, many are rounded.
Nasa chose the Gale Crater as the landing site because it contains a large mountain in the middle, called Mount Sharp, where observations from orbiting space probes indicate the rocks have been formed by sedimentary processes, which are likely to preserve some geological signs of life.
Dr Grotzinger said that clay and sulphate minerals have been detected on the slopes of this mountain and these types of rocks are good preservers of carbon-based organic chemicals that are the potential ingredients for life.
Observations of the crater from orbit have also revealed the existence of an alluvial fan formed by watery material that had been washed down from the rim of the crater. Images show streaks of channels where large amounts of water once flowed.
“A long-flowing steam can be a habitable environment. It's not our top choice as an environment for preservation of organics, though. We're still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment,” Dr Grotzinger said.
Curiosity is armed with toolbox of sophisticated instruments for analysing the chemical and physical composition of rocks but it is not designed to test for direct evidence of Martian life, he said. However, Nasa hopes that the rover will provide important scientific clues that could lead to the discovery of fossilised life forms, if they do indeed exist on Mars.