For the first time, direct evidence of a flowing stream has been discovered on Mars, NASA announced on Thursday.
Previously the only strong evidence of a large body of water having once existed upon Mars were indirect, such as images captured from orbit. Now, just a few weeks into its two year mission, the Curiosity rover has discovered and photographed a dried-up riverbed. These latest observations are a confirmation that the Red Planet was once indeed the site of rivers and streams, just like Earth is.
The pictures come from an examination of two areas of rocky outcrops, named “Hottah” and “Link”. The observations were a follow up on previous hints from another outcrop, exposed by a thruster exhaust as Curiosity touched down.
Both outcrops contain rounded rocks embedded in stone, forming what are known as ‘rock conglomerates’. The gravels in these conglomerates range both greatly in size, from that of a golf ball to as small as a grain of sand, and shape, some being smoothed and round and others being sharp edged and angular.
“Hottah looks like someone jack-hammered up a slab of city sidewalk, but it’s really a tilted block of an ancient streambed,” said Mars Science Laboratory Project Scientist John Grotzinger of the California Institute of Technology in Pasadena.
The images captured clearly show a gravel bed in which the stones have been eroded over a long period of time to form the pebbles much like the familiar, smooth riverbed stones of our streams. From the size and shape of these stones, Curiosity’s science co-investigator William Dietrich of the University of California, Berkeley has been able to deduce that the stream was probably flowing of a speed of around three feet per second, and somewhere between ankle and hip deep. The stones are too big to have been moved by wind, and many are rounded, eroded by their movement in the stream.
A possible next step for the rover is to test the riverbed and analyse the elemental composition of the material holding the rock conglomerate together. This would allow further conclusions to be drawn about the environment that caused these formations. And since the stones of the conglomerate were transported from above the rim of the crater, any analysis of these would allow lessons to be learned about the broader regional geography.
The discovery site lies between the north rim of the Gale Crater and the base of Mount Sharp, a mountain within the crater. Earlier imagery shows an alluvial fan of lake sediment washed down the rim of the crater, with many channels dividing it, just uphill of Curiosity’s discovery. This abundance of channels is suggestive of continuous or repeated flow of water over a long time, not just a one-off or even just over a few years.
The Gale Crater was chosen as the landing site for the Curiosity rover because of the presence of the central mountain, Mount Sharp, which, going by previous observations, was formed by sedimentary processes. This is useful because it is a fairly non-destructive form of mountain formation and so geological indicators of life are more likely to have been preserved.
“A long-flowing stream can be a habitable environment,” said Grotzinger. “It is 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.”
During the two-year primary mission of the Mars Science Laboratory, researchers will use Curiosity’s 10 scientific instruments to investigate as to whether areas in Gale Crater have ever offered environmental conditions favourable for microbial life. This is a promising start to what will hopefully be an insightful two years.