Detailed analysis and review have borne out researchers’ initial interpretation of pebble-containing slabs that Nasa’s Mars rover Curiosity investigated last year: They are part of an ancient streambed.
The rocks are the first ever found on Mars that contain streambed gravels. The sizes and shapes of the gravels embedded in these conglomerate rocks – from the size of sand particles to the size of golf balls – enabled researchers to calculate the depth and speed of the water that once flowed at this location.
“We completed more rigorous quantification of the outcrops to characterise the size distribution and roundness of the pebbles and sand that make up these conglomerates,” said Rebecca Williams of the Planetary Science Institute, Tucson, Ariz., lead author of a report about them in the journal Science. “We ended up with a calculation in the same range as our initial estimate last fall. At a minimum, the stream was flowing at a speed equivalent to a walking pace — a metre, or so, per second — and it was ankle-deep to hip-deep.”
Three pavement-like rocks examined with the telephoto capability of Curiosity’s Mast Camera (Mastcam) during the rover’s first 40 days on Mars are the basis for the new report. One, “Goulburn”, is immediately adjacent to the rover’s “Bradbury Landing” touchdown site. The other two, “Link” and “Hottah,” are about 50 and 100 metres to the southeast.
The larger pebbles are not distributed evenly in the conglomerate rocks. In Hottah, researchers detected alternating pebble-rich layers and sand layers. This is common in streambed deposits on Earth and provides additional evidence for stream flow on Mars. In addition, many of the pebbles are touching each other, a sign that they rolled along the bed of a stream.
The atmosphere of modern Mars is too thin to make a sustained stream flow of water possible, though the planet holds large quantities of water ice. Several types of evidence have indicated that ancient Mars had diverse environments with liquid water. However, none but these rocks found by Curiosity could provide the type of stream flow information published in Science. Curiosity’s images of conglomerate rocks indicate that atmospheric conditions at Gale Crater once enabled the flow of liquid water on the Martian surface.
Source: Mars Science Laboratory | Nasa
