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.
NASA’s Curiosity rover found evidence for an ancient, flowing stream on Mars at a few sites, including the rock outcrop pictured here, which the science team has named “Hottah” after Hottah Lake in Canada’s Northwest Territories. It may look like a broken sidewalk, but this geological feature on Mars is actually exposed bedrock made up of smaller fragments cemented together, or what geologists call a sedimentary conglomerate. Scientists theorize that the bedrock was disrupted in the past, giving it the titled angle, most likely via impacts from meteorites. Image Credit: NASA/JPL-Caltech/MSSS
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 characterize 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 this week. “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 meter, or three feet, 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 165 and 330 feet (50 and 100 meters) to the southeast. Researchers also used the rover’s laser-shooting Chemistry and Camera (ChemCam) instrument to investigate the Link rock.
These stereo views (above and below) from Curiosity shows a rock called “Link,” which bears rounded pebbles that provide evidence about vigorous flow of water in a stream on ancient Mars. The scene appears three dimensional when viewed through red-blue glasses with the red lens on the left. The scale bar at lower right is 5 centimeters (2 inches). Link contains rounded gravel fragments, or clasts, up to a couple inches (few centimeters) in size, in a matrix of white material. Many gravel-sized rocks have eroded out of the outcrop onto the surface, particularly in the left portion of the frame. Water transport is the only process capable of producing the rounded shape of clasts of this size. Curiosity’s Mast Camera instrument (Mastcam) acquired component images of this scene on the 27th Martian day, or sol, of the mission (Sept. 2, 2012). The Mastcam has two cameras, a telephoto right eye (Mastcam 100) with a 100-millimeter-focal-length lens, and a moderately wide-angle left eye (Mastcam 34) with a 34-millimeter lens. This stereo image combines images from each eye. The name Link is derived from a significant rock formation in the Northwest Territories of Canada, where there is also a lake with the same name. Scientists enhanced the color in the version above to show the Martian scene as it would appear under the lighting conditions we have on Earth, which helps in analyzing the terrain. A version closer to the raw color recorded by the camera on Mars (adjusted for stereo effect) is can be seen below. Image Credit: NASA/JPL-Caltech/MSSS
“These conglomerates look amazingly like streambed deposits on Earth,” Williams said. “Most people are familiar with rounded river pebbles. Maybe you’ve picked up a smoothed, round rock to skip across the water. Seeing something so familiar on another world is exciting and also gratifying.”
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.
“Our analysis of the amount of rounding of the pebbles provided further information,” said Sanjeev Gupta of Imperial College, London, a co-author of the new report. “The rounding indicates sustained flow. It occurs as pebbles hit each other multiple times. This wasn’t a one-off flow. It was sustained, certainly more than weeks or months, though we can’t say exactly how long.”
The stream carried the gravels at least a few miles, or kilometers, the researchers estimated.
This annotated set of images compares the Link outcrop of rocks on Mars (left) with similar rocks seen on Earth (right). The image of Link, obtained by NASA’s Curiosity rover, shows rounded gravel fragments, or clasts, up to a couple inches (few centimeters), within the rock outcrop. Erosion of the outcrop results in gravel clasts that fall onto the ground, creating the gravel pile at left. The outcrop characteristics are consistent with a sedimentary conglomerate, or a rock that was formed by the deposition of water and is composed of many smaller rounded rocks cemented together. A typical Earth example of sedimentary conglomerate formed of gravel fragments in a stream is shown on the right. The image highlights a piece of gravel that is about 0.4 inches (1 centimeter) across. It was selected as an example of coarse size and rounded shape. Rounded grains (of any size) occur by abrasion in sediment transport, by wind or water, when the grains bounce against each other. Gravel fragments are too large to be transported by wind. At this size, scientists know the rounding occurred in water transport in a stream. Scientists enhanced the color in the Mars image to show the scene as it would appear under the lighting conditions we have on Earth, which helps in analyzing the terrain. The Link outcrop was imaged with the 100-millimeter Mast Camera on Sept. 2, 2012, which was the 27th sol, or Martian day of operations. Image Credit: NASA/JPL-Caltech/MSSS and PSI
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 this week. Curiosity’s images of conglomerate rocks indicate that atmospheric conditions at Gale Crater once enabled the flow of liquid water on the Martian surface.
During a two-year prime mission, researchers are using Curiosity’s 10 science instruments to assess the environmental history in Gale Crater on Mars, where the rover has found evidence of ancient environmental conditions favorable for microbial life.
Source: Jet Propulsion Laboratory (JPL)