Mission scientists discussed a blindingly white rock Curiosity ran over at the 44th Lunar and Planetary Science Conference (LSPC) this week. The rock, dubbed “Tintina”, was broken open after the rover ran over it during a short drive on January 17. The unusually bright rock oversaturated Curiosity’s cameras, which were calibrated to take pictures of the more washed-out yellows and reds of Mars. “”The first time we tried to take the image, it saturated the detector, because we had no idea we’d have something so bright,” said Jim Bell (Arizona State University).
The rock was especially unusual to Curiosity’s MastCam, which is capable of estimating water content based on a property called reflectance. Reflectance is a measure of how a rock reflects incoming light and is useful for measuring the amount of water present within the rock. The reflectance signal from Tintina indicated that the rock was largely a block of hydrated minerals.
MastCam’s observations were followed up by the Russian-built Dynamic Albedo of Neutrons (DAN) instrument, which shoots pulses of neutrons at rocks to probe for hydrogen. The neutrons are slowed and reflected by hydrogen, which is typically bound to water. The DAN instrument is capable of measuring water present in rocks down to a concentration of 0.1% and to a depth of up to 2m below the surface. DAN also found a strong signal that water was present in the rock.
The hydrated mineral in this case is likely calcium sulfate, which was found in the drill samples from John Klein, which is located nearby. The calcium sulfate likely formed as a large vein that has since weathered out of a nearby outcrop. The veins probably formed while the sediments were submerged or soaked in groundwater, which would have increased in alkalinity as the climate dried. So what exactly is a hydrated mineral?
When some minerals form, their crystal lattice traps small amounts of water. In Tintina’s case calcium sulfate (a salt commonly known as gypsum) forms as water evaporates and increases the concentration of calcium and sulfate ions. When the concentration becomes high enough, the ions begin precipitating out of the water to form the salt. Calcium sulfate’s mineral structure allows it to incorporate large amounts of water within it. That water then shows up on instruments like MastCam and DAN.
So, where did that water come from?
Based on data from John Klein, it seems likely that Gale Crater was once a freshwater lake. But where did that water come from? The answer may lie in Peace Vallis, an alluvial fan that formed as a river spilled over the side of the crater and fanned out across its floor.
One scientist at the LPSC, Marisa Palucis (UC-Berkley), presented evidence that the river was the result of snowmelt washing off the highlands to the south. That interpretation was developed by studying the shape and behavior of the river channels from Mars Reconnaissance Orbiter data. Based on the Mars Reconnaissance Orbiter data, along with Curiosity’s studies of the pebbles deposited in Peace Vallis, it seems likely that the river was a fairly sizable stream, up to 1m deep and flowing at a velocity of several meters per second.
Crater counts by another scientist, Dr. John Grant (National Air and Space Museum), indicate that Peace Vallis is young compared to much of Gale Crater’s floor (young being a relative term – the feature formed between 3.4 and 3 billion years ago). Curiosity has an extremely bright future ahead of it, but the rover is currently in preparation for solar conjunction, which will hamper radio communications between Earth and Curiosity.
The rover has suffered a series of computer problems in recent weeks, the most recent occurring on March 18. The problem was caused by a flipped-bit error that was created by a radiation strike. The flipped bit caused two files to be appended together. In turn, the larger than expected file caused the rover’s systems to fail an internal check. The failed check caused Curiosity to go into safe mode, but engineers quickly fixed the problem. The problem was not serious, and the rover has already resumed operations.
IMAGE: Tintina rock, visible light and reflectance. NASA/JPL-Caltech/MSSS