New measurements by Curiosity at Gale Crater show that Martian sediments contain almost 2% water by weight, says a study released today in Science. These measurements, taken by Curiosity’s Sample Analysis at Mars instrument (SAM), were taken in November 2012 at a site named Rocknest. Rocknest, a sandy patch near Curiosity’s landing site, was one of the first ports-of-call for the rover upon arriving at Mars.
The Science study, lead by Laurie Leshin (Polytechnic Institute), is the first detailed analysis of SAM readings at Rocknest. SAM cooked its samples at a temperature of 835 degrees C before measuring the gases that baked out. Those measurements showed that the sand at Rocknest contained 2% water by weight. To put that in perspective, if future astronauts could collect all of the water from the sediment, they would produce 33 liters from every cubic meter of dirt they processed.
Isotopic measurements taken by SAM also suggest that the sediments at Rocknest are being sponged up by the atmosphere, says Leshin. The evidence is a high ratio of deuterium to hydrogen. Deuterium is an isotope of hydrogen that contains one neutron. This makes it approximately twice as heavy as hydrogen. As the Martian atmosphere slowly escaped into space, more deuterium than hydrogen remained behind. This left Martian surface water with much more deuterium than water belched out by Martian volcanoes. The results suggest that over time, the sediments sponged up water from the Martian atmosphere, says Leshin.
Obstacles to Manned Spaceflight
Despite holding a significant amount of water, future space missions will likely be unable to do much with it. First, it’s difficult to extrapolate the results from Gale Crater to Mars as a whole. The rocks at Gale Crater were deposited in water, which is somewhat atypical for the Martian surface. Nearly 2/3 of the Martian surface is basalt, volcanic rock that contains a lot less water than sedimentary rocks. This is due to the fact that mineral grains in volcanic rocks are intergrown, leaving no pore space for water to collect inside the rock.
Further, the water that is contained within these sediments will be difficult to extract. The reason why can be shown with a simple home experiment. Take a cup of sand and record the amount of water you put in. Do this until the sand is completely soaked, with a little bit of water puddling on the top. Now, turn the cup to let the water leak out and measure the amount of water you have. You’ll notice that you only get out about a third of the water you put in. Why? Because the sand particles will have a film of water clinging to them thanks to surface tension. The finer the sediment, the more water you leave behind. More can be removed by pumping, but at the trace levels found by Curiosity, even that will be unextractable.
Further adding to the difficulty of extracting that water is that clay, a very fine sediment that forms the majority of rocks in Gale Crater. Clays that form from weathering of volcanic rocks like those found on Mars, are generally hydrophilic. The chemical composition of these clays attracts water and prevents it from leaving. The crystalline structure found in clay minerals also provides more space to stash away water.
In practice, to extract the water found by Curiosity, future astronauts would need to bake the sediments and collect the water vapor evaporating from the rocks. This would require extra equipment like an oven, as well as the tools and equipment necessary to operate it. As a result, it quickly becomes more practical to simply bring all the water needed from Earth, since reprocessing will already be necessary in the first place for the astronauts’ Mars-bound travel supply.
Image credit: NASA/JPL-Caltech/MSSS