Water Ice and Soil Samples on Mars - by Deborah Bass
Phoenix landed on May 25, 2008 in the icy northern plains of Mars.› Full caption
We’ve been steadily learning about what it takes to run this thing called the Phoenix lander. As expected, not everything has gone exactly as planned. But that in its own way was planned — we work to maintain flexibility in our schedule and our design, so that we can absorb new things that happen without throwing the whole team into a tizzy!
So what have we been doing?
The Robotic Arm Camera on Phoenix captured this
image underneath the lander on the fifth Martian day
of the mission. The abundance of excavated smooth
and level surfaces adds evidence to a hypothesis
that the underlying material is an ice table covered
by a thin blanket of soil.
› Full caption
The really big thing so far is that the Phoenix team discovered what is believed to be water ice beneath the surface under the lander. Computer models suggested that the ice would be several inches beneath the surface and, in fact, that is where we found it! We watched some soil lumps fall apart over several days (in a set of images taken to “monitor change”) and concluded that what was holding the lumps together was ice. After a few days exposed to the Martian atmosphere the cementing agent sublimed — in other words, it changed from a solid to a gas without ever being a liquid. If it had been, say, salts that were holding the lumps together, exposure to the atmosphere over several days wouldn’t have made a difference.
This conclusion about the ice has been arrived at rather carefully. First we saw some bright patches under the lander that had been exposed by the thruster engines during landing. We couldn’t do much with those patches, so we just noted them as “light-toned, forward-reflecting material.” We tried to come up with different hypotheses to explain the bright patches that might be consistent with something other than water ice — like frozen hydrazine fuel that we brought with us, or salt patches, or just lighter-toned rock! We took pictures in different wavelengths and decided that the light-toned material had the right reflective properties of water. We also scraped down a few inches and found the same light-toned material as we were seeing just beneath the lander. Then the team looked at the cloddy soil.
Small clumps of Martian soil were delivered to the MECA wet chemistry experiment. › Full caption
The wet chemistry experiment in one of the lander’s instruments called the Microscopy, Electrochemistry and Conductivity Analyzer, or MECA, also found salts in the soil samples. Salts are only formed when water has been present! So that is another indicator that there was abundant water in this region of Mars. What are these salts? They appear to be chemicals containing sodium, magnesium, potassium and chlorine. The soils were found to be alkaline, with a pH greater than 7 — similar to soils in the upper dry valleys of Antarctica.
This animation shows a sprinkle test,
where the scoop on the Robotic Arm
is vibrated so material gently falls to the target below.
› Full caption
But, like I said, everything hasn’t been totally smooth. The team discovered that the Martian soil is lumpy and sticks together. That made the first sample difficult to deliver! So the team thought about how to make the process easier, and we figured out various ways to break up the lumps. We tried three methods: de-lumping, sprinkling and agitation.
De-lumping refers to shaking the acquired material in the scoop by running a Dremel-like tool that vibrates the entire scoop, breaking up clumps. Then there is sprinkling: By running the rasp while slightly tipping the scoop, the team can command Phoenix to send a small shower and sift particles down into the TEGA (Thermal and Evolved-Gas Analyzer ) and MECA instruments rather than dumping a whole load of clumped-up dirt onto each instrument. As for agitation, the TEGA instrument has a method to shake itself — it has an agitator which shakes the sample loose if anything has stuck to its entry port. The sprinkle and agitation methods have been routinely adopted for sample delivery.
The neat consequence of this is that it solves what had always been our worry about how to deliver the same sample to each instrument for comparison of science results. The sprinkle delivery method enables us to put a large sample into the scoop and deliver part of it to MECA microscopy, part to MECA wet chemistry and part to the TEGA instrument. Same sample problem: solved!!
When life gives you lemons, make lemonade! Or in this case, Marsade!
