24 March 2009
Unfortunately I missed the earliest sessions today because I had to drive down to Johnson Space Center to get a badge. I am going to be working there for four weeks after LPSC and another five weeks later in the summer, characterizing rock samples and shooting them with a laser, so I needed a badge to be able to do that work. I got back to the conference just in time for Bill Boynton’s talk about the evidence for Carbonates at the Phoenix landing site.
He presented results from TEGA, the Thermal Evolved Gas Analyzer, which is a set of 8 ovens that are used to heat a sample up to ~1000 degrees C and analyze the gases that are created. When compounds undergo a phase change, they tend to absorb energy without increasing in temperature. Think of ice in a glass of icewater; the system doesn’t start warming up until the last bit of ice has melted. Until that point, any additional energy goes toward the phase change between solid and liquid rather than warming up the mixture. TEGA operates on the same principle: by calculating how much energy is required to heat a sample it can detect phase changes. It also sends any gases created during heating to a Gas Chromatograph Mass Spectrometer to be analyzed.
During analysis, there was a significant release of carbon dioxide at high temperatures, indicating the decomposition of calcium carbonate (the same material that makes up limestone on Earth). Calculations show that if the carbonate was formed purely due to atmospheric humidity, it would be much less than 1% of the soil, but the TEGA results require something like 3%-5%, indicating that the carbonates formed in water.
Another interesting talk was from Delphine Nna Muondo, who talked about the use of laser pulses to simulate impact shocks. I will be using pulsed lasers for my upcoming research so it was interesting to see how another research group is using the same type of laser for very different purposes. Their work was focused on determining the chemistry induced by impacts, which they simulated with laser pulses. Laser pulses have the advantage over high-speed gun experiments that the can deliver energy equivalent to 100 km/s impacts, much higher than what can be achieved with actual impactors. Also, laser pulses are easily repeatable, and there is no contamination of the target by the impactor. The disadvantages of using lasers to simulate impacts are that natural impacts have longer-lasting shock waves, and they couple their energy to the target differently. Nonetheless, Muondo showed that laser pulses do induce some chemistry, which may explain the presence of some organics in the outer solar system.
Later in the day, one of the most interesting talks was one from Nilton Renno, discussing the possibility of liquid H2O at the Phoenix lander site. He suggested that the odd growths observed on the lander’s leg may have been extremely salty water droplets. Salts are very common on Mars and Phoenix showed that the soil was rich in perchlorates, which can lower the freezing point of water down to -75 degrees C. He suggested that daily variations in surface temperature, which oscillate above and below -57 C, would cause a layer of very salty water to be concentrated just beneath the surface. During landing, Phoenix’s rockets blasted through the soil and uncovered ice, and in the process “splashed” this brine onto the lander’s legs. The uncovered ice began to sublimate, and the water vapor then was absorbed by the concentrated brine droplets, causing them to grow! The growth slowed down toward the end of the mission because the exposed ice was no longer sublimating and providing water vapor.
The talk was pretty similar to one which I reported on back in December at AGU. I am of pretty much the same opinion; that it sounds like a plausible argument to me, but that it may not be as compelling as Renno thinks since the Phoenix team hasn’t been shouting this result from the rooftops.