LPSC: Saltwater at the Phoenix landing site?

It's the Phoenix sessions at the Lunar and Planetary Science Conference (LPSC) that received the most advance buzz, because of a press release issued by Nilton Renno, a researcher at the University of Michigan, to the effect that those mysterious blobs on the lander legs are actually liquid drops of saltwater. The announcement refers to this LPSC abstract (PDF format).

The LPSC abstract is coauthored by quite a long list of researchers: Brent Bos, David Catling, Ben Clark, Line Drube, David Fisher, Walter Goetz, Stubbe Hviid, H. Uwe Keller, Jasper Kok, Sam Kounaves, Kristoffer Leer, Mark Lemmon, Morton Madsen, Wojciech Markiewicz, John Marshall, Chris McKay, Manish Mehta, Miles Smith, Peter Smith, Carol Stoker, and Suzanne Young. I immediately recognize lots of these names -- you have the mission's PI, the lead scientists for several of the experiments, and several other very well-respected and senior Mars researchers. This isn't some crackpot's presentation. At the same time, I just couldn't bring myself to post anything about this announcement until it was actually presented at LPSC and I could begin to hear some of the reaction from people whose names were not on the coauthor list.

So I looked for someone to help me out with some impressions from the meeting, and Keri Bean offered her assistance. She's an undergrad at Texas A&M who worked with Mark Lemmon on the operation of Phoenix' camera, and an active member (indeed, a founder) of the social networking community of Mars missions. (She Twitters as aggieastronaut.) She attended Renno's talk and actually had a copy of his slide presentation in front of her as she wrote the following report. Before I get in to that, I wanted to post an animation of the images of the lander legs that are the basis for the abstract. These weren't taken from the raw images website; they represent the calibrated data, the archived versions stored on the Planetary Data System. Many thanks to Gordan Ugarkovic for his aid in converting them from their archived format. If you would like to look at the original data for yourself, here is a Zip file (2MB) containing the archive-format files (32-bit PDS formatted images) and also the converted versions that I worked with, in 16-bit PNG format.

The hottest Phoenix topic recently has been an abstract submitted to the Lunar and Planetary Conference about evidence for liquid water on Mars. The planetary science community, especially the Phoenix science team, are actively debating this subject. I attended Nilton Renno's presentation Monday afternoon, and he was kind enough to let me borrow his presentation to hopefully explain his idea and clean up any misconceptions.

First off, get rid of the image in your head of pools of pretty blue water that you could swim in sitting on the surface. If you must think of an Earth analogy, you'll get pretty close with the Great Salt Lake in Utah, except much saltier, in much less quantities, and not on the surface. Specifically, the liquid water suggested in the paper is not pure liquid water, but instead a brine. Brine is liquid water that has a very high salt concentration, either approaching or at saturation. Brines have been suggested to exist on Mars for more than 30 years.

Phoenix's Thermal and Electrical Conductivity Probe (TECP) showed us that at the landing site, the humidity is high near the surface and decreases with height. We also know Phoenix's landing site has salts, thanks to the results from the analytical instruments, the Wet Chemistry Laboratory and Thermal and Evolved Gas Analyzer. A simulation experiment was done to measure the influence of Phoenix's landing thrusters on the terrain. Pressures increased to several times Earth's atmosphere, and temperatures reached between 1000-1200 Kelvin (that's 727- 927 degrees Celsius, or 1340 to 1700 degrees Fahrenheit!) With this much force and heating, it was suspected that the thrusters would easily brush away the top layer of soil, exposing the ice underneath the lander. Sure enough, upon landing, ice was exposed, and "Holy Cow" was born.

For salty liquid water to exist above the surface, conditions have to be right. The temperatures monitored by Phoenix mostly stayed between -20 to -95 degrees Celsius. The pressure also mostly stayed between 7 and 8 hectopascals, or millibars. The surface pressure could permit liquid water, if the temperatures increased. To demonstrate this, play around with the ideal gas law, PV=nRT, the equation taught in high school chemistry courses. P stands for pressure, V for volume, n for the moles of gas present, R is the ideal gas constant, and T for temperature. To make the sides equal, each side needs to balance the other. R and n are essentially constants, so if you change the pressure or volume, the temperature has to compensate. If you increase the pressure, temperature also has to increase, and vice versa. So in our case, the lower the pressure, the lower the temperature can be. While that is for water vapor, it's essentially the same concept for working with liquids.

If you're less equation inclined, think of the trucks that drive around after an ice storm, depositing sand or salt on the roads. The salt combines with the ice, and the salt-ice mixture has a lower freezing point than the outside air, even if it is below zero degrees Celsius. So the salt ice begins to melt, forming a liquid saltwater mixture. Also consider snow, which typically doesn't start to form until cloud temperatures drop below -2 degrees Celsius (28 degrees Fahrenheit) because clouds aren't made of pure water vapor.

Now bring this back to Mars. Brines can be a liquid at a much lower temperature than pure liquid water. How do they think Martian brines get their high salt concentration? The water comes and goes as part of the freeze and thaw cycle of the polar region, leaving the salts behind. Over a very long period of time, the salts concentrate and form a thin layer and/or small pockets near the surface.

Now take a look on the lander leg of Phoenix. There seems to be a bunch of little spheroids hanging out on the leg. These spheroids were monitored over the course of the mission. The drops near the top of the strut hardly change overall, due to them being closer to the lander, and thus warmer. The drops further down the leg move, merge, and even drip over time. These little spheroids also change brightness by 10-15% over the mission. They increase just before moving or dropping, which is consistent with the dissolution (removal) of salts. The centers of the spheroids are darker, which is consistent with exothermic chemical reactions. Exothermic reactions are those that release heat over the course of the mission. Also, based on the conditions, there would be a critical size these spheroids can reach. None of the spheroids observed was observed to reach that size. There was one that got close, but dripped before actually reaching that size.

The thermodynamic evidence focuses on the potential that these spheroids are formed by deliquescence. Deliquescence is the process by which a substance (in our case, the salts) takes water vapor out of the atmosphere and forms a liquid solution. The idea that deliquescence was happening on Phoenix's legs is supported by several observations. First, the spheroids only grew on dark material (which was speculated to be a muddy mixture). Second, the growth rate of the spheroids matched the rate predicted if you assume deliquescence is happening. Third, the spheroid that dripped stopped growing, suggesting the salts went with the water. The spheroids grew in size on Phoenix's leg, which is warmer than the sublimating ice below. Phoenix's leg was warmer due to the heat generated by all the electronics on board. The spheroids also started sublimating away towards the end of the mission when it was considerably colder, because the frozen brine would sublimate as the salts separated from the water.

The first question that comes to mind is "Why don't they think the spheroids are made of ice, not liquid water?" They argue that ice particles wouldn't have formed in spheroids, they would have formed a thin, uniform layer, much like the frost coating seen later in the mission. For the spheroids to be ice at the observed weather conditions, the humidity would have to be higher than 100%. Also, toward the end of the mission, when frost was abundant at the landing site, ice spheroids should have grown in volume rather than shrinking. Ice couldn't form on the lander leg unless the leg was colder than the ice, but engineering data returned from the lander shows warmer temperatures.

The next question that's most often asked is "Couldn't the thrusters' composition have contaminated the landing site?" The answer is that Phoenix definitely disturbed her landing site; however, there is no evidence Phoenix chemically altered the site. If any ice was melted by the thrusters, it would have quickly turned into a vapor and not have turned into a liquid. After landing, several containers were vented, and all were on the opposite side from the spacecraft from where the robotic arm's workspace, and thus also the leg that showed these spheroids. The engineering data doesn't show that there was any hydrazine left to vent, and had there been, it would have been a solid at Phoenix's site due to the low temperature. Any byproduct of the hydrazine would not have caused the spheroids either.

The conclusions of Renno's presentation were that the layered ice in the Dodo-Goldilocks trench (99% pure water ice, very bright white) and in the Snow White trench (approximately 30% ice, 70% soil, blends into the soil color) suggest that this brine layer is present near the surface in undisturbed areas.

Does this mean that there really is a brine layer beneath the surface in the polar regions of Mars? That still remains to be seen. Scientific hypotheses, especially ones with such significant implications for geology, geochemistry, habitability, etc., are meant to be debated and scrutinized. We probably won't know the real answer for many years to come.