Ryan Anderson • Sep 16, 2008
3rd MSL Landing Site Workshop: Engineering and Geobiology
This article originally appeared on Ryan Anderson's "The Martian Chronicles" blog and is reposted here with permission.
We covered a LOT today, so I have decided to split things up. This post will cover the talks in the morning and then I will give each site its own post.
Today started off with presentations from some of the engineers and managers on the mission. They updated us on the rover's status (it it making lots of progress, but still has a long way to go!), and informed us that all seven potential landing sites are safe as far as the engineers are concerned. They told us this over and over in various ways with various pieces of evidence. The bottom line was: this workshop is about SCIENCE.
After the introductory talks, we spent the morning hearing about aspects of geobiology from several terrestrial geologists. The goal of this mission is to asses the habitability (past and present) of Mars, and therefore it is important to know what geologic locations are best at preserving evidence of life, assuming it is/was there.
First was Roger Buick who gave a great talk about the earliest records of life on Earth. The philosophy was, we should understand how hard it is to find evidence of early life here, and keep the lessons learned in mind when we go to mars. He pointed out that Mars may actually be better than earth at preserving life because there is no plate tectonics to destroy the ancient crust. He talked about six kinds of evidence of life that one can look for:
- "Dead bodies" (fossils)
- Preserved remains of biological activity - Think of Pompeii: the people were killed but their city was left as evidence. Same idea, but with microbes.
- Organic molecules - "If you were to take Grotzinger [the project scientist] and bury him in the ground for 3 billion years, you would still be able to work out his cholesterol levels."
- "Atomic fossils" (isotopes)
- Biominerals (minerals that form due to life)
- Microbial remineralization
These signatures are preserved best (on Earth) in fine-grained, sedimentary rocks, especially in long-lived aqueous environments. On Mars, Buick speculated that you would want to look for a site with diverse fine-grained sedimentary rocks, a long-lived body of low-acidity water with rapid formation of minerals (such as salts forming when lakes or oceans dry up). Also desirable is if the place hasn't been disturbed and has only recently been exposed to the surface and has well understood geology.
The next presentation was given by Roger Summons about the preservation of biomarkers on Earth. a biomarker is basically an organic molecule or set of molecules that indicate that life was involved in the chemistry. These can include complex, patterned structures like DNA or protein that are made of simple building blocks. Also important are the configurations in which you find certain types of molecules, sometimes called "handedness" or "chirality". The common analogy is that a glove can be left or right-handed and still be a glove. The same idea holds for some molecules. The interesting thing is that biology often only uses one possible "handedness". This is true of amino acids which only have two possibilities, but also is true of cholesterol which has 256 potential configurations! You can also look for chunks of larger molecules that you know are made by life.
Summons said that organics are best preserved when isolated, concentrated, formed at the same time as minerals, and buried in fine-grained sediment. On Mars, biological organics, if there are any, would be best preserved in low temperature sedimentary environments such as places that form clays, evaporites, and silica.
The third geobiology-related talk was given by Nick Tosca, who shared the results of some calculations that he had done about the origin of life in very salty water. He said that there are two ways that microbes deal with salt and both basically amount to minimizing the difference in saltiness between the inside of the cell and its surroundings. The adaptations that allow this are just that: an adaptation. Tosca made the point that it is pretty unlikely for life to originate in highly saline water. His calculations showed that the "water activity" in brines on mars was very low, meaning that they would have been nasty places for life. For site selection, he recommended choosing a site that had a favorable environment for prebiotic chemistry, had water for a long time, but once it dried out, remained dry.
Next, Lisa Pratt talked about how good phyllosilicates (clays) are at preserving life. She showed us a lot of data from clays on Earth, and pointed out that in standing water, you get less preservation of organics the deeper the water. Basically, most of the life is at the surface, but as organics drift down to the bottom, the longer they spend in the water, the more time they have to react with things. In open marine settings, less than 1% of organics get preserved on earth. On the other hand, in lakes, up to 10% can be preserved!
Alan Howard then talked to us about geomorphology and how you can tell what the environment was like when the sedimentary rocks were formed. He emphasized the concept of "source to sink". That is, you want to know how sediment was produced, how it was transported, how it was deposited, how it may have been altered, and how it has become exposed to the surface again. With that in mind, he suggested that sites in closed basins would be the easiest to understand.
We also heard from Jeff Bada, who talked about how well sulfate minerals might preserve organic molecules. He pointed out that radiation on the surface would destroy any biomarkers, as could oxidating chemicals. Using amino acids as a "typical" organic molecule, he evaluated how good different minerals were at preserving them and found that sulfates did the best job. Salts were also good and clays were only good if the organic molecules were deposited in non-oxidizing conditions. He recommended that we go to a site with a variety of minerals.
Finally, John Grotzinger, the project scientist, summarized things and reminded us of the main criteria to keep in mind while we discuss all of the sites. The four criteria are:
- Geologic Diversity - Are there a lot of interesting things at the landing site?
- Geologic Context - Do we understand the site? Can we fit it into a larger story about Mars?
- Habitability - Could life have once survived here? (Note: this is different from asking whether life could originate here)
- Preservation - Assuming that there was life, would evidence of its existence be preserved?
I will stop here for now. In the afternoon, we talked about the first two landing sites: Miyamoto Crater and South Meridiani. I've decided to give each site it's own blog post though, so stay tuned!
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