Brad Thomson, a former grad school classmate of mine, was kind enough to write down this summary of the Day 2 discussions of individual possible site candidates. There were about 30 sites presented over all three days, and they were divided into thematic groups: "Layers in Craters," "Meridiani Planum," "Chasmata," "Chaos, Valles, and Volcanoes," and "Salts, Sulfates, and Clays." I'll have analysis for you later on how the scientists wound up voting for or against these various sites. If you're curious about what they look like, check out the MSL landing site webpage, where you can download almost everybody's presentations. --ESL
by Brad Thomson
Wednesday was the second full day of the MSL (Mars Science Laboratory) site selection workshop. Unlike the Mars Exploration Rover mission, which featured two golf-cart-sized landers, this time we have only one giant, Volkswagen beetle-size. So at the end of this process we have to pick our favorite place on Mars, not our favorite two places.
After each cluster of presentations, there was a round of voting by the audience on the quality of the science that could be done at each site. Using each of four criteria listed below, each person cast one of three possible votes: green, yellow, or red.
- Ability to characterize the geology and geochemistry
- Evidence for a habitable environment
- Preservation of biosignatures
- Ability to assess biological potential with MSL payload
If you haven't guessed by reading the voting criteria, the principal science goal of the MSL missionis not geology -- it is to "assess whether Mars ever had an environment capable of supporting microbial life." Therefore, all of the proposals focused on finding sites with not just water, but water that existed in a low-energy depositional environment that has a high potential for archiving and preserving potential biological signatures. For example, an erosional channel cut by a fast-moving stream is a less relevant environment from an astrobiological perspective than a depositional lakebed setting, even though both cases involve water in contact with rock and sediment.
The recent addition of OMEGA and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) near-infrared spectroscopic data to the mix of tools available for studying Mars have greatly expanded our view of minerals, particularly altered minerals, on the Martian surface. It seems that no site will move forward to the next stage of the site selection processes without the orbital detection of altered minerals. In general, altered minerals require the presence of water, so they provide an independent means of confirming the picture provided by the surface morphology and geology alone.
The morning session begun with a continuation of the theme from yesterday afternoon, "Layers in Craters."
Carlton Allen spoke first about a crater in southwest Arabia Terra (recently christened Vernal Crater) that may have once contained a lake. Linda Kah presented another crater in West Arabia that contains about 500 meters of rhythmically bedded strata. The proposed landing ellipse is outside of the crater of interest, so here the rover would have to negotiate a path down the 10-degree slopes of the crater walls. Sharon Wilson and Eldar Noe Dobrea spoke next about a potential landing site Terby Crater, which lies on the northern margin of the Hellas Basin. Some 2.5 kilometers of layered sedimentary layers are exposed within Terby, which may have a lacustrine (lake) or loess (airfall dust) origin. Eldar presented some interesting evidence of hydrated minerals at the site. Finally, Liliya Posilova presented a potential site near a crater in Meridiani Planum. Although this group of talks was about individual craters of interest, it serves as a bridge to the next batch of talks, focused on landing sites in Meridiani Planum.
After the voting was concluded, we moved on to the next batch of sites proposed in Meridiani Planum. Opportunity, the MER rover formerly known as MER-B, has been trundling across the Meridiani plains since 2004. But it has only been able to access a relatively small portion of the total stratigraphic section, about 7 meters. So many of these talks proposed sites that will explore a different portion of the stratigraphy than the packet of layers that were analyzed (or more accurately, continues to be analyzed) by Opportunity.
Jean-Pierre Bibring, the Principal Investigator (PI) of the OMEGA instrument on the European Mars Express mission, presented multiple sites in Meridiani Planum. Bibring's focus was on sites that contain or are near a boundary between sulfate-rich terrain (like that currently being explored by the MER rover) and phyllosilicate-rich terrain detected by OMEGA. Mike Malin, head of Malin Space Science Systems, spoke next about a site in Northern Meridiani and billed it as one of the few places on Mars that is like "parking lot," that is, flat at all length scales and relatively rock-free. Some additional sites may be retained through the next round of down-selection if they are so called "safe-havens," so this is not a bad strategy. Sandra Wiseman spoke about a potential site in "Runcorn" crater, a provisional name, that is at southern end of Meridiani near the Opportunity site. Following an OMEGA clay detection in this region, CRISM observations have shown additional evidence for a phyllosilicate-rich layer that is capped by another layer. Horton Newsom spoke next about a site in West Meridiani that is close to the previous ellipse. Here there is a possible inverted (that is, now positive relief) channel that shows interesting spectroscopic evidence for a chlorite. This latter detection is a recent result from the THEMIS instrument team led by Phil Christensen. Finally, Brian Hynek presented a potential landing site in East Meridiani that is topographically and stratigraphically lower than hematite-rich plains currently being explored by Opportunity. Although CRISM data does not yet cover this site, material with a similar texture to nearby clay-rich deposits are exposed within the ellipse, which hints at the role of water.