Well, after three days of fascinating science and heated discussion, the fifth and final MSL landing site workshop has come to a close, and the consensus is -- that all of the sites are pretty darn interesting.
We spent all afternoon on Wednesday [May 18] compiling a summary of information about each of the sites, broken down into the overarching hypothesis, the pros, the cons, and the uncertainties for each site. By the end of the day, most of the cons had been bumped into the "uncertainties" category, and all the sites still looked good. As a service to you, my loyal readers, and to organize my own thoughts on the sites, I am going to take the draft of this summary that is currently available on the marsoweb site, translate it from science-speak into English as much as I can. Unfortunately at the time that I wrote this the final draft had not been posted on the site (the link that claims to be the final draft was actually to draft 3) so I might be missing some of the things we added or changed on Wednesday.
I tried to condense things a bit, and removed some points that I thought were false, irrelevant or so obvious that they didn't merit their own bullet. I also added things, especially possible cons, that some people brought up which weren't captured in draft available online. I tried to be fair, but all of this tweaking and filtering on my part probably introduces my own bias. I will say that after the workshop, my mental ranking of the sites was much less clear than it was going in. That is, the science at all of the sites is good enough based on what I saw at the meeting that it's hard for me to say which one would be best.
Hypothesis: The remnant of a delta at Eberswalde crater and other layered rocks in the crater record the evolution of a habitable lake. The fine-grained minerals deposited by the delta concentrated and preserved organic molecules.
- There is a delta, meaning that sediment from a known area outside the crater was dumped into a standing body of water.
- The lowest layers of deltas are great for concentrating and preserving organic molecules, making them good targets for MSL. Lake-bed deposits would also be good for preserving organic molecules and evidence that the crater was once habitable.
- We could learn a lot about the conditions at Eberswalde and the climate on Mars when the delta formed.
- Based on the shape of some of the fan-shaped features at Eberswalde, the delta and the lake were probably long-lived.
- There are interesting targets in the ellipse, including possible hydrothermal veins, hills of ejecta from Holden crater, and exposed Eberswalde floor material.
- There is no evidence of a shoreline in the crater. Could the lake have been ice-covered?
- The source of water and sediment for Eberswalde might be ejecta from the Holden impact causing ice to melt and flow into Eberswalde.
- The delta looks like it formed after most of the planet had dried out. Is a younger age bad if you're looking for evidence of habitability and organic molecules?
- Even though there is good stuff in the landing ellipse, it is a long drive to the primary objective: the big delta.
- The delta at Eberswalde does not represent a significant portion of martian history.
Hypothesis: The layers in the central mound preserve a record of habitable environments over a long period of Martian history.
- The layered mound, with beds containing clay and sulfate minerals, is a thick record of changing environments.
- The geometry of some of the layers is most consistent with formation in a lake or flowing water.
- You land on and immediately get to study a fan of water-transported material from the ancient crater walls.
- Having a thick pile of layered rock which records many changes means that MSL can test lots of different environments on Mars for evidence of habitability.
- The layers of the mound can be traced a long distance, so what we learn with MSL can be extended to the rest of the mound.
- The type of clay minerals seen in the mound suggest that they have not seen high temperatures and pressures, which bodes well for protecting any organic materials those clays might hold. The sulfates in the mound might also preserve organics.
- There is a hard, fractured surface in the ellipse, and the pattern of fractures suggest it might be a playa, which is a good place to look for organics.
- Most hypotheses and observations suggest that the mound used to be a lot more extensive, but it has eroded back to what we see now. Still, there was one (somewhat controversial) hypothesis that the mound "formed as a mound" of minerals precipitated out of water.
- The total amount of time recorded in the mound is unknown, but the lower mound seems to be constrained to the early Hesperian based on crater counting.
- We can't be sure from orbit whether the layered rocks were deposited in a lake, or in some other less-habitable setting. (This is true at all sites.)
- The source of water is unknown. The channels on the rim are probably not big enough, and likely formed late, so groundwater could be involved.
- Even though there is good stuff in the landing ellipse, it is a long drive to the primary objective: the phyllosilicate and sulfate-bearing lower mound layers.
- Tying the stratigraphy of the mound to the rest of Mars might be difficult, since there are no obvious remnants of the mound outside the crater.
- Even though Gale has interbedded clays, it is mostly sulfate-bearing rocks. A vocal minority were concerned that this makes it too similar to Meridiani.
Hypothesis: Holden crater preserves evidence of an evolving system of flowing water and a crater lake. This system was a sustained, habitable environment.
- Holden's alluvial fans and layered rocks make up one of the largest and best preserved alluvial systems on Mars.
- The targets can be related to the global stratigraphy of Mars since lots of crater counting has been done on the Holden area.
- It's a diverse site, with fan sediments, clay-bearing layered rocks, flood deposits, and blocks of rock blasted into Eberswalde when the Holden crater formed.
- The layered rocks might be lake beds, which are good for preserving organic molecules.
- There are examples of all the key targets inside the ellipse, although better examples of some require a drive outside the ellipse to the south.
- The outcrops of impact-crushed boulders (mega-breccia) in Holden may suggest an impact-induced hydrothermal system.
- Other than the fact that the layered rocks have nice, continuous layers, there isn't much evidence that can help to tell how they formed from orbit. MSL could figure this out in-situ.
- The layered rocks may be relatively young. Same uncertainty as Eberswalde: is that a bad thing for life?
- The various science targets might record times spanning from very early mars to relatively recent.
- Has a lot of similarities to the other two crater sites, Eberswalde and Gale, but less stratigraphy than Gale and less evidence that the layered rocks formed in a lake than Eberswalde.
- Lots of ripples in the landing ellipse might make traversing across the ripples time-consuming.
Hypothesis: Mawrth Vallis records the geologic processes during early Martian history, when water was abundant and altered the rocks to form a wide variety of clay minerals. Because it is such an old portion of the crust that has been subjected to so much water, Mawrth would help us understand habitability on early Mars.
- Mawrth exposes the oldest rocks of the four candidate sites, so exploring these rocks would allow MSL to study evidence for processes that were active on early Mars.
- The rock record at Mawrth may be from a time that is not recorded in Earth's rocks, but which was crucial for the evolution of life.
- Mawrth has the strongest spectral signatures for clays on Mars that we know of, and the layers of clay minerals seen at Mawrth extend over a broad region of Mars.
- MSL would land on the primary target: the diverse hydrated minerals, and the changes in spectrally dominant minerals are close together, so MSL wouldn't have to drive far.
- Phyllosilicates are good at trapping organic molecules when conditions are right. Both of the major clay mineral types should be investigated for organics.
- The presence of phyllosilicates does not guarantee good preservation of organics (despite the obsession with clay minerals throughout this landing site selection process).
- We don't know much about how the rocks at Mawrth were deposited, or what environments they might record. It's not clear whether the mineral stratigraphy matches with the physical stratigraphy, or was overprinted later by alteration. We would probably need to land at Mawrth to find out.
- The amount of water, where it came from (atmosphere or ground) and how long it was present are all unknown.
- The relationship between the rocks at Mawrth and the huge Oyama crater just to the west is not fully known. We aren't sure if the dark capping unit in the ellipse is Oyama ejecta.
- The importance of impacts in forming the rocks is not clear, but given the age of the rocks they probably played a significant role in depositing the rocks and damaging them.
- There was no consensus that the environment in which the rocks formed could be understood based on rock texture and composition changes since Mawrth is so beat up by impacts, lacks context and may even lack traceable sedimentary layers.
- Oddly enough, Mawrth may have been exposed to too much water, which can be bad for preserving organic molecules. (There's no guarantee that this isn't a problem for other sites too.)
As I said above, I'm probably missing some points here, but this should at least give some idea of what the result of the public workshop was, filtered through my editing and biases. On Thursday the MSL science team met and talked about this some more, but I wasn't there and they are sworn to secrecy, so we will have to wait until the final decision is made by NASA HQ before finding out what happened during the Thursday meeting.