How is site selection done?
A group of relevant scientist convene several times over several years to discuss where best to land a planned Mars rover or lander. The attendees of the workshop are typically the Principal and Co-Investigators from MSL, Principal and Co-investigators from major experiments expected to be used to certify the sites, and many scientists unaffiliated with MSL.
For picking the landing site of Curiosity 5 workshops were held. In these landing site workshops the science requirements of the MSL mission were weighed against the engineering requirements and the safety of the landing sites.
The discussion recognised the need for a strategy to ensure that the science value of the landing site is not unnecessarily compromised when faced with late-breaking threats to Entry, Descent and Landing (EDL)or degradation in rover surface performance. Also the scientificially interesting sites need to meet the nominal engineering constraints.
The question was raised whether having two rovers on the same side of Mars sending their data back to earth would give problems as Opportunity and Spirit were operational for most of the time the selection discussions where being held. The conclusion was: it would not. As the final site, Gale Crater is neither close to Gusev Crater nor Meridiani Planum, where Opportunity still operates, it will not be an issue indeed.
The First Landing Site Workshop was held end of May in 2006 in Pasadena, California. The objective of the workshop was to hear about the complete range of proposed landing sites for MSL and learn how each fared with respect to stated mission, planetary protection, and engineering constraints. A rank ordering of the proposed sites for use in future targeting for imaging by the Mars Reconnaissance Orbiter (MRO) and other orbital assets was one of the results of the workshop. All the proposed sites were intended to remain under consideration until new data show them to be in violation of engineering or planetary protection constraints or introduce significant risk to Curiosity in achieving her mission science objectives. A wide variety of sites were proposed at the workshop. A total of 33 general sites were proposed to be further discussed that incorporated 94 landing ellipses that span a considerable range of latitudes and elevations on Mars. The sites farthest north and south are at 23 degrees North and 57 degrees South.
The second landing site workshop also took place in Pasadena, CA, one year later in October. The 33 sites that emerged from the first workshop were discussed, as were some new ones, due to new data coming through Mars satellites like MRO. The main four criteria while evaluating the landing sites:
* 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?
The third landing site workshop was held in Monrovia, Ca in September 2008. Updates were given on how far the assembling of the MSL rover had progressed; there still was a long way to go before the rover would be flight ready.
At this workshop 7 potential landingsites (see image on the right) were discussed that were deemed safe from an engineering point of view. Also there was further discussion on what geological locations are best at preserving evidence of life, assuming it once was present on Mars.
Emphasis was given on how hard it is to find the earliest records of life on Earth. This means that finding life on Mars with no more than a robot and its limited resources of instruments and movability (compared to a human researcher in the field) will be that much harder. The good news is that as Mars has no plate tectonics, that destroys a good deal of fossil records on Earth, it might actually be more likely that Mars has preserved evidence of life.
We are looking for the following signs:
- “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 you therefore most probably 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 fourth landing site workshop was held in Monrovia, CA, from September 27-29, 2010. The meeting had over 150 participants from the science community and the MSL science teams on all three days of the workshop. The workshop objective was strongly focused on discussion of the science merits of the four remaining candidate landing sites. The goal of the workshop was to develop summary statements regarding their science merits as measured against major science objectives of the MSL mission. The workshop participants agreed unanimously that all four of the sites represent acceptable science targets for exploration by Curiosity. This lead to the conclusion that a fifth and final workshop was needed to finally decide where to land the next Mars rover. A call was send out for suggestions from the science community of specific targets for study at each of the four landing sites.
The fifth and final workshop took place in May 2011 again in Monrovia, California. Even after all these days of discussions all agreed that all of the four proposed landing sites are very interesting. By going about it very methodically, compiling pros and cons of each site. After 3 days of heated discussions this finally led to the Gale Crater being selected as the landing site for MSL
The four landing sites that were discussed in the last two workshops (Holden crater, Gale crater, Mawrth Vallis and Eberswalde crater) all four represent compelling locations where MSL can significantly advance our knowledge of the conditions and potential habitability of Mars. Gale Crater was already under discussion as landing site for the Spirit rover, but at that time the precision of landing a rover on Mars was not as great as today. The wide and long landing ellipse that Spirit would have needed made landing on the crater floor of Gale Crater too risky and Gale Crater was rejected as landing site for Spirit. 8 years later this is no longer true for Curiosity.
All four remaining sites clearly possess high science merit and were deemed to be generally safe for landing by the MSL project.
- Eberswalde Crater contains a delta with phyllosilicates, a potentially habitable environment that is particularly favorable to the preservation of organic materials.
- Holden Crater contains finely layered phyllosilicates suggesting deposition in quiet fluvial or lacustrine setting with a well
- Mawrth Vallis exposes an ancient preserved layered stratigraphic section providing an opportunity to characterize early wetter conditions in the Noachian.
- Gale Crater offers access to diverse stratigraphy, including interbedded sulfates and phyllosilicates in a 5 km high mound that reflects deposition during changing environmental conditions.
The coming days we will discuss the pros and cons of each of the proposed four landing sites.