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Mars Exploration Rovers Landing Sites: First Cut
If you're going to send machinery to Mars, there's an obviously important question to address: Where to land? The answer depends on both science (where can we learn the most about Mars wetter, better past?) and practical constraints (what terrain is smooth enough so that the rover-bearing spacecraft will make a safe landing?)
The SETI Institute's Nathalie Cabrol, a Principal Investigator working out of NASA Ames Research Center, talks about landing sites that have made the "First Cut" for consideration by the two rover missions scheduled to be launched in 2003.
The Red Planet beckons but some parts beckon more seductively than others.
High-Priority Landing Sites for the 2003 Mars Exploration Rovers: First Cut
Nathalie A. Cabrol Principal Investigator, SETI Institute
On January 24 and 25th, the First Landing Site Workshop for the 2003 Mars Exploration Rovers (MER) was held at NASA Ames Research Center. The goal of the workshop was to downsize a list of 29 sites proposed by the planetary science community to a few sites that will be fully investigated by the Mars Global Surveyor's MOC camera (now orbiting Mars) in the coming weeks as potential candidates for MER A and MER B.
After two days of discussion, the list of 29 was reduced to 8 high-priority candidates. The region containing hematite (a course-grained, iron-bearing mineral that is formed either in shallow standing bodies of water or via the action of water leaching on minerals. Hematite implies the former existence of liquid water.) provoked considerable consensus in the research community, and unless unexpected obstacles are discovered in the MOC images that will be acquired in the coming weeks, one of the rovers should land there in 2004.
All 8 candidate-sites will be fully covered by high-resolution MOC reconnaissance images in order to assess their potential for science and their safety, as the sites for MER must provide the best trade-off between engineering constraints and science return. They also need to satisfy a criterion of spatial separation from each other that will facilitate communication and rover operations during the mission. Following this survey campaign, the number of sites will be downsized again next September. In the end, only two final sites and their back-ups will remain.
The designated 8 are extremely interesting and have each a high scientific potential, everybody will agree on that point. Unfortunately, some other promising sites, like Apollinaris Patera or Durius Vallis had to be dropped because of the abundance of dust revealed by the TES data. The designated 8 sites are:
- The hematite region (3 possible sites)
- Melas Chasma, Valles Marineris
- Gale crater
- Gusev crater
- Isidis Planitia
- Elysium Planitia
The presence of impact crater paleolakes in this first cut is significant for many reasons. Impact crater lakes have been studied and promoted as high science return landing sites for years now by several researchers, including Edmond Grin and myself, both of the SETI Institute. When the 2001 mission was still a lander, the constraints on latitude did not permit the inclusion of Gusev Crater which is 5 degrees too far south to be explored. But following the loss of the two 1999 missions - Climate Orbiter and Polar Lander - the 2001 lander changed into an orbiter mission and the 2003 Athena became the twin MER mission.
In the meantime, the Mars Global Surveyor MOC camera was unveiling the secrets of Mars. What had been suspected at Viking resolution was revealed in its great beauty: there had been lakes in craters, and countless sedimentary layers were discovered in impact basins (see Malin and Edgett, 2000 Science). It is highly probable that not all the layers have a lacustrine (lake) origin: wind and volcanic activity are also candidates, but for some of the craters, the geological and morphological context, such as the presence of an inflowing channel, argue for the lacustrine case. Suddenly, Gale crater became a new Mars "star" with a central mound finely layered by the work of sedimentation, erosion, and the sculpting hand of time [Fig. 1].
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Figure 1 Details of the layers in Gale crater (222,5W/5S). Mars geologic and climatic history are preserved in this sedimentary record. If one of the rover lands in Gale, it will be south of this region, in a less rugged terrain, where layering is still accessible but landing safe. (MOC image, courtesy NASA JPL/MSSS) |
Unfortunately, a few weeks before the workshop, new engineering considerations were eliminating Gale and Gusev once again. This time, the latitude was not the guilty party, but the landing ellipse. It had became too large at the considered latitudes to include a possible safe landing in either the 160-km diameter Gale or the 170-km Gusev craters. Therefore, when I started my presentation on the interest of selecting landing sites in impact craters for 2003, I had not much hope for a positive result: Gale and Gusev were out because of the ellipse; Schiaparelli Crater was out because it is too high (the minimal altitude requirement is -1.3 km and Schiaparelli is -0.9); Henry Crater (a beauty) was eliminated because of the landing ellipse too. This 170-km crater is locate 10N Latitude and there the landing ellipse is a little bit larger than 300 km.
To be provocative, I aso proposed Aram Chaos [Fig. 2], a 275-km basin, where altitude, latitude, and ellipse were fine. The only problem was the chaos itself; definitely too rough for MER! My presentation was during the second day. During the first day, discussion had been going on between the engineers and the scientists. We know that we have to live with the engineering constraints, and that if we do not land safely we will not have a mission. However, at some point, these constraints become really too limiting for science, and the planetary science community argued about that. This is when I made the remark that Mars was not flatBut my colleagues stood their ground too, and added more arguments. Several of them also advocated ancient crater lakes. Once again Gale and Gusev were mentioned, and also Boeddicker and an unnamed crater in the Meridiani region. By the end of the day, all the ancient crater lakes we had presented were still there and making the first cut: Gale and Gusev as priority-targets for MOC along with the Valles Marineris, Elysium, Isidis, and the Hematite sites, and Boeddicker and the unnamed crater as back-ups in case the two primary craters are deemed too dangerous on the basis of the new images.
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Figure 2 Aram Chaos (MOLA data). 21.5W/2.5N. Aram is probably the most complete of all sites. It shows evidence of lacustrine activity, a delta, high-albedo deposits, and a hematite deposit that can be put in the perspective on the fluvio-lacustrine history. Unfortunately, the site is too rugged for the MER missions. The frequency and size of the blocks would be highly hazardous for the mission. |
Personally, I think that the remaining sites are ALL absolutely beautiful, well thought-through and would make excellent landing sites. The reason I am particularly happy about Gusev and Gale is somewhat obvious, but it is not only because I have been spending a lot of time studying and publishing about them. The reason is knowledge and what can be learned from such site. Ancient lakes are time machines. They are receptacles where material from various origins (lakes, rivers, volcanoes, wind) are collected. They are open books left for us to decipher about the evolution of Mars, its cycle and its changes. In craters like Gale and Gusev that are at least 3.5 billion years old, a real Rosetta Stone is waiting for us up there.
How much we will learn will depend on how far we can go and how long the mission can last. But lakes have one more quality: on Earth, they are known to favor the inception, development, and preservation (via fossils) of life. If life ever appeared on Mars, lakes are certainly among the best places with hydrothermal sites to investigate. It is then important that ancient lakes are included in the first cut of this selection. The road is still long before the final selection, and in the coming months the MOC images will help us decide where to go. Safety will certainly prevail but all 8 sites present great promise of science harvest. Wherever it is, the public and the science community will enjoy wonderful landscapes and, I hope, some distant and now silent lakeshores.
Footnote: All details concerning the 8 sites and back-ups can be found at: http://marsoweb.nas.nasa.gov/landingsites