Martian Canyonlands: Candor Chasma. Image credit: NASA.At Candor, their preferred site, parallel rock layers were exposed in the sloping sides of a 1.3-kilometer-tall mesa standing at the bottom of the four-kilometer-deep canyon. If the MSR lander could set down within a five-kilometer landing ellipse atop the mesa, then a seven-kilometer round-trip traverse would permit sampling of some of the layers. Recalling their 1977-1978 study, which assumed a more capable (and more costly) rover, they noted that a "much longer traverse — more than 200 km — would allow the full thickness of rock layers (~4 km) in the canyon walls to be sampled."
The MSWG's fifth report, the first of the six prepared by members of the MSWG Sample Acquisition Team, looked at the availability of rocks on Mars with emphasis on the equatorial Central Latitude Belt, which spanned between 30° N and 30° S. The report's author, University of Houston geologist E. King, explained that celestial mechanics and MSR lander engineering constraints would probably dictate that the Belt contain the first MSR landing site.
The twin Viking landers had had trouble collecting small rocks on Mars, King noted. This had led some to suggest that what looked like rocks at the Viking sites were in fact soft "clods" of martian dirt. If correct, then this hypothesis would mean that rocks were rare on Mars, which would in turn eliminate the primary motivation for an MSR mission; that is, to collect rocks.
King reported that his "evaluation of all of the presently available relevant data" had eliminated this concern "completely" for large parts of Mars, including for the Central Latitude Belt. Especially encouraging were data from the Viking orbiter Infrared Thermal Mapping (IRTM) experiment, which mapped thermal inertia (that is, how long it takes a given surface to become cool at night). Rocky surfaces need longer to cool down than do dusty surfaces.
Viking IRTM data indicated that much of the Central Latitude Belt has thermal inertias as high as 12. "It is very difficult to construct a reasonable model of the martian surface that has a thermal inertia of more than about 3 that does not have a substantial percentage of the surface area covered with rocks," King wrote.
He attributed the Vikings' inability to collect small rocks to inadequacies in the Viking sampler design. After it scooped a sample containing small rocks, controllers on Earth commanded the sampler to turn upside-down and shake for up to two minutes to sieve out dust. King noted that shaking the sampler caused its lid to flap open as much as an inch. This would allow any pebbles it contained to escape.
He advocated collecting rock samples in the form of drilled cores, since drilling could penetrate past any weathered rock rinds. Drilling could also collect uniform cylindrical samples that could be handled easily and stored efficiently in the MSR spacecraft.
King was ambivalent about the need for mobility in an MSR mission; he wrote that, if the objective of the mission were to collect fresh igneous rocks, and if the MSR landing site were similar to the Viking landing sites, then little mobility would be necessary. He added that, while it might be prudent to "build in some additional mobility as a margin of safety and to afford additional possibilities for sample collection. . .such provisions [had to be] traded off against lander science and returned sample weight."
USGS geologist H. Moore wrote the sixth MSWG report, which constituted a tour of the landscape within view of the Viking 1 and Viking 2 lander cameras. Viking 2 landed in Utopia Planitia, near the large impact crater Mie, a region more northerly than Viking 1's site in Chryse Planitia. Like King, Moore wrote that Viking 1 rocks were varied (there were 30 types) and tended to be smaller than Viking 2 rocks. The Viking 2 rock population, for its part, appeared to be dominated by ejecta from Mie.
Moore then described hypothetical rover traverses at the two sites. In each, the rover would visit 17 sampling stations, traverse about 100 meters, and range up to 20 meters from its lander.The boulder named "Big Joe" at the Viking 1 landing site in Chryse Planitia. Image credit: NASA.At the Viking 1 site, the rover would collect samples of cloddy soil, crunchy "duricrust" material, an active dune, and drift material, as well as 10-centimeter-long cores from bedrock outcrops, layered rocks, dark and light rocks, a pink rock, rocks formed by asteroid impacts, and gray-hued "Big Joe" (the largest rock near the lander). The rover at the Viking 2 site would collect samples of "inter-rock drift" material, a "drift dunelet," thick crust near a rock, and small rocks, along with cores from a coarsely pitted rock, planar and rounded rocks, a banded rock, the "massive" and pitted ends of one angular rock, and a ventifact (a rock scratched and carved by wind-blown dust and sand).
Moore estimated that the rover would spend between six and eight days traversing and collecting for each station. Each traverse would thus last from 102 to 136 days. The total mass of samples collected on each traverse would total about two kilograms.
The seventh MSWG report sought to estimate the number of crystalline rocks — that is, volcanic rocks such as basalt — at the Viking landing sites and to plan traverses that would adequately sample them. Its authors, R. Arvidson, E. Guinness, S. Lee, and E. Strickland, geologists in the Department of Earth and Planetary Sciences at Washington University in St. Louis, Missouri, argued that any rock larger than about 10 centimeters in diameter at the Viking sites was a good candidate for being crystalline.
Such rocks, they added, cover 9% of the Viking 1 site and 17% of the Viking 2 site. The former, they wrote, included bedrock exposures and at least four soil types, while the latter included two soil types and no bedrock. They pointed out that, while a sampler arm could probably reach a crystalline rock at either site, it would not be able to sample all of the available materials. For that reason, they proposed that MSR landers at the Viking sites should each deploy a "mini-rover."
The Viking 1 site was "such an interesting place," the Washington University team wrote, that they had planned for it a 40-meter traverse with seven sampling stations (with an option to extend to 50 meters and 10 stations). The basic traverse would collect 10-centimeter core samples from three rocks and four soil samples. The extended traverse would sample two more rocks, including Big Joe, and would gather a total of five soil samples, including very red soil from atop Big Joe.
The Viking 2 site, by contrast, featured minimal variety, so the Washington University team's traverse there would cover only 25 meters and seven stations. The mini-rover would collect four soil samples and core samples from three rocks.
N. Nickle of JPL's Flight Projects Planning Office authored the eighth MSWG report, which was titled Requirements for Monitoring Samples. The report was published originally as a JPL Interoffice Memorandum dated 20 October 1978. Nickle wrote that the "scientific integrity of the returned Martian samples is of prime importance." "Scientific integrity," he explained, meant "the preservation of the physical and chemical state of the acquired samples."
To maintain the scientific integrity of the samples collected during the minimum MSR mission, Nickle recommended that they be kept 20° C cooler than the estimated minimum temperature they had experienced on Mars, and that they be sealed within a container with martian air at typical martian surface pressure. In addition, he recommended that the samples be exposed to no more galactic cosmic and solar radiation than they had been on Mars, and to no magnetic field stronger than Earth's natural field.
The minimum MSR mission sought to control cost in part by avoiding science instrumentation not required for sample collection. In the MSWG's ninth report, J. Warner of NASA's Johnson Space Center (JSC) in Houston, Texas, looked at low-mass, low-power MSR science instruments designed to "provide adequate information to select samples."
His candidate instrument suite included a steerable imager, a reflectance spectrometer, a chemical analyzer on a boom, a boom-mounted densitometer, and a tool for measuring hardness (this might, Warner suggested, be made a function of the sample scoop; the Viking arm and claw had been used to scratch and chip at rocks to judge their hardness).
Warner also prepared the tenth and last report of the Site Selection and Sample Acquisition Study, which he titled A Returned Martian Sample. In it, he looked at the form the minimum MSR sample should take. He looked at two different landing site types: a Viking-like site "laden with a variety of rocks and soils" and a hypothetical "smooth plains site."
The JSC geologist cited Moore's report when he wrote that, at a Viking-like site, an adequate sample could be "obtained on a traverse of a few hundred meters that never leaves the field of view of the lander." He estimated that an atmosphere sample, a soil core, nine rock cores, four small rock fragments, two duricrust samples, and six scoops of soil would adequately represent a Viking-like site. Together these samples would have a mass of 4.1 kilograms.
An eight-month, 15-station traverse could adequately sample a rock-poor smooth plains site, Warner wrote. The rover would range widely over the smooth terrain. Sampling stations would occur at "obstructions" (for example, craters). The rover would drill two or three rock cores and collect one rock fragment at each station, scoop soil at every other station, and collect duricrust at every fifth station. Adding a soil core and an atmosphere sample would bring the total sample mass to 5.7 kilograms if two rock cores were collected and 6.9 kilograms if three cores were collected.SourcesMars Sample Return: Site Selection and Sample Acquisition Study, JPL Publication 80-59, Neil Nickle, editor, NASA Jet Propulsion Laboratory, 1 November 1980.
Detailed Reports of the Mars Sample Return Site Selection and Sample Acquisition Study, JPL 715-23, Volumes I-X, Mars Science Working Group Mars Sample Return Study Effort, NASA Jet Propulsion Laboratory, November 1980.
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