Implementing a strategy for Mars exopaleontology

摘要

A critical step in implementing a strategy to explore for evidence of past or present Martian life and/or prebiotic chemistry is to locate accessible surface outcrops of aqueously-formed sedimentary deposits, especially fine- grained, clay-rich detrital sediments, water-lain volcanic ash deposits, and chemical precipitates which provide especially favorable environments for microbial fossilization. We are presently limited in our site selection efforts by a lack of high spatial resolution remote sensing data at wavelengths that can provide information about surface mineralogy. Globally-distributed compositional data will be obtained at an average spatial resolution of approximately 3 km/pixel by the Thermal Emission Spectrometer instrument launched on the Global Surveyor orbiter in 1996. This will provide a basis for initial targeting of sites for a landed rover missions in 2001 and '03 which will cache samples for potential Earth return in '05. During '05, a sample return vehicle will be sent to one of the previous landed mission sites based on what is learned from those missions during in situ rover investigations. To optimize planning for rover missions to explore for evidence of past life, we need to attain outcrop-scale spatial resolution in the range approximately 100 to 300 m/pixel. This will be required to precisely locate sedimentary deposits of the right mineralogy (i.e. rock types most favorable for preserving a fossil record of past life) at landing sites and be able to reach them with rovers during nominal mission times. The earliest opportunity to obtain high spatial resolution orbital mapping of mineralogy is the 2001 opportunity. This data is needed as early as possible in the '01 mission to influence the landing site selection for the '03 rover mission. Once deposits of exopaleontological interest have been identified from orbit, we must deliver well-equipped, highly mobile science laboratories to the highest priority sites to carry out in situ mineralogical and organic analysis of rocks. To guarantee accessibility to the right samples, we will need to improve (1) landing precision, (2) rover mobility and (3) sample acquisition systems. Ideally, landing precision will match the minimum traverse distances needed for rovers to reach their intended targets within nominal mission times. The landing precision and mobility requirements will vary with each mission, depending on the size of the targeted deposits and terrain trafficability. But some of the highest priority exopaleontological targets (e.g. hydrothermal) are likely to be quite small (few kms~2), requiring rover mobility (and equivalent landing precision) in the range of 5 - 10 kms. Under present mission scenarios, rovers in 2001 and '03 will need to screen landing sites for the most promising rocks using in situ analytical methods. Once targeted, rocks will need to be sampled and cached for potential return during the 2005 opportunity. In addition to providing samples for potential Earth return, rovers in '01 and '03 will also gather crucial mineralogical information for ground truthing orbital data. Because sample return payloads are likely to be very small (several hundred grams), sub-sampling of larger rocks will be necessary to ensure that we obtain the most promising materials for return to Earth. In screening rocks to subsample, spectral methods that combine both mineralogical and organic analysis hold the greatest advantages for exopaleontology. However, such analyses will need to be carried out on freshly exposed rock surfaces, requiring rovers that are capable of coring, grinding, and/or breaking rocks. The need for rovers to expose fresh rock surfaces and to efficiently subsample larger rocks will require substantial improvements in sample acquisition and handling systems.