The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Preventing the Forward Contamination of Mars
the standard for all subsequent Mars soft landers to date, although tests also showed that the purified hydrazine would contaminate Mars landing sites with ammonia (50 to 500 ppm), N2 (5 to 50 ppm), and possibly a small amount of water (quantity not measured). The large amounts of ammonia trapped in the soil would make interpretation of organic analyses more difficult.1 The results of the rocket exhaust assessment also led to the conclusion that, for the Viking mission, the combined effects of plume gases, surface heating, surface erosion, and gas composition resulting from the use of retrorockets would not interfere with the planned biology investigation (Husted et al., 1977).
The rocket exhaust study conducted for the Viking mission also addressed gas dissipation from the soil after landing (actually release and diffusion), because measuring the chemical composition of the atmosphere (especially the presence of less abundant species to accuracies of 100 ppm) was another mission objective. The Viking Molecular Team set as criterion that exhaust gas contamination at a concentration above 10 ppm be permitted to exist for no more than 2 days after landing. That period was considered to be sufficient for diffusion of the plume gases (actually calculated to be 2.7 days, but any minor wind or air movement will decrease the amount of time) (Husted et al., 1977).
No subsequent comprehensive analyses for retrorocket contamination have been performed. The Viking assessment results have served as the ad hoc basis for all subsequent NASA Mars soft-lander missions, that is, Mars Polar Lander, Mars ’01 (not flown), and Phoenix (in development). Furthermore, the use of solid retrorockets on Pathfinder and MER A/B, which emit very different exhaust products (including aluminum oxide and carbonaceous compounds from pyrolysis of the ethylene propolyne diene monomer rubber case insulation), has not been assessed as a potential source of contamination because those landers came to rest quite a distance from the location of the terminal retrorocket burn.
POTENTIAL CONTAMINANTS FROM FUTURE MISSIONS
NASA’s Mars Exploration Program has planned a series of landing missions that will follow the Phoenix Lander mission in 2007 (see Chapter 3). The Mars Science Laboratory (MSL) mission in 2009 will be a nuclear-powered rover with an order-of-magnitude more payload (50 kg, 10 instruments) and enhanced capability as compared with the MERs. MSL will employ a unique “sky-crane” landing system that will allow it to lower the rover on a tether to the surface while hovering on retrorockets and then fly away from the landing site (see Figure G.1). Follow-on landers such as the Astrobiology Field Laboratory and Mars Sample Return (MSR) missions will most likely use this sky-crane landing system. The sky crane is expected to use hydrazine retrorockets as did Viking, but it may have throttled engines rather than pulsed engines, which would reduce potential disturbances of soil.2 Nonetheless, the sky crane will require larger loads of hydrazine propellant for their payloads (200 kg compared with 85 kg for Viking) and will at least temporarily contaminate the atmosphere surrounding the landing site.
The first stage of the planned Mars Ascent Vehicle (MAV) of MSR missions poses another possibility for contamination of the rocket site and atmosphere. Current designs use a solid motor first stage that ignites at the surface and burns to an altitude of approximately 15 km. Whether or not the resultant contamination would be tolerable, given that the sample would already have been collected, would likely depend on whether any subsequent surface investigations were planned to be conducted after the ascent stage launch and on the nature of the atmospheric contamination by the rocket during launch.
According to Husted et al., “The presence of ammonia complicates both the experiment preparation for Martian chemical analysis and the interpretation of the returned data. The primary concern is the potential reaction of ammonia with simple organics to form nitrogen-containing organics which would add to the difficulty in interpretation. Further, these new compounds could be of a biological nature and of the type considered primary for a life searching mission” (Husted et al., 1977, p. 22).
Soil disturbances are what one would expect from the blast of the descent engine exhausts, that is, dust and small particles becoming airborne and surface excavation, among other factors. The exhaust from pulsed engines can be much stronger than that from continuously burning engines, which can be throttled to lower thrust levels.