scientific. In 2001, MEPAG was asked to define a Mars exploration strategy that embodied a series of alternative pathways that could be chosen based on anticipated discoveries. In undertaking this task, MEPAG was operating under explicit instructions from the Office of Management and Budget to devise at least one pathway that did not include a Mars sample-return mission. In other words, the AFL was conceived as a scientific response to an anticipated budgetary and political climate that would preclude the return of sample from Mars to Earth in the decade 2011-2020. Despite its origins, an appropriately instrumented AFL has important astrobiological potential as either a stand-alone mission or, with appropriate phasing, as a means to exploit scientific discoveries made by earlier missions. At this writing, the AFL mission is not well enough defined to allow detailed discussion of the astrobiological results that would be obtained.
The Mid Rovers are conceived as being more capable than the Mars Exploration Rovers but less complex, costly, and heavy than the Mars Science Laboratory. Their principal purpose is to serve as geological explorers, that is, to evaluate the geological context of specific sites and search for organic compounds at targets identified by prior missions. As currently envisaged, NASA’s goal is to fly two rovers for a cost approximately equal to that of the Mars Science Laboratory. The Mid Rovers would be equipped with a modest yet capable payload and utilize an entry, descent, and landing system capable of placing the spacecraft with a landing ellipse <100 km long. This mission concept postdates the publication of the SSE decadal survey.
The Mars Long-Lived Lander Network (ML3N) is envisaged as a global grid of small landers designed to make coordinated measurements of geophysical and meteorological phenomena for an extended period, possibly several martian years. High-priority objectives for such a network, as outlined in the solar system exploration decadal survey, include the following:9
Determine the planet’s internal structure, including its core;
Elucidate the composition of the surface and near-surface layers and investigate their oxidizing properties;
Measure the thermal and mechanical properties of the surface;
Conduct extensive synoptic measurements of the atmosphere and weather;
Establish the isotopic composition of atmospheric gas and their potential variability; and
Investigate surface-atmosphere volatile exchange processes.
The geophysical goals would be addressed via passive seismometers and heat-flow probes. The seismic goals will require a minimum of three stations. The meteorological goals can be addressed via measurements of pressure, temperature, relative humidity, atmospheric opacity, and wind velocity. Humidity sensors are particularly important from an astrobiological perspective because they would track the flux of water vapor into and out of the regolith with time of day and season, providing important insight into the water budget on Mars. The meteorological goals would, ideally, require a dozen or more stations distributed so that the maximum distance between any pair of observing sites is no more than a planetary radius. The inclusion of mass spectrometers in instrument packages will permit high-precision, long-lived chemical and isotopic atmospheric analysis of the chemical dynamics of C, H, and O at Mars’s surface. Time variability of isotopic compositions can be interpreted in terms of sources, sinks, and reservoirs of volatiles, and atmospheric evolution.
1. D.J. McCleese (ed.), Robotic Mars Exploration Strategy: 2007-2016, JPL 400-1276, Jet Propulsion Laboratory, Pasadena, Calif., 2006.