Astrobiological Assessment of Current Mars Mission Architecture
The Mars Exploration Program mission architecture has undergone revision approximately every 2 to 3 years in response to results obtained by ongoing and new missions and to the changing funding profiles available for future missions. The current Mars architecture, undergoing final revision as a joint effort of the NASA Jet Propulsion Laboratory Mars program office and the community-based Mars Exploration Program Assessment Group (MEPAG), provides plans up through the 2016 launch opportunity. Here, the committee describes, from the perspective of achieving astrobiology goals, the ongoing missions, those in development for launch through the end of this decade, and those in the planning stages for the next decade.
Missions that are operating at Mars and continuing to return data as of this writing include Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, and the two Mars Exploration Rovers (Spirit and Opportunity).
The active instruments on board Odyssey are the Gamma-Ray Spectrometer (GRS), Neutron Spectrometer (NS), High-Energy Neutron Detector (HEND), and the THEMIS instrument (Thermal Emission Imaging System, which includes a mapping infrared spectrometer and a visible imaging system). Both are providing measurements of high astrobiological relevance.
The GRS is obtaining maps of elemental abundance, including mapping of Cl and S abundances that are potentially related to aqueous processes. It also maps the H abundance that relates to polar-cap and high-latitude ground ice and to chemically bound water or transient ground ice at low latitudes. Silicon abundance is of less astrobiological relevance but can be used to map the coming and going of the seasonal polar caps, which is a strong
boundary constraint on the current climate. The NS and HEND are both sensitive to near-surface H and are being used to map ground ice at high latitudes and bound water or ice at lower latitudes.
THEMIS is mapping surface composition using infrared multispectral imaging. This is able to identify surface mineralogy, which provides strong constraints on geological processes and, in particular, on places where aqueous processes have been relevant. In the visible mode, it is imaging morphological features that constrain the geological history, which is relevant to volcanism and tectonism, as well as water-related processes. In both modes, it is mapping the polar caps, which again provides constraints on the present climate and on how to extrapolate it to other epochs. THEMIS also is mapping surface physical properties using temperature measurements and, with both infrared and visible imaging, is being used to help understand potential landing sites for the Phoenix and Mars Science Laboratory missions.
Mars Express is the European Space Agency’s first Mars orbiter. Its instrument complement includes a high-resolution stereo camera, a visible/infrared mineralogical mapping spectrometer, a subsurface sounding radar altimeter, Fourier and ultraviolet/infrared spectrometers for atmospheric studies, and an energetic neutral-atoms analyzer for studying the properties of the upper atmosphere.
The mineralogical mapping spectrometer—known as its French acronym, OMEGA—is able to map surface composition using reflectance spectroscopy by identifying characteristic absorption features. In its early mission phases, it was able to identify sulfate minerals on the surface that are strong indicators of aqueous geochemical processes, especially as constrained by the in situ measurements of the rover Opportunity.
The instruments designed to study the upper atmosphere are able to measure ion abundance. It has thus been possible to demonstrate that the martian upper atmosphere is being lost at present, although the net loss rate and the relationship to upper-atmospheric and solar-input processes have not been measured.
The radar sounding experiment uses long-wavelength radar that can penetrate as much as kilometers below the surface. It is able to identify subsurface structure that is related to the layering in the polar caps (Figure 6.2) and in sediments associated with impact basins.
The high-resolution stereo camera can be used to map morphology that tells researchers about ongoing geological processes. Of special interest has been the recent history of aqueous processes and of volcanism.
Mars Reconnaissance Orbiter
The Mars Reconnaissance Orbiter (MRO) is just beginning its primary science mission at this writing, and only a few preliminary results have been obtained to date. The instruments on board are a high-resolution camera (HiRISE), a high-resolution imaging spectrometer (CRISM), and a shorter-wavelength radar that can provide higher-vertical-resolution information on subsurface structure and on possible subsurface liquid water. Each of these measurements will be making significant contributions to landing-site selection for Phoenix and MSL, and each provides information that is of high value to astrobiology science objectives.
Mars Exploration Rovers
The rovers, Spirit and Opportunity, have been operating for more than 3 years and have returned a tremendous wealth of science data. Both rovers carry a panoramic imaging system (Pancam), a miniature thermal-emission spectrometer (Mini-TES), a Mossbauer spectrometer, an alpha particle x-ray spectrometer (APXS), a microscopic imager, and a rock abrasion tool (RAT). This instrument package has proved capable of characterizing astrobiologically relevant materials at landing sites selected for their high potential for having had liquid water—i.e., evaporates at Meridiani Planum and aqueously altered basaltic rocks at the Gusev crater. Not only have the rovers’ instruments returned a wealth of pertinent data confirming aqueous processes at both landing sites, but both spacecraft also continue to explore the surrounding terrain and are expected to remain operational for the foreseeable future.
MISSIONS IN DEVELOPMENT AND PLANNING
The Mars exploration architecture proposed by NASA envisages the launch of a mission to Mars at every possible launch opportunity, that is, every 26 months. The missions considered for the period from 2007 to 2016 are as follows:1,2
2007, Phoenix (the first competitively selected Mars Scout);
2009, Mars Science Laboratory;
2011, Mars Scout (the second competitive selection for flight);
2013, Mars Science and Telecommunications Orbiter; and
2016, Astrobiology Field Laboratory or two Mid Rovers or the Mars Long-Lived Lander Network.
Phoenix and the Mars Science Laboratory are both in phases C/D of their development. NASA selected two Mars Scout concepts for phase-A studies in early 2007. All the other missions listed above are in pre-phase-A concept-study development at the present time.
An additional mission likely to be flown in this same period is the European Space Agency’s ExoMars rover, which is currently scheduled for launch during the 2013 launch opportunity.
The Phoenix mission, scheduled for launch in August 2007, is the first of NASA’s principal-investigator-led, competitively selected Mars Scout missions. The importance of the Scout program to Mars exploration rests in its ability to address high-priority science goals related to unexpected discoveries and in the opportunity they provide for maintaining program balance. These factors led the solar and space exploration (SSE) decadal survey to rank the Mars Scout program as the highest-priority activity in the small Mars mission category.3
When Phoenix lands on Mars in May 2008, it will begin a program of investigations specifically designed to measure volatiles (especially water) and complex organic molecules in the arctic plains of Mars, where the Mars Odyssey orbiter has discovered evidence suggesting ice-rich soil very near the surface. The science objectives of Phoenix are as follows:
Understand the water cycle and its interactions with the atmosphere and the regolith;
Determine the recent history of water and its role in shaping the surface; and
Assess whether or not the landing site is a habitable zone by looking for organics and other biogenic elements.
These objectives will be addressed via an instrument package which includes the following: a stereoscopic imager (SSI) and a descent imager (MARDI); a thermal- and evolved-gas analyzer (TEGA); a microscopy, electro-chemistry, and conductivity analyzer (MECA); and a meteorological station (MET). Samples to be analyzed by TEGA and MECA will be collected with the assistance of a camera-equipped robotic arm.
Mars Science Laboratory
The Mars Science Laboratory is an advanced rover mission designed to follow the highly successful Mars Exploration Rovers, Spirit and Opportunity. The primary goal of the mission is to assess Mars’s potential as a past or present abode of life, that is, to determine whether Mars ever was, or is still today, an environment able to support microbial life. The specific scientific objectives are as follows:
Determine the nature and inventory of organic carbon compounds;
Inventory the chemical building blocks of life (i.e., carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur);
Identify features that may represent the effects of biological processes;
Investigate the chemical, isotopic, and mineralogical composition of the martian surface and near-surface geological materials;
Interpret the processes that have formed and modified rocks and regolith;
Assess long-timescale (i.e., 4-billion-year) atmospheric evolution processes;
Determine the present state, distribution, and cycling of water and carbon dioxide; and
Characterize the broad spectrum of surface radiation, including galactic cosmic radiation, solar proton events, and secondary neutrons.
These goals will be addressed by a comprehensive suite of experiments including a panoramic mast camera (MastCam), a microscopic imager (MAHLI), and a descent imager (MARDI); an alpha particle x-ray spectrometer (APXS), a laser-induced-breakdown spectrometer and microimaging camera (ChemCam), a gas chromatograph-mass spectrometer and tunable laser spectrometer system (SAM), and an x-ray diffraction/x-ray fluorescence instrument (CheMin); an environmental radiation monitor (RAD) and a neutron spectrometer (DAN); and a meteorological package (REMS).
Although the MSL mission was not well defined at the time the SSE decadal survey was drafted, its importance to addressing key Mars science goals was recognized and this mission was determined to be the highest-priority medium-cost Mars mission for the decade 2003-2013. Since then the scope and cost of the mission have grown significantly.
The combination of MSL’s highly capable science payload, its long expected lifetime, and its use of as-yet-untested entry, descent, and landing systems have led some observers to suggest that it would be prudent to send two. Indeed, NASA’s 2005 Roadmap for the Robotic and Human Exploration of Mars recommends that two MSL spacecraft should be launched “to ensure mission success and maximize the science return.”4 Such an approach might be an appropriate risk-reduction strategy. However, its implementation at such a late stage in the development of a large and complex mission seemed ill advised, irrespective of its financial implications for the rest of the Mars program.5
Mars Scout 2011
NASA proposes to launch the second Mars Scout missions no later than January 2012. NASA released an announcement of opportunity for this mission in early May 2006 and, 9 months later, selected two candidates for additional studies, the Mars Atmosphere and Volatile Evolution mission (MAVEN) and The Great Escape. Given the competitive nature of the Scout program, the detailed scientific goals and capabilities of these two orbiters remain proprietary. Nevertheless, it is understood that both spacecraft are designed to address questions relating to the composition and evolution of the martian atmosphere, in general, and the structure and dynamics of the upper atmosphere and ionosphere, in particular. As such it is likely that both MAVEN and The Great Escape are responsive to the scientific goals of the Mars Upper Atmosphere Orbiter, a high-priority mission identified in the SSE decadal survey report.6 NASA plans to select one of these two candidates for flight implementation in January 2008. In addition to selecting these two spacecraft missions, the Mars Scout program is also funding three U.S. teams providing scientific and/or instrumental contributions to the ESA’s ExoMars rover mission.
Mars Science and Telecommunications Orbiter
The Mars Science and Telecommunications Orbiter (MSTO) is envisaged as being comparable in size, scope, and cost with the Mars Reconnaissance Orbiter and capable of addressing a broad range of scientific objectives associated with the study of Mars’s atmosphere and space-plasma environment. Its scientific goals and instrument complement are only partially defined at the moment. Science goals endorsed in the recently completed study by MEPAG’s Mars Science and Telecommunications Orbiter Science Analysis Group include the following:7
Determine the interaction of the solar wind with Mars;
Determine diurnal and seasonal variations of Mars’s upper atmosphere and ionosphere;
Determine the influence of the crustal magnetic field on ionospheric processes;
Measure thermal and nonthermal escape rates of atmospheric constituents and estimate the evolution of the martian atmosphere;
Measure composition and winds in the middle atmosphere; and
Address in detail the issue of methane and other trace gases in the bulk atmosphere.
The selection in early 2007 of two Scout concept missions, both addressing the structure and dynamics of Mars’s upper atmosphere, as candidates for the 2011 launch opportunity has cast some uncertainty as to the scientific scope of the mission to be flown in 2013. Since MAVEN and The Great Escape both address some of the goals listed above, the exact scientific scope of the 2013 orbiter is currently being reevaluated. Whatever the outcome of the current rescoping, this spacecraft will likely have a secondary role as a telecommunications relay to enhance the data return from surface missions such as MSL (if it is still operating) and subsequent landed missions. The dual science and mission-support role of MSTO presents issues concerning the selection of appropriate orbits. The different science goals would likely benefit from different orbits, and the telecommunications goal would require yet another orbit. Approaches could involve either compromises on the orbit or changing orbits midway through the mission.
ExoMars is the first flagship mission in the ESA’s Aurora program of robotic and human exploration of the planets. It is a highly ambitious project involving a rover, an instrumented lander, and, possibly, an orbiting communications relay.8 The rover was conceived as being of roughly the same size as a Mars Exploration Rover, but having the ability to carry an MSL-class payload. The science goals of ExoMars are the following:
Search for signs of past and present life on Mars;
Characterize the geochemistry of and distribution of water in the near-surface regolith;
Measure the geophysical characteristics of the martian environment; and
Identify possible surface hazards to future human missions.
These goals will be addressed by Pasteur, a comprehensive set of scientific instruments mounted on the rover, and by a separate geophysics/environmental package on the landing platform. ExoMars is currently scheduled for launch in 2013. The ambitious Pasteur package consists of remote-sensing, contact, and analytic instruments supported by a complex sample-handling system and a drill capable of extracting samples from depths of 1 to 2 meters. The latter may be particularly important if putative organic compounds, preserved in the subsurface regolith from the harsh oxidizing surface conditions, are to be studied. Instruments of particular relevance to astrobiology include a Raman/laser-induced-breakdown spectroscope, an organics and oxidants detector, a gas chromatograph-mass spectrometer, and gas and antibody-based microarray organics detectors. The results, positive or negative, from the Pasteur instruments capable of detecting organic molecules at very high sensitivity (parts per billion and better) may be key to future astrobiological studies of Mars. Overall, ExoMars has great scientific potential, and its timing relative to the Mars Science Laboratory and the Astrobiology Field Laboratory may prove particularly fortuitous.
Astrobiology Field Laboratory
The Astrobiology Field Laboratory (AFL) mission is conceived as being a highly capable rover derived from the Mars Science Laboratory. Its principal goals would be to assess the biological potential of sites, interpret the paleo-climate record, and search for biosignatures of ancient and modern life. This mission concept postdates the publication of the SSE decadal survey, and it is important to note that its origins are more programmatic than
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.
Mars Long-Lived Lander Network
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.
2. D.W. Beaty, M.A. Meyer, and the Mars Advanced Planning Group, 2006 Update to Robotic Mars Exploration Strategy: 2007-2016, Unpublished white paper, posted November 2006 by the Mars Exploration Program Analysis Group (MEPAG) at http://mepag.jpl.nasa.gov/reports/index.html.
3. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 84 and 200.
4. NASA, Science Mission Directorate, A Roadmap for the Robotic and Human Exploration of Mars, p. ES-3, NASA Headquarters, Washington, D.C., 2005.
5. National Research Council, Review of Goals and Plans for NASA’s Space and Earth Sciences, The National Academies Press, Washington, D.C., 2006, p. 11.
6. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 7, 83, 200.
7. MEPAG’s study of MSTO is available at <mepag.jpl.nasa.gov/reports/MSTO_SAG_reports.doc>.
8. J. Vago, B. Gardini, G. Kminek, P. Baglioni, G. Gianfiglio, A. Santovincenzo, S. Bayón, and M. van Winnendael, “ExoMars: Searching for Life on the Red Planet,” ESA Bulletin 126:16-23, 2006.
9. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, p. 7.