1. Geophysics Investigations on Mars

This article, Martian Geology Investigations, is Part 3 of the results of the Human Exploration of Mars Science Analysis Group (HEM-SAG) which are presented in five papers in this issue of the Journal of Cosmology (Levine et al., 2010a,b,c,d,e).

Mars geophysics science objectives fall into two broad categories: planetary scale geophysics (1000s of km), and what might be called “exploration geophysics,” which addresses regional (10s-100s km) or local scales (<10 km). The first involves characterizing the structure, composition, dynamics and evolution of the Martian interior, while the second addresses the structure, composition and state of the crust, cryosphere, hydrologic systems, and upper mantle. Here we describe how these objectives could be met through investigations carried out on human missions, as determined by the Human Exploration of Mars Science Analysis Group (HEM-SAG). HEM-SAG was chartered by the Mars Exploration Program Analysis Group (MEPAG) in 2007, and co-chaired by James B. Garvin (NASA Goddard Space Flight Center) and Joel S. Levine (NASA Langley Research Center). The MEPAG science investigations relevant to Mars geophysics are listed in Tables 1 and 2.

We assume here that no robotic missions to Mars before 2025 address the science issues in a complete way. For example, we assume that no network mission would be flown. We do this to be conservative, in order to make as complete a set of human exploration-related geoscience investigations and activities as possible. Clearly if future robotic missions address Mars geophysics topics, the human mission activities must be revisited.

In general, Mars geophysics would be well served by landing sites and traverses identified by the Geology Panel. Figure 1 shows four of the 58 sites considered by HEM-SAG. These sites would span the planet, and offer a sampling of Mars' remarkable geologic diversity. The Chasma Boreale site (CB) offers access to an immense stratigraphic column of polar-layered deposits presumably stretching far back into the Amazonian. The Nili Fossae site (NF) sits on the edge of the hemispheric dichotomy boundary and provides access to Noachian/Hesperian-age fluvial features. The Centauri Montes site (CM), on the eastern rim of the giant Hellas impact basin, contains features ranging from Noachian basin rim materials to Amazonian/Hesperian outflow channels to Amazonian debris aprons and recent gully changes, hinting at the possibility of nearsurface water. The Arsia Mons site (AM) sits on the western flank of the volcano and provides access to putative Amazonian-age glacial deposits and comparatively young lava flows. Each site offers the opportunity to address multiple geophysics investigations. We will revisit these sites and plausible geophysical exploration strategies later.

2. Planetary Scale Geophysics: Structure, Composition, Dynamics, and Evolution of the Martian Interior

To characterize the structure and dynamics of Mars' interior and determine the chemical and thermal evolution of the planet, physical quantities such as density and temperature with depth, composition and phase changes within the mantle, the core/mantle boundary location, thermal conductivity profile and the three-dimensional mass distribution of the planet must be determined. To determine the origin and history of the planet's magnetic field, we must discover the mineralogy responsible for today's observed remnant magnetization, and understand how and when the rocks bearing these minerals were emplaced.

The measurement requirements for planetary scale geophysics present some drivers for Mars exploration architectures. A key driver is the need to instrument the planet at appropriate scales: for example, global seismic studies rely on widely separated stations so that seismic ray paths passing through the deep mantle and core could be observed. This need would translate into multiple, widely separated landing sites for the first human missions. If only a single landing site would be selected and revisited, then far less information about the planet's interior would be obtained. As can be seen in Fig. 1, the three low-latitude sites would provide a reasonable planetary-scale network, and would also enable heat flow measurements in diverse crustal/lithospheric settings: the volcanic Tharsis rise, the Isidis wall/dichotomy boundary, and the rim of the Hellas basin. However, these sites do not fall into the region of intense remnant crustal magnetization, so it is important to consider a location in the mid- to high latitudes of the southern hemisphere near 180E longitude.

3. Exploration Geophysics: Structure, Composition, State of Near-Surface Crust

To characterize the structure, composition, and state of Mars' near-surface crust both local and regional subsurface information must be obtained. A wide variety of exploration geophysics techniques exist that provide such information. For example, sounding for aquifers could be accomplished through electromagnetic techniques, and layering in sedimentary units could be determined through reflection seismology. Magnetic surveys carried out at landing sites tell us about the spatial scales of crustal magnetization, and tie in to local and regional geology for context.

4. Human Science Reference Missions (HSRM): Geophysics

Geophysics measurement requirements span three disparate spatial scales, depending on the science to be done. At the largest scales (1000s of km), characterizing the interior of Mars would require a widely spaced network of at least 3 emplaced central geophysics stations, one at each landing site. At regional scales (10s-100s km), characterizing crustal structure, magnetism, and other objectives require mobility to emplace local networks around a landing site. Finally, at local scales (~10 km), mobility is key to performing traverse geophysics, and in carrying out investigations (such as seismic or EM sounding) at specific stations along a traverse. The central geophysics stations and the regional scale networks would be emplaced and left to operate autonomously after the human crew departs. Traverse and station geophysics would be carried out only during the human mission, unless this can be done robotically after completion of the human mission.

Central geophysical stations at each landing site would include passive broadband seismic, heat flow, precision geodesy, and passive low-frequency electromagnetic instrumentation. Satellite geophysics stations would include the nodes of a regional seismic array and vector magnetometers. Along the traverses, experiments would be performed at sites of interest. These would include active EM sounding for subsurface aquifers, active seismic profiling to establish structure with depth, and gravity measurements. Ground penetrating radar and neutron spectroscopy along the traverse track would help map out subsurface structure and hydration state/ice content for the near-subsurface.

Based on the geophysics science objectives, multiple sites would meet the investigation needs and geologic settings. A list of these sites, prioritized, is summarized in Table 3; three sites at widely separated locations would be required to address global questions concerning the interior of Mars. Table 3 lists instrumentation relevant to geophysics investigations, and provides estimates of mass and power.

5. Centauri Montes Site

Figure 2 shows the Centauri Montes mission landing site and traverses. The permanent, autonomous, central landing site geophysics station is shown by the green square. During traverses, permanent satellite geophysics stations (red triangles) are emplaced. The active-gully location is denoted by the cyan diamond. At selected points along a traverse, gravity, electromagnetic sounding, active seismic experiments would be done (crosses).

The Centauri Montes site would provide a location for addressing multiple geophysics objectives. First, it is one of three sites for global seismic monitoring. Heat flow measurements for this highlands site could be compared to, for example, such measurements in the large volcanic Tharsis province, if the Arsia site is also chosen.

Figure 3 shows the Centauri Montes site geologic traverse plan with superposed symbols denoting geophysics central station (green square), and satellite stations (red triangles) forming part of the local/regional seismic network and locations of electromagnetic observatories. Exploration targets at this site would include recent gullies (possibly liquid water), ancient Noachian Hellas basin rim constructs, Amazonian debris aprons, and other features associated with geologically recent climate change. The figure shows several traverses, each requiring an extended period of exploration. During these traverses, specific sites would be selected for indepth geophysical exploration. The right panel zooms in on one part of the blue traverse, showing two stations (red crosses) where detailed geophysical exploration could be done. Active reflection seismology and EM sounding, for example, might be carried out to explore in detail the subsurface structure of these lobate debris aprons.

While traversing, some kinds of measurements could be made to map out subsurface structure and state. For example, ground penetrating radar and neutron spectroscopy would provide cuts of near surface layering (with sufficient dielectric contrast) and bulk hydrogen estimates as a function of position along the traverse. Perhaps most important, geophysical methods could be used to sound the subsurface along the rim of the gullied crater, thus providing information about the presence or absence of an aquifer as a potential gully source. This combination of surface prospecting during traverse and fixed, temporary sounding sites is illustrated in Fig. 4. Here the rover team first would explore the crater rim, stopping at promising sites to temporarily emplace geophysics instrumentation such as electromagnetic sounding and active seismic systems to characterize the subsurface. Results from these surveys would help determine the most promising location(s) to drill.

6. Nili Fossae — Isidis Site

The Nili Fossae site would be another location for addressing multiple geophysics objectives. Again, it is one of three sites for global seismic monitoring. Heat flow measurements for this dichotomy boundary site, far from late Amazonian volcanic activity, provide another important tie-point for interior structure, composition and dynamics. The stratigraphy of depositional fans, possible lake-bottom deposits, shoreline breaches and other features could be explored.

7. Arsia Mons Site

The Arsia Mons site would open the exploration of the most important volcanic province on Mars. Again, it would be one of three sites for global seismic monitoring. Heat flow measurements for this Tharsis rise site, where extensive volcanism has occurred since the Noachian, would certainly improve our knowledge of interior structure, composition and dynamics. Figure 6 shows the proposed locations of the base and satellite stations for geophysics investigations. In this case the satellite stations may include additional heat flow experiments, searching for evidence of late stage dike intrusion, if cooling time is not too short (<10⁵ yr). With a local seismic network, identify seismic velocity anomalies associated with deep magmatic bodies. Active (reflection) seismic studies at many local sites along the traverses would help reveal the history of ash deposits and lava flows. They may also reveal the presence of ice at depths consistent with Late Amazonian deposition and subsequent sublimation in current obliquity and climate conditions.

8. Crustal Magnetism Site

The most intense remnant crustal magnetic fields are found in the region between Terra Sirenum and Terra Cimmeria, at mid- to high southern latitudes centered on 180E. This region is thus a prime target for shedding light on the history of an early Mars dynamo, the nature of the crustal magnetization process, and attendant implications for Mars' early thermal history. The left panel of Fig. 7 shows the filtered radial magnetic field component observed by Mars Global Surveyor at the mapping altitude of 400 km. Superposed on this map are the locations of gullies identified in MGS/MOC and MRO/HiRISE. The right panel shows a HiRISE image of one of these gullies (outlined by a black circle in the left panel, 47.2S 176.8E), on a south-facing interior crater wall. This crater is within ~100 km of a radial magnetic field reversal seen in MGS/MAG-ER aerobraking data. The region of intense crustal magnetization contains many gullies; like the Centauri Montes site, it affords many opportunities for study of plausibly icecored formations and episodic surface water flow (if this is the operative gully formation mechanism). Thus, a landing site in the vicinity of these features would permit investigation of diverse science areas: crustal magnetism (including possibly the identification of magnetic minerals in ancient Noachian rocks), gullies and their relationship to a (once) ice-rich mantle or subsurface aquifer, a record of Amazonian climate change in the mantle deposits, and the astrobiological potential of subsurface habitats.

9. MEPAG Geophysics Investigations

In the following table, the first column lists the MEPAG Goal III investigations, the second column identifies the relevant geophysics techniques (in no particular order) used to address the objectives of each investigation. For example, a seismic network provides S and P wave travel times, from which ray paths and velocities are determined. Models of interior composition and structure must be consistent with these measurables.