GEMOC ARC National Key Centre

Paul Morgan¹, Suzanne Smrekar², and David R. Barnett³¹

1. GEMOC Macquarie

2. Jet Propulsion Laboratory, California Institute of Technology

3. Lockheed Martin Astronautics

The NASA New Millennium Program Mars Microprobe Project, designated as Deep Space 2 (DS2), includes the recording and transmission back to Earth of subsurface temperature data from the Mars regolith as a function of time. As this mission has a very high-latitude study site, close to the South Pole of Mars between 73 and 77o latitude, where very low environmental temperatures are expected (< -40°C), temperatures will be recorded for only one to two days. The mission lifetime is essentially constrained by power requirements: the site is too far from the equator for use of solar-electric power, and low environmental temperatures limit battery life. Such constraints do not apply at lower latitude study sites, however, and missions could be designed for these latitudes with a lifetime of a Mars year (687 days) or more. Temperature data from the DS2 probes will be used primarily to define the thermal relaxation of the entry heating of the probes from which the thermal properties of the regolith at the landing site will be determined. We do not anticipate that the probe will have re-equilibrated with background regolith temperatures before the end of the mission. Similar temperature data from longer missions, however, have multiple uses.

Microprobe technology was originally conceived and designed for subsurface thermal measurements in planetary regoliths with the purposes of measuring thermal properties of the regolith penetrated by the microprobes and to measure subsurface temperatures. These data were sought for three primary purposes. Thermal property data are useful for constraining the compaction and composition of the regolith, particularly with respect to the presence of water ice. Water is commonly considered to be a requirement for life, and thus, detection of subsurface water or water ice is an important pre-requisite for astrobiological studies on Mars, and for constraining sites for the search for life.

Regolith thermal conductivity is also an essential parameter in understanding heat transfer in the regolith. The background vertical temperature gradient in the regolith, after the decay of the initial penetration heating pulse, can be combined with the thermal conductivity to determine the heat flow through the regolith. With sufficient measurements of heat flow, the thermal regime of the planet and the energy that drives volcanism and tectonism may be examined. Thermal gradients in the outer layer of a planet control lithospheric thickness, and thus provide important information concerning the mechanics of the lithosphere. Deductions possible from these data include: i) estimates of the strength of the lithosphere, relating to the possibility of modern intraplate tectonics on Mars; ii) the strength profile in the lithosphere, relating to the maximum depth of brittle failure and marsquake hypocenters; and iii) depths at which solidus temperatures are approached in the Martian mantle, relating to geologically recent Martian volcanism.

Transient thermal perturbations in the regolith are related to climate. Seasonal surface temperature variations penetrate the regolith as thermal waves with strong attenuation of their high-frequency components. Thus, subsurface temperature profiles provide a low-pass filtered record of recent temperature history. Such data would be valuable supplements to surface temperature studies as part of a climate network study. The wavelength of propagation of thermal waves into a conductive medium is controlled by the thermal properties of the medium. Studies of the subsurface thermal profile will thus provide independent information from which regolith thermal properties can be determined.

Subsurface temperatures are very sensitive to non-conductive heat-transfer components, and the seasonal exchange of volatiles between the atmosphere and the regolith may have a signature in subsurface temperature. In general, very low convective heat fluxes dominate over conductive heat transfer in low thermal conductivity media, and, combined with any latent heat effects associated with the volatile fluxes, should be easily detectable if present. Volatile fluxes are expected only in the upper few tens of millimeters of the regolith, and although they may modify the transfer of surface temperature variations to depth, both should still be measurable.

We have been conducting experiments to increase the depth coverage of subsurface temperature measurements with a DS2-like probe. The DS2 probes include temperature sensors only in the fore and aft body sections with nothing in between. We have successfully integrated temperature sensors into the flexible tether that connects the surface and subsurface portions of the probe in order to make a number of temperature measurements over the depth of penetration of the probe. Simulations at low pressures and room temperature suggest that these temperature sensors accurately record the wall temperature of the hole formed by penetration of the forebody even when not in contact with the wall. As a backup, however, we are also investigating the use of mechanical systems to place the temperature sensors in contact to with the penetration-hole wall.

Even with the small size and mass of the DS2 microprobes, instrumentation required for the thermal studies is only a fraction of the available scientific payload, and the low data requirements of the thermal studies allows opportunities for other data return. The extension of the use of microprobes for studies in addition to thermal studies is limited only by the requirement of impact-resistant small components. The small and lightweight design of the microprobes would allow ten to fifteen Deep Space 2-like probes to be attached to a single delivery vehicle, jettisoned to land at different locations on the planet. Such a deployment would be ideal for network science, such as seismology and meteorology. Subsurface thermal studies would provide ideal additional data from probes designed primarily for these other purposes, or from any other microprobe missions.