Planetary Science Institute
Thermal Inertia Web Site
Thermal inertia maps
2007 Global MGS-TES Maps (Putzig and Mellon 2007b)
2003 Global MGS-TES Map (Putzig et al. 2005)
2000 Global MGS-TES Map (Mellon et al. 2000)
What is thermal inertia?
Thermal inertia is defined as [SI units]:where:
Putzig (2006) has proposed tiu as a derived SI unit for thermal inertia, which is used here for the 2007 mapping results and associated publications.
As the name implies, thermal inertia represents the ability of a material to conduct and store heat, and in the context of planetary science, it is a measure of the subsurface's ability to store heat during the day and reradiate it during the night. While compositional differences (ie, mineralogy) will have some effect, for a terrestrial planetary surface such as that of Mars, I will depend predominantly on the physical properties of the near surface materials such as particle size, degree of induration (ie, cementation of grains), rock abundance, and exposure of bedrock (rocks will have a much higher thermal inertia than sand or dust - that is, it takes longer to heat rocks up during the day and to cool them off at night).
How are MGS-TES thermal inertias calculated?
The Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) measures emitted infrared energy from which one can infer brightness temperature and surface kinetic temperature. These temperatures are driven by a number of factors such as albedo, dust opacity, atmospheric pressure, and thermal inertia, the last of which is the key surface property controlling the diurnal temperature oscillations. We use a coupled surface and atmospheric thermal model of Mars to generate a lookup table of surface and brightness temperatures for a complete set of seasons, latitudes, times of day, atmospheric pressures, surface albedos, dust opacities, and thermal inertias. This table is then used to derive the surface thermal inertia from the MGS-TES temperature measurements at 3 km resolution for the entire surface of the planet.How should one interpret MGS-TES thermal inertias?
TES-derived thermal inertias represent a complex combination of surfaces within each detector's field of view (about 3 km). Rocks, sand, dust, and bedrock may all contribute to the observed temperatures and the thermal inertias derived from them. Surface temperature variations are sensitive to the properties of the upper few centimeters of the surface. Therefore, thermal inertia is a measure of the geological properties of the surface on a this same depth scale.
- Lowest thermal inertia (purple to dark blue colors in the maps) probably represents loose, fine surface dust and very few rocks.
- Medium thermal inertia (green to yellow) could represent a combination of coarser loose particles, crusted fines, a fair number of scattered rocks, and/or perhaps a few scattered bedrock outcrops.
- Higher thermal inertia (red to white) could be a combination of: coarse sand, dune sand, strongly-crusted fines, abundant rocks, and/or scattered bedrock exposures.
- Continuous bedrock over the full pixel would be off scale, e.g. a thermal inertia perhaps two or three times the present maximum of 800 SI units.
What are the uncertainties in MGS-TES thermal inertias?
Excessive atmospheric dust and water ice clouds are probably the largest contributors to uncertainties in the mapped thermal inertias shown. Quantitative information about the presence of airborne dust and clouds has not yet been incorporated. A nominal dust load and no clouds are assumed in deriving thermal inertias. Many other factors may contribute to the uncertainty, including model-dependent and data-dependent errors. These are complex and difficult to assess. We estimate the uncertainty to be better than about 6% (see Mellon et al., 2000).
References and Further Reading
Christensen, P. R., and H. J. Moore, The Martian Surface Layer, in Mars (eds. H. H. Kieffer, B. M. Jakosky, C. W. Snyder, and M. S. Matthews), University of Arizona Press, Tucson, 686-729, 1992.
Jakosky, B. M., M. T. Mellon, H. H. Kieffer, and P. R. Christensen, The Thermal Inertia of Mars from the Mars Global Surveyor Thermal Emission Spectrometer, J. Geophys. Res. 105, 9643-9652, 2000.
Mellon, M. T., B. M. Jakosky, H. H. Kieffer, and P. R. Christensen, High-Resolution Thermal-Inertia Mapping from the Mars Global Surveyor Thermal Emission Spectrometer, Icarus 148, 437-455, 2000.
Putzig, N. E., Thermal inertia and surface heterogeneity on Mars, Ph. D. dissertation, University of Colorado, Boulder, 2006.
Putzig, N. E., M. T. Mellon, Thermal behavior of horizontally mixed surfaces on Mars, Icarus, 191, 52-67, doi: 10.1016/j.icarus.2007.03.022, 2007a.
Putzig, N. E., M. T. Mellon, Apparent thermal inertia and the surface heterogeneity of Mars, Icarus, 191, 68-94, doi: 10.1016/j.icarus.2007.05.013, 2007b.
Putzig, N. E., M. T. Mellon, K. A. Kretke, and R. E. Arvidson, Global thermal inertia and surface properties of Mars from the MGS mapping mission, Icarus 173, 325-341, doi: 10.1016/j.icarus.2004.08.017, 2005.
Last update: 2018 November 6