Bruce M. Jakosky

Mars Global Surveyor Thermal Emission Spectrometer experiment: Investigation description and surface science results

The Thermal Emission Spectrometer (TES) investigation on Mars Global Surveyor (MGS) is aimed at determining (1) the composition of surface minerals, rocks, and ices; (2) the temperature and dynamics of the atmosphere; (3) the properties of the atmospheric aerosols and clouds; (4) the nature of the polar regions; and (5) the thermophysical properties of the surface materials. These objectives are met using an infrared (5.8- to 50-μm) interferometric spectrometer, along with broadband thermal (5.1- to 150-μm) and visible/near-IR (0.3- to 2.9-μm) radiometers. The MGS TES instrument weighs 14.47 kg, consumes 10.6 W when operating, and is 23.6×35.5×40.0 cm in size. The TES data are calibrated to a 1-σ precision of 2.5−6×10−8 W cm−2 sr−1/cm−1, 1.6×10−6 W cm−2 sr−1, and ∼0.5 K in the spectrometer, visible/near-IR bolometer, and IR bolometer, respectively. These instrument subsections are calibrated to an absolute accuracy of ∼4×10−8 W cm−2 sr−1/cm−1 (0.5 K at 280 K), 1–2%, and ∼1–2 K, respectively. Global mapping of surface mineralogy at a spatial resolution of 3 km has shown the following: (1) The mineralogic composition of dark regions varies from basaltic, primarily plagioclase feldspar and clinopyroxene, in the ancient, southern highlands to andesitic, dominated by plagioclase feldspar and volcanic glass, in the younger northern plains. (2) Aqueous mineralization has produced gray, crystalline hematite in limited regions under ambient or hydrothermal conditions; these deposits are interpreted to be in-place sedimentary rock formations and indicate that liquid water was stable near the surface for a long period of time. (3) There is no evidence for large-scale (tens of kilometers) occurrences of moderate-grained (>50-μm) carbonates exposed at the surface at a detection limit of ∼10%. (4) Unweathered volcanic minerals dominate the spectral properties of dark regions, and weathering products, such as clays, have not been observed anywhere above a detection limit of ∼10%; this lack of evidence for chemical weathering indicates a geologic history dominated by a cold, dry climate in which mechanical, rather than chemical, weathering was the significant form of erosion and sediment production. (5) There is no conclusive evidence for sulfate minerals at a detection limit of ∼15%. The polar region has been studied with the following major conclusions: (1) Condensed CO2 has three distinct end-members, from fine-grained crystals to slab ice. (2) The growth and retreat of the polar caps observed by MGS is virtually the same as observed by Viking 12 Martian years ago. (3) Unique regions have been identified that appear to differ primarily in the grain size of CO2; one south polar region appears to remain as black slab CO2 ice throughout its sublimation. (4) Regional atmospheric dust is common in localized and regional dust storms around the margin and interior of the southern cap. Analysis of the thermophysical properties of the surface shows that (1) the spatial pattern of albedo has changed since Viking observations, (2) a unique cluster of surface materials with intermediate inertia and albedo occurs that is distinct from the previously identified low-inertia/bright and high-inertia/dark surfaces, and (3) localized patches of high-inertia material have been found in topographic lows and may have been formed by a unique set of aeolian, fluvial, or erosional processes or may be exposed bedrock.

Mars' volatile and climate history

There is substantial evidence that the martian volatile inventory and climate have changed markedly throughout the planets history. Clues come from areas as disparate as the history and properties of the deep interior, the composition of the crust and regolith, the morphology of the surface, composition of the present-day atmosphere, and the nature of the interactions between the upper atmosphere and the solar wind. We piece together the relevant observations into a coherent view of the evolution of the martian climate, focusing in particular on the observations that provide the strongest constraints.

The distribution and behavior of Martian ground ice during past and present epochs

Mars undergoes significant oscillations in its orbit, which will have a pronounced effect on its climate and, in particular, on the behavior of subsurface water ice. We explore and map the behavior of ice in the Martian near-surface regolith over the past 1 m.y. using a diffusion and condensation model presented in an earlier paper, with two primary modifications to include orbitally induced variations in insolation and atmospheric water abundance. We find that the past behavior of ground ice differs significantly from that at the present epoch, primarily the result of high-amplitude oscillations in obliquity (presently 25°). In midlatitude and equatorial regions, ground ice will condense from atmospheric water during times of higher obliquity, filling the top few meters of the regolith with significant amounts of ice. At an obliquity of 32°, ground ice becomes stable globally. During times of lower obliquity, ground ice will sublime and diffuse back into the atmosphere, dessicating the regolith to a depth of about 1 to 2 m equatorward of 60° to 70° latitude. In the high-latitude regions these oscillations are considerably subdued. Below this depth of cyclic saturation and dessication a long-term stability of ice exists in some geographic regions. We present a map of the distribution of ice expected at the present epoch. Cyclic exchange of water between the global regolith and polar regions will have significant implications for surface geology and the polar layered deposits.

The Clementine Mission to the Moon: Scientific Overview

In the course of 71 days in lunar orbit, from 19 February to 3 May 1994, the Clementine spacecraft acquired just under two million digital images of the moon at visible and infrared wavelengths. These data are enabling the global mapping of the rock types of the lunar crust and the first detailed investigation of the geology of the lunar polar regions and the lunar far side. In addition, laser-ranging measurements provided the first view of the global topographic figure of the moon. The topography of many ancient impact basins has been measured, and a global map of the thickness of the lunar crust has been derived from the topography and gravity.

Geographic variations in the thermal and diffusive stability of ground ice on Mars

To investigate the stability of ground ice within the top several meters of the Martian regolith, time-dependent models of the thermal and diffusive behavior of the regolith have been developed. The geographic distribution of thermal inertia and albedo as well as the latitudinal variation in insolation have been included in calculations of surface and subsurface temperatures between ±60° latitude. Ground ice was found to be stable where the annual mean surface and subsurface temperatures were below the atmospheric frost point. This generally occurs poleward of the mid-latitudes. The latitude poleward of which ground ice is stable varies by about 20° to 30° from one longitude to another. Geographic variations in thermal inertia and albedo are the primary factors controlling regional variations in ice stability. Calculations of temperatures at high and low obliquity suggest that ground ice would be stable globally at high obliquity and would not be stable between ±60° latitude at low obliquity. Thermally driven diffusion of atmospheric water vapor within the regolith was modeled accounting for both ordinary molecular and Knudsen transport and equilibrium between ice, vapor, and adsorbed phases. Atmospheric water vapor was found to be able to supply the top few meters of the regolith with ice in regions where the annual mean surface temperature was below the atmospheric frost point. Ice was found to begin condensing in as short as 1000 Martian years. Rapid accumulation of ice in the pore space of the upper layers of the regolith acted to choke transport to lower layers and slow the diffusion process. Even so, after 105 Martian years, in some cases, as much as 30% to 40% of the available pore space accumulated ice. The total amount of subsurface ice ranged from a few to more than 25 g/cm2 within the top few meters in regions of stability. Ground ice was found to form below a depth where the annual average vapor pressure over ice was equal to the annual average atmospheric vapor pressure near the surface. Atmospheric vapor was not found to accumulate as ice below a depth where the seasonal temperature oscillations gave way to the geothermal gradient. The time scales for condensation of ground ice were found to be comparable to that of orbital oscillations suggesting that the present geographic distribution of ground ice may depend on the orbital history of Mars.

Evidence for magmatic evolution and diversity on Mars from infrared observations

Compositional mapping of Mars at the 100-metre scale with the Mars Odyssey Thermal Emission Imaging System (THEMIS) has revealed a wide diversity of igneous materials. Volcanic evolution produced compositions from low-silica basalts to high-silica dacite in the Syrtis Major caldera. The existence of dacite demonstrates that highly evolved lavas have been produced, at least locally, by magma evolution through fractional crystallization. Olivine basalts are observed on crater floors and in layers exposed in canyon walls up to 4.5 km beneath the surface. This vertical distribution suggests that olivine-rich lavas were emplaced at various times throughout the formation of the upper crust, with their growing inventory suggesting that such ultramafic (picritic) basalts may be relatively common. Quartz-bearing granitoid rocks have also been discovered, demonstrating that extreme differentiation has occurred. These observations show that the martian crust, while dominated by basalt, contains a diversity of igneous materials whose range in composition from picritic basalts to granitoids rivals that found on the Earth.

The NASA Astrobiology Roadmap.

The NASA Astrobiology Roadmap provides guidance for research and technology development across the NASA enterprises that encompass the space, Earth, and biological sciences. The ongoing development of astrobiology roadmaps embodies the contributions of diverse scientists and technologists from government, universities, and private institutions. The Roadmap addresses three basic questions: How does life begin and evolve, does life exist elsewhere in the universe, and what is the future of life on Earth and beyond? Seven Science Goals outline the following key domains of investigation: understanding the nature and distribution of habitable environments in the universe, exploring for habitable environments and life in our own solar system, understanding the emergence of life, determining how early life on Earth interacted and evolved with its changing environment, understanding the evolutionary mechanisms and environmental limits of life, determining the principles that will shape life in the future, and recognizing signatures of life on other worlds and on early Earth. For each of these goals, Science Objectives outline more specific high-priority efforts for the next 3-5 years. These 18 objectives are being integrated with NASA strategic planning.

Chaotic obliquity and the nature of the Martian climate

Recent calculations of the Martian obliquity suggest that it varies chaotically on timescales longer than about 107 years and varies between about 0 and 60°. We examine the seasonal water behavior at obliquities between 40 and 60°. Up to several tens of centimeters of water may sublime from the polar caps each year, and possibly move to the equator, where it is more stable. CO2 frost and CO2-H2O clathrate hydrate are stable in the polar deposits below a few tens of meters depth, so that the polar cap could contain a significant CO2 reservoir. If CO2 is present, it could be left over from the early history of Mars; also, it could be released into the atmosphere during periods of high obliquity, causing occasional periods of more-clement climate.

The history of Martian volatiles

The behavior of water and other volatiles on Mars is key to understanding the evolution of the climate. The early climate played a fundamental role in producing the observed surface morphology and possibly in enabling the existence of an early biosphere. Geochemical and isotopic data can be used to infer the history of volatiles. On the basis of the isotopic data from the atmosphere and from components of the surface (as measured in meteorites that come from Mars), there appear to be at least two reservoirs of volatiles, one that has undergone exchange with the atmosphere and has been isotopically fractionated, and a second that is unfractionated and may represent juvenile gases. The fractionation of the atmospheric component has occurred primarily through the escape of gas to space. In addition, the atmospheric gases have mixed substantially with crustal reservoirs of volatiles. Such exchange may have occurred in aqueous or hydrothermal environments. The history of escape to space, as driven by the properties of the Sun through time, is consistent with the surface geomorphology. Together, they suggest an early environment that was substantially different from the present one and the evolution through time to a colder, dryer climate.

The persistence of equatorial ground ice on Mars

In the current Martian climate, ground ice is unstable in the equatorial regions and, if present, would undergo sublimation and diffusive loss to the atmosphere. Previous studies suggest that the ice table (the uppermost occurrence of ice in the regolith) would continuously recede throughout geologic history. We present new models of the behavior of ice in the Martian equatorial regolith which predict that porous interstitial ice will persist at relatively shallow depths for geologically long periods of time. The persistence of interstitial ice is due to recondensation of water vapor as it diffuses toward the surface, encountering colder temperatures. We discuss the implications for the formation of rampart craters and debris aprons.