Engineering topics
Publications and source records attributed to Christensen, P. R..
Methods refined and adapted from the TES investigation are used to develop a surface-atmosphere separation strategy for THEMIS image analysis and atmospheric temperature and opacity retrievals. Additional information is contained in the original extended abstract.
A gradation of surface units represents either (1) an influx of basaltic sediment from southern highlands, deposited on andesitic volcanics, or (2) incompletely weathered basalt marking the geographic extent of submarine alteration of basaltic crust. Additional information is contained in the original extended abstract.
Emissivity spectra of Sinus Meridiani, Mars suggest that the hematite consists of platy particles that occur as consolidated, schistose lenses or loose, platy particles. This platy hematite may have originated as a result of burial metamorphism. Additional information is contained in the original extended abstract.
TopHat is a balloon-borne instrument designed to operate on the top of a balloon. From this location, the experiment could efficiently observe using a clean beam with extremely low sidelobes. The experiment was designed to scan a large portion of the sky directly above it and to map the anisotropy of the Cosmic Microwave Background (CMBR) and thermal emission from galactic dust. The instrument used a one meter class telescope with a five band single pixel radiometer spanning the frequency range from 150-600 GHz. The radiometer used bolometric detectors operating at approx. 250 mK. Here, we will report on the flight of the TopHat experiment over Antarctica in January, 2001 and describe the scientific goals, the operation, and in-flight performance.
The intent of this paper is to show the relationships for Mars among albedo, thermal inertia, roughness inferred from MOLA pulse width spread data, and geology inferred from photogeological analyses. Mapping of surface units using these parameters and approaches, in combination with analysis of hyperspectral image data from ISM, TES, OMEGA, and CRISM observations, will maximize our understanding of the distribution and nature of surface units on the red planet. Results will directly impact the selection of landing sites that exhibit geological records needed to understand planetary habitability.
The Thermal Emission Spectrometer (TES) instrument is a Fourier transform Michelson interferometer operating with 10 or 5 cm(exp -1) sampling in the thermal infrared spectral region from 1700 to 200 cm(exp -1) (-6 to 50 micrometers) where virtually all minerals have characteristic fundamental vibrational absorption bands. The TES data used in this paper are among the 6 x 10(exp 7) spectra collected during the early mapping phase of the Mars Global Surveyor (MGS) mission from southern hemisphere winter to early summer (aerocentric longitude, L(sub s), 107 deg to 297 deg. The methodology for separating the surface and atmospheric components of the radiance from Mars, which allows detailed analysis and interpretation of surface mineralogy, is described in previous paper. Additional information is contained in original extended abstract.
Crystalline hematite on Mars has been mapped using the MGS TES. Two major, and several minor areas of significant accumulation are identified. We favor precipitation models involving Fe-rich water, providing direct mineralogic evidence for large-scale water interactions.
The Thermal Emission Spectrometer on board the Mars Global Surveyor has observed "White Rock" and the data do not indicate the presence of evaporite minerals. We suggest it is a deposit of compacted or weakly cemented aeolian sediment.
The Mars Global Surveyor (MGS) spacecraft was launched from Cape Kennedy in November 1996. MGS was put into orbit around Mars in September of 1997 and has since been sending back data from a suite of instruments, including the Thermal Emission Spectrometer (TES). The TES instrument is an interferometric spectrometer designed to map the surface mineralogy of Mars by measuring the midinfrared emitted radiation over the spectral region of about 1600 to 200 cm(exp -1) (about 6 to 50 microns). This mineralogically sensitive technique utilizes the characteristic intra- and inter-molecular vibrations of minerals that are manifested in the midinfrared spectra. These spectral "fingerprints" are unique because they are dependent upon chemical composition, crystal structure, crystal orientation, and other factors. Midinfrared spectral data received from the MGS-TES instrument have indicated the presence of a large deposit of hematite (alpha-Fe2O3) in Sinus Meridiani, Mars. This hematite ore body, that is accompanied by basalt, is really extensive, encompassing an area about 350 by 500 km. To better understand the geologic context of this large deposit, a detailed laboratory spectroscopic investigation was conducted using more than 20 hematite samples so that their spectra could be compared to the martian spectra. The samples included red and gray polycrystaline hand samples, gray single-crystal hand samples, and red and gray fine- and coarse-grained particulates. The laboratory analyses provided thermal emissivity spectra that, when compared to the hematite emissivity spectra from Mars, suggest the Sinus Meridiani hematite is possibly an exposure of oriented hematite grains. These grains are likely coarser that 10 microns (and may be much larger) and gray in color. The characteristic of oriented grains is suggested by the apparent crystal axis-dependence of the energy emitted from the surface of Mars. The strong degree of crystal alignment exhibited in the emissivity spectra of Mars suggests that these oriented hematite crystals most likely occur as bedrock of aligned specular hematite grains (possibly schistose in texture) or as a secondary coating on bedrock, rather than as discrete particles. We are investigating the nature of this vast hematite deposit in order to understand better the geologic setting and infer past conditions and geological evolution on Mars.
The Mars Global Surveyor (MGS) spacecraft was launched from Cape Kennedy in November 1996. MGS was put into orbit around Mars in September of 1997 and has since been sending back data from a suite of instruments, including the Thermal Emission Spectrometer (TES). The TES instrument is an interferometric spectrometer designed to map the surface mineralogy of Mars by measuring the midinfrared emitted radiation over the spectral region of approximately 1600 to 200 per centimeter (appjroximately 6 to 50 microns). This mineralogically sensitive technique utilizes the characteristic intra- and inter-molecular vibrations of minerals that are manifested in the midinfrared spectra. These spectral "fingerprints" are unique because they are dependent upon chemical composition, crystal structure, crystal orientation, and other factors. Midinfrared spectral data received from the MGS-TES instrument have indicated the presence of a large deposit of hematite (alpha-Fe2O3) in Sinus Meridiani, Mars. This hematite ore body, that is accompanied by basalt, is areally extensive, encompassing and area approximately 350 by 500 km. To better understand the geologic context of this large deposit, a detailed laboratory spectroscopic investigation was conducted using more than 20 hematite samples so that their spectra could be compared to the martian spectra. The samples included red and gray polycrystaline hand samples, gray single-crystal hand samples, and red and gray fine- and coarse-grained particulates. The laboratory analyses provided thermal emissivity spectra that, when compared to the hematite emissivity spectra from Mars, suggest the Sinus Meridiani hematite is possibly an exposure of oriented hematite grains. These grains are likely coarser that 10 microns (and may be much larger) and gray in color The characteristic of oriented grains is suggested by the apparent crystal axis-dependence of the energy emitted from the surface of Mars. The strong degree of crystal alignment exhibited in the emissivity spectra of Mars suggests that these oriented hematite crystals most likely occur as bedrock of aligned specular hematite grains (possibly schistose in texture) or as a secondary coating on bedrock, rather than as discrete particles. We are investigating the nature of this vast hematite deposit in order to understand better the geologic setting and infer past conditions and geological evolution on Mars.
The Mars Surveyor Program 2001 Mission will carry instruments on the orbiter, lander and rover that will support synergistic observations and experiments to address important scientific questions regarding the local bedrock and soils. The martian surface is covered in varying degrees by fine materials less than a few mms in size. Viking and Pathfinder images of the surface indicate that soils at those sites are composed of fine particles. Wheel tracks from the Sojourner rover suggest that soil deposits are composed of particles <40 mm. Viking images show that dunes are common in many areas on Mars and new MOC images indicate that dunes occur nearly everywhere. Dunes on Mars are thought to be composed of 250-500 microns particles based upon Viking IRTM data and Mars wind tunnel experiments. If martian dunes are composed of sand particles > 100 microns and soils are dominated by <10 micron particles, then where are the intermediate grain sizes? Have they been wom away through prolonged transport over the eons? Were they never generated to begin with? Or are they simply less easy to identify because do they not form distinctive geomorphic features such as dunes or uniform mantles that tend to assume superposition in the soil structure?
Information on the thermal structure of the Martian atmosphere has been obtained using several different techniques, including in situ sounding from decent probes, radio occultations and infrared thermal emission measurements from orbiters, and earth based full-disk microwave measurements. Knowledge of the 3- dimensional atmospheric temperature field and its temporal variation provides an important observational approach to the study of the dynamics of the atmosphere. Comparison of the observed thermal structure with results from a Mars General Circulation Model (MGCM) can be used to refine the model and provide greater insight into the physical processes controlling the dynamics of the atmosphere. Direct assimilation of the temperature observations into an MGCM can also be used to address similar goals. In addition, relatively simple diagnostic models can be applied to the data to directly obtain information on winds and other meteorological properties of the atmosphere. In this overview, we concentrate primarily on the structure retrieved from thermal emission measurements. Additional information is contained in the original extended abstract.
Dust coatings on rocks obscured the underlying rock surfaces at the Mars Pathfinder landing site and made it difficult to obtain pristine visible/near-infrared reflectance spectra and chemical measurements. Such dust coatings also affect the ability of thermal infrared (TIR) spectra (such as that provided by TES and future lander (mini-TES) and orbiter multispectral imaging systems (THEMIS) to reveal accurately the mineralogy of rocks and soils. Similarly, high shock pressures from impact events cause crystalline disorder in important minerals such as pyroxene and feldspar that also affects their TIR spectra and complicates spectral interpretation of shocked impact materials. Additional information is contained in the original extended abstract.
The oxide mineral hematite alpha-Fe2O3) is present on Mars as evidenced by spectroscopy using visible/near infrared (VNIR) and midinfrared (MIR) wavelengths. The physical nature of this surficial hematite varies from fine-grained, crystalline red hematite (red-Hm) in certain martian bright regions, to coarser-grained, crystalline gray hematite (gray-Hm) in the Sinus Meridiani equatorial site discovered in the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) data. The VNIR spectra require that the red hematite is present in amounts subordinate to a second ferric pigment, which accounts for the general shape and position of the martian ferric absorption edge. By analogy with palagonitic tephra and synthetic samples, the pigment is a nanophase ferric oxide, possibly nanophase hematite (np-Hm). The purpose of this abstract is to document the physicospectral properties of hematite in the VNIR (0.35 to 2.2 mm) and MIR (5 to 25 mm) with respect to the terms "nanophase" and "crystalline", "red" and "gray", and "fine-grained" and "coarse-grained". We will show that different "types" of hematite (np-, red-, and gray-Hm) have different spectral responses in the VNIR and MIR regions and that these differences are important for inferring the types of hematite and their relative proportions from remotely sensed spectra of Mars. Because hematite is an alteration product that forms through many different geologic processes, determination of the type of hematite provides important clues about the style and diversity of martian weathering processes. Additional information is contained in the original extended abstract.
Successful operation of the Mars Global Surveyor spacecraft, beginning in September 1997, has permitted extensive infrared observations of condensation clouds during the martian southern summer and fall seasons (184 deg less than L(sub s) less than 28 deg). Initially, thin (normal optical depth less than 0.06 at 825/ cm) ice clouds and hazes were widespread, showing a latitudinal gradient. With the onset of a regional dust storm at L(sub s) = 224 deg, ice clouds essentially vanished in the southern hemisphere, to reappear gradually after the decay of the storm. The thickest clouds (optical depth approx. 0.6) were associated with major volcanic features. At L(exp s) = 318 deg, the cloud at Ascraeus Mons was observed to disappear between 21:30 and 09:30, consistent with historically recorded diurnal behavior for clouds of this type. Limb observations showed extended optically thin (depth less than 0.04) stratiform clouds at altitudes up to 55 km. A water ice haze was present in the north polar night at altitudes up to 40 km; this probably provided heterogeneous nucleation sites for the formation of CO2 clouds at altitudes below the 1 mbar pressure level, where atmospheric temperatures dropped to the condensation point of CO2.
The presence of discrete condensate clouds on Mars is certainly not a new discovery, having been observed through most of the documented history of telescopic monitoring. Furthermore, spacecraft data have been used to study discrete cloud features in the Martian atmosphere in greater detail, e.g., morphology, seasonal occurrence. Condensate clouds, specifically discrete water ice clouds, appeared to be regarded as fairly common but, with the possible exception of the polar regions, generally uninteresting from a climatological point of view. However, recent observations indicate that in addition to their large spatial scale, the water ice clouds may in fact play a more prominent role in the Martian climate. In this paper, we wish to examine the spatial and temporal variations of the cloud belt optical depth, as well as the microphysical characteristics of the water ice particles themselves.
The Thermal Emission Spectrometer (TES) instrument on the Mars Global Surveyor (MGS) mission has discovered a remarkable accumulation of crystalline hematite ((alpha-Fe2O3) that covers an area with very sharp boundaries approximately 350 by 350-750 km in size centered near 2 S latitude between 0 and 5 W longitude (Sinus Meridiani). Crystalline hematite is uniquely identified by the presence of fundamental vibrational absorption features centered near 300, 450, and >525/cm, and by the absence of silicate fundamentals in the 1000/cm region. Spectral features resulting from atmospheric CO2, dust, and water ice were removed using a radiative transfer model. The spectral properties unique to Sinus Meridiani were emphasized by removing the average spectrum of the surrounding region. The depth and shape of the hematite fundamental bands show that the hematite is crystalline and relatively coarse grained (>5-10 micron). Diameters up to and greater than 100s of micrometers are permitted within the instrumental noise and natural variability of hematite spectra. Hematite particles <5-10 micron in diameter (either as an unpacked or hard-packed powders) fail to match the TES spectra. The spectrally-derived areal abundance of hematite varies with particle size from approximately 10% for particles >30 micron in diameter to 40-60% for unpacked 10 micron powders. The hematite in Sinus Meridiani is thus distinct from the fine-grained (diameter <5-10 micron), red, crystalline hematite considered, on the basis of visible, near-IR data, to be a minor spectral component in Martian bright regions like Olympus-Amazonis. Sinus Meridiani hematite is closely associated with a smooth, layered, friable surface that is interpreted to be sedimentary in origin. This material may be the uppermost surface in the region, indicating that it could be a late-stage sedimentary unit, or it could be a layered portion of the heavily cratered plains units. We consider five possible mechanisms for the formation of coarse-grained, crystalline hematite. These processes fall into two classes depending on whether they require a significant amount of near-surface water: (1) chemical precipitation that includes origin by (a) precipitation from oxygenated, Fe-rich water (iron formations), (b) hydrothermal extraction and crystal growth.
Part of the task of interpreting IR spectral features observed by MGS/TES due to surface minerals requires distinguishing those IR signatures from atmospheric signatures of gas and dust. Surface-atmosphere separation for MGS/TES depends on knowledge of the retrieved temperature profile. In turn, the temperature retrieval Erom the observed data depends on molecular parameters including 15 micron CO2 line shape or line intensities which contribute to defining the Mars synthetic radiative transfer model. Using a simple isothermal, homogeneous single layer model of Pinnock and Shine, we find the ratio of (the error in degrees Kelvin of the retrieved temperature profile) to (the percentage error in the absorption coefficient) (deg K/percent) to be 0.4 at 200K. This ratio at 150K and 250K is 0.2 and 0.6, respectively. A more refined model, incorporating observed MGS/TES retrieved temperature profiles, the TES instrumental resolution and the most recent molecular modelling, will yield an improved knowledge of this error sensitivity. We present results of such a sensitivity study to determine the dependence of temperature profiles inverted from MGS/TES on these and other molecular parameters. This work was supported in part by NASA's Mars Data Analysis Program.