Geology topics

H.U. Keller

Publications and source records attributed to H.U. Keller.

8 recordsLinked to original sources

USGS2012

Dawn at Vesta: testing the protoplanetary paradigm

The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta's south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta's crust by melting of a chondritic parent body. Vesta's mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.

Science↗

USGS2005

Rain, winds and haze during the Huygens probe's descent to Titan's surface

The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

Nature↗

USGS2004

Scientific objectives and selection of targets for the SMART-1 Infrared Spectrometer (SIR)

The European SMART-1 mission to the Moon, primarily a testbed for innovative technologies, was launched in September 2003 and will reach the Moon in 2005. On board are several scientific instruments, including the point-spectrometer SMART-1 Infrared Spectrometer (SIR). Taking into account the capabilities of the SMART-1 mission and the SIR instrument in particular, as well as the open questions in lunar science, a selection of targets for SIR observations has been compiled. SIR can address at least five topics: (1) Surface/regolith processes; (2) Lunar volcanism; (3) Lunar crust structure; (4) Search for spectral signatures of ices at the lunar poles; and (5) Ground truth and study of geometric effects on the spectral shape. For each topic we will discuss specific observation modes, necessary to achieve our scientific goals. The majority of SIR targets will be observed in the nadir-tracking mode. More than 100 targets, which require off-nadir pointing and off-nadir tracking, are planned. It is expected that results of SIR observations will significantly increase our understanding of the Moon. Since the exact arrival date and the orbital parameters of the SMART-1 spacecraft are not known yet, a more detailed planning of the scientific observations will follow in the near future. ?? 2004 Elsevier Ltd. All rights reserved.

Planetary and Space Science↗

USGS1999

Overview of the Mars Pathfinder Mission: Launch through landing, surface operations, data sets, and science results

Mars Pathfinder successfully landed at Ares Vallis on July 4, 1997, deployed and navigated a small rover about 100 m clockwise around the lander, and collected data from three science instruments and ten technology experiments. The mission operated for three months and returned 2.3 Gbits of data, including over 16,500 lander and 550 rover images, 16 chemical analyses of rocks and soil, and 8.5 million individual temperature, pressure and wind measurements. Path‐finder is the best known location on Mars, having been clearly identified with respect to other features on the surface by correlating five prominent horizon features and two small craters in lander images with those in high‐resolution orbiter images and in inertial space from two‐way ranging and Doppler tracking. Tracking of the lander has fixed the spin pole of Mars, determined the precession rate since Viking 20 years ago, and indicates a polar moment of inertia, which constrains a central metallic core to be between 1300 and ∼2000 km in radius. Dark rocks appear to be high in silica and geochemically similar to anorogenic andesites; lighter rocks are richer in sulfur and lower in silica, consistent with being coated with various amounts of dust. Rover and lander images show rocks with a variety of morphologies, fabrics and textures, suggesting a variety of rock types are present. Rounded pebbles and cobbles on the surface as well as rounded bumps and pits on some rocks indicate these rocks may be conglomerates (although other explanations are also possible), which almost definitely require liquid water to form and a warmer and wetter past. Air‐borne dust is composed of composite silicate particles with a small fraction of a highly magnetic mineral, interpreted to be most likely maghemite; explanations suggest iron was dissolved from crustal materials during an active hydrologic cycle with maghemite freeze dried onto silicate dust grains. Remote sensing data at a scale of a kilometer or greater and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catstrophic floods, which are relatively dust free. The surface appears to have changed little since it formed billions of years ago, with the exception that eolian activity may have deflated the surface by ∼3–7 cm, sculpted wind tails, collected sand into dunes, and eroded ventifacts (fluted and grooved rocks). Pathfinder found a dusty lower atmosphere, early morning water ice clouds, and morning near‐surface air temperatures that changed abruptly with time and height. Small scale vortices, interpreted to be dust devils, were observed repeatedly in the afternoon by the meteorology instruments and have been imaged.

Journal of Geophysical Research E: Planets↗

USGS1999

Observations of Phobos, Deimos, and bright stars with the Imager for Mars Pathfinder

The Imager for Mars Pathfinder (IMP) was used to observe several objects during the Martian night. The satellites, Phobos and Deimos, were observed on two occasions each, through the IMP geological filters covering the wavelength range 440 nm to 1 μm. The observations were converted to geometric albedo using triaxial ellipsoid models of the satellites and phase functions derived from Viking Orbiter images. The spectral slopes over the full wavelength range were 7.9(±0.5)% (100 nm) −1 and 9.6(±0.6)% (100 nm) −1 , respectively, referenced to 600 nm. In the Deimos spectra, some evidence for decreased reddening toward the trailing hemisphere was found. The geometric albedoes of Phobos and Deimos were found to be 0.065 (±0.010) and 0.068 (±0.009), respectively, averaged over 440 and 600 nm, in good agreement with previous measurements. The nighttime optical depth was investigated using observations of stars. A mean value of 0.56 (±0.09) was determined from measurements at different airmass. A possible maximum in the optical depth near 0200 local time was found, which may result from condensation during the night. A measurement of the egress of Phobos from eclipse was made. Modeling of the light curve gave a scale height for the scatterers of 10–15 km. The exact time of the eclipse reappearance over the limb could be reconstructed from the measurements and was in reasonable accord with predictions, although a small modification to the predicted position of Phobos of 6.8 (±6.0) km would have produced better agreement.

Journal of Geophysical Research E: Planets↗

USGS1999

The color of the Martian sky and its influence on the illumination of the Martian surface

The dust in the atmosphere above the Mars Pathfinder landing site produced a bright, red sky that increases in redness toward the horizon at midday. There is also evidence for an absorption band in the scattered light from the sky at 860 nm. A model of the sky brightness has been developed [ Markiewicz et al. , this issue] and tested against Imager for Mars Pathfinder (IMP) observations of calibration targets on the lander. The resulting model has been used to quantify the total diffuse flux onto a surface parallel to the local level for several solar elevation angles and optical depths. The model shows that the diffuse illumination in shadowed areas is strongly reddened while areas illuminated directly by the Sun (and the blue forward scattering peak) see a more solar-type spectrum, in agreement with Viking and IMP observations. Quantitative corrections for the reddening in shadowed areas are demonstrated. It is shown quantitatively that the unusual appearance of the rock Yogi (the east face of which appeared relatively blue in images taken during the morning but relatively red during the afternoon) can be explained purely by the changing illumination geometry. We conclude that any spectrophotometric analysis of surfaces on Mars must take into account the diffuse flux. Specifically, the reflectances of surfaces viewed under different illumination geometries cannot be investigated for spectral diversity unless a correction has been applied which removes the influence of the reddened diffuse flux.

Journal of Geophysical Research E: Planets↗

USGS1997

The imager for Mars Pathfinder experiment

The imager for Mars Pathfinder (IMP), a stereoscopic, multispectral camera, is described in terms of its capabilities for studying the Martian environment. The camera's two eyes, separated by 15.0 cm, provide the camera with range‐finding ability. Each eye illuminates half of a single CCD detector with a field of view of 14.4×14.0° and has 12 selectable filters. The ƒ/18 optics have a large depth of field, and no focussing mechanism is required; a mechanical shutter is avoided by using the frame transfer capability of the 512×512 CCD. The resolving power of the camera, 0.98 mrad/pixel, is approximately the same as the Viking Lander cameras; however, the signal‐to‐noise ratio for IMP greatly exceeds Viking, approaching 350. This feature along with the stable calibration of the filters between 440 and 1000 nm distinguishes IMP from Viking. Specially designed targets are positioned on the Lander; they provide information on the magnetic properties of wind‐blown dust, measure the wind vectors, and provide radiometric standard reflectors for calibration. Also, eight low‐transmission filters are included for imaging the Sun directly at multiple wavelengths, giving IMP the ability to measure dust opacity and potentially the water vapor content. Several experiments beyond the requisite color panorama are described in detail: contour mapping of the local terrain, multispectral imaging of the surrounding rock and soil to study local mineralogy, viewing of three wind socks, measuring atmospheric opacity and water vapor content, and estimating the magnetic properties of wind‐blown dust. This paper is intended to serve as a guide to understanding the scientific integrity of the IMP data that will be returned from Mars starting on July 4, 1997.

Journal of Geophysical Research E: Planets↗

USGS1997

Results from the Mars Pathfinder camera

Images of the martian surface returned by the Imager for Mars Pathfinder (IMP) show a complex surface of ridges and troughs covered by rocks that have been transported and modified by fluvial, aeolian, and impact processes. Analysis of the spectral signatures in the scene (at 440- to 1000-nanometer wavelength) reveal three types of rock and four classes of soil. Upward-looking IMP images of the predawn sky show thin, bluish clouds that probably represent water ice forming on local atmospheric haze (opacity ∼0.5). Haze particles are about 1 micrometer in radius and the water vapor column abundance is about 10 precipitable micrometers.

Science↗