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M. H. Carr

Publications and source records attributed to M. H. Carr.

At least 37 records · Page 2Linked to original sources
USGS2000

Meter-Scale Characteristics of Martian Channels and Valleys

Mars Global Surveyor images, with resolutions as high as 1.5 m pixel, enable characterization of martian channels and valleys at resolutions one to two orders of magnitude better than was previously possible. A major surprise is the near-absence of valleys a few hundred meters wide and narrower. The almost complete absence of fine-scale valleys could be due to lack of precipitation, destruction of small valleys by erosion, or dominance of infiltration over surface runoff. V-shaped valleys with a central channel, such as Nanedi Vallis, provide compelling evidence for sustained or episodic flow of water across the surface. Larger valleys appear to have formed not by headward erosion as a consequence of groundwater sapping but by erosion from water sources upstream of the observed sections. The freshest appearing valleys have triangular cross sections, with talus from opposing walls meeting at the center of the valley. The relations suggest that the width of the valleys is controlled by the depth of incision and the angle of repose of the walls. The flat floors of less fresh-appearing valleys result primarily from later eolian fill. Several discontinuous valleys and lines of craters suggest massive subsurface solution or erosion. The climatic implications of the new images will remain obscure until the cause for the scarcity of fine-scale dissection is better understood. ?? 2000 Academic Press.

Icarus↗
USGS2000

Results of the Galileo solid state imaging (SSI) experiment

We present a brief synopsis of the nature of SSI data that was taken during the nominal Galileo mission. Significant results are briefly described. These include evidence for geologic activity on Europa that supports the hypothesis of liquid water under a thin ice shell; a demonstration of the ubiquitous presence of a thick crust and high temperature silicate volcanism on Io; a demonstration that, together with results from other experiments, the molecular composition in Jupiter's atmosphere depends on local meteorology; the resolution of lightning flashes and their association with cloud type; the identification of cloud structures at or below the 4-bar level that are presumably composed of water; the location and morphology of visible aurorae in Jupiter's Northern hemisphere; improved geodetic control nets for the Galilean satellites; measurement of the sizes and shapes of Metis, Adrastea and Amalthea; new features of the main and gossamer rings; discovery of visible Io-glow and surface hotspots especially at the sub- and anti-jove regions on Io; evidence for solid state convection in Europa's ice shell and its probable non-synchronous rotation; the discovery of visible Europa-glow; evidence that tectonism has dominated the resurfacing of bright grooved terrain on Ganymede; the discovery of a thick blanket of dark material on Callisto's surface and that mass-wasting and sublimation are significant degradation processes on this satellite.

Advances in Space Research↗
USGS2000

North-south geological differences between the residual polar caps on Mars

Polar processes can be sensitive indicators of global climate, and the geological features associated with polar ice caps can therefore indicate evolution of climate with time. The polar regions on Mars have distinctive morphologic and climatologic features: thick layered deposits, seasonal CO2 frost caps extending to mid latitudes, and near-polar residual frost deposits that survive the summer. The relationship of the seasonal and residual frost caps to the layered deposits has been poorly constrained, mainly by the limited spatial resolution of the available data. In particular, it has not been known if the residual caps represent simple thin frost cover or substantial geologic features. Here we show that the residual cap on the south pole is a distinct geologic unit with striking collapse and erosional topography; this is very different from the residual cap on the north pole, which grades into the underlying layered materials. These findings indicate that the differences between the caps are substantial (rather than reflecting short-lived differences in frost cover), and so support the idea of long-term asymmetry in the polar climates of Mars.

Nature↗
USGS1999

Does Europa have a subsurface ocean? Evaluation of the geological evidence

It has been proposed that Jupiter's satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H 2 O layer about 100 km thick but cannot determine the current physical state of this layer (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull‐apart bands, chaos, ridges, surface frosts, topography, and global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Europa; the model favored here indicates an average age of ∼50 Myr. Searches for evidence of current geological activity on Europa, such as plumes or surface changes, have yielded negative results to date. The current existence of a global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within Europa today.

Journal of Geophysical Research: Planets↗
USGS1999

Retention of an atmosphere on early Mars

The presence of valley networks and indications of high erosion rates in ancient terrains on Mars suggest that Mars was warm and wet during heavy bombardment. Various processes that could occur on early Mars were integrated into a self-consistent model to determine what circumstances might lead to warm temperatures during and at the end of heavy bombardment. Included were weathering and burial of CO 2 as carbonates, impact erosion, sputtering, and recycling of CO 2 back into the atmosphere by burial and heating. The models suggest that despite losses from the atmosphere by weathering and impact erosion, Mars could retain a 0.5 to 1 bar atmosphere at the end of heavy bombardment partly because weathering temporarily sequesters CO 2 in the ground and protects it from impact erosion while the impact rate is declining and impact erosion is becoming less effective. Because of the low output of the early Sun, surface temperatures can be above freezing only for a very efficient greenhouse, such as that suggested by Forget and Pierrehumbert [1997]. With weak greenhouse models, temperatures are below freezing throughout heavy bombardment, and such a large amount of CO 2 is left in the atmosphere at the end of heavy bombardment that it is difficult to eliminate subsequently to arrive at the present surface inventory. With strong greenhouse models, temperatures are well above freezing during heavy bombardment and drop to close to freezing at the end of heavy bombardment, at which time the atmosphere contains 0.5 to 1 bar of CO 2 . This can be largely eliminated subsequently by sputtering and low-temperature weathering. Such a model is consistent with the change in erosion rate and the declining rate of valley formation at the end of heavy bombardment. Conditions that favor warm temperatures at the end of heavy bombardment are an efficient greenhouse, low weathering rates, low impact erosion rates, and a smaller fraction of heat lost by conduction as opposed to transport of lava to the surface.

Journal of Geophysical Research E: Planets↗
USGS1999

Bright dunes on mars

Seasonal changes observed on the surface of Mars can in part be attributed to the transport of geological materials by wind. Images obtained by orbiting spacecraft in the 1970s showed large wind-formed features such as dunes, and revealed regional time-varying albedos that could be attributed to the effects of dust erosion and deposition. But the resolution of these images was insufficient to identify different types and sources of aeolian materials, nor could they reveal aeolian deposits other than large dunes or extensive surface coverings that were redistributed by dust storms. Here we present images of Mars with up to 50 times better resolution. These images show that martian dunes include at least two distinct components, the brighter of which we interpret to be composed of relatively soft minerals, possibly sulphates. We also find large areas of the martian surface that have several metres or more of aeolian mantle lacking obvious bedforms.

Nature↗
USGS1998

Europa: Initial Galileo Geological Observations

Images of Europa from the Galileo spacecraft show a surface with a complex history involving tectonic deformation, impact cratering, and possible emplacement of ice-rich materials and perhaps liquids on the surface. Differences in impact crater distributions suggest that some areas have been resurfaced more recently than others; Europa could experience current cryovolcanic and tectonic activity. Global-scale patterns of tectonic features suggest deformation resulting from non-synchronous rotation of Europa around Jupiter. Some regions of the lithosphere have been fractured, with icy plates separated and rotated into new positions. The dimensions of these plates suggest that the depth to liquid or mobile ice was only a few kilometers at the time of disruption. Some surfaces have also been upwarped, possibly by diapirs, cryomagmatic intrusions, or convective upwelling. In some places, this deformation has led to the development of chaotic terrain in which surface material has collapsed and/or been eroded. ?? 1998 Academic Press.

Icarus↗
USGS1998

Mountains and Calderas on Io: Possible Implications for Lithosphere Structure and Magma Generation

The combination of Voyager images and newly acquired Galileo images with low illumination and resolutions ranging from 2 to 6 km/pixel now allows determination of the global distribution of mountains and volcanic centers on Io. The mountains generally do not have characteristics typical of terrestrial volcanic landforms, they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes. Relative elevations, determined by shadow measurements and stereoscopy, indicate that mountains in the newly imaged area range in elevation up to at least 7.6 km. The origin of the mountains remains uncertain. Some appear to be multitiered volcanic constructs; others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes; yet others show extensive tectonic disruption. Volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes. The low latitudes have one volcanic center per 7 ?? 104km2, and, on average, the centers are spaced roughly 250 km apart. The global distribution of high mountains suggests that the lithosphere over most of Io is thick. Although the thickness cannot be calculated, the previously suggested 30 km appears reasonable as a lower limit. The high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere. The slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on Io, coupled with the high rate of magma production, will likely result in larger melt fractions than is typical for source regions on Earth. The variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface. This mixture will be carried downward through the lithosphere as a consequence of the 0.5-1.5 cm/yr resurfacing rates. During descent, the more volatile components will tend to be driven off early, but complete or near-complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains. ?? 1998 Academic Press.

Icarus↗
USGS1998

The Shape of Io from Galileo Limb Measurements

Galileo CCD images of the limb of Io provide improved data for determining the shape of this synchronously rotating satellite. The best ellipsoidal fit is within 0.3 km of the best equilibrium fit of 1829.7, 1819.2, 1815.8 km. The shape is consistent with substantial mass concentration in a core and with gravity measurements from tracking of the Galileo spacecraft. The surface of Io is largely plains and isolated peaks, with little long-wavelength topography over 1 km in amplitude. ?? 1998 Academic Press.

Icarus↗
USGS1998

Evidence for a subsurface ocean on Europa

Ground-based spectroscopy of Jupiter's moon Europa, combined with gravity data, suggests that the satellite has an icy crust roughly 150 km thick and a rocky interior. In addition, images obtained by the Voyager spacecraft revealed that Europa's surface is crossed by numerous intersecting ridges and dark bands (called lineae) and is sparsely cratered, indicating that the terrain is probably significantly younger than that of Ganymede and Callisto. It has been suggested that Europa's thin outer ice shell might be separated from the moon's silicate interior by a liquid water layer, delayed or prevented from freezing by tidal heating; in this model, the lineae could be explained by repetitive tidal deformation of the outer ice shell. However, observational confirmation of a subsurface ocean was largely frustrated by the low resolution (>2 km per pixel) of the Voyager images. Here we present high-resolution (54 m per pixel) Galileo spacecraft images of Europa, in which we find evidence for mobile 'icebergs'. The detailed morphology of the terrain strongly supports the presence of liquid water at shallow depths below the surface, either today or at some time in the past. Moreover, lower- resolution observations of much larger regions suggest that the phenomena reported here are widespread.

Nature↗
USGS1998

Geological evidence for solid-state convection in Europa's ice shell

The ice-rich surface of the jovian satellite Europa is sparsely cratered, suggesting that this moon might be geologically active today. Moreover, models of the satellite's interior indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water layer. But the mechanisms of interior heat loss and resurfacing are currently unclear, as is the question of whether Europa has (or had at one time) a liquid water ocean. Here we report on the morphology and geological interpretation of distinct surface features-pits, domes and spots-discovered in high-resolution images of Europa obtained by the Galileo spacecraft. The features are interpreted as the surface manifestation of diapirs, relatively warm localized ice masses that have risen buoyantly through the subsurface. We find that the formation of the features can be explained by thermally induced solid-state convection within an ice shell, possibly overlying a liquid water layer. Our results are consistent with the possibility that Europa has a liquid water ocean beneath a surface layer of ice, but further tests and observations are needed to demonstrate this conclusively.

Nature↗
USGS1997

High-temperature hot spots on Io as seen by the Galileo solid state imaging (SSI) experiment

High-temperature hot spots on Io have been imaged at ∼50 km spatial resolution by Galileo's CCD imaging system (SSI). Images were acquired during eclipses (Io in Jupiter's shadow) via the SSI clear filter (∼0.4–1.0 µm), detecting emissions from both small intense hot spots and diffuse extended glows associated with Io‧s atmosphere and plumes. A total of 13 hot spots have been detected over ∼70% of Io–s surface. Each hot spot falls precisely on a low-albedo feature corresponding to a caldera floor and/or lava flow. The hot-spot temperatures must exceed ∼700 K for detection by SSI. Observations at wavelengths longer than those available to SSI require that most of these hot spots actually have significantly higher temperatures (∼1000 K or higher) and cover small areas. The high-temperature hot spots probably mark the locations of active silicate volcanism, supporting suggestions that the eruption and near-surface movement of silicate magma drives the heat flow and volcanic activity of Io.

Geophysical Research Letters↗
USGS1997

Martian drainage densities

Drainage densities on Mars range from zero over large areas of volcanic plains to 0.3–0.5 km −1 locally on some volcanoes. These values refer to geologic units, not to drainage basins, as is normal for terrestrial drainage densities. The highest values are close to the lowest terrestrial values derived by similar techniques. Drainage densities were determined for every geologic unit portrayed on the 1:15,000,000 geologic map of Mars. Except for volcanoes the geologic unit with the highest drainage density is the dissected Noachian plains with a drainage density of 0.0074 km −1 . The average drainage density for Noachian units is 0.0032 km −1 , for Hesperian units is 0.00047 km −1 , and for Amazonian units is 0.00007 km −1 , excluding the volcanoes. These values are 2–3 orders of magnitude lower than typical terrestrial densities as determined by similar techniques from Landsat images. The low drainage densities, despite a cumulative record that spans billions of years, indicate that compared with the Earth, the channel-forming processes have been very inefficient or have operated only rarely or that the surface is extremely permeable. The high drainage density on volcanoes is attributed to a local cause, such as hydrothermal activity, rather than to a global cause such as climate change.

Journal of Geophysical Research E: Planets↗
USGS1996

Channels and valleys on Mars: Cold climate features formed as a result of a thickening cryosphere

Large flood channels, valley networks, and a variety of features attributed to the action of ground ice indicate that Mars emerged from heavy bombardment around 3.8Gyr ago, with an inventory of water at the surface equivalent to at least a few hundred meters spread over the whole planet, as compared with 3 km for the Earth. The surface water resided primarily in a porous, kilometers thick, megaregolith created by the high impact rates. At the end of heavy bombardment a rapid decline in erosion rates by a factor of 1000 suggests a major change in the global climate. It is proposed that at this time the climate became similar to today's and that this climate has been maintained throughout the rest of Mars' history. The various drainage features represent an adjustment of the distribution of water to the surface relief inherited from the period of heavy bombardment and to a thickening of the cryosphere as the heat flow declined. The valley networks formed mostly at the end of heavy bombardment when erosion rates were high and climatic conditions permitted an active water cycle. They continued to form after heavy bombardment when the cryosphere started to form by a combination of episodic flooding and mass-wasting aided by the presence of liquid water at shallow depths. As the cryosphere thickened with declining heat flow, water could no longer easily access the surface and the rate of valley formation declined. Hydrostatic pressures built below the cryosphere. Eruptions of groundwater became more catastrophic and massive floods resulted, mainly in upper Hesperian time. Flood sources were preferentially located in low-lying, low-latitude areas where the cryosphere was thin, or near volcanoes where a thinner than typical cryosphere is also expected. Floods caused a drawdown in the global water table so that few formed in the second half of Mars' history. The floodwaters pooled in low-lying areas, mostly in the northern plains. Some of the water may still be present as thick ice deposits, some has been lost to space, particularly during periods of high obliquity.

Planetary and Space Science↗
USGS1996

Galileo's first images of Jupiter and the Galilean satellites

The first images of Jupiter, Io, Europa, and Ganymede from the Galileo spacecraft reveal new information about Jupiter's Great Red Spot (GRS) and the surfaces of the Galilean satellites. Features similar to clusters of thunderstorms were found in the GRS. Nearby wave structures suggest that the GRS may be a shallow atmospheric feature. Changes in surface color and plume distribution indicate differences in resurfacing processes near hot spots on lo. Patchy emissions were seen while Io was in eclipse by Jupiter. The outer margins of prominent linear markings (triple bands) on Europa are diffuse, suggesting that material has been vented from fractures. Numerous small circular craters indicate localized areas of relatively old surface. Pervasive brittle deformation of an ice layer appears to have formed grooves on Ganymede. Dark terrain unexpectedly shows distinctive albedo variations to the limit of resolution.

Science↗
USGS1996

Scientific objectives of human exploration of Mars

While human exploration of Mars is unlikely to be undertaken for science reasons alone, science will be the main beneficiary. A wide range of science problems can be addressed at Mars. The planet formed in a different part of the solar system from the Earth and retains clues concerning compositional and environmental conditions in that part of the solar system when the planets formed. Mars has had a long and complex history that has involved almost as wide a range of processes as occurred on Earth. Elucidation of this history will require a comprehensive program of field mapping, geophysical sounding, in situ analyses, and return of samples to Earth that are representative of the planet's diversity. The origin and evolution of the Mars' atmosphere are very different from the Earth's, Mars having experienced major secular and cyclical changes in climate. Clues as to precisely how the atmosphere has evolved are embedded in its present chemistry, possibly in surface sinks of former atmosphere-forming volatiles, and in the various products of interaction between the atmosphere and surface. The present atmosphere also provides a means of testing general circulation models applicable to all planets. Although life is unlikely to be still extant on Mars, life may have started early in the planet's history. A major goal of any future exploration will, therefore, be to search for evidence of indigenous life.

American Astronautical Society, Scientific Technol↗
USGS1994

Galileo multispectral imaging of the north polar and eastern limb regions of the moon

Multispectral images obtained during the Galileo probe's second encounter with the moon reveal the compositional nature of the north polar regions and the northeastern limb. Mare deposits in these regions are found to be primarily low to medium titanium lavas and, as on the western limb, show only slight spectral heterogeneity. The northern light plains are found to have the spectral characteristics of highlands materials, show little evidence for the presence of cryptomaria, and were most likely emplaced by impact processes regardless of their age.Multispectral images obtained during the Galileo probe's second encounter with the moon reveal the compositional nature of the north polar regions and the northeastern limb. Mare deposits in these regions are found to be primarily low to medium titanium lavas and, as on the western limb, show only slight spectral heterogeneity. The northern light plains are found to have the spectral characteristics of highlands materials, show little evidence for the presence of cryptomaria, and were most likely emplaced by impact processes regardless of their age.

Science↗