G. Neukum

Cratering Chronology and the Evolution of Mars

Results by Neukum et al. (2001) and Ivanov (2001) are combined with crater counts to estimate ages of Martian surfaces. These results are combined with studies of Martian meteorites (Nyquist et al., 2001) to establish a rough chronology of Martian history. High crater densities in some areas, together with the existence of a 4.5 Gyr rock from Mars (ALH84001), which was weathered at about 4.0 Gyr, affirm that some of the oldest surfaces involve primordial crustal materials, degraded by various processes including megaregolith formation and cementing of debris. Small craters have been lost by these processes, as shown by comparison with Phobos and with the production function, and by crater morphology distributions. Crater loss rates and survival lifetimes are estimated as a measure of average depositional/erosional rate of activity.

Cassini Observes the Active South Pole of Enceladus

Cassini has identified a geologically active province at the south pole of Saturns moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at ∼55°S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturns E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

Evidence for a subsurface ocean on Europa

Ground-based spectroscopy of Jupiters 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 Europas 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 Europas thin outer ice shell might be separated from the moons 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.

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

It has been proposed that Jupiters satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O 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.

The Galileo Solid-State Imaging experiment

The Solid State Imaging (SSI) experiment on the Galileo Orbiter spacecraft utilizes a high-resolution (1500 mm focal length) television camera with an 800 × 800 pixel virtual-phase, charge-coupled detector. It is designed to return images of Jupiter and its satellites that are characterized by a combination of sensitivity levels, spatial resolution, geometric fiedelity, and spectral range unmatched by imaging data obtained previously. The spectral range extends from approximately 375 to 1100 nm and only in the near ultra-violet region (∼ 350 nm) is the spectral coverage reduced from previous missions. The camera is approximately 100 times more sensitive than those used in the Voyager mission, and, because of the nature of the satellite encounters, will produce images with approximately 100 times the ground resolution (i.e., ∼ 50 m lp-1) on the Galilean satellites. We describe aspects of the detector including its sensitivity to energetic particle radiation and how the requirements for a large full-well capacity and long-term stability in operating voltages led to the choice of the virtual phase chip. The F/8.5 camera system can reach point sources of V(mag) ∼ 11 with S/N ∼ 10 and extended sources with surface brightness as low as 20 kR in its highest gain state and longest exposure mode. We describe the performance of the system as determined by ground calibration and the improvements that have been made to the telescope (same basic catadioptric design that was used in Mariner 10 and the Voyager high-resolution cameras) to reduce the scattered light reaching the detector. The images are linearly digitized 8-bits deep and, after flat-fielding, are cosmetically clean. Information ‘preserving’ and ‘non-preserving’ on-board data compression capabilities are outlined. A special “summation” mode, designed for use deep in the Jovian radiation belts, near Io, is also described. The detector is ‘preflashed’ before each exposure to ensure the photometric linearity. The dynamic range is spread over 3 gain states and an exposure range from 4.17 ms to 51.2 s. A low-level of radial, third-order, geometric distortion has been measured in the raw images that is entirely due to the optical design. The distortion is of the pincushion type and amounts to about 1.2 pixels in the corners of the images. It is expected to be very stable.We discuss the measurement objectives of the SSI experiment in the Jupiter system and emphasize their relationships to those of other experiments in the Galileo project. We outline objectives for Jupiter atmospheric science, noting the relationship of SSI data to that to be returned by experiments on the atmospheric entry Probe. We also outline SSI objectives for satellite surfaces, ring structure, and ‘darkside’ (e.g., aurorae, lightning, etc.) experiments. Proposed cruise measurement objectives that relate to encounters at Venus, Moon, Earth, Gaspra, and, possibly, Ida are also briefly outlined. The article concludes with a description of a ‘fully distributed’ data analysis system (HIIPS) that SSI team members intend to use at their home institutions. We also list the nature of systematic data products that will become available to the scientific community. Finally, we append a short ‘historical’ note outlining the responsibilities and roles of institutions and individuals that have been involved in the 14 year development of the SSI experiment so far.

Discovery of a flank caldera and very young glacial activity at Hecates Tholus, Mars

The majority of volcanic products on Mars are thought to be mafic and effusive. Explosive eruptions of basic to ultrabasic chemistry are expected to be common, but evidence for them is rare and mostly confined to very old surface features. Here we present new image and topographic data from the High Resolution Stereo Camera that reveal previously unknown traces of an explosive eruption at 30° N and 149° E on the northwestern flank of the shield volcano Hecates Tholus. The eruption created a large, 10-km-diameter caldera ∼350 million years ago. We interpret these observations to mean that large-scale explosive volcanism on Mars was not confined to the planets early evolution. We also show that glacial deposits partly fill the caldera and an adjacent depression. Their age, derived from crater counts, is about 5 to 24 million years. Climate models predict that near-surface ice is not stable at mid-latitudes today, assuming a thermo-dynamic steady state. Therefore, the discovery of very young glacial features at Hecates Tholus suggests recent climate changes. We show that the absolute ages of these very recent glacial deposits correspond very well to a period of increased obliquity of the planets rotational axis.

Geologic evolution and cratering history of Mercury

Abstract Among the terrestrial planets, Mercury is the smallest and has the highest bulk density. Mercury exhibits a lunar-like surface, shaped by impact basins and craters. Rapid cooling and contraction as well as tidal despinning have resulted in a large inventory of tectonic scarps and faults visible on the surface. With plans for new orbiter missions to this intriguing planet taking shape, this paper presents a summary of our current knowledge on Mercurys geology and cratering history. On the basis of improved data on asteroid populations and crater scaling, we updated the time stratigraphic sequence for the planet and made new estimates for the time of formation of impact basins such as Tolstoj and Caloris, which generally are now thought to be younger than in previous estimates. In order to advance our understanding of the geology of the planet, imaging experiments on future missions must fill the gap in the global coverage left by the Mariner spacecraft, and increase the global multispectral spatial resolution to at least 100 m / pixel . Locally, the image resolution must reach approx. 10 m / pixel . Also, stereo topographic models with global and local resolutions of 200 and 20 m , respectively, are required.

Galileo Multispectral Imaging of the North Polar and Eastern Limb Regions of the Moon

Multispectral images obtained during the Galileo probes 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.

High-resolution, digital photogrammetric mapping: A tool for Earth science

When photogrammetric techniques are used to map surface topography both high-resolution digital elevation models (DEM) and ortho-images can be produced simultaneously (Figure 1). In ortho-images, displacements resulting from the interplay between the surface relief, imaging geometry, and the exterior orientation of the imaging system have been eliminated by differential rectification, making use of the relief information of the DEM. Ortho-images thus provide the metric properties of a map and complement the continuous description of surface topography represented in the DEM. Airborne photogrammetry is a powerful tool for monitoring mountain areas characterized by topography-related natural hazards such as landslides, avalanches, and lava flows on volcanoes. However, its use has been limited in the past due to the intensive data acquisition and processing needed to achieve high accuracy. This situation has been considerably improved by applying (1) a digital, high-resolution sensor system for photogrammetric data acquisition in combination with (2) direct sensor orientation techniques, replacing the use of abundant ground control points (GCP) with direct measurements of the sensors position and attitude. Knowledge of the sensor orientation for the time of exposure is a prerequisite for point determination, since stereophotogrammetry derives the position of visible surface points in a three-dimensional reference system by using the metric properties of overlapping images acquired from different viewpoints.