Gerhard Neukum

Imaging of Titan from the Cassini spacecraft

Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titans nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titans history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150–200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved.

Ages of mare basalts on the lunar nearside

The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size-frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size-frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size-frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5–2.0 b.y., starting at about 3.9–4.0 b.y. and ceasing at ∼2.0 b.y., (2) most basalts erupted during the late Imbrian Period at about 3.6–3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin-filling volcanism probably started within ∼100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre-Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.6–3.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3–3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin-filling volcanism (1.5–1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even ∼1.0 b.y. longer than in Tranquillitatis and Humboldtianum.

Galileo encounter with 951 gaspra: first pictures of an asteroid.

Galileo images of Gaspra reveal it to be an irregularly shaped object (19 by 12 by 11 kilometers) that appears to have been created by a catastrophic collisional disruption of a precursor parent body. The cratering age of the surface is about 200 million years. Subtle albedo and color variations appear to correlate with morphological features: Brighter materials are associated with craters especially along the crests of ridges, have a stronger 1-micrometer absorption, and may represent freshly excavated mafic materials; darker materials exhibiting a significantly weaker 1-micrometer absorption appear concentrated in interridge areas. One explanation of these patterns is that Gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas.

Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars' equator

It is thought that the Cerberus Fossae fissures on Mars were the source of both lava and water floods two to ten million years ago. Evidence for the resulting lava plains has been identified in eastern Elysium, but seas and lakes from these fissures and previous water flooding events were presumed to have evaporated and sublimed away. Here we present High Resolution Stereo Camera images from the European Space Agency Mars Express spacecraft that indicate that such lakes may still exist. We infer that the evidence is consistent with a frozen body of water, with surface pack-ice, around 5° north latitude and 150° east longitude in southern Elysium. The frozen lake measures about 800 × 900 km in lateral extent and may be up to 45 metres deep—similar in size and depth to the North Sea. From crater counts, we determined its age to be 5 ± 2 million years old. If our interpretation is confirmed, this is a place that might preserve evidence of primitive life, if it has ever developed on Mars.

Lunar impact basins: New data for the western limb and far side (Orientale and South Pole‐Aitken Basins) from the first Galileo flyby

Compositional aspects of impact basin materials can be analyzed using multispectral image data acquired by the Galileo solid state imaging (SSI) experiment during the December 1990 lunar encounter. These data provide important information on the spectral properties of the western lunar limb and parts of the far side. The SSI images cover the wavelength range 0.4–1.0 μm, allowing measurement of spectral slope and estimation of the strength of the 1 μm absorption due to iron in the mafic minerals olivine and pyroxene. Among deposits of the 930-km-diameter Orientale basin, exterior ejecta comprising the Hevelius Formation is relatively homogeneous and spectrally similar to mature Apollo 16 soils, suggesting an upper crustal source. The centrally located Maunder Formation is distinct from the younger mare basalts but comparable to the Hevelius Formation in its spectral reflectance properties, supporting an interpretation as basin impact melt. The Montes Rook Formation, located in an annulus between the Maunder and the Hevelius, shows a slightly stronger mafic absorption and may be the deepest crustal material excavated. The distal Orientale deposits show local mafic enhancements (in the Schiller-Schickard and Mendel-Rydberg regions) interpreted to represent pre-Orientale mare deposits, or cryptomaria, intermixed with overlying basin ejecta. In this case, maria of sizes comparable to those presently observed were widespread in this region before the Orientale impact. Mixing-model analyses are consistent with the ballistic erosion and sedimentation model for ejecta emplacement in the distal regions beyond the continuous ejecta deposit. On the southern lunar farside, a huge area with an enhanced mafic absorption corresponds to the interior and rim of the pre-Nectarian South Pole-Aitken impact basin, 2000–2500 km in diameter. The anomaly is interpreted to be due to several factors, including excavation into the more mafic lower crust, and the presence of extensive early volcanic fill (cryptomare), similar to that seen in ancient basins such as Smythii and Australe. These results show that although basin-forming events are an important factor in producing lateral heterogeneities in crustal composition, and in modifying preexisting deposits (such as cryptomaria), the majority of material in even the largest basins was excavated from crustal levels. Our results suggest a gradational vertical crustal stratigraphy consisting of an uppermost mixed crustal layer of anorthosite, basin ejecta, and cryptomaria deposits (generally corresponding to the megaregolith), an upper crustal layer of anorthosite, and a lower more noritic layer. Many of the basic questions remaining from this study could be addressed by global high-resolution geochemical and mineralogical data obtained by polar orbiting spacecraft.

Orientation and distribution of recent gullies in the southern hemisphere of Mars: Observations from High Resolution Stereo Camera/Mars Express (HRSC/MEX) and Mars Orbiter Camera/Mars Global Surveyor (MOC/MGS) data

Geologically recent small gullies on Mars display morphologies consistent with erosion by water or by debris flows. Suggested formation models are divided into two main categories: (1) groundwater or (2) melting of near-surface ice/snow sourced from the atmosphere. We have measured location and orientation and recorded the local contexts of gullies to constrain the likely models of gully formation. More than 22,000 Mars Orbiter Camera Narrow Angle (MOC NA) and >120 Mars Express High Resolution Stereo Camera (HRSC) images in the southern hemisphere were searched for gullies. Discrete gullied slope sections with consistent orientation were recorded rather than individual gullies. Slope setting (impact crater, valley wall, etc.), location, and orientation were recorded for each slope section. More than 750 MOC images with gullies (>900 distinct gullied slope sections) and more than 40 HRSC images (>380 distinct gullied slope sections) were identified. From both MOC and HRSC, gullies were found to be most common between 30 and 50 degrees latitude and to have an overall pole facing preference. The preferred gully orientation for HRSC is southeast rather than south in MOC, owing to illumination effects that make gullies difficult to detect on south- to southwest-facing slopes in HRSC. In both MOC and HRSC surveys, higher-latitude gullies show less preference for pole facing than those at mid latitudes. Both data sets produced similar results, demonstrating that our data are reliable. We suggest that the observed latitudinal and orientation distributions of gullies show that insolation and atmospheric conditions play a key role in gully formation.

Mars Express HRSC Data Processing - Methods and Operational Aspects

Automated procedures for ground processing of Mars Express HRSC data have been developed and are applied systematically immediately after download in order to provide calibrated data sets as well as photogrammetric image and 3D data products within a short time frame. Multi-spectral orthoimages in scales of 10 - 50 m/pixel and digital terrain models of 200 m raster width are generated within days even for large orbits covering areas of several hundred thousand square kilometers. An even higher image resolution of up to 2.3 m/pixel provided by HRSCs Super Resolution Channel (SRC) extends the potential of the HRSC camera experiment.

Large-scale spring deposits on Mars?

We present a large-scale spring hypothesis for the formation of various enigmatic light-toned deposits (LTDs) on Mars. Layered to massive LTDs occur extensively in Valles Marineris, chaotic terrains, and several large craters, in particular, those located in Arabia Terra. Most of these deposits are not easily explained with either a single process or multiple ones, either in combination or occurring sequentially. Spring deposits can have a very wide range of internal facies and exhibit complex architectural variations. We propose the concept of large-scale spring deposits for explaining LTDs on Mars. Stable volcano-tectonic settings, such as the ones typical on Mars, are compatible with a large-scale, long-term, multistage formation of spring deposits. The large-scale spring deposit model can explain the formation of LTDs with a common process, although active in different times and locations, compatible with coeval local or regional processes and deposits, such as volcaniclastic ones. LTDs, if formed as spring deposits derived from subsurface fluids, could potentially offer favorable conditions both to life and to the fossilization of past life forms.

Images from Galileo of the Venus Cloud Deck

Images of Venus taken at 418 (violet) and 986 [near-infrared (NIR)] nanometers show that the morphology and motions of large-scale features change with depth in the cloud deck. Poleward meridional velocities, seen in both spectral regions, are much reduced in the NIR In the south polar region the markings in the two wavelength bands are strongly anticorrelated. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large-scale planetary waves. No optical lightning was detected.

Automated Crater Detection, A New Tool for Mars Cartography and Chronology

An automated crater detection algorithm is presented which exploits image data. The algorithm is briefly described and its application demonstrated on a variety of different Martian geomorphological areas and sensors (Viking Orbiter Camera, Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), and High Resolution Stereo Camera (HRSC)). We show assessment results through both an intercomparison of automated crater locations with those from the manually-derived Mars Crater Consortium (MCC) catalogue and the manually-derived craters. The detection algorithm attains an accuracy of 70 to 90 percent and a quality factor of 60 to 80 percent depending on target sensor type and geomorphology. We also present crater detection results derived from HRSC images onboard the ESA Mars Express on a comparison between manually-determined Size-Frequency Distributions (SFDs) and those derived fully automatically. The approach described appears to offer great potential for chronological research, geomatic and geological analysis and for other purposes of extra-terrestrial planetary surface mapping.