Frank C. Chuang

A Closer Look at Water-Related Geologic Activity on Mars

Water has supposedly marked the surface of Mars and produced characteristic landforms. To understand the history of water on Mars, we take a close look at key locations with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter, reaching fine spatial scales of 25 to 32 centimeters per pixel. Boulders ranging up to ∼2 meters in diameter are ubiquitous in the middle to high latitudes, which include deposits previously interpreted as finegrained ocean sediments or dusty snow. Bright gully deposits identify six locations with very recent activity, but these lie on steep (20° to 35°) slopes where dry mass wasting could occur. Thus, we cannot confirm the reality of ancient oceans or water in active gullies but do see evidence of fluvial modification of geologically recent mid-latitude gullies and equatorial impact craters.

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.

Geologic mapping of Europa

Galileo data enable the major geological units, structures, and surface features to be identified on Europa. These include five primary units (plains, chaos, band, ridge, and crater materials) and their subunits, along with various tectonic structures such as faults. Plains units are the most widespread. Ridged plains material spans a wide range of geological ages, including the oldest recognizable features on Europa, and appears to represent a style of tectonic resurfacing, rather than cryovolcanism. Smooth plains material typically embays other terrains and units, possibly as a type of fluid emplacement, and is among the youngest material units observed. At global scales, plains are typically mapped as undifferentiated plains material, although in some areas differences can be discerned in the near infrared which might be related to differences in ice grain size. Chaos material is composed of plains and other preexisting materials that have been severely disrupted by inferred internal activity; chaos is characterized by blocks of icy material set in a hummocky matrix. Band material is arrayed in linear, curvilinear, wedge-shaped, or cuspate zones with contrasting albedo and surface textures with respect to the surrounding terrain. Bilateral symmetry observed in some bands and the relationships with the surrounding units suggest that band material forms by the lithosphere fracturing, spreading apart, and infilling with material derived from the subsurface. Ridge material is mapped as a unit on local and some regional maps but shown with symbols at global scales. Ridge material includes single ridges, doublet ridges, and ridge complexes. Ridge materials are considered to represent tectonic processes, possibly accompanied by the extrusion or intrusion of subsurface materials, such as diapirs. The tectonic processes might be related to tidal flexing of the icy lithosphere on diurnal or longer timescales. Crater materials include various interior (smooth central, rough inner, and annular massif) and exterior (continuous ejecta) subunits. Structural features and landforms are shown with conventional symbols. Type localities for the units are identified, along with suggestions for portraying the features on geological maps, including colors and letter abbreviations for material units. Implementing these suggestions by the planetary mapping community would facilitate comparisons of maps for different parts of Europa and contribute to an eventual global synthesis of its complex geology. On the basis of initial mapping results, a stratigraphic sequence is suggested in which ridged plains form the oldest unit on Europa, followed by development of band material and individual ridges. Band materials tend to be somewhat older than ridges, but in many areas the two units formed simultaneously. Similarly, the formation of most chaos follows the development of ridged plains; although chaos is among the youngest materials on Europa, some chaos units might have formed contemporaneously with ridged plains. Smooth plains generally embay all other units and are late-stage in the evolution of the surface. C 1 craters are superposed on ridged plains but are crosscut by other materials, including bands and ridges. Most c2 craters postdate all other units, but a few c2 craters are cut by ridge material. C3 craters constitute the youngest recognizable material on Europa.

Landform degradation and slope processes on Io: The Galileo view

The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on Io. We recognize four major slope types observed on a number of intermediate resolution (∼250 m pixel−1) images and several additional textures on very high resolution (∼10 m pixel−1) images. Slopes and scarps on Io often show evidence of erosion, seen in the simplest form as alcove-carving slumps and slides at all scales. Many of the mass movement deposits on Io are probably mostly the consequence of block release and brittle slope failure. Sputtering plays no significant role. Sapping as envisioned by McCauley et al. [1979] remains viable. We speculate that alcove-lined canyons seen in one observation and lobed deposits seen along the bases of scarps in several locations may reflect the plastic deformation and “glacial” flow of interstitial volatiles (e.g., SO2) heated by locally high geothermal energy to mobilize the volatile. The appearance of some slopes and near-slope surface textures seen in very high resolution images is consistent with erosion from sublimation-degradation. However, a suitable volatile (e.g., H2S) that can sublimate fast enough to alter Ios youthful surface has not been identified. Disaggregation from chemical decomposition of solid S2O and other polysulfur oxides may conceivably operate on Io. This mechanism could degrade landforms in a manner that resembles degradation from sublimation, and at a rate that can compete with resurfacing.

Geological history of the Tyre region of Europa: A regional perspective on Europan surface features and ice thickness

Galileo images of the Tyre Macula region of Europa at regional (170 m/pixel) and local (∼40 m/pixel) scales allow mapping and understanding of surface processes and landforms. Ridged plains, doublet and complex ridges, shallow pits, domes, “chaos” areas, impact structures, tilted blocks and massifs, and young fracture systems indicate a complex history of surface deformation on Europa. Regional and local morphologies of the Tyre region of Europa suggest that an impactor penetrated through several kilometers of water ice to a mobile layer below. The surface morphology was initially dominated by formation of ridged plains, followed by development of ridge bands and doublet ridges, with chaos and fracture formation dominating the latter part of the geologic history of the Tyre region. Two distinct types of chaos have been identified which, along with upwarped dome materials, appear to represent a continuum of features (domes-platy chaos-knobby chaos) resulting from increasing degrees of surface disruption associated with local lithospheric heating and thinning. Local and regional stratigraphic relationships, block heights, and the morphology of the Tyre impact structure suggest the presence of low-viscosity ice or liquid water beneath a thin (several kilometers) surface ice shell at the time of the impact. The very low impact crater density on the surface of Europa suggests that this thin shell has either formed or been thoroughly resurfaced in the very recent past.

Large mass movements on Callisto

Galileo images reveal the presence of mass movement deposits within impact craters on Callisto. Eleven such deposits were found in 830 candidate craters imaged at sufficient resolutions (86–280 m/pixel) for their identification. All of the deposits are located within impact craters, and their sources appear to be crater wall material. The morphologies of the Callistan deposits are similar to terrestrial mass movements and rock glaciers. Rock glaciers involve ductile flow of ice and rock, but in the Callistan environment, water ice is likely to undergo brittle deformation. Consequently, rock glaciers are unlikely analogs for the features on Callisto. Three morphologies are observed: one blocky, one slump-like, and nine lobate deposits. Blocky deposits are characterized by massive blocks on the crater floor, while slump-like deposits appear as debris piles along the base of the crater wall. Lobate deposits are 1.7–9.9 km long, average ∼90 m in thickness, and have tapered or semicircular terminations. Callistan lobate deposits are morphologically similar to rapidly emplaced dry-rock avalanches on Earth. Their significant thickness and steep frontal margins suggest that they may have behaved similarly to Bingham plastic material with a critical yield strength. The Callistan deposits have yield strengths similar to those estimated for terrestrial dry-rock avalanches. The blocky deposit appears similar to terrestrial block glide deposits, in which large blocks detach and slide downslope. Individual blocks within the blocky deposits may be up to 1 km across and stand 30–60 m high. Callistan slump-like deposits are similar to terrestrial slumps, which fail along rotational planes. Mass movements on Callisto are not confined to craters; they have also been observed along the base of scarps, knobs, and ridges. Three of the Callistan deposits may have been impact triggered because of the proximity of an impact crater near the source. First-order analyses of the ground force generated by impacts show that two of the candidate deposits are capable of being impact triggered. The Callistan mass movements are probably set into motion by a combination of sublimation of ice, which undermines a near-surface layer of “lag” material, and seismic “triggering” associated with nearby impact events.

Multiple surface wetting events in the greater Meridiani Planum region, Mars: Evidence from valley networks within ancient cratered highlands

Morphological characterization of valley networks in three exposures of ancient cratered highlands (Nhc1) in the greater Meridiani Planum region yields insight into the Martian aqueous history. From our mapping, key regional differences are apparent in fine-scale valley network attributes including morphologic type, planimetric form, density, and links to candidate paleolakes. This information, combined with crater retention age (inferred exposure age), provides new details on the relative timing and nature of aqueous processes in the region. Newly identified pitted-type valley networks have morphological similarity to terrestrial pitted landforms in an evaporite setting. We interpret the pitted valley networks to reflect late-stage groundwater processes concentrated along the former fluvial conduits. Evidence from this study indicates that localized reactivation of valley networks occurred during or after exhumation of eastern Nhc1 unit.