Daniel C. Berman

Elysium Planitia lava flows: Crater count chronology and geological implications

Mars Global Surveyor (MGS) and Viking images allow analysis of chronological and geological relations among lava flows in southern Elysium Planitia, based on crater populations. MGS and Viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces. There is no evidence of substantial dust mantling on the young flows, and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves. The youngest flows have some of the lowest crater densities that we have seen on Mars, some <1% of lunar mare values, at crater diameters below about 180 to 500 m. We infer that thin, scattered volcanic flows, some tens of meters thick, have been emplaced atop stratigraphic units within the last 100 Myr, and possibly within the last 10 Myr. Volcanism is thus a continuing process in the recent geologic history of Mars, and this must constrain geophysical models of the planet.

Martian outflow channels: How did their source aquifers form, and why did they drain so rapidly?

Catastrophic floods generated ~3.2 Ga by rapid groundwater evacuation scoured the Solar System’s most voluminous channels, the southern circum-Chryse outflow channels. Based on Viking Orbiter data analysis, it was hypothesized that these outflows emanated from a global Hesperian cryosphere-confined aquifer that was infused by south polar meltwater infiltration into the planet’s upper crust. In this model, the outflow channels formed along zones of superlithostatic pressure generated by pronounced elevation differences around the Highland-Lowland Dichotomy Boundary. However, the restricted geographic location of the channels indicates that these conditions were not uniform Boundary. Furthermore, some outflow channel sources are too high to have been fed by south polar basal melting. Using more recent mission data, we argue that during the Late Noachian fluvial and glacial sediments were deposited into a clastic wedge within a paleo-basin located in the southern circum-Chryse region, which was then completely submerged under a primordial northern plains ocean. Subsequent Late Hesperian outflow channels were sourced from within these geologic materials and formed by gigantic groundwater outbursts driven by an elevated hydraulic head from the Valles Marineris region. Thus, our findings link the formation of the southern circum-Chryse outflow channels to ancient marine, glacial, and fluvial erosion and sedimentation.

Fresh exposures of hydrous Fe‐bearing amorphous silicates on Mars

We have discovered relatively fresh exposures of a hydrated, amorphous material along the wall rock slopes in Coprates Chasma, Mars. Visible and near-infrared reflectance spectra extracted from the deposits exhibit broad absorptions around 1.42, 1.94, and 2.25 µm that are most consistent with laboratory spectra of nanophase hydrated Fe-rich allophane and Fe-rich opal. The three absorptions, especially the 1.4 µm band, have the strongest hydration signatures yet detected on Mars by orbital data, suggesting high-water content material that is relatively fresh and has not altered or lost water since its formation or exposure. Age dating from crater size-frequency distributions of the Fe-rich allophane/opal deposits yields ages of ~50–100 Myr, consistent with a young exposure time and minimal time for dehydration. Either the Fe-rich allophane/opal represents an older material already contained within the wall rock that has been more recently exposed, or it represents a younger material formed during more recent aqueous activity.

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.

THE HAMO-BASED GLOBAL GEOLOGIC MAP OF CERES FROM NASA’S DAWN MISSION

This abstract discusses current results from the 1:2.5M-scale High Altitude Mapping Orbit (HAMO)-based global geologic mapping effort of Ceres using image, spectral and topographic data from the Dawn mission. Mapping base materials include the Dawn Framing Camera (FC) HAMO mosaic and individual images (∼140 m/pixel), the global HAMO DTM (137 m/pixel) derived from FC stereo images, and FC color mosaics (0.44-0.96 μm). These data are used to identify contacts and features, and for unit characterization. Geologic units are discriminated primarily by differences in albedo and surface texture; FC color images are used to spectrally constrain and characterize units. The map displays contacts and linear features (e.g., structures) represented by polylines, and singular features (e.g., albedo spots) represented by points. Because of map scale, only geologic units greater than 100 km2 in area, impact craters greater than 20 km in diameter, and linear features greater than 20 km in length are shown. Through geologic mapping we have defined several widespread units: cratered terrain, smooth material, and units of the Urvara/Yalode system. Cratered terrain forms the largest unit exposed on Ceres and contains rugged surfaces derived largely from the structures and deposits of impact features. This unit includes the oldest terrains exposed on Ceres, but the geologic materials likely consist of crustal materials mixed with impact materials. Smooth material forms a large deposit of nearly flat-lying to hummocky plains that fill and surround Kerwan basin, and embay the cratered terrain. Geologic materials related to the Urvara and Yalode basins consist of floor, rim, and ejecta deposits. Urvara ejecta consists of a rugged and a smooth facies; Yalode ejecta is distinguished by its smooth and rolling to stucco-like texture. Stratigraphic relations show that ejecta deposits and structures from Urvara superpose Yalode, indicating it is younger. Impact craters are the most prevalent features on the surface of Ceres, and appear to have caused most of the visible modification of the surface [1]. Impact craters exhibit sizes ranging from the limits of resolution to larger impact basins such as Urvara (170 km), Yalode (260 km), and Kerwan (284 km). Impact craters also exhibit a range of preservation states. Many craters of all sizes appear morphologically “fresh” to moderately degraded, with nearly circular rims that are raised above the surrounding terrain. Small fresh craters (<15 km) display simple bowl shapes, whereas larger fresh craters display steep walls and flat (sometimes fractured) floors [2], and most contain hummocky or irregular-shaped deposits on their floors. Many craters exhibit irregularly shaped, sometimes scalloped, rim structures, and contain debris lobes on their floors, suggesting instability in surface materials [1]. We are currently engaged in crater-based age dating, determining superposition relations, and using these to interpret Ceres chronostratigraphy, which will be presented at EGU. Support of the Dawn Instrument, Operations, & Science Teams is acknowledged. This work is supported by grants from NASA, DLR and MPG.