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The volcanic processes that formed Vallis Schr teri are not well understood. Vallis Schr teri, located on the Aristarchus Plateau, is the largest rille on the Moon, and it displays three key morphologic components: the Cobra Head, the 155-km-long primary rille, and the 240-km-long inner rille. Observations of terrestrial eruptions are applied here to help explain the morphologic relationships observed for Vallis Schr teri. The Cobra Head, a 10-km-wide source vent surrounded by a 35-kmdiameter and 900-m-high low shield, might have been constructed from fl ows, spatter, and pyroclastic deposits erupted during lava fountain events, similar to the early stages of the vent at Pu u O o in Hawaii and the fi nal morphology of Bandera crater, a cinder cone in New Mexico. The vent fed an initial sheet fl ow controlled by preeruption topography. A channel formed within this sheet fl ow was the foundation for the primary rille, which deepened through construction and thermomechanical erosion by lava. The inner rille is confi ned to the fl at fl oor of the primary rille and is characterized by tight gooseneck meanders. This rille crosscuts the distal scarp of the primary rille and extends toward Oceanus Procellarum. This enigmatic relationship can be explained through backup, overfl ow, and diversion of the lava into a new rille that eroded into the margin of the primary rille. Similar backup, overfl ow, and redirection of the lava fl ow were observed during the 1984 Mauna Loa eruption in Hawaii. Analysis of the fi nal morphology of lunar rilles provides key information about lunar volcanic processes and insight into the local stratigraphy.

A field of small volcanic vents south of Pavonis Mons was mapped with each vent assigned a two-dimensional data point. Nearest neighbor and two-point azimuth analyses were applied to the resulting location data. Nearest neighbor results show that vents within this field are spatially random in a Poisson sense, suggesting that the vents formed independently of each other without sharing a centralized magma source at shallow depth. Two-point azimuth results show that the vents display north-trending alignment relationships between one another. This trend corresponds to the trends of faults and fractures of the Noachian-aged Claritas Fossae, which might extend into our study area buried beneath more recently emplaced lava flows. However, individual elongate vent summit structures do not consistently display the same trend. The development of the volcanic field appears to display tectonic control from buried Noachian-aged structural patterns on small, ascending magma bodies while the surface orientations of the linear vents might reflect different, younger tectonic patterns. These results suggest a complex interaction between magma ascension through the crust, and multiple, older, buried Tharsis-related tectonic structures.

International audience; This study focuses on the volcanism in Syria Planum, located at the center of the Tharsis bulge at an altitude of 6 to 8 km above Mars datum. Syria Planum was previously recognized as a center for the tectonic activity of Tharsis, but not as a major locus for volcanic activity, despite its centrality over the bulge. Using high-resolution images from the high resolution stereo camera on Mars Express combined with Mars Observer Laser Altimeter data, we have characterized a volcanic system that reveals a number of very interesting aspects of Mars volcanism. We identified a swarm of tens of coalesced shallow volcanic edifices, typically 10–30 km diameter, 0.1–0.2 km high, and with slopes around 0.5°. These characteristics are similar to those of small shield volcanoes found in Iceland. In addition, an intermediate-sized volcano, which is the source of lava flows that extend over >200 km, is observed west of this shield swarm. Our study characterizes a previously unrecognized volcanic assemblage on Mars which appears to be much more developed than was documented before, in terms of morphology, inferred origin, and periodicity of eruption. The estimated lava flux of the Syria Planum volcanoes is of the same order as the lava flux of Tharsis Montes. These characteristics suggest that Syria Planum experienced a very specific style of volcanism, which we dated to the Hesperian period.

[1] We calculated average and median slope values and produced hypsometric curves for the subaerial Hawaiian shield volcanoes using Shuttle Radar Topography Mission (SRTM) digital elevation models. The SRTM-derived average and median slope values display similar trends to published results based on other topographic data products, showing an increase in slope relative to volcano age. Shadows in the radar data over high relief regions reduce the slope values calculated for older volcanoes that have undergone significant erosion as compared to nonradar topographic data, suggesting that SRTM data are less sensitive to studies of erosion. Both the 1 and 3 arc sec SRTM data provide comparable results showing that the nearly global 3 arc sec data can be used to conduct the same studies for other volcanoes. Hypsometric analyses show that the Hawaiian shields maintain consistent upward and lateral growth until alkalic capping and subsidence alter the relief-surface area ratio. Changes in the morphology of the Hawaiian volcanoes are in part related to decreased magma production rates at older shields. The internal dynamics act to decrease the steadiness and duration of magma delivery to the surface causing a decrease in buffered eruptions and lava tube formation, all of which contribute to an increase in slope and relief-surface area ratios. Terrain analyses combined with mapping can provide insight into the development of remote volcanoes or volcanoes on other planets for which remotely sensed data exist but less is known about their internal and eruptive history.

[1] We mapped lava flows on the Tharsis Montes (Arsia Mons, Pavonis Mons, and Ascraeus Mons) using High Resolution Stereo Camera images that centrally transect each shield from north to south, covering ∼20% of each shield's surface. These data were co-registered to Mars Orbiter Laser Altimeter, Thermal Emission Imaging System, and Mars Orbiter Camera data, enabling lava flow structures and vents to be consistently differentiated across each volcano. Lava flow and vent abundances and relationships are used to provide new insight into the late Amazonian eruptive history of the Tharsis Montes. The volcanoes are divided into their main flanks, rift aprons, and small-vent fields. Where present on the main flanks, channel-fed flows always embay tube-fed flows, indicating a change from long-lived, stable tube-forming eruption conditions to shorter-lived, less stable channel-forming eruption conditions. Superposition relationships suggest that main flank and rift apron development were likely separated by an eruptive hiatus. The rift aprons, as compared to the main flanks, show higher abundances of tube- and channel-fed flows, and embayment of tube-fed flows by channel-fed flows is less consistent. Several trends from the Arsia Mons, to Ascraeus Mons, southwest rift aprons and small-vent fields were identified, including increased tube abundance, median slope, and number of satellitic eruptive vents and a decrease in channel- to tube-fed flow ratios, apron volumes, and maximum apron elevations. These trends suggest that the most recent volcanic activity at the Tharsis Montes might have originated from a single, shared magma source, possibly marking a change in magma production style from main flank construction.

[1] Lava flow mapping was conducted on a north-south transect of Olympus Mons using the European Space Agency's Mars Express High Resolution Stereo Camera (HRSC) image H0037. The HRSC image was coregistered to Mars Orbiter Laser Altimeter, Thermal Emission Imaging System, and Mars Orbiter Camera data, enabling lava flow structures to be differentiated and mapped consistently across the shield. Because different structures develop as a result of different effusive conditions, their abundance and distribution provide insight into the eruptive history of a shield volcano. Results show that lava channels are the dominant flow structure, whereas tabular sheets are most common beyond the basal scarp. A hummocky unit dominates the summit area and likely represents a combination of (1) volcanic lava flows, (2) pyroclastic deposits, (3) a dust mantle, and (4) frozen volatiles, all of which have been suggested to exist on Olympus Mons in the past. Lava fans are typically associated with lava tubes, indicating that they represent tube outbreaks as was previously suggested as one possible formation mechanism. No vents were identified, suggesting that major rift zones have not developed on the north or south flank. Younger channel-fed flows typically embay older tube-related flows, which they outnumber by a ratio of 5:1. Therefore Olympus Mons likely experienced a change in eruptive style from longer-lived, stable, tube-forming eruptions to shorter-lived, less stable, channel-forming eruptions in the late Amazonian. A similar trend exists for the Hawaiian volcanoes in which a decrease in the magma production rate drives a change to dominantly channel forming eruptions associated with increased shield age.

[1] The origins of the surficial materials in the geologic units surrounding the Martian southern polar region have been poorly constrained on the basis of pre-Mars Global Surveyor (MGS) data and studies. MGS studies suggest that these units are the remnant of volatile loss from an originally massive volatile-rich debris blanket or the result of fluidized slurries resulting from magma/volatile interactions or impact shaking. We use Mars Orbiter Laser Altimeter data to examine a region adjacent to the south polar layered terrain at 72°–79°S and 230°–275°E, generally equivalent to the mapped Parva member of the Dorsa Argentea Formation (DAF). The pedestal and “ghost” impact crater morphologies in this area suggest that extensive deposits of loosely consolidated materials have been removed from this region. The Parva member is thus likely to be the remaining debris blanket from the deflated remnant of an unprotected deposit that was originally similar to the buried DAF deposits in the adjacent Cavi member. Crater counts indicate that the Parva member is of Hesperian age and overlies an older Noachian surface, likely the highland cratered terrain (Npl1). If regional deflation began in the Hesperian and continues through today, the region has been exposed to erosion rates of 1.3–1.6 × 10−7 m/yr. However, if deflation started later than the assumed times or ceased in the Amazonian, when deposition of the polar layered deposits began, erosion rates as high as 2.–5.2 × 10−7 m/yr might have existed. These erosion rates are within the range of published Martian nonbedrock erosion rates of 10−8–10−5 m/yr.