P44C-01 INVITED
Mars Tectonics
Mars tectonics centers on Tharsis, an enormous elevated volcanic and tectonic bulge that is surrounded by radial extensional grabens and rifts and concentric compressional wrinkle ridges that together deform the entire western hemisphere and northern plains. Deformation in the eastern hemisphere is more localized in and around large impact basins and volcanic provinces. Extensional structures are dominantly narrow grabens (several km wide), although larger (100 km wide), deeper rifts are also present. Compressional structures are dominated by wrinkle ridges, interpreted to be folds overlying blind thrust faults, although larger compressional ridges and lobate (thrust fault) scarps have also been identified. Mapping of extensional structures in deformed regions that have rich stratigraphies shows that structures on Mars formed during 5 main stages, with about half forming during the Noachian >3.8 Ga, indicating that tectonic activity peaked early and generally decreased with time. Wrinkle ridge formation peaked in the Hesperian (both around Tharsis and in the eastern hemisphere), suggesting an overprint and modulation by global compressional cooling stresses. Lithospheric deformation models resulting from elastic-shell loading show that loading over the scale of Tharsis (large relative to the radius of the planet) is dominated by membrane stresses and produces the concentric extensional stresses around the periphery and the radial compressional stresses closer in that are needed to explain the radial grabens and rifts and concentric wrinkle ridges. Because elastic-shell models based on present-day gravity and topography can explain the observed distribution and strain of radial and concentric tectonic features, the basic lithospheric structure of the province has probably changed little since the Noachian and elastic support of the Tharsis load by a thickening lithosphere has been the dominant geodynamical mechanism. The origin of Tharsis required a positively buoyant mantle region accompanied by voluminous partial melting, of which a core-mantle plume is one possibility. This enormous volcanic load produced a moat around it, which shows up most dramatically as a negative gravity ring, and an antipodal bulge that contributes to the first-order shape and gravity field of the planet. If the load is composed of basaltic magmatic products, water released with the magma would be equivalent to a global layer up to 100 m thick, which might have enabled an early warm and wet martian climate.
P44C-02 INVITED
Arabia Terra: A partial multi-ring structure around the Borealis impact basin on Mars?
The crust of Mars can be divided into four large-scale physiographic provinces: the southern highlands, northern lowlands, Tharsis, and Arabia Terra. Tharsis is a volcanic province superimposed over the ancient crustal dichotomy. The elliptical shape of the dichotomy boundary and bimodal crustal thickness distribution between the highlands and lowlands suggest that the dichotomy formed in a giant impact, with the northern lowlands representing the ancient Borealis impact basin. This leaves Arabia Terra as the one large-scale physiographic province on Mars whose origin still defies explanation. Arabia Terra has been commonly grouped with the southern highlands because of its ancient cratered surface, though it exhibits lower topography and a thinner crust than typical of the highlands. Arabia Terra is separated from both the highlands and the lowlands by distinct breaks in slope, with the southern edge lying approximately parallel to the northern edge at a distance from the Borealis basin center of approximately 1.57 times the basin radius. A strong topographic similarity to multi-ring structures around other giant impact basins suggests a similar origin for this province. Multi-ring structures can be identified on the basis of either geologic characteristics, such as scarps and massifs, or the long-wavelength topographic structure. For the largest and most ancient basins, only this large-scale topographic signature is preserved. Azimuthally averaged topographic profiles through a number of lunar and martian multi-ring basins reveal a diagnostic structural commonality: an outer ring is expressed as an inwards facing slope at ~1.4 basin radii, which transitions into a gently sloping to concave upwards bench of lower topography before reaching the main basin rim. Based on this simple structural definition, Arabia Terra can be classified as a partial multi-ring structure around the Borealis basin. Preliminary results of finite element models of the collapse of the transient cavity following the impact support the ring-tectonics model of basin ring formation, in which inwards flow of weak deep crustal and mantle material exerts a drag on the brittle lithosphere, leading to tectonic failure at the outer ring scarp. This inwards flow is supported by models of the crustal thickness in the Arabia Terra region, which reveal a northwards (toward the basin center) shift of the dichotomy boundary at the Moho relative to the topographic boundary. If Arabia Terra is indeed a multi-ring structure around the Borealis basin, it will provide a unique opportunity for investigating the structure and formation of multi-ring basins.
P44C-03 INVITED
Radar evidence for ice in lobate debris aprons in the mid- latitudes of Mars
Martian "lobate debris aprons" (LDAs) are masses of material 100s of m thick up to 10s of km wide that occur adjacent to escarpments in certain mid-latitude regions of Mars. Their morphology has led many workers to hypothesize that ice played an important role in their formation and subsequent evolution. Data from the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter indicate that LDAs indeed consist predominantly of ice. SHARAD signals penetrate the LDA material to depths up to 1 km. A reflection is typically observed at a time delay consistent with detection of an interface between the LDA material and the pre-depositional substrate. The echo strength of the basal reflections is strong, indicating minimal attenuation of the signal, which is consistent with a water-ice-dominated composition. The largest populations of ice-cored LDAs are in the Deuteronilus Mensae (40-51° N, 14-35° E) and Eastern Hellas (40-46° N, 100- 108° E) areas. At these latitudes, water ice is unstable at the surface. LDAs appear to be mantled by a relatively thin (<10 m) debris layer that protects the ice core from sublimation. Mass wasting of slopes of massifs, valley and crater walls maintains this debris mantle and explains the local preservation of LDAs adjacent to these slopes. LDAs are likely the remnants of much larger ice sheets and glaciers from an epoch in Martian history when substantial precipitation occurred at the mid-latitudes. The water ice currently preserved in LDAs likely represents the largest reservoir of near surface H2O outside of the polar regions. Their presence at the mid-latitudes make them intriguing targets for in situ exploration and possible resource utilization.
P44C-04
Connecting Spacecraft Observations to Planetary Evolution
Since the lead up to the Apollo era, observations made by spacecraft have been defining our knowledge of both the present state and geological histories of terrestrial bodies. We will discuss how our understanding of the histories of these bodies has been reshaped by the discoveries enabled by the return of new spacecraft to these planets over the last two decades, from the Magellan mission to Venus to the ongoing MESSENGER mission to Mercury. The geology observed at the surfaces of the Moon and terrestrial planets is the result of external, surficial, and internal processes as well as their complex interactions. A critical element in developing understanding of how terrestrial planets and moons as a whole evolve is connecting these spacecraft observations to the processes governing the behavior of their interiors through time. We will discuss how insights from spacecraft data derived from a broad range of techniques and resolutions have been and are continuing to be leveraged to constrain modeling of planetary heat loss, lithospheric and crustal evolution, mantle convection and magnetic field generation. Important examples are drawn from the crustal plateaus and resurfacing of Venus as well as the development of the Tharsis plateau on Mars. Finally, we will focus on how observations from MESSENGER, both to-date and yet-to-come, will help us unravel Mercury's internal evolution.
P44C-05 INVITED
Assessing the History of Water On Mars Through Global Analysis of Valley Networks
Valleys on Mars are thought to have been formed by precipitation and surface runoff as well as groundwater processes, and the relative contributions of these mechanisms have long been debated. Topography from MOLA, along with images, has shed new light on valley formation. These datasets show many more valleys are present than previously observed. In an updated global map, >4 times as many valleys have been identified, totaling a summed length 2.5 times greater than earlier estimates. Drainage densities of networks are almost always much higher. Most of the valleys have characteristics consistent with formation by precipitation, including dendritic form; meandering channels that occasionally exhibit braiding, and terracing; tributaries reaching up to drainage divides; and high stream order. Additionally, some valley networks appear to be in different stages of preservation, indicating multiple periods of formation spread over up to 400 million years. As already understood, most of the valleys occur on Noachian terrains. In terms of age, ~84% of valleys in the new global map of networks lie entirely on Noachian terrains (>3.7 Ga ago), 10% cross into or are entirely contained in Hesperian-aged surfaces (3.7-3.0 Ga) and 6% occur on Amazonian units (<3.0 Ga). This is a shift to younger ages from previous work; for example, Carr (1995) mapped roughly 92% of all valleys on Noachian terrain. The newly-mapped Noachian-age valleys are constrained in elevation and have a rough Gaussian distribution centered around 1500 m. Phillips et al., (2001) showed that most of these valleys follow the long-wavelength topography set-up by the development of the Tharsis bulge, helping constrain its age to the Noachian epoch. The younger valleys, while comprising a small part of the total, developed more evenly across all elevations. These valleys formed after climatic conditions favored precipitation and stable surface water on a global scale and they probably originated from processes such as volcanism or asteroid impacts that could force local climate change. All valleys with an age <2.9 Ga occur on the Tharsis Rise and most of these likely had a hydrothermal origin. In summary, new global analysis of valley networks has pointed to punctuated times of precipitation in the Noachian, with a shift to local formation in more recent epochs.
P44C-06
Dissected Mantle Terrain on Mars: Formation Mechanisms and the Implications for Mid- latitude Near-surface Ground Ice
Determining the present and past distribution of surface and subsurface ice on Mars is critical for understanding the volatile inventory and climatic history of the planet. An analysis of a latitude-dependent layer of surface material known as the dissected mantle terrain can provide valuable insight into the distribution of ice in the recent past. The dissected mantle terrain is a surface unit that occurs globally in the mid-latitude of Mars. This unit is characterized by a smooth mantle of uniform thickness and albedo that is draped over the existing topography. This smooth mantle is disaggregated and dissected in places resulting in a hummocky pitted appearance. We propose that the mid-latitude dissected terrain results from collapse of a dusty mantle into the void left from desiccation of an underlying ice-rich (pure or dirty ice) layer. During period(s) of high obliquity, it is possible for ice to become stable at lower latitudes. Due to lack of direct solar insolation, surface ice deposits will preferentially accumulate on pole-ward facing slopes first. A mantle of dust and dirt is then deposited on top of these ice-rich deposits. As the climate changes, desiccation of the now buried ice leads to collapse of the overlying dusty layer resulting in a hummocky pitted appearance. This theory is supported by the pole-ward preference for the dissection pits as well an increase in dissection with increasing latitude. A study of the global distribution of the mid-latitude dissected terrain can provide invaluable clues towards unlocking the distribution of ice in the recent past. An analysis of HiRISE images and MOLA data indicate that the distribution of dissection pits varies from one region to the next. Knowing the distribution of ice in conjunction with ice stability modeling can provide a global view of the climate and orbital history of Mars at the time these features formed.