Planetary Sciences [P]

 CC:716B  Wednesday  0800h

Planetary Sciences General Contributions III

Presiding:  J Arkani-Hamed, University of Toronto; B Banerdt, NASA, Jet Propulsion Laboratory


Polar Wander of Mars: Evidence from Giant Impact Basins

* Arkani-Hamed, J (, Department of Physics University of Toronto, 60 St. George Street, Toronto, ON M5S1A7, Canada

We investigate the polar wander of Mars in the last about 4 Gyr, since shortly before the formation of Tharsis bulge. We show that many of the 20 giant impact basins recently reported by Frey (2008) trace great circles on Mars and propose that the circles were the prevailing equators of Mars at the impact times. Four different tests are conducted based on the probability calculations and fitting planes to randomly chosen combinations of the basins to support our hypothesis. We identify three different rotation poles of Mars, besides the present one. Accordingly, Tharsis bulge was initially formed at ~50 degree north latitude. It then moved toward southwest and passed the equator while rotating counter clockwise due to influence of the two newly forming volcanic structures, Alba Patera and Elysium Rise. The formation of giant impact basins Argyre, Hellas, Isidis and Utopia and subsequent density perturbations due to the formations of mass concentrations (mascons) in Argyre, Isidis, and Utopia basins, and surface masses of volcanic mountains such as Ascraeus, Pavonis, Arsia and Olympus, caused further polar wander which rotated Tharsis bulge clockwise and move it slightly to the north to arrive at its present location. The complex polar motion of Mars suggests that the lithosphere was not strong enough on a global scale to prevent Tharsis crossing the equator, though it may have cooled and become strong later.


The Surface of Venus is Saturated With Ancient Impact Structures, and its Plains are Marine Sediments

* Hamilton, W B (, Dept. of Geophysics, Colorado School of Mines, Golden, CO 80401, United States

Conventional interpretations of Venus are forced to fit dubious pre-Magellan conjectures that the planet is as active internally as Earth and preserves no ancient surface features. Plate tectonics obviously does not operate, so it is commonly assumed that the surface must record other endogenic processes, mostly unique to Venus. Imaginative systems of hundreds of tiny to huge rising and sinking plumes and diapirs are invoked. That much of the surface in fact is saturated with overlapping large circular depressions with the morphology of impact structures is obscured by postulating plume origins for selected structures and disregarding the rest. Typical structures are rimmed circular depressions, often multiring, with lobate debris aprons; central peaks are common. Marine-sedimentation features are overlooked because dogma deems the plains to be basalt flows despite their lack of source volcanoes and fissures. The unearthly close correlation between geoid and topography at long to moderate wavelengths requires, in conventional terms, dynamic maintenance of topography by up and down plumes of long-sustained precise shapes and buoyancy. A venusian upper mantle much stronger than that of Earth, because it is cooler or poorer in volatiles, is not considered. (The unearthly large so-called volcanoes and tessera plateaus often are related to rimmed circular depressions and likely are products of impact fluidization and melting.) Plains-saturating impact structures (mostly more obvious in altimetry than backscatter) with diameters of hundreds of km are superimposed as cookie-cutter bites, are variably smoothed and smeared by apparent submarine impact and erosion, and are differentially buried by sediments compacted into them. Marine- sedimentation evidence includes this compaction; long sinuous channels and distributaries with turbidite- channel characteristics and turbidite-like lobate flows (Jones and Pickering, JGSL 2003); radar-smooth surfaces and laminated aspect in lander images; and widespread minor structures with neither terrestrial volcanic analogues nor plausible volcanic explanations. Broad tracts of polygonal reticulations 100 m to 5 km in diameter have dimensional and geometric terrestrial analogues in the polygonal faulting shown by 3-D reflection-seismic surveys of dewatered fine-grained sediments in marine basins. Impact-comminuted basaltic crust may dominate the fine sediment. Vast numbers of small low so-called shield volcanoes have geometric analogues in terrestrial mud volcanoes, not magmatic constructs. Less than half of the 1000 small misnamed pristine craters, the only venusian craters accepted by all as of impact origin, in fact are pristine. The rest are variably eroded, their craters partly filled by sediments that often display polygonal faulting, and their aprons partly covered by sediments of surrounding plains. All gradations are displayed between these structures and the more modified but otherwise similar structures from which they are arbitrarily and inconsistently separated. Lunar analogy dates the thousands of large venusian craters, 300-2000 km in rim diameter, as older than 3.8 Ga. Marine sedimentation began before late-stage accretion was complete. The nominally pristine craters are commonly assumed to be younger than 1 Ga but may go back to 3.8 Ga. Venusian oceans persisted long after that, without stillstands sufficient for development of global shorelines and shelves, before complete greenhouse evaporation, deep desiccation, and top-down metamorphism of sediments.


Origin and Emplacement of Melt Deposits at the Lunar Impact Crater Tycho

* Veillette, D (, Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada
Osinski, G (, Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON N6A 5B7, Canada

The Tycho impact crater is an 85 km diameter complex crater thought to be of Copernican age (50 to 100 Ma) [1]; evidence of its young age is provided by the rays that spread outwards radially [1]. In addition to the large coherent melt sheet in the crater interior, impact melt ponds are also abundantly observable in and around the Tycho impact crater; they range from the lowest topographic area within the crater, to isolated slumped portions along the crater walls and on the crater rim. They have also been observed well beyond the crater as far as 74.8 km away (0.88 crater radii) [2]. In this study, we investigate the nature, origin, and emplacement of impact melt materials within and around the Tycho impact crater, using a GIS database of lunar imagery and terrestrial analogue studies. It is notable that the majority of the melt ponds are observed in the eastern sector of the crater (within and outside of the rim), with the highest melt pond concentration within the crater in the east-northeast corner. This geographic distribution is thought to originate from the obliquity of the impact [1, 2]. Emplacement of melt along the crater rim occurred on top of the ballistic ejecta. Thus, melt emplacement and ponding must have occurred following the deposition of the ballistic ejecta blanket during the modification stage of crater formation (within minutes after the initial impact). Around the Tycho melt ponds, there are signs of fluidic flow patterns, indicating the melt flowed in a viscous state shortly after deposition; there are also cooling cracks, which form as the melt contracts while cooling rapidly. As the impact melt has a tendency to follow topography, it will inevitably accumulate in areas of lowest elevation [2]. But what is the emplacement mechanism(s) for these melt ponds? The evidence above suggests these melt bodies were emplaced as flows outwards from the crater centre during the modification stage of crater formation. Refs: [1] Morris, A.R., et al., 2000, Impact melt distribution and emplacement on Tycho: a new look at an old question: LPSC. [2] Hawke, B.R. and Head, J.W., 1977, Impact melt on lunar crater rims: In: Impact and Explosion Cratering, p. 815-841.


Inferred Stratigraphy of the Letronne Crater Area, From Fused Lunar Data Sets

* Antonenko, I (, University of Western Ontario, Department of Earth Sciences 1151 Richmond St.,, London, ON N6A 5B7, Canada
Osinski, G (, University of Western Ontario, Department of Earth Sciences 1151 Richmond St.,, London, ON N6A 5B7, Canada

A wealth of lunar remote sensing data is currently available in electronic format, with more anticipated from recent and current missions (eg. Kaguya, Chandrayaan-1). Advances in data fusion and georeferencing techniques allow all these data sets to be combined in novel ways, providing new insights into the stratigraphy of the Moon. We combined Clementine UVVIS, NIR, and Laser Altimeter data with Lunar Orbiter (LO) images and geological maps of the Humorum and Letronne areas. Clementine UVVIS and NIR data were further processed to provide standard colour ratio composites and FeO estimates. Iron content in the highlands south of Letronne crater was found to decrease radially away from the rim and correspond well with the hummocky ejecta texture apparent in the LO images. Because this iron anomaly extends almost 1 crater radius away from Letronne and is located in a topographic high, we do not believe it is due to lateral gardening transport, especially since nearby highland protrusions into Oceanus Procellarum show only minimal down slope movement. We suggest this hummocky, iron-rich material indicates that the Letronne impact excavated pre-existing mare material prior to being flooded by later basalts. Furthermore, an impact crater within Letronne excavates a lower iron (12% FeO) material from a depth of approximately 350m, indicating that the surface mare (17% FeO) is very shallow here and separated from any potential older mare by a unit of significantly lower iron content.