Effective January 1, 2011, LPI seminars will be held on Fridays.
LPI seminars are held from 3:30–4:30 p.m. in the Lecture Hall at USRA, 3600 Bay Area Boulevard, Houston, Texas. Refreshments are served at 4:30 p.m. For more information, please contact Georgiana Kramer (phone: 281-486-2141; e-mail: firstname.lastname@example.org) or Patricia Craig (phone: 281-486-2144; e-mail: email@example.com). A map of the Clear Lake area is available here. This schedule is subject to revision.
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In my PhD work, I have focused on the study of Saturn's icy satellites Dione and Rhea using data acquired in the infrared spectral range by the Cassini/VIMS imaging spectrometer. The surfaces of the main Saturnian icy moons are composed primarily by water ice, with a minor percentage of non-water-ice material whose composition is still debated and whose distribution is not uniform across the satellites’ surface. The differences in contaminants’ composition, water-ice abundance and regolith grain size are revealed by variations in spectral profiles, which are bounded both to exogenic (micrometeoroids and particles coming from rings or interplanetary dust) and endogenic (cry-volcanism, tectonic activity) processes. The only way to discern between them and, in turn, to understand how each satellite evolved, is to investigate the distribution of contaminants and water-ice on the moons’ surfaces. In order to identify different terrain units on the two satellites’ surface we applied the Spectral Angle Mapper (SAM) classification technique to Dione’s and Rhea’s hyperspectral images acquired by VIMS in the infrared range. On a relatively limited portion of the surface of Dione and Rhea we first identified nine and eight spectral endmembers respectively, corresponding to as many terrain units, which mostly distinguish for water ice abundance and ice grain size. We then used these endmembers in SAM to achieve a comprehensive classification of the entire surface. The analysis of the infrared spectra returned by VIMS shows that different regions of Dione and Rhea have variations in water ice bands depths, in average ice grain size, and in the concentration of contaminants, such as CO2 and hydrocarbons, which are clearly connected to morphological and geological structures. Generally, the spectral units that classify optically dark terrains are those showing suppressed water ice bands, a finer ice grain size and a higher concentration of carbon dioxide. Conversely, spectral units labeling brighter regions have deeper water ice absorption bands, higher albedo and a smaller concentration of contaminants. Finally, we performed a comparison between Rhea and Dione, to highlight different magnitudes of space weathering effects in the icy satellites as a function of the distance.
2014 marks 25 years since NASA last launched a mission to the planet Venus. Analysis of the geologic history of the planet has progressed since the initial post-Magellan flurry, and a couple of relatively mature world views of the planet's geologic history have developed. I will discuss a handful of key observations that constrain the big picture, and I will evaluate how compatible each observation is with the existing world views. I will argue that we cannot rule out either viewpoint with existing data, and I will discuss what new data would be most effective for distinguishing between existing hypotheses.
The returned Apollo and Luna sample collections present an incomplete view of lunar geology because of their restricted geographic coverage. Lunar meteorites are thus an important resource for lunar science, as they provide a more global sampling of the lunar crust than that available from the Apollo and Luna samples. In particular, they likely provide samples of the farside feldspathic highlands, which are not represented in the returned sample collections. However, the lunar meteorites are of limited utility due to their uncertain provenance and geologic context. In this talk, I will present results from laboratory studies of feldspathic lunar meteorite samples and from global remote sensing studies of the lunar surface. By combining these approaches, we place new constraints on the provenance of these samples. While this approach does not identify specific “source craters”, it provides regional-scale geologic context for these important samples.
The geological record of the earliest history of the Moon is poorly preserved as a result of the heavy impact bombardment of the surface prior to 3.7 Ga. However, the signatures of early lunar evolution are preserved in the subsurface. Recent data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission is providing a view of the lunar subsurface at unprecedented resolution. Linear gravity anomalies reveal a population of ancient igneous intrusions that likely formed during an early period of thermal expansion of the Moon, providing an important constraint on lunar formation. Later intrusive activity was dominated by the formation of circular or arcuate dikes within the ring structures surrounding the major impact basins. In the absence of ring dikes, the gravitational signatures of tectonic offsets across the rings reveal the nature of the basin ring tectonics. The largest magmatic-tectonic structure revealed by GRAIL is a quasi-rectangular set of linear density anomalies ~2500 km in diameter, encompassing the Procellarum region on the lunar nearside. The gravitational signatures of the Procellarum border structures are consistent with volcanically flooded rift valleys, formed by extension driven by the gradual cooling and contraction of the Procellarum KREEP terrain. These and other observations from GRAIL are shedding new light on the early history of Earth's nearest neighbor.
The notion of a dry Moon has recently been challenged by the discovery of high water contents in lunar apatites and in melt inclusions within olivine crystals from two pyroclastic glasses. However, these water contents were determined on lithologies that are rare on the lunar surface. We measured the Zn content, a highly volatile element, of mineral and rock fragments in lunar soils collected during Apollo missions, which average over the surface of the Moon. We show here that the Moon is significantly more depleted in Zn than the Earth. Combining Zn with existing K and Rb data on similar rocks allows us to anchor a new volatility scale based on the bond energy of non-siderophile elements in their condensed phases. Extrapolating the volatility curve to H shows that the bulk of the lunar interior must be dry (≤1 ppm). This contrasts with the water content of the mantle sources of pyroclastic glasses, inferred to contain up to ~40 ppm water based on H2O/Ce ratios. These observations are best reconciled if pyroclastic glasses derive from localized water-rich heterogeneities in a dominantly dry lunar interior.
The Diviner Lunar Radiometer, onboard the Lunar Reconnaissance Orbiter, is the first multispectral thermal instrument to globally map the surface of the Moon. Diviner’s unprecedented and growing dataset is revealing the extreme nature of the lunar thermal environment, thermophysical properties, and surface composition. In this talk I will address each of these three topics, with emphasis on my contributions to the surface compositional and thermophysical investigations. Additionally, I will describe the legacy of Diviner for future opportunities to explore other airless solar system bodies using thermal emission techniques.
Volcanism is a common occurrence on planetary bodies, including asteroids as well as rocky and icy planets and moons, and on some worlds is both the dominant mode of heat transport to planetary surfaces and the main resurfacing mechanism. Dr Mitchell will discuss the dynamics of volcanic eruptions, how planetary environments modulate the expression of volcanism, and how volcanic features can be used to give insights into the planetary interior.
The Moon likely formed as a result of a giant impact between proto-Earth and another large body. The timing of this event and the subsequent lunar differentiation timescales are actively debated. New high-precision Nd isotope data for Apollo mare basalts are used to evaluate the Low-Ti, High-Ti and KREEP mantle source reservoirs within the context of lunar formation and evolution. The resulting models are assessed using both reported 146Sm half-lives (68 and 103 Myr). The linear relationship defined by 142Nd-143Nd systematics does not represent multi-component mixing and is interpreted as an isochron recording a mantle closure age for the Sm-Nd system in the Moon. Using a chondritic source model with present day μ142Nd of -7.3, the mare basalt mantle source reservoirs closed at 4.45+10-09 Ga (t½ 146Sm = 68 Myr) or 4.39+16-14 Ga (t½ 146Sm = 103 Myr). In a superchondritic, 2-stage evolution model with present day μ142Nd of 0, mantle source closure ages are constrained to 4.41+10-08 (t½ 146Sm = 68 Myr) or 4.34+15-14 Ga (t½ 146Sm = 103 Myr). The lunar mantle source reservoir closure ages <4.5 Ga may be reconciled in 3 potential scenarios. First, the Moon formed ca. 4.55 to 4.47 Ga and small amounts of residual melts were sustained within a crystallizing lunar magma ocean (LMO) for up to c. 200 Myr from tidal heating or asymmetric LMO evolution. Second, the LMO crystallized rapidly after early Moon formation. The later Sm-Nd mantle closure age represents resetting of isotope systematics. This may have resulted from a global wide remelting event. Third, the Moon formed later than currently favored models indicate, such that the lunar mantle closure age is near or at the time of lunar formation. While current Earth-Moon formation constraints cannot exclusively advocate or dismiss any of these models, the fact that U-Pb ages and Hf isotopes for Jack Hills zircons from Australia are best explained by an Earth that re-equilibrated at 4.4 Ga or earlier following the Moon-forming impact, does not favor a later forming Moon. If magma oceans crystallize in a few million years as currently advocated, then a global resetting, possibly by a large impact at 4.40 to 4.34 Ga, such as that which formed the South Pole Aitken Basin, best explains the late mantle closure age for the coupled Sm-Nd isotope systematics presented here.
Understanding how shallow reservoirs store and redirect magma is critical for deciphering the relationship between surface and subsurface volcanic activity on the terrestrial planets. In this talk I will demonstrate how elastic models provide useful insight into the mechanics of magma reservoir inflation and rupture, and hence into related and commonly observed volcanic phenomena such as edifice growth, circumferential intrusion, radial dyke swarm emplacement and caldera formation. Based on finite element model results, the interplay between volcanic elements – including magma reservoir geometry, host rock environment, mechanical layering, and edifice loading – dictates the overpressure required for rupture, the location and orientation of initial fracturing and intrusion, and the associated surface uplift. Model results are either insensitive to, or can readily incorporate, material and parameter variations characterizing different planetary environments, and they also compare favorably with predictions derived from rheologically complex, time-dependent formulations for a surprisingly diverse array of volcanic scenarios. These characteristics indicate that elastic models are a powerful and useful tool for exploring many fundamental questions in planetary volcanology.
Despite its small size, the icy surface of Enceladus shows evidence for geologic and tectonic diversity that rivals its larger outer planet satellite companions. In this talk, I will present the wide array of tectonic structures observed on the surface, discuss the possible formation mechanisms, and the implications of the diverse tectonics for the geologic history of Enceladus. I will then focus on one structure in particular, revealed by topography to be a large (~200 km long) normal fault. Using forward mechanical modeling of fault-related topography, I determine important fault characteristics at depth. I then apply flexure modeling to estimate the elastic thickness of the ice shell at the time of fault formation. I will conclude with estimates of the local heat flux at the time of fault formation and what this implies about the tectonic history and ice shell properties of Enceladus.
The MSL rover Curiosity landed on Mars on August 6, 2013, just over a Mars year ago, on the floor of Gale Crater. Since then, Curiosity has traversed many kilometers and made significant scientific discoveries on its path toward Mt. Sharp, the central mound of Gale Crater. Allan Treiman, a co-I on the CheMin instrument on Curiosity, will give a summary of what Curiosity has been doing, its ultimate goals, and some of the human processes behind its daily planning.
Impact basins are the largest type of impact structure in the Solar System, but also the rarest and least understood. The Moon's surface shows evidence of at least 40-50 of these basins (impact structures greater than 300 km in diameter), some of which are fairly well-preserved. Here, numerical modeling is used to investigate the formation and structure of these basins. The modeling constrains impact conditions for two of the largest basins, South Pole-Aitken and Orientale, highlights similarities between basin and smaller-scale crater formation, and demonstrates the great importance of target temperature on the basin formation process.
In its flybys of Mercury in 1974–75, the Mariner 10 spacecraft identified smooth plains deposits across the ~45% of the planet it observed, raising the prospect that effusive volcanism had occurred on the innermost planet. The provenance of these deposits remained uncertain, however, until the three flybys of the MESSENGER spacecraft in 2008–09, during which almost the entire surface of Mercury was imaged. MESSENGER showed the smooth plains to be widespread and the majority to be volcanic in nature. The latter inference was based on superposition relations indicative of the sequential embayment of impact basins and ejecta, regional-scale spectral homogeneity but color variation, partially buried impact structures, and deposit thicknesses of hundreds to thousands of meters. Observations made after MESSENGER was inserted into orbit about Mercury in 2011 indicate that smooth plains occupy some 27% of the planetary surface. In this talk, I will review some of the most salient aspects of Mercury’s effusive volcanic character, including expansive northern plains, ghost craters, and lava channels, as well as its chemical composition, emplacement history, and what this volcanism can tell us of the planet’s thermal evolution.
Crustal deformation such as faulting, fracturing, and folding has long been recognized as a major control on fluid and gas transport within earth’s crust, creating fast flow pathways for the migration of groundwater and potential contaminants in some cases, while becoming barriers and traps for oil and gas in others. Understanding how faults and fractures grow, link, and evolve is critical to understanding fault network connectivity pathways for fluid and volatile migration. However, the role of heterogeneity in the crust – which varies both laterally and within stratigraphic layers – in the growth and linkage of fracture networks is not well understood. Additionally, the role of pressurized fluids in creating and/or reactivating existing fractures, such as magmatic intrusion and hydraulic fracturing, is even less well characterized. This talk will focus on numerical models, laboratory experiments, and field investigations performed to characterized fault and fracture connectivity, with application to contaminant transport, magma intrusion, and hydraulic fracturing. These techniques, such as displacement versus length and fault connectivity analyses, have application to several solar system bodies, ranging from volcanic-tectonic interactions on Mars and Venus to geysering at Enceladus and Europa.
Impact cratering has played a crucial role in the surface development of the inner planets. Constraining the timing of this bombardment history is important in understanding the origins of life and our planet’s evolution. Plate tectonics, active volcanism, and vegetation hinder the preservation and identification of existing impact craters on Earth. Providing age constraints on these elusive structures will provide a deeper understanding of our planet’s development. To do this, (U-Th)/He thermochronology and in situ 40Ar/39Ar laser microprobe geochronology are used to provide ages for the Haughton and Mistastin Lake impact structures, both located in northern Canada. Planetary surface missions, like one designed to explore and sample an impact crater, require the integration of engineering constraints with scientific goals and traverse planning. The inclusion of in situ geochemical technology, such as the handheld X-ray fluorescence spectrometer (hXRF), into these missions will provide human crews with the ability to gain a clearer contextual picture of the landing site and aid with sample high-grading. The introduction of hXRF technology could be of crucial importance in identifying high priority sampling targets. In addition to enhancing planetary field geology efforts, hXRF deployment could also have real implications for enriching terrestrial field geology. Ongoing efforts in hXRF development, including a case study using results from the 2010 NASA Desert RATS (Research and Technology Studies) field test, will be discussed, as well as an overview of continuing fieldwork at the December 1974 flow at Kilauea Volcano, HI.
The existence of volcanic activity on Mars in the last 200 million years is demonstrated by both the low crater densities on volcanos such as Olympus Mons and by radiometric dating of the shergottite meteorites. This implies the existence of adiabatic decompression melting and thus an actively convecting mantle. On the other hand, the preservation of isotopically distinct reservoirs that formed in the first 100 million years of solar system history has been interpreted by some investigators as evidence that the martian mantle cannot be convecting. This apparent paradox can be resolved by considering the effects of geographic isolation of isotopic reservoirs and of inefficient convective mixing, which together allow geochemical reservoirs to be preserved within a convecting martian mantle.
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