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.
Join the LPI-Seminars mailing list to receive email notifications about upcoming LPI Seminars. To join the mailing list please send an email to:
The Association of Space Explorers Committee on Near-Earth Objects (NEOs) and its Panel on Asteroid Threat Mitigation have prepared a decision program to aid the international community in organizing a coordinated response to asteroid impact threats. The program is described in the ASE’s report, Asteroid Threats: A Call for Global Response, which will be considered by the United Nations Committee on the Peaceful Uses of Outer Space in its 2009 sessions. The findings and recommendations of this report are presented, along with some of the major implications of the complex decision-making involved in developing a coordinated international response to the technical and policy challenge of protecting the Earth from NEO impacts. The effort to produce a working international agreement faces fiscal and bureaucratic obstacles, but the ASE report has spurred discussion at the international level and is a practical starting point for dealing with the NEO hazard.
Analogue studies play an important role in Mars research and exploration – especially with regard to understanding the role of permafrost and periglacial processes to cold region geomorphology, hydrology and geophysical investigations. Many periglacial and volcanic landforms are observed in Alaska, such as pingos, polygons, rock glaciers, thermokarst, volcanoes and ash deposits. In this talk I will provide an overview of how the nature and complexity of these Alaskan permafrost environments -- which include water-rich sub- and intra-permafrost layers, brine, massive ground ice, and the effects of a variety of weathering processes -- are relevant to Mars. I will conclude by discussing the results of a recent geophysical investigation of the cryological structure of a pingo using ground penetrating radar, DC resistivity sounding, seismic refraction, very low frequency (VHF) electromagnetic, helicopter borne electromagnetic (HEM), transient electromagnetic (TEM), and surface Nuclear Magnetic Resonance (NMR) techniques. An open (hydraulic) system pingo is an ice-cored mound or hill, ranging from several meters to several hundred meters in diameter, which forms in response to sub-, or intra- permafrost artesian water pressure. Before the water can reach the ground surface, it freezes, forming a core of relatively pure ice. The study of this and other Alaskan cold-climate analogues is likely to provide important insights into the origin of, and optimal techniques for investigating, similar landforms on Mars.
Phyllosilicate and sulfate hydrated minerals were first definitively identified on Mars from orbit by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument on board Mars Express. Global mapping showed that sheet silicates are widespread but largely found in terrains of Noachian age while sulfates were localized in the region of Valles Marineris, Aram Chaos, Meridiani, and dunes in the northern plains. In contrast to sulfates, phyllosilicate formation requires moderate to high pH and high water activity. A major hypothesis evolving from this discovery is that the conditions necessary for phyllosilicate formation were specific to the Noachian, the earliest era in Mars' history and sulfate formation evolved subsequent to that as Mars experienced massive global change. I will explore this hypothesis incorporating new high spatial resolution, precision pointing, and nested observations by the Context Imager (CTX), Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), and the High Resolution Imaging Sci-ence Experiment (HiRISE)) on the Mars Reconnaissance Orbiter (MRO). These data provide enhanced capabilities to analyze surface mineralogy across the planet and determine the nature and geologic setting of hydrated mineral deposits. Analysis of the diversity of phyllosilicates, associated hydrated minerals, and their geologic setting based on integrated OMEGA-CRISM-MRO analysis will be covered and the implications considered.
James Oberg, a former Mission Control rendezvous specialist at JSC and an authority on the Soviet/Russian space program, will discuss what is officially and unofficially known and suspected about 'Fobos-Grunt' and the announced (but in doubt) October 2009 launch date. In 1996, Oberg sleuthed out the actual impact area of the last Russian Mars mission, 'Mars-96'. He is currently 'space consultant' for NBC News, and leads VIP tour groups to the Baykonur Space Center; his home page is www.jamesoberg.com.
Supernovae (SNe) are important producers of interstellar dust, particularly in the very early Universe, but the amount and types of dust formed is uncertain. A relatively new source of information on SN dust comes from presolar stardust - tiny mineral grains found in primitive meteoritic materials with isotopic compositions indicating that they formed in previous generations of red giant stars and SNe and survived processing in interstellar space and the early solar system. This talk will describe how presolar supernova grains are identified and how they provide valuable information on nucleosynthesis and dust production in exploding stars. Moreover, there is evidence that a majority of presolar SN grains in meteorites formed in a single supernova, with potentially important implications for the origin of the Solar System.
The Hadean Eon (4.5-4.0 Ga) is the dark age of Earth history; there is no known rock record from this period. However, detrital zircons as old as ~4.4 Ga provide unprecedented insights into this formative phase of Earth history. Geochemical records from these ancient zircons are interpreted to reflect an early hydrosphere and felsic crust, "wet" granite melting in a low heat-flow environment, and even plate boundary interactions - in contrast with the traditional view of an uninhabitable, hellish world. Scenarios are explored with a view to reconciling this record with our knowledge of conditions then extant in the inner solar system.
One of the key scientific legacies of the Apollo Program is the concept of a severe period of impact bombardment ~4 Ga that is commonly called the Lunar Cataclysm. Post-Apollo evidence suggests the bombardment implicated in the Lunar Cataclysm may have affected all of the terrestrial planets and, thus, may instead be representative of an Inner Solar System Cataclysm. The hypothesis has survived several tests, but the existing data is still insufficient to prove the point. Because the concept is fundamental to the evolution of the Earth-Moon system and the remainder of the Solar System, testing this concept further is the nation’s highest lunar science priority. The interface between that science requirement and future lunar research, including crewed missions, will be explored.
From May 25 to November 2, 2008 the Phoenix Mission conducted laboratory experiments in the polar plains of Mars. The team discovered that there is ample evidence to conclude that the soils have been wet enough in the recent past to allow the formation of Ca-carbonate. Since liquid water is the primary ingredient for a habitable zone, we are attempting to understand the climate changes over the past 10 Myr that could have led to wetter and warmer conditions than those that we observe today. Could we have discovered a region that is periodically habitable?
The GENESIS mission concept grew out of the successful analysis of solar wind from foils deployed by the Apollo astronauts. The GENESIS spacecraft traveled to the Earth-Sun L1 point and exposed collectors to three different types of solar wind from 2001 to 2004. The payload also included an electrostatic concentrator to provide higher fluence samples for oxygen isotopic analyses. As the first mission ever to return samples from beyond the Moon, the GENESIS capsule crashed on the Utah salt flats upon re-entry, shattering many of the delicate solar-wind collectors. But in spite of the crash, we now have data on all of the primary science objectives! The primary objectives of GENESIS were to determine solar isotopic ratios of oxygen, nitrogen, and noble gases, for which in most cases no precise data existed. Fundamental to this approach was to determine whether solar wind is isotopically fractionated from the photosphere. I will address this question, and will then proceed to describe the newly determined isotopic ratios mentioned above, including the holy grail: the solar oxygen composition. I will give a status on other potential isotopic measurements. Finally, a summary of solar-wind elemental abundances determined by GENESIS will be given in the context of photospheric abundances and other solar wind measurements.
The transient population of near-Earth asteroids represents the largest part of the population of Solar System objects that can impact the Earth. For the first time in history, humankind has the capacity to consider how to prevent a catastrophic collision of a near-Earth object with the Earth, and the audacity to imagine that it can do so. In pursuit of this goal, scientists have concentrated efforts to identify and characterize the near-Earth object population. In the past decade, our knowledge of the number of near-Earth asteroids, physical attributes, and relationship to main-belt asteroid classes has increased. Understanding the delivery mechanisms for asteroid fragments to near-Earth space and impact delivery to the Earth probes Solar System evolution mechanisms. I will review our current state of knowledge about the near-Earth asteroids, and their origins in the Solar system.
A major challenge in solid Earth geophysics and geochemistry is to decipher the lithologic and mineralogic omplexity of the Earth’s mantle. Geophysical (seismological) observations on one hand provide a present-day snapshot of the internal structure and heterogeneity of the Earth’s interior but shed little light on the emporal evolution and mechanisms of formation of deep-seated chemical heterogeneities. On the other and, geochemical (isotope and trace element measurements) observations of mantle-derived samples provide nsight into time-evolution and differentiation of various chemical systems in the Earth’s interior but shed little light on the physical location or petrologic characteristics of such reservoirs. To integrate geochemical and geophysical observations of the Earth’s mantle, it is thus important to relate various geochemical mantle eservoirs with their lithologic characteristics, which potentially can be resolved using a geophysical approach. In this presentation, I will look at global scale relations between radiogenic isotopes and major lements in oceanic island basalts (OIBs). Based on the natural OIB compositions, I will demonstrate that mantle isotopic end members likely have distinct major element compositions. Melting processes (pressure, emperature and degree of melting) do not appear to be the sole control of major element concentrations of primary basalts and long, time integrated history of various parent-daughter elements appear to be coupled to major element and/or volatile heterogeneity in the mantle source.
The recent discovery of methane in the martian atmosphere hints at a planet not nearly as dormant as once thought. Being a reduced gas in an oxidizing atmosphere, its presence essentially requires an active or recently active emission source, as the photochemical lifetime of CH4 is only 350 years. A positive detection must require recent, if not current, replenishment. Recent findings by Mumma et al. have positively identified multiple spectral lines of methane with both a spatial and temporal variability. This variability suggests both local source regions and seasonal variations in the source strength. To understand more about the nature of these methane signals, we apply the Mars Weather Research and Forecasting (MarsWRF) general circulation model to simulate the development of atmospheric plumes derived from passive tracers over several time periods and for several different tracer lifetimes and initial conditions. We find that previous methane detections do not strictly require a short lifetime for methane, and that the source of the observed signals must have been either active at the time of the observations, or very recently extinguished. The observed signals are more consistent with regional emission than a tightly localized plume. These results shed further insight into the possible size and origin of the sources that may be responsible for the observed methane signals.
Early in the history of our solar system, comets and asteroid impacts brought water and organic compounds to rocky planets. This delivery process supplied much of the inventory of light elements at the surfaces of terrestrial planets and likely contributed building blocks essential for the origin of life on Earth. Whether organic compounds break apart upon the shock of impact, survive impact, or combine to form larger molecules during delivery is poorly understood. Also of interest is whether the distribution of material following impact would yield concentrations of material that would promote further chemical evolution after delivery. This talk will present an overview of work that addresses these aspects of extraterrestrial delivery through modeling and experiment. I will describe 3D hydrocode models and show movies that track the phase-state of water during comet-earth and asteroid-earth collisions over a range of impact angles and velocities. These modeling results can be used to infer the survivability of organic compounds and liquid water over a range of impact scenarios for comet-Earth and asteroid-Earth collisions. Next, I will describe and summarize shock chemistry experiments conducted using gas guns and share results that focus on comet and asteroid analogs. These results will be described in the context of the flux of astromaterials and water to the prebiotic Earth.
The internal structure of the Earth consists of a series of concentric and interacting layers: the metallic inner and outer cores, the silicate mantle and the continental and oceanic crusts. Two giant heat engines, represented by convective processes in the mantle and the outer core, cooled the Earth since its formation as a planet, transporting primordial and radiogenic heat to the surface. The vigourous cooling of the core results in the generation of the Earth's magnetic field in the liquid outer core through the geodynamo mechanism. The oldest magnetized rocks on Earth are 3.7 billion years old, proving that the Earth's internal magnetic field has existed for at least that long. On the other hand, recent high-pressure mineral physics experiments reveal a high(» 4000 K) present-day temperature at the core mantle boundary (CMB) horizon and a high temperature drop between the core and the mantle, indicating that the core has not cooled very much since its formation. This leaves us with the important question: How is it possible to maintain a convective regime such that the magnetic field has existed for this long, and while maintaining a slow rate of core cooling? Some possible solutions to this paradox invoke the presence of radioactive potassium in the core or some strict mantle control on the thermal regime of the core. The presence of 40K in the core alleviates the energetic requirements for maintaining an active dynamo, creating more entropy available for the geodynamo, related to the Ohmic dissipation. Conversely, the lowermost part of the mantle may be compositionally distinct from rest of the mantle, due to iron enrichment or due to a hidden reservoir rich in incompatible elements. If so, the lowermost mantle may act as a thermal insulator for the core, and the CMB heat flow will be reduced, but a thermal regime such that the dynamo mechanism can produce a magnetic field will be maintained. To analyze the effects of these possible geochemical scenarios, I study the thermal evolution of the planet by coupling 2D axi-symmetric mantle convection simulations with energy balance models for the core. Further, a model for the main magnetic field evolution can be derived from the entropy equations of the core. The results show that the presence of at least 300 ppm potassium in the core will maintain a continuous and robust magnetic field throughout geological time, and may increase the temperature at the CMB 1 to about 3900 K. In the second scenario, the insulating layer located at the top of the core proves to be a distinct possibility to achieve high modern-day core temperatures and an old inner core in a thermal evolution model. The magnetic field is present for most of the Earth's evolution. The most successful model contains about 5% additional internal heating in the mantle basal layer and yields final temperature at the top of the core of 3946 K, with the generation of magnetic field for more than 3.5 billion years.
Currently, most analysis of images collected by orbiters around other planets proceeds manually. Interesting features, such as craters, fissures, gullies, and volcanoes are annotated in a painstaking process. We have developed an automated method to detect and classify these features of interest ("landmarks"), as well as to detect new, previously unseen types of features. In an evaluation on dark slope streaks, dust devil tracks, and craters, the classifier achieved an accuracy of 94%. Automated landmark identification can be useful both onboard a remote spacecraft and in ground-based processing on the Earth. In an onboard setting, salient landmarks can be detected and catalogued as they are observed, providing a highly compressed summary of the region under study (e.g., "five craters, two gullies, and 37 sand dunes" along with their locations). On the ground, gigabyte archives of past images can be analyzed and annotated with meta-data indicating the existence and location of different landmark types. These annotations can enable a content-based search facility that will permit the easy retrieval of images that contain a specific feature of interest. Ultimately, these catalogues of surface features can enable automated change detection, by tracking landmarks over time.
Valley networks are often cited as the best evidence that the climate of Mars was different in the past. However, for decades there has been considerable debate as to just what this means. Some scientists have suggested that valley networks were formed by groundwater sapping and that perhaps geothermal heating could provide the mechanism for releasing the water to the surface. Others have suggested that valley networks formed by rainfall and surface runoff, and that the early martian climate must have been very earthlike. As a geologist at the Smithsonian Institution, Bob Craddock has been following this debate for over twenty years. He will discuss how water behaves differently on Mars, the nature of the martian surface, where valley network occur and why, and what we currently think valley networks are telling us about the history of water--and possibly life--on the Red Planet.
The notion that highly volatile elements, especially hydrogen, were completely evaporated away during the catastrophic heating events that formed the Moon has changed. New evidences of indigenous water in the primitive lunar volcanic glasses indicate the presence of a deep source within the Moon relatively rich in volatile1. Here we report new volatile data (C, H2O, F, S, Cl) for over 200 individual Apollo 15 lunar glasses with composition ranging from very low to low Ti contents (sample 15427,41; 15426,138; 15426,32). Our new SIMS detection limits (~0.13 ppm C; ~0.4 ppm H2O, ~0.05 ppm F, ~0.21 ppm S, ~ 0.04 ppm Cl by weight determined by the repeated analysis of synthetic forsterite located on each sample mount), represent at least 2 orders of magnitude improvement over previous analytical techniques. After background correction the volatile contents are 0-0.3 ppm for C; 0-71 ppm for H2O; 1.6-60 ppm for F; 58-885 ppm for S; and 0-3 ppm for Cl. Our new values represent an increase in the volatile concentrations by a factor of 2 from previously reported data1. Two outstanding features of the data are the significant correlation among H2O, C, Cl, F and S contents, and the clear relationship between the volatile and the major element contents of the glasses. D/H ratios measured in the lunar in the glasses range from +700” to +5400” and are inversely correlated with water contents. Part of the D enrichment may result from insitu spallation from interactions with solar and galactic cosmic rays. At the lowest H2O contents, spallogenic D can potentially account for half of the abundance of D in the interiors of the glass beads based on estimated D production rates2, but the production rates would have to be underestimated by a factor of 100 to account for the D/H ratios of glasses with high H2O. The D/H ratios of magmatic water in H2O -rich glasses are thus undisputably fractionated from terrestrial values. The data are consistent with kinetic fractionation of D from H during post-eruptive degassing, from a pre-eruptive H2O-D/H composition similar to terrestrial basalts, provided that H and D diffuse as protons/deuterons. Alternatively, the high D/H ratio of the water in the glasses could be inherited from gas condensed within the Moon from a residual atmosphere surrounding the proto-lunar disk after the giant impact. 1. Saal et al. (2008) Nature 454, 192-195. 2. Merlivat et al. (1976) LPSC v. 7, 649-658
Several recently acquired Antarctic ureilite meteorites display evidence for impact smelting (melting with concomitant FeO reduction and carbon oxidation) focused within pigeonitic pyroxene. Ureilites, the 2nd most common variety of achondrite, are extremely depleted asteroidal-mantle peridotites, with typically about 2/3 olivine, 30% pyroxene (mostly pigeonite), ~3 wt% carbon (mostly graphite and/or amorphous matter that is probably impact-disrupted graphite), and generally zero observable feldspar. Late-stage reduction of ureilite olivine, with reduction strongly concentrated in grain rims, has long been recognized as a characteristic feature of this meteorite type. But the impact smelting described here was focused in pyroxene, and was virtually as effective in the cores of grains as in their rims. The exemplar of this phenomenon is LAR04315, an unusually high-shock (mosaicized olivine) ureilite. The pyroxene of LAR04315 (pigeonite, about 40% of the rock) features evenly scattered tubular pores, typically ~30×10 µm as sliced in thin sections (10 µm is the tube diameter, 30 µm merely the typical apparent length); and constituting ~15% of the internal volume of each pigeonite grain. The pigeonites also show uneven extinction, albeit each grain recovered into rough optical continuity, after the impact; and a telling compositional diversity. Very rare patches that escaped smelting, as manifested by sharp optical features and lack of porosity, show in typical ureilite fashion a tight clustering of composition (En74Wo10), but most of the pyroxene is reduced (lower in FeO). Assuming that the overall change was pure FeO removal (averaging out major scatter in Wo among the smelted px), the final pyroxene’s average composition was reduced to ~En79Wo11; and its FeO content was reduced from an original 10.2 wt% to ~6.4 wt%. By-products of the smelting reaction include COx gas and Fe-metal; and, if FeSiO3 is the smelted silicate, SiO2. We see vestiges of the COx in the high porosity. LAR04315 features Fe-metal at an unprecedentedly high level (2.9 vol%) for a ureilite. This ureilite also features remarkably abundant SiO2 phase(s), often lengthy-tubular in shape and associated with felsic glasses in interstitial portions of the pre-impact texture. However, much of the felsic glass and SiO2 formed by smelting may have been entrained with the COx gas that must have blown out of this material during the brief interval of smelting immediately after the impact. The pyroxene-selective impact smelting of LAR04315 and several other ureilites (come to the talk!) will help to elucidate the evolution of the ureilite asteroid(s), and possibly also the origin of another, somewhat controversial, shock-metamorphic material: maskelynite. There may even be more fundamental implications for the generic process of shock melting.
The Aristarchus Plateau is a fault-bounded crustal block on the Moon, possibly uplifted by the Imbrium basin impact. It also contains a high density of volcanic structures, including the largest pyroclastic deposit on the Moon and numerous sinuous rilles. Gravity observations show that the plateau itself is isostatically compensated, but is surrounded on its eastern and southern margins by dense subsurface material. This talk will show how gravity and topography observations, when interpreted in combination with photogeology and geochemical constraints, can lead to an improved understanding of the volcanic and impact history of this unique lunar province.
One of the most important challenges facing all observational astronomers lies in interpreting how signatures in telescopic data translate to the physical properties of their astronomical targets. Through Monte Carlo modeling of the gas jets in Comet Hale-Bopp, we discovered the source of the coma morphology. Hapke modeling of visible/Near-IR data of asteroid Itokawa led us to accurately predict its surface properties prior to the arrival of Hayabusa. Modeling of the mid-infrared spectrum of comet 9P/Tempel 1 in its quiescent state yielded results in stark contrast to those collected of the ejecta plume released by Deep Impact. This raised the question: how might the DI impact, or collisions comets may have enduring throughout their lifetimes, play a role in altering the signatures we obtain telescopically compared with direct measurements (e.g. Stardust and laboratory samples)? I have initiated a laboratory program to investigate the chemical, mineralogical, and spectral effects that impacts have had on comets throughout their histories. Experiments are being conducted at the NASA Johnson Space Center Experimental Impact Laboratory. Targets include refractory components found in comet dust. Analyses of the pre-and post-impact materials show distinctive changes. Spectral changes have been measured using a Fourier Transform Infrared Spectrometer (5 - 15 µm), demonstrating significant effects on slope, band depths, and band shifting. Structural and shock-induced effects of dust have been analyzed though Transmission Electron Spectroscopy, and have been compared directly with TEM images of several Stardust grains that display evidence of shock, suggesting past collisions. Results from Stardust are tantalizingly similar to lab experiments.
While modern SETI experiments are often highly sensitive, reaching detection limits of 10-25 watts/m2-Hz in the radio, and ~102 photoelectrons in the optical, the enormous interstellar distances imply that if extraterrestrial societies are using isotropic or broad-beamed transmitters, the power requirements for their emissions are enormous. Indeed, isotropic transmissions to the entire Galaxy, sufficiently intense to be detectable by our current searches, would consume power comparable to the stellar insolation of an Earth-size planet. In this seminar we consider how knowledge can be traded for power, and how, and to what degree, astronomical accuracy can reduce the energy costs of a comprehensive transmission program by putative extraterrestrials. We also consider why there is reason to think that if SETI is ever going to pick up a signal from the cosmos, that is likely to happen in the next two dozen years.
Maps of the basalt units that make up the lunar maria based on their pristine compositions and discrete unit ages are being created using Clementine high-resolution multispectral data. Small, immature impacts that have penetrated the ubiquitous regolith, exposing the underlying bedrock are analyzed to determine the true composition of the mare basalts - as they were soon after eruption, prior to contamination by impact gardening and space weathering. The resultant maps will characterize the composition and relative ages of discrete mare basalt units, as interpreted using established spectroscopic and crater counting techniques. With sample and meteorite data providing geochemical constraints, the relationships between discrete compositional units can be used to model their petrogenesis and source characteristics over the basin's eruptive history on a regional scale. The global map of pristine mare basalt units can be used collectively to assess mantle evolution in the Lunar Magma Ocean.
The cosmic-ray exposure (CRE) age of a meteorite measures the time a meteorite has been exposed to cosmic rays in space. Since cosmic rays only penetrate a few meters, it is generally assumed that meteorites were buried deep enough on their parent body that CRE started when the meteorite was liberated from the parent body, and the CRE age thus represent the transfer time from parent body to Earth. As this assumption appears to be true for most meteorite types, the CRE ages provide the ground truth for understanding the delivery mechanism of meteorites from the asteroid belt, the Moon and Mars to Earth. As an introduction, I will give an overview of the CRE ages of many types of meteorites, including irons, ordinary and carbonaceous chondrites, HED meteorites, ureilites, lunar and Martian meteorites and discuss their implications for the collisional history of their parent bodies and the delivery mechanism of meteorites to Earth. In the second part of the talk I will focus on the exposure history of large chondrites, say with a pre-atmospheric diameter larger than a meter. Detailed cosmogenic nuclide studies of large chondrites like Jilin (H5), Gold Basin (L4-6), QUE 90201 (L/LL5) and Jiddat al Harassis 073 (L6) have shown that they are much more likely to show a complex exposure history, i.e., a combination of exposure on an asteroid surface, followed by exposure in space as a meter-sized body. I will discuss the exposure histories of these chondrites based on combined cosmogenic noble gas and radionuclide concentrations and discuss possible explanations why large chondrites are more likely to show complex exposure histories and what we can learn from them.
Gas hydrates are solid crystalline, container compound materials formed from a cage structure of water molecules hosting gas molecules in structural voids. A weak van der waals bonding stabilizes the structure. During formation, the chemical reaction of hydrate formation strongly rejects dissolved ionic and very small suspended sold materials to a much greater extent than ice, whose crystallization is a phase change involving no other materials than water molecules. All the hydrate cages do not have to be filled for the hydrate structure to be stable. Almost all gases can form hydrate. Each has a unique field of pressure and stability, although many of the common gas hydrate stability fields overlap considerably. When the concentration of any hydrate former rises to a certain level of saturation, hydrate will form. Hydrate formation can alter local pH; when hydrate forming materials, for instance H2S and SOx, the solid material extracts the materials into the almost chemically inert hydrate form. When hydrate forms from a mixture of gases, they are incorporated into the hydrate according to their preference so that the resulting hydrate has a different ratio of gas components than the mixture from which it formed. This is a little-known natural process of gas separation. On earth, the economically interesting natural gas hydrate forms in marine aqueous environments when natural gas (mainly methane) flux in water rises to an appropriate level in its field of P-T stability. This sequesters methane, which is an important greenhouse gas, from reaching the atmosphere or otherwise affecting the CO2 balance and may be an important mechanism within Gaia. In other locales, such as in a gas pipeline, when dissolved water reaches an appropriate concentration in the gas, unwanted hydrate formation takes place. Hydrate physical chemistry and the abundance of hydrate forming materials can be anticipated throughout the universe wherever the three conditions for hydrate formation (pressure-temperature-concentration) occur. Likely hydrate occurrences could be on Titan, the proposed hidden sea beneath the ice crust of Encaledus, and particularly in the upper crust of Mars, amongst other locales in our solar system. Natural gas hydrate may confer a resource-rich character to planets and moons previously characterized as resource-poor, and be a key to acceleration of human exploration of the solar system.
In addition to a high-resolution synthetic-aperture radar imager (SAR), a lower resolution scatterometer and a profiling altimeter, the Cassini Radar includes a passive mode: the radiometry channel that is able to observe simultaneously with, or separately from, the active measurements. Over the last 4 years, almost all Titan’s surface 2.2 cm-microwave emission has been observed in this operational mode. Among these measurements, ~35% of the surface has been observed near closest approach enabling the construction of a high-resolution mosaiced map of the brightness temperature corrected to normal incidence. The emissivity and the reflectivity of surface features are generally anticorrelated. Correlation of the radiometry data acquired near closest approach, with active radar, addresses a number of compositional and geological questions. In this seminar we will present the theoretical background and main results of the joint analysis of the passive and active Cassini Radar data. In particular, we’ll show that backscatter from some bright regions of Titan cannot be explained by existing models. We will explore one candidate for the anomalous backscatter that has a plausible geological basis for bright channels recently observed.
Interplanetary dust particles collected in the stratosphere are fragments of asteroids and comets. Compositional evidence indicates that IDPs are among the most primitive (unaltered) materials available for laboratory study. Compared to meteorites, IDPs are quite small—on the order of a few tens of microns in size at most—and thus require high sensitivity, high spatial resolution techniques for analysis. Noble gases in IDPs are dominated by contributions from implanted solar wind, presumably acquired during space exposure when particles were in transit from their parent objects. An important goal of noble gas analyses of IDPs is estimating their space exposure histories, as the duration of exposure is sometimes diagnostic of the parent body origin of individual particles. This talk will review our efforts to measure and interpret noble gas compositions of individual IDPs, and will discuss evidence linking some particles to specific classes of parent objects.
Among the terrestrial planets, Mars is most Earth-like in the nature of its hydrosphere. However, there are many aspects of the martian hydrosphere that limit comparison. Stagnation. Cold conditions and absence of plate tectonics to cycle wet sediment into the lithosphere results in static deep groundwater system with drier host rock. Slow reaction rates, disequilibrium assemblages, and pathway-dependent reactions are likely more prevalent on Mars than on Earth. Minerals formed in the martian hydrosphere that would have limited “lifespans” on Earth (e.g., smectites, zeolites) may persist on Mars indefinitely. Severe obliquity cycles. Massive redistribution of ice on Mars follows obliquity excursions of a magnitude not seen on Earth. Subglacial and periglacial processes are thus relatively more significant on Mars. Low water/rock ratios. Fluvial geomorphology on Mars points to high-energy environments with evidence of persistent standing bodies of water elusive. At dispersed and low water/rock ratios the mineralogy of the hydrosphere is likely dominated by soluble salts. However, evidence of abundant clay minerals suggests higher water/rock ratios in the past. A challenge to understanding the martian hydrosphere is in development of credible models for what appears to have once been a much wetter environment and building testable conceptual models of mineralogy that could have formed in that wetter hydrosphere.
The “fingerprint” linking Martian meteorites to Mars is the noble gas and nitrogen signature in the shergottite meteorites (particularly in shock-produced glass in Elephant Moraine 79001) that matches the Martian atmosphere as measured by the Viking landers. However, soon after that match was identified, Ott and Begemann (1985) noticed that the nakhlite meteorites contain a noble gas component that matches the isotopic composition of the Martian atmosphere, but not the elemental composition, suggesting that some elemental fractionation process has occurred. Allan Hills 84001 matches the nakhlites. Several mechanisms for this fractionation have been suggested, including incorporation during aqueous alteration, changes in the atmospheric elemental composition over timescales ranging from a season to millions or billions of years, fractionation during shock implantation, incorporation of noble gases into clathrates, and Martian desert weathering. Some of these mechanisms have now been eliminated, others are currently being experimentally tested, and some can potentially be tested by the Mars Science Laboratory.
The composition, structure and processes of the deep interior of Mars hold the keys to unlocking many of the fundamental questions regarding the formation, differentiation and subsequent history of that planet, and terrestrial planets in general. Geophysics and geochemistry, working together, provide the most powerful tools for unlocking the secrets of a planetary interior. In this talk I will address the geophysical investigation of the Martian interior, which has had an auspicious beginning and a frustrating subsequent history. (The geochemical investigation of the interior of Mars has followed an eerily parallel trajectory, but that is the subject of another talk.) Immediately following the remarkably successful geophysical investigation of the Moon by Apollo in the early 1970s, a pair of seismometers was included on the Viking missions to Mars. However an unfortunate combination of technical compromises and bad luck conspired to render this experiment largely ineffective. Even more unfortunately, this turned out to be the high water mark thus far in the geophysical exploration of the solar system beyond the Moon (the only meager relief has come from the analysis of gravity and rotation, which forever labor under Stokes’ curse). The only other seismometers launched toward Mars flew (briefly) 20 years later on the Soviet Mars ’96 mission, until an upper stage failure cut that adventure tragically short. However there have been many plans to put geophysical instrumentation on the surface of Mars in the past two decades, some of which came tantalizingly close to fruition. With the upcoming Decadal Survey (which is performed every ten years or so) and complete abandonment and rebuilding of the long-term, 20-year Mars Program Architecture (which is performed every ten months or so), it is instructive to revisit the reasons why investigations of the interior of Mars are (still) a high scientific priority, and to understand the roadblocks to implementing them. The latter include both technical (e.g., instrumentation, data volumes, number and distribution of stations, lifetime) and psychological/programmatic (e.g., the uncertain level of seismic activity coupled with the Viking negative result, misalignment with short-term agency goals, scientific ignorance of decision makers, community self-selection, oscillating views on international collaboration) barriers.
What is the noble gas composition of the solar nebula? To answer this question was one of the major objectives of sample return Genesis mission. As noble gases are highly depleted in solids and do not show suitable lines in the photospheric spectrum, the solar wind is the best proxy to determine the noble gas composition of the photosphere that is considered to have largely preserved solar nebula composition. I present our results on the isotopic and elemental composition of noble gases in the bulk solar wind. Genesis allowed us for the first time to measure Kr and Xe directly in the solar wind. Genesis also collected solar wind from different regimes separately. The composition of the solar wind regimes sheds light on potential fractionation processes between photosphere and solar wind, important to ultimately deduce the photospheric composition of noble gases.
The two most abundant volatile components in the Earth’s mantle are water and carbon. The presence of either in an upwelling peridotite will cause the formation of a low-degree, volatile-enriched melt at higher pressures and lower temperatures than would be expected for volatile-free peridotite. While interaction between the volatiles is expected, carbon tends to flux melting at higher pressures than water. A full understanding of the effects of volatiles on the chemistry of melting therefore requires experiments conducted across the range of possible volatile compositions and at the temperatures and pressures of interest for melting driven by each. Because of experimental challenges, investigation into the low-carbon end-member of this system has been limited. I will present a series of successful melting experiments from 1375°C and 3 GPa; conditions within the garnet field that should be of interest for typical terrestrial water-driven melting, which have produced hydrous, low-carbon glasses in equilibrium with garnet peridotite. The main effect of the addition of water to this system is to increase the stability of solid olivine relative to garnet and pyroxene. These experiments show interesting behavior in a number of elements, particularly for Manganese, and suggest potential methods for identifying the contribution of hydrous melting on the Earth.
The vision of the red soil of Mars from the Viking landers in the late 1970’s has led to a long and exciting effort to explain the nature of surficial materials on the martian surface, and to understand the role of impact craters and water on Mars, including hydrothermal systems and formation of lakes. This has led to studies by the speaker of terrestrial analog impact craters on five continents and of Mars with data from the surface and from orbit. An explosion of new data includes chemical data from the Mars Odyssey Gamma Ray Spectrometer, mineralogical data from the THEMIS, OMEGA and CRISM instruments, and other high resolution imaging data. The new data are providing exciting information on impact processes and related aqueous and hydrothermal activity, and the origin and nature of surficial materials on Mars. The importance of impact craters is influencing the question of where to land our robots, leading to the decision to send one of the MER rovers to Gusev Crater. Most of the candidate sites for the Mars Science Laboratory (MSL), the next rover mission, are also located in large craters. In the near future, MSL will provide chemical data from martian materials and sediments on fluid mobile elements like lithium, which will tell us even more about the role of water in martian geochemical processes. The lead role of LPI in the outreach effort for the ChemCam instrument on MSL, and other science team involvement will ensure that LPI is heavily involved in the next leap forward in our exploration of the red dirt on Mars.
Galactic cosmic rays (GCR) provide information on the solar neighborhood during the sun’s motion in the galaxy. Although GCR were discovered almost a century ago, their origins remain unclear. There is considerable evidence for GCR acceleration by shock waves of supernova in active star-forming regions (OB associations) in galactic spiral arms. Supernova remnants are among the few galactic sources that can satisfy the large energy requirements. During times of passage into star-forming regions an increase in the GCR-flux is expected. Recent data from the Spitzer Space Telescope (SST) are shedding new light on the structure of the Milky Way and of its star-forming-regions in spiral arms. Records of past GCR flux variations can be found in solar system detectors. Iron meteorites with exposure times of several hundred million years have long been studied as potential detectors. Variable concentration ratios of GCR-produced stable and radioactive nuclides with varying half-lives were reported; the most recent indicated a 38% GCR-flux increase. Two of the potential flux recorders which were recently improved (81Kr-Kr and 129I-129Xe) have the special property of self-correction for shielding at depth. The 81Kr-Kr method uses Kr isotope ratios, while stable 129Xe is the decay product of the radionuclide 129I, which is produced by secondary neutrons on Te. We will discuss GCR reactions on planet Earth, where radionuclides 10Be, 26Al, and 36Cl, and the stable isotopes 21Ne and 22Ne are produced in subsurface rocks by GCR secondaries. These data permit studies of eroding surfaces. Recent calculations of production rates in Si, including muon reactions using cross-sections of analogous reactions, give production rate ratios 21Ne/10;Be which change with depth. This quantity is expected to be useful for obtaining erosion rates for various geological settings and surface covers. Documented core samples from a quartz dyke were obtained for detailed calibrations of these production ratios.
The interpretation of oxygen isotope signatures of meteorite groups and particularly the basaltic achondrites (HED group) splits those groups into those showing evidence of parent body processes (mass dependent fractionation processes) and those showing nebular processes (non-mass dependent fractionation). The 'rule of thumb' is that samples falling on a mass dependent trend (slope ~0.52) share a parent body while those that do not are from different bodies. The improved precision in the available oxygen data, has led to a 'fashion' of splitting objects that are otherwise similar into groups from spatially distinct parent bodies. The alternative that grossly similar objects share a parent body, despite their distinct oxygen signatures, is currently unfashionable. Simply, there are scattering models and there are clumping models. Mass-dependent fractionation implies a closed system with regard to oxygen isotope exchange. Traditionally this is considered to be a signature of planet or a parent body that behaves as a closed system. This constraint is unnecessary and indeed leads to misinterpretation of the data. A planet or planetoid may indeed be a closed system for oxygen (under appropriate circumstances), however this is by no means necessary. Numerous closed systems may exist within a single planetoid – magma plumbing systems are an obvious example, but there is little direct evidence that planetoid scale exchange is, in fact, common. Exploring the clumping approach leads to interesting options that may be more consistent with the totality of geological/meteoritical information. Scenarios of heterogeneous oxygen signatures within a parent planetoid are easily envisaged in the context of early solar system accretion (~first 10 My) and the onset of planetary differentiation. Accretion and differentiation are not discrete steps in the evolution of a planetoid. They occur together. Given evidence of extended accretion through “late” bombardment times, it is clear that they must be contemporaneous. In this context, unless a process that actively homogenizes the oxygen in a planetoid can be unequivocally identified, we should expect Δ17Ο heterogeneity within the body. The accreting material on any parent body must not be assumed to be homogeneous particularly in the earliest stage of accretion – yet this is a necessary assumption for distinct Δ17Ο signatures on each parent body, in the absence of global homogenization. The variation of Δ17Ο may in fact reflect the time dimension rather than the spatial dimensions. In other words Δ17Ο may be a stratigraphic indicator rather than a geographic one. The data from purported Vestan samples will be explored in this context and an accretionary stratigraphy of planetoid 4 Vesta offered.
The Hadean Eon is widely regarded as the most geodynamically vigorous period in the history of our planet. It has been variously inferred that during this time the Earth collided with a Mars-sized-object, formed a deep magma ocean, grew the first continents, suffered withering bombardment, and witnessed the emergence of life. In terms of ‘hard’ geochemical evidence, however, ! the record from these earliest times is extremely limited, and our understanding of these events is rudimentary at best. The oldest firmly dated terrestrial rock is only 4.06 Gyr old, so there is no rock record to inform us about processes that occurred during the prior 500 Myr of Earth history . Prof. Watson will provide a summary of the evidence gleaned recently from pre-4.0 Ga zircons from Western Australia supporting the view that continental crust was already fully developed, plate tectonic-style recycling in full swing, and (possibly) liquid water at or near the Earths surface. The Hadean zircons retain a record of: 1) crystallization temperatures similar to present day water-saturated grani! toids; 2) Hf isotopes indicative of substantial crustal development; 3 ) mineral inclusions corresponding to partial melting of pelitic materials; and 4) overgrowths of metamorphic zircon indicating contemporaneous 3.97-3.94 Ga thermal events affecting the crust. This information may bear on the development of habitable conditions on earliest Earth.
The late Eocene (ca. 35.3 Ma) Chesapeake Bay impact structure (CBIS) in southern Virginia is one of the largest and best preserved “wet” impact structures on Earth. The 85-km-wide structure formed in a layered target of seawater over sediments and crystalline rocks, and it has the shape of an inverted sombrero with a deep central crater surrounded by a shallower annular trough. It is the apparent source of the North American tektite strewn field and contains salty ground water of public concern in an area of urban growth. The CBIS is of special interest for understanding the consequences of an impact in a nearshore continental shelf environment and, by analogy, in layered and wet settings on other planetary bodies. Although well preserved beneath a blanket of postimpact sediments, the CBIS has no surface outcrops and can be sampled only by drilling. The International Continental Scientific Drilling Program (ICDP) - U.S. Geological Survey (USGS) Eyreville cores through the central-crater moat provide one of the most complete geologic sections ever obtained from an impact structure. This 1766-m section consists (upward) of basement-derived crystalline blocks with dikes of polymict impact breccia, melt-poor polymict impact breccias with large cataclastic gneiss blocks, melt-rich suevites with intercalated clast-rich impact melt pods, sand with an amphibolite block and lithic boulders, a 275-m slab of granitic rocks, sedimentary breccias, and post-impact strata. Scientific results include insights into the evolution of the structure, influence of its marine target, and temporal relations among shock metamorphism, impact melts, ground surge, the ejecta plume, collapse stages, rock and sediment avalanches, ocean-resurge debris flows, and the transition to normal shelf sedimentation.
Global maps of hydrogen near the surface of Mars, interpreted as mass fraction of water-equivalent hydrogen, WEH, have been generated from neutron and gamma-ray leakage fluxes from Mars. Although these data provide an unambiguous indicator of WEH, quantitative details of its magnitude and burial depth depend on the regolith model used to interpret the data. Presently, this model assumes a surface cover layer having one-to-two percent mass fraction of WEH having thickness, D, covering a semi-infinite ‘permafrost’ deposit containing water mass fraction [H]. Although general characteristics of these maps compare favorably with other observations at high latitudes, no comparisons have been possible at low to mid latitudes. Recent HiRISE observations of white deposits uncovered temporarily by five recent small caters between 43° and 55° N latitude, 150° to 190° E longitude [Byrne et al., LPSC 40, Abstract 1831, 2009], provide unambiguous disagreement with both the neutron and gamma-ray model results. In this talk we develop a new model that gives consistent results with the new HiRISE observations. Use of this model shows that large areas in Arcadia and Amazonis contain high-grade water ice deposits that are buried less than one meter below the surface.
Lunar crustal materials are pervasively magnetized in spite of the absence of a present-day core dynamo. Correlative evidence (concentrations of anomalies antipodal to young large basins) and modeling suggests that most of the observed magnetization can be explained in terms of transient fields generated in large impacts. However, many older (early Nectarian) basins have strong magnetic anomalies at their centers that are difficult to explain by impacts alone. Similar anomalies are present near the centers of some terrestrial impact craters and were formed by remanent magnetization in the presence of the Earth's field. In addition, recent studies of lunar samples suggest that a core dynamo may have existed near 4.2 Gyr, which may be consistent with the observed central anomalies of early Nectarian basins. Overall, although the jury is still out, the evidence indicates that both impact processes and an early core dynamo may have caused the observed magnetization. Many of the strongest individual anomalies correlate with unusual curvilinear albedo markings of the Reiner Gamma class. Possible origins for these markings that are consistent with the magnetic evidence will be discussed. In particular, the mini-magnetosphere model and current evidence for the existence of lunar mini-magnetospheres will be discussed.
The talk will give an overview of the technique of estimating crater retention ages from crater counts, and development of the crater "isochron diagram." Recent discoveries of 10-20 m craters forming on Mars in the last few years, support the formation rates used in the isochron diagram. Crater retention ages for Martian igneous units in the 1970s (and recently) agree beautifully with Martian meteorite ages reported subsequently for Martian meteorites...with one possible caveat. The concept of a solar-system wide cataclysm 3.9 Gy ago gained suppport Graham Ryder's discovery of an intense peak in impact melts at 3.9 Gy ago, and from the recent "Nice dynamical model" of planet formation. Other studies, however, raise questions. Asteroid and lunar meteorite data do not confirm a unique event at 3.9 Gy. All data show a paucity of earlier impact melts. Whether a cataclysm appeared or not, however, it appears that impact melts, sampled at planetary surfaces, should decrease with age before ~ 4.0 Gy, due to regolith evolution effects. The enigmatic cataclysm issue remains of extreme importance in understanding evolution of worlds in our planetary system.
One of the primary objectives of planetary scientists is to understand the thermal and dynamic evolution of the interiors of rocky planets. Do rocky planets lose their heat primarily by conduction or by solid-state convection? If the latter, what is the nature of convection (e.g., stagnant lid or subduction) and how might this have changed with time? How is the compositional differentiation of a planet linked to its internal dynamics? The best way to answer these questions is to quantify various intensive variables (such as T and composition) within the mantle itself. However, this is not straightforward because basaltic magmas, although ultimately generated from the mantle, cool and differentiate during their ascent and emplacement into the crust. In this talk, I will present new developments in estimating the temperatures, pressures, oxygen fugacity and major-element compositions of the mantle source regions of primitive basalts on Earth (and other rocky planetary bodies) and the implications of the results. In particular, I show that there may have been a time window in Earth’s history where decompression melting, associated with solid-state whole mantle convection, generated dense liquids that sank into the lower mantle. These liquids would be enriched in incompatible elements and noble gases, providing one way of generating a primitive-like and undegassed reservoir by whole-mantle convection, which was previously thought to be impossible. I will also challenge many of our paradigms related to arc magmatism. It is widely thought that arc basalts are generated by fluxing of hydrous fluids (solidus depression), leading to the general view that the sub-arc mantle wedge is cooler and more oxidized than the rest of the mantle. I will show that in general, the sub-arc mantle wedge is not anomalous in these parameters, which implies that decompression melting may still be the dominant driver of melting in arcs and that mobilization of fluids or sediments from the subducting slab do not oxidize the mantle significantly. The high oxygen fugacities of arc lavas are the result of open-system processes during differentiation and therefore bear no direct relationship to the mantle source. These observations have implications for interpreting apparent correlations between redox indicators and radiogenic isotopes in Martian meteorites.
Chondritic, porous interplanetary dust particles (CP IDPs) are the most primitive samples of extraterrestrial material available for laboratory analysis. These ~10 micrometer CP IDPs are unequilibrated aggregates of mostly submicron, anhydrous grains of a diverse variety, including olivine, pyroxene, glass, and sulfide. These individual grains in these CP IDPs are coated by layers of carbonaceous material, typically ~100 nm thick, which holds the grains together. Carbon XANES maps of ultramicrotome sections from several CP IDPs were obtained by X-ray Absorption Near-Edge Structure (XANES) spectroscopy using the Scanning Transmission X-Ray Microscope (STXM). Cluster analysis, which compares spectra from each pixel in the map and identifies groups of pixels exhibiting similar spectra, indicates most of the carbonaceous grain coatings are organic and have very similar C-XANES spectra, with the two strongest absorption features resulting from the C=C and C=O functional groups. This organic matter coats the individual grains, implying an assembly sequence beginning with grain formation, followed by the emplacement of the organic coating, and finally the assembly of the primitive dust particles. Thus, the organic grain coatings in the primitive CP IDPs appear to have formed prior to the aggregation of the most primitive dust of our Solar System, indicating that these grain coatings are the oldest surviving samples of the pre-biotic organic matter in our Solar System. The thickness and C-XANES spectrum for the coatings on all grains in an individual CP IDP are very similar, independent of the mineralogy of the underlying grain. This indicates that mineral specific catalysis (e.g., the Fischer-Tropsch process), one of the widely accepted models for organic formation in the early Solar System, was not the production mechanism for the primitive, pre-biotic organic matter that coats the grains in the CP IDPs. This result is consistent with the alternative model, that primitive organic matter was produced by irradiation of carbon-bearing ices that condensed on the grain surfaces.
Basic physical data on the density, porosity, strength, and thermal properties of meteorites are essential to understand and model many aspects of small solar system objects, including the physical state of asteroids, thermal histories of outer solar system bodies, the spin and evolution of comets, and the nature of bolides. Density and porosity are the best-determined values for meteorites, and have been reasonably well characterized for all meteorite types; our latest results will be reviewed here. Thermal properties have only been measured for a few meteorite samples; new research in this area is ongoing and should provide data useful for understanding the thermal response of asteroids and the thermal evolution of asteroids and small moons. Future work should also include strength measurements, essential for understanding both impact phenomena and terrestrial bolides. We can also begin to discuss typical error in the laboratory data and caveats in applying these data to astrophysical situations.
The centerpiece of my Ph.D research is a catalogue I have compiled of intermediate-sized (20-100 km diameter) volcanic edifices and calderas on Venus and Mars, which experience very different surface conditions. The catalogue has been used to identify what environmental factors are most influential in determining the distribution of intermediate volcanic landforms of different morphologies both within and between planets, and whether their distribution is consistent with neutral buoyancy theory. My research has also incorporated the study of the formation conditions and subsequent modification of Icelandic rootless cones, the results of which have been correlated to analogue features that exist on Mars. Data from the MARSIS instrument has also been studied, the aim of which is to constrain the Martian crustal volatile inventory, which would be used to address uncertainties regarding the extent to which Martian crustal volatiles may have interacted with magma in the past, and the effect this would have had on volcanic processes and morphologies. However, the negative result returned by MARSIS has led to an independent estimation of the depth to which MARSIS is penetrating in the Martian crust.
The process for selecting the landing site for the 2011 Mars Science Laboratory began in late 2005 and has involved consideration of more than 50 candidate landing sites. Early landing site activities included definition and refinement of mission science objectives and engineering constraints on landing and mobility that have influenced the latitude, elevation, and relief of terrain upon which MSL will target and can land upon. The primary scientific goal of the Mars Science Laboratory (MSL) is to assess the present and past habitability of the Martian environments accessed by the mission. Habitability is defined as the potential of an environment to support life, as we know it. Such assessments require integration of a wide variety of chemical, physical, and geological observations. In particular, MSL will assess the biological potential of the regions accessed, characterize their geology and geochemistry at all appropriate spatial scales, investigate planetary processes that influence habitability, including the role of water, and characterize the broad spectrum of surface radiation. To enable these investigations, MSL will carry a diverse payload capable of making environmental measurements, remotely sensing the landscape around the rover, performing in situ analyses of rocks and soils, and acquiring, processing, and ingesting samples of rocks and soils into onboard laboratory instruments. Highly ranked candidate landing sites contain evidence suggestive of a past or present habitable environment and . are expected to preserve geological, chemical, and/or biological evidence for habitability that should be accessible to, and interpretable by the MSL investigations. The science community has been involved in every step of the site selection process and has suggested many of the candidate sites under consideration and participated in their evaluation at a series of open workshops. The top candidate landing sites for MSL will be discussed relative to how they might achieve these mission science goals. The process for considering new candidate sites and the timeline for selection of the final landing site will also be presented.
Although Moon is usually said to be "volatile-free", lunar basalts are often vesicular with mm-size bubbles. The vesicular nature of many lunar basalts suggests that they contain some initial gas content. A recent publication by Saal et al. (2008) estimated volatile concentrations in lunar basalts (though the estimate has been changing with time; personal communications with Saal). We model the growth of bubbles in lunar basalts using available surface tension, solubility, viscosity and diffusivity data and examining the role of various parameters. We also show a need for experimental determination of critical parameters relevant to lunar basaltic melts. Because lunar atmospheric pressure is essentially zero, the confining pressure on bubbles is supplied by surface tension and the overlying melt column. For a bubble with a diameter of 1 mm, the surface tension pressure is about 1200 Pa. Hence, by investigating bubble size, it is possible to estimate the minimum partial pressure of gases. A melt column of 0.2 m would provide a pressure of about 1000 Pa. Due to low volatile concentrations in lunar basaltic melt, bubbles only form at a shallow depth in the melt. Hence, vesicular lunar basalts must have formed at very shallow depth. Due to low volatile concentrations in lunar basalt, bubble nucleation is expected to be extremely difficult. Efficient nucleation sites must be available for bubble growth. Some findings from the modeling include: (a) Due to low confining pressure as well as low viscosity, even though volatile concentration is very low, bubble growth rate is extremely high, much higher than typical bubble growth rate in terrestrial melts. Hence mm-size bubbles can easily form once a bubble nucleates. (b) Because the pertinent pressures are low, pressure due to surface tension often plays a main role in lunar bubble growth, contrary to terrestrial cases. (c) Time scale to reach equilibrium bubble size increases as the confining pressure increases.
Copyright © 2016 - Lunar and Planetary Institute