Planetary Science Seminar - spring-2016
Ceres's surface: its composition and implications for Ceres evolution
March 31, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Eleanora Ammannito - UCLA
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Ceres's surface: its composition and implications for Ceres evolution
As the Dawn spacecraft started its encounter with dwarf planet Ceres in early 2015, the IR spectrometer confirmed previous observation of a low albedo surface (0.035) and the presence of bands at 3.1, 3.3-3.5, 3.9microns. In addition, a band at 2.72 micronS has been clearly identified. The best fit of Ceres' average spectrum over the infrared range is obtained with a combination of phyllosilicates, ammoniated clays, carbonates and a dark material such as magnetite. These components are present everywhere across the surface although with different relative abundances. The retrieved composition indicates that during its evolution Ceres experienced an aqueous alteration process that was global and complete. In addition, its surface enrichment in ammonium bearing species poses questions about its origin, formation and the conditions in the very early solar nebula.
The Frequency Spectrum of Tidal Heating
April 7, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Matt Walker - UCLA
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The Frequency Spectrum of Tidal Heating
Tidal friction can be a significant source of internal heat, especially in smaller bodies which may have already lost their primordial heat. Energy dissipated in this way drives internal evolution as well as orbital evolution, allowing us to reconstruct a planet's past as well as predict its future state. Classic studies find a very simple expression to arrive at the global tidal heating rate for any body having orbital eccentricity and/or spin obliquity. I will explain some major assumptions to this formulation, dealing with the internal structure of the body as well as the frequency at which it is forced. In this, I will show that this simple expression is probably neglecting important terms which describe greater dissipation rates than classically described.
Analyzing Surface Structures on Europa: A Physical Analogue Modeling Approach
April 12, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Erin Leonard - UCLA
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Analyzing Surface Structures on Europa: A Physical Analogue Modeling Approach
The existence of global oceans on some icy satellites—Europa and Enceladus, for example—implies the presence of a ductile warm ice layer. However, the role of such a ductile layer in controlling icy-surface deformation has never been systematically investigated nor quantified. We aim to address this issue by combining previous observations from geomorphological mapping of surface features on icy bodies with a unique two-layer analogue model containing an overlying brittle layer and a ductile creeping layer. Although analogue models have been widely used for tectonic studies on Earth, they have only rarely been adapted to the studies of the icy-surface deformation. Using the analogue experimental approach and analyzing the effects of a subsurface ductile layer, we will gain understanding of different formation mechanisms for surface features and aid in reconstructing the resurfacing history of icy satellites such as Europa.
Tropical Precipitation on Slowly Rotating Planets
April 14, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Sean Faulk - UCLA
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Tropical Precipitation on Slowly Rotating Planets
One of the most prominent features of the Earth's large-scale circulation in low latitudes is the intertropical convergence zone (ITCZ), where tropical precipitation is concentrated in a relatively narrow latitudinal band that moves seasonally. On Earth, the ITCZ is limited to low latitudes; however on Mars and Titan, it has been argued that analogous convergence zones can migrate significantly off the equator into the summer hemisphere, perhaps even reaching the summer pole in the case of Titan. Previous studies of the ITCZ’s extent have focused primarily on thermodynamics, particularly emphasizing its collocation with maximum surface temperature and its response to local surface heat capacity. Here, we focus on the dynamical mechanisms controlling ITCZ migrations, examining the ITCZ’s extent through the perspective of the momentum budget and offering a new view on the seasonal weather patterns of terrestrial planets.
Powering a Magnetic Field on Earth but not Venus
April 21, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Joseph O'rourke - Caltech
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Powering a Magnetic Field on Earth but not Venus
Earth's global magnetic field arises from vigorous convection within the liquid outer core. Paleomagnetic evidence suggests that the geodynamo has operated for at least 3.4 billion years. Available power sources in standard models include secular cooling and compositional convection driven by the solidifying inner core's expulsion of light elements. But recent experiments and theory imply that the thermal conductivity of iron is two or three times larger than typically assumed in these models. This presents a problem: a large increase in the conductive heat flux implies that the inner core is less than one billion years old, yet sustaining the geodynamo with thermal convection alone during earlier epochs is difficult. I will demonstrate that the precipitation of magnesium-bearing minerals from the core could serve as an alternate power source. A small amount of magnesium partitions into core alloy in the high-temperature aftermath of giant impacts. Transport of magnesium as oxide or silicate across the core/mantle boundary is an order of magnitude more efficient per unit mass as a source of buoyancy than inner-core growth. Even if Venus suffered giant impacts like Earth, however, stagnant-lid convection in the mantle would limit core cooling enough to kill a dynamo.
CANCELLED
April 28, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Marisa Palucis - Caltect
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The hydro-geomorphic history of Gale crater
The Curiosity rover was sent to Gale crater where we are in a unique position to determine the extent to which topography can tell us the evolutionary history of a place on another planet, since our hypotheses can actually be tested with ground-based observations. Here I report results of rover-based investigations coupled with satellite-derived information, which suggests alternating wet-and-dry periods of varying duration and magnitude. Sedimentary sequences discovered along the rover’s traverse show evidence for an ancient lake early in Gale’s history, but the size and duration of this lake is difficult to discern from the stratigraphy. Eventually, there was a period of drying and infilling of the crater with wind-derived sediments, before erosion formed the central mound. Post-formation of the mound there were a sequence of large lakes (the deepest being 700 m) that required at least 30 m/yr of runoff to maintain the last lake stand. After the lakes disappeared, reduced hydrologic activity continued, evidenced by a period of fan building (including the Peace Vallis fan). This sequence of events suggests an episodic shift through time from relatively wet regional conditions to a drier environment with local runoff.
Energetic Electrons at Ceres: Implications for a Bow Shock
May 5, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Mickey Villarreal - UCLA
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Energetic Electrons at Ceres: Implications for a Bow Shock
During Survey Orbit, Dawn’s Gamma Ray and Neutron Detector (GRaND) observed distinctive, short-lived enhancements in its exterior scintillators. Corresponding enhancements in counts in the Bismuth Germanate (BGO) scintillator were absent during this period, indicating that neither the particles responsible for the bursts in the exterior scintillators nor their progeny reached the BGO. The source of the enhancements was likely swift electrons that penetrated directly into the exterior scintillators or interacted with surrounding materials to make bremsstrahlung. If it was the latter, then the electron energies corresponding to these bursts were between 20-100 keV. These electron bursts were seen on three successive orbits over the course of a week and each time were located in the same part of the orbit in a solar wind oriented coordinate system. We postulate that these bursts were associated with the formation of a temporary bow shock associated with either a transient atmosphere or an electrically conducting Ceres interior.
Ceres in the light of Dawn: One year of geological observations and the case for near-surface ground
May 5, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Kynan Hughson - UCLA
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Ceres in the light of Dawn: One year of geological observations and the case for near-surface ground ice
On 6 March 2015 NASA’s Dawn mission became the first spacecraft to visit the dwarf planet Ceres, the largest asteroid, and the only large asteroid having a density so low (2.161 g/cm3) that it requires the presence of appreciable water. We report spectroscopic (both reflectance and neutron) and geomorphological evidence for near-surface ground ice on Ceres in the form of ubiquitous flow features, pits, reduced neutron counts, and the direct detection of water. The observed flow features exist on a morphological continuum between two endmembers: (1) thick, domical flows with well-developed snouts, and (2) thin, sheeted, multi-lobed flows with very long runouts. We interpret these features to be similar to ice-cored or ice-cemented flows, and ballistically emplaced fluidized ejecta respectively. A large population of morphologically intermediate flows also exists; they have been interpreted as being a mix of long-runout landslides and fluidized ejecta emplaced as surface flows.
Saturn's rings seen by Cassini spacecraft
May 12, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Estelle Deau - JPL
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Saturn's rings seen by Cassini spacecraft
NASA/ESA's Cassini spacecraft entered Saturn orbit in 2004 after a travel of seven years. Now in its 12th year orbiting Saturn, Cassini has completed more than 200 orbits around the planet with the enigmatic rings. Indeed, Saturn’s rings are one of the most prominent and dramatic features of the solar system, however, their origin and age are still matter of debate. Clues to the evolution of the rings exist in the complex structure the ring particles. While called "particles", the individual ring constituents can be pretty big: ranging from micron-sized dust to tens of meters. The Composite Infrared Spectrometer (CIRS) and the Imaging Science Subsystem (ISS) on Cassini have observed the rings in various geometries and an almost full season. These measurements provide informations on the micro-structure of the rings, such as albedo, composition and thermal inertia of the ring regolith. Comparisons of CIRS and ISS data at a specific geometry called the "opposition" are also very valuable for the macro-structure of the rings. This opposition effect has revealed details about the spatial distribution of the ring particles. In addition, optical and thermal infrared data have revealed interesting behaviors in the C ring plateaus. These structures are enigmatic and remain so far unexplained by current models. I will provide an overview of the ring system and the observations of Saturn’s rings made with CIRS and ISS and discuss their implications for understanding the evolution of the ring system.
Ocean Worlds from the Inside Out
May 19, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Steve Vance - JPL
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Ocean Worlds from the Inside Out
I will describe possible geochemical drivers for habitability in icy ocean worlds, focusing on Jupiter’s moon Europa. Europa may be special for having high oxidant fluxes to its ocean, coupled with abundant and reliable seafloor reductants. Larger ocean worlds like Ganymede are geophysically intriguing, and may be more astrobiologically compelling than typically thought. I will describe some recent progress in relevant aqueous chemistry and modeling of large ocean worlds that suggests high-pressure ices need not preclude water rock interaction and material transport in deep ocean worlds.
Crystallization of Terrestrial Cores
May 26, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Anne Pommier - UCSD
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Crystallization of terrestrial cores: Insights from high-pressure experiments and thermochemical modeling with application to Mars.
The decline of Mars’ global magnetic field some 3.8-4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. This talk will investigate the crystallization regimes of the Martian core by combining thermochemical modeling and high-pressure experiments. In a first part, a systematic study of the thermochemical evolution of the core compatible with all available constraints and the estimated thermal, chemical and structural properties will be presented. In a second part, the effect of redox conditions on the structure and dynamics of the Martian deep interior will be presented, based on high- pressure and high-temperature phase-equilibria experiments. The scenario that reproduces best geodetic observations and Mars’ magnetic history involves top- down crystallization (so-called iron snow regime).
Exploring Europa: A Potentially Habitable World
June 2, 2016
noon - 12:50 p.m.
Slichter 3853
Presented By:
- Bob Pappalardo - JPL
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Exploring Europa: A Potentially Habitable World
Jupiter's moon Europa may be a habitable world. Galileo spacecraft data suggest that a global ocean exists beneath its frozen ice surface. A paucity of large craters argues for a young surface and recent geological activity, and magnetometry implies that a salty ocean persists today. Europa's ocean and surface are inherently linked. Tidal deformation of the floating ice shell generates stresses that fracture and deform the surface to create ridges and bands. Dark spots, domes, and chaos are probably related to tidally driven ice convection and partial melting. Europa's activity may permit the "ingredients" necessary for life to be present within the satellite's ocean. After many years of study, NASA recently selected a highly capable suite of remote sensing and in situ instruments for a mission to explore Europa and investigate its habitability through multiple close flybys. The mission will interrogate the moon’s ice shell, ocean, composition, and geology including any current activity. This presentation will summarize both our state of knowledge about Europa and the synergistic science potential of NASA’s mission to explore Europa and investigate its habitability.