Spring 2016

April 1, 2016

Speaker: Thomas Barclay, NASA Ames

Title: The Effect of Giant Planet on Terrestrial Planet Formation

April 8, 2016

Speaker: Megan Shabram, NASA Ames Research Center

Title: Implications for Planet Formation from Population Inference of Kepler-Planet-Candidates and Eclipsing Binaries

Abstract: The Kepler Space Science Mission has revolutionized our understanding of planetary system architectures, and the diversity of planet bulk densities. From Kepler, we can begin to characterize the distribution of stars and planets to tease out relationships between planet properties and host star properties in a robust statistical manner. The results of such investigations can constrain proposed planet formation theories. For my dissertation research I analyzed three particular sub-populations observed by Kepler that are well suited for hierarchical inference to characterize their population properties. First, I investigated the eccentricity distribution for a sample of short-period planet candidates from Kepler, where both the transit and occultation are observed for each system. This sub-sample lends a rare opportunity for tractable inference of its eccentricity distribution, exposing at least two populations within the eccentricity distribution and potential correlations of the eccentricity with host star metallicity and planet radius. Secondly, I investigated the mass-radius-eccentricity relation for a sample of near-resonant planet-pairs from Kepler. This study has the potential to greatly improve upon previous research of constraining the mass-radius relation for small planets. Furthermore, I explored the period-eccentricity distribution of eclipsing binary stars from Kepler. We find that ∼72% of EBs below ∼11 days are very circularized, where as ∼87% of EBs above ∼11 days can take on a wide range in eccentricity values including some with significant eccentricities

April 15, 2016

Speaker: Jonathan Glen, USGS

Title: Developing intelligent autonomous Unmanned Aerial Systems (UAS) for the the Earth Sciences

April 22, 2016

Speaker: Marko Gacesa, NASA Ames

Title: Non-thermal escape in planetary and exoplanetary atmospheres

Abstract: Non-thermal processes play a key role in understanding complex interactions between the upper layers of planetary atmospheres, or exospheres, and solar radiation and plasma. They are responsible for formation of non-hydrogenic hot planetary coronae and their escape to space and play a key role in understanding the evolution of the Martian atmosphere over time – which is a subject of an ongoing study by NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission. Describing energy deposition and transport driven by such processes requires detailed knowledge of high-temperature cross sections for the participating atomic and molecular species. These quantities are typically not well-known.

A non-thermal process of interest for evolution of the Martian atmosphere and water inventory in time is collisional escape of neutral atmospheric species driven by non-thermal collisions with hot oxygen atoms, produced by dissociative recombination of oxygen molecular ions and precipitation of solar wind ions. In this talk, I will present a model of this process and its role in the escape of neutrals from Mars based on detailed state-to-state transport cross sections for O+O, O+He, O+H2, and O+HD. Even though this study will be presented for Mars, the same physical mechanisms are present in atmospheres of other planets and satellites, such as Titan, and may have played an important role in evolution of exoplanetary atmospheres immersed in hotter stellar environments in current or previous epochs.

April 29, 2016

Speaker: Jan Verhoeven, UCSC

Title: The Compressional Beta Effect: A Source of Zonal Winds in Planets?

Abstract: Giant planets like Jupiter and Saturn feature strong zonal wind patterns on their surfaces. Although several different mechanisms that may drive these jets have been proposed over the last decades, the origin of the zonal winds is still unclear.
Here, we explore the possibility that the interplay of planetary rotation with the compression and expansion of the convecting fluid can drive multiple deep zonal jets by a compressional Rhines-type mechanism, as originally proposed by Ingersoll (1982). In a certain limit, this deep mechanism is shown to be mathematically analogous to the classical Rhines mechanism possibly operating at cloud level. Jets are predicted to occur on a compressional Rhines length that depends on the angular velocity, the typical density scale height and the jet velocity. Two-dimensional numerical simulations using the anelastic approximation reveal that this mechanism robustly generates jets of the predicted width, and that it typically dominates the dynamics in systems deeper than one compressional Rhines length. Potential vorticity staircases are observed to form spontaneously and are typically accompanied by unstably stratified buoyancy staircases. The mechanism only operates at large rotation rates, exceeding those typically reached in three-dimensional simulations of deep convection in spherical shells.  Applied to Jupiter and Saturn, the compressional Rhines scaling reasonably fits the available observations. Interestingly, even weak vertical density variations such as those in the Earth core can give rise to a large number of jets, leading to fundamentally different flow structures than predicted by the Boussinesq models typically used in this context.

May 6, 2016

Speaker: Walter Mooney, USGS

Title: Structure and Evolution of the Afro-Arabian Rift

May 13, 2016

Speaker: Kerry Key, UCSD

Title: Marine electromagnetic imaging of plate boundary fluids at subduction zones

Abstract: The recent adoption of marine electromagnetic (EM) methods by the hydrocarbon exploration industry has driven technological innovations in acquisition hardware and modeling software that have created new opportunities for studying plate boundary structure at subduction zones. Because the bulk electrical resistivity measured by EM surveys depends on crustal porosity and hence fluid content, EM data provide unique constraints on the crustal hydration of the incoming oceanic plate, fluids released after the plate is subducted, and fluid flux through the overlying forearc crust and mantle. Since water is also thought to play an important role in regulating subduction earthquake processes and frictional behavior along the plate boundary, EM data may reveal new insights into subduction zone earthquakes and modes of slip along the plate boundary. This presentation discusses results from recent EM surveys of the Middle America Trench and the Cascadia Subduction Zone. At the Middle America Trench offshore Nicaragua, we deployed 54 seafloor EM receivers along a 280 km profile across the trench in the first controlled-source EM survey of a subduction zone. We broadcasted controlled-source EM signals by deep-towing a low-frequency electric dipole transmitter close to the seafloor along the entire survey profile, generating diffusive EM waves that traveled through the crust and uppermost mantle before being measured by the receiver array. The data reveal a significant increase in crustal porosity along plate bending faults at the trench outer rise, and image a continuous zone of low resistivity porous sediments carried down with the subducting plate to at least 25 km from the trench. At the Cascadia Subduction Zone, we used the deeper sensing passive-source magnetotelluric method for a 3D amphibious EM survey deploying 71 offshore stations and 75 onshore stations to study the nature of the seismogenic locked zone and the down-dip transition zone where episodic tremor and slip originates. Preliminary 3D inversions of the data reveal significant lateral conductivity variations along the plate boundary and in the overlying forearc, suggesting that along-strike variations in pore fluid content may explain observed patterns of plate-locking.

May 20, 2016

Speaker: Brooks Proctor, USGS

Title: Frictional melting during laboratory stick-slip events: Implications for post-slip fault strength

May 27, 2016

Speaker: David Sparks, Texas A&M

Title: Insights into slip in fluid-filled faults from grain-scale models