Winter 2016

Winter 2016

Tuesday Afternoons at 4:00 PM
Thimann Lecture Hall, Room 001

January 12, 2016

Speaker: Seth Burgess, USGS

Title: Using geochronology to link magmatism with environmental change from the Pleistocene to the Permian

Hosts:Karen Petersen

January 19, 2016

Speaker: Thorn Lay, UCSC

Title: The Continuing Enigma of Deep Focus Earthquakes

Hosts:Galen Gorski

January 26, 2016

Speaker: Bruce MacFadden, University of Florida and Florida Museum of Natural History

Title: Miocene biodiversity in the New World tropics:
A once-in-a-century opportunity along the Panama Canal

Hosts:Kerri Johnson

February 2, 2016

Speaker: Kevin Walsh, Southwest Research Institute

Title: Building Terrestrial Planets - Planetsimals to Planets

Abstract: Models of each stage of planet growth have largely been successful at
identifying the important physics in different regions of space at
different times. The earliest stages are dominated by small-body
interactions, where km-size bodies collide with each other but also
interact with the gaseous solar nebula. Later, the gravitational
interaction between planetary embryos on long timescales becomes more
important. However, splitting the problem into different stages, each
modeling subsets of the total physics, has an inherit weakness -
planet growth does not progress at the same speed at all
locations. Rather the inner regions of the Solar System may have many
gravitationally interacting planetary embryos, while outer regions are
still dominated by small-body collisions.

Using a particle-based code that models the fragmentation, accretion
and dynamical evolution of a large number of planetesimals through the
entire growth process, one can avoid the pitfalls of the classical
piece-wise approaches. This technique finds that growth timescales
that are far more dependent on distance from the Sun than previously
expected. This leads to giant collisions between Mars-size planetary
embryos at 1 AU before Ceres-size embryos have formed beyond 2 AU. We
use these results to test classical models of Terrestrial Planet
formation and some of the fundamental assumptions built into most
modern models.

Hosts:Mikael Witte

February 3, 2016

(held in E&MS A340)

Speaker: Kevin Walsh, Southwest Research Institute

Title: Turning Asteroids Inside Out

Abstract: Space missions are headed off to collect and return samples from the
surfaces of asteroids. The most accessible asteroids, and those being
visited, are km-sized near-Earth asteroids. This population consists
of transient visitors to the inner Solar System whose current
properties do not immediately reveal where they formed. Therefore
studying the lifecycle of these small asteroids is important for
interpreting any returned samples. The near-Earth asteroids bulk
physical properties, their shape and spin, are suggestive of dramatic
evolution and of behaving, and re-shaping, like collections of
gravitationally bound granular material. By modeling their bulk
structure with discrete element particle codes we can study numerous
evolutionary processes - spinup by thermal forces and also tidal
disruption during close encounters with terrestrial planets. We will
show how these two effects can fully re-shape an asteroid and even
disrupt them to the point of turning them inside-out. These processes
appear to be dominant players in the geology small asteroids as found
by previous space missions and by the studies of upcoming mission
targets. The same tools used to study the granular mechanics of the
entire bodies are also being deployed to help optimize the sampling
strategy to be used by NASA's OSIRIS-REx spacecraft.

February 9, 2016

Speaker: Marisa Palucis, California Institute of Technology

Title: Quantitative analysis of landforms to reconstruct past hydrologic conditions on Earth and Mars

Abstract: The amount of water runoff needed to evolve landscapes is rarely assessed. Empirical studies correlate erosion rate to runoff or mean annual precipitation, but rarely is the full history of a landscape known such that it is possible to assess how much water was required to produce it. While this may not seem to be of primary importance on Earth where water is commonly plentiful, on Mars the amount of water necessary to drive landscape evolution is a key question. A combination of remote sensing, field studies, and large-scale experimentation gives insight into planetary processes that cannot be obtained through imagery alone. Analyses of LiDAR data, stratigraphic mapping, and cosmogenic dating at Meteor Crater (Arizona, USA) reveal that after the impact 50,000 years ago runoff from the crater walls entrained loose debris, forming debris flows that swept to the canyon floor. A series of laboratory experiments in a 4 m diameter vertically rotating drum suggests that fine material, likely generated in the impact, and deposited with the coarse debris on the lower portion of the crater wall, is key to this bulking up process, as flows cut across the deposits. Cosmogenic dating of levee deposits indicates that debris flows ceased in the early Holocene, synchronous with regional drying. Our analysis suggests that depositional landforms may record only a small fraction of the total runoff, though much more runoff did occur, as evidenced by lake deposits on the crater floor and Holocene fluvial activity. This work is then applied to the Mars Science Laboratory (MSL) landing site, where using remote sensing techniques, we calculated that runoff required to produce the nearby Peace Vallis fan is more than 600 m, perhaps as much as 6000 m, indicating a hydrologic cycle that likely lasted 1000’s of years. Based on topographic data, the fan likely spilled into a topographic low suggesting that the light-toned fractured terrain within this topographic low corresponds to the distal deposits of the fan. In such a setting lacustrine deposits were predicted and later confirmed by observations from the rover.

Hosts:Esteban Chaves Sibaja

February 10, 2016

(held in E&MS A340)

Speaker: Marisa Palucis, California Institute of Technology

Title: The hydro-geomorphic history of Gale crater and implications for future work

Abstract: 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. The goal of the mission is to explore a 5 km thick central mound, the base of which shows a vertical sequence of spectral signatures indicating clay minerals overlain by gypsum, suggesting a rich history of Mars climate and habitability. 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 Peace Vallis). This sequence of events suggests an episodic shift through time from relatively wet regional conditions to a drier environment with local runoff. The exact timing of these lakes is unknown, as our modeling shows that none of the hydrogeomorphic features (e.g. fans and deltas) are large enough to be dated. My future plans include: 1) collaborate with climate modelers to explore possible climate conditions on Mars that could sustain the observed lake levels, 2) document rover observations that will be made on a key deposit I have mapped from satellite imagery as a stacked fan/delta deposit, 3) develop a runoff-erosion model to constrain rates, sources, and timing of water to Gale, and 4) use these types of analyses to aid in the decision-making process of where to land the Mars 2020 rover.

February 16, 2016

Speaker: Hao Cao, California Institute of Technology

Title: A Magnetic Perspective on Mercury's Enigmatic Interior

Abstract: The recently concluded MESSENGER mission has revealed surprising facts about Mercury. The silicate mantle of Mercury was found to occupy only the outermost 15% of the planet’s radius, leaving the inner 85% of the radius for the iron core. The iron core of Mercury is at least partially liquid, but the size of a solid inner part is highly uncertain. The surface of Mercury is sulfur rich and iron poor, suggesting a highly reducing condition during core formation of Mercury. The internal magnetic field of Mercury is equatorially asymmetric: the field in the northern hemisphere is approximately three times stronger than that in the southern hemisphere. Such a significant north-south asymmetry resembles none of the magnetic fields of other known active planetary dynamos.

In this presentation, I will show that the peculiar north-south asymmetry in Mercury’s magnetic field can be reconciled with extensive iron snow within Mercury’s liquid iron core and the slow rotation of Mercury. I will then show that the radius of the solid inner core inside present-day Mercury is likely smaller than 1000 km and could be detected indirectly with high-spatial-resolution magnetic field measurements near Mercury's north pole. I will conclude by discussing how future measurements from BepiColombo will advance our understanding of Mercury.

Hosts:Kyle Young

February 17, 2016

(held in E&MS A340)

Speaker: Speaker: Hao Cao, California Institute of Technology

Title: Probing the Interior Dynamics of Jupiter and Saturn with Gravity and Magnetic Fields

Abstract: The inner working of solar system gas giant planets remain elusive after decades of exploration. One lasting debate concerns the nature of east-west zonal flows observed on the cloud level of these planets with amplitude on the order of 100 m/s: an observational fact is yet to be established about whether these flows are shallow atmospheric dynamics or surface expression of deep interior dynamics. The upcoming gravity and magnetic field measurements to be carried out by the Juno mission and the Cassini Grand Finale provide a great opportunity to establish such an observational fact. In this presentation, I will first describe a critical assessment of the applicability of the thermal wind equation (TWE) in forward calculating the gravity field associated with deep zonal flows. Our analysis shows that the applicability of the TWE in calculating the wind-induced gravity moments depends crucially on retaining the non-sphericity of the background density and gravity. I will then turn to the magnetic field signals associated with deep zonal flows. Using mean field dynamo theory (MFDT), we show that poloidal magnetic fields spatially correlated with deep zonal flows are expected. The amplitude of the wind-induced poloidal magnetic fields is determined by the strength of the background magnetic field, the amplitude of the shear in the zonal flow, as well as the amplitude of the dynamo alpha-effect. I will conclude by discussing how the upcoming Juno and Cassini Grand Finale measurements will contribute to building a dynamical picture of the interiors of Jupiter and Saturn.

February 23, 2016

Speaker: Caleb Fassett, Mount Holyoke College

Title: Landform Evolution Rates on the Moon and Mercury

Abstract: When we look at the Moon or Mercury, we see densely cratered surfaces, and it is tempting to think that these airless bodies experience no ongoing erosion or topographic change.  However, slow landform evolution is occurring, which, over the course of billions of years, becomes a significant process.  By studying the degradation of impact craters, we can infer both the rate at which landform evolution occurs and better understand the underlying processes causing topographic change.  The derived rates can then be used to estimate the age of craters, surface units, and other landforms based on their topography.  In addition, a comparison with similar measurements on Mercury suggests that it experiences much faster landscape evolution rates than the Moon.

Hosts: Ana Martinez Fernandez

February 24, 2016

(held in E&MS A340)

Speaker: Caleb Fassett, Mount Holyoke College

Title: Glaciation in the Mid-Latitudes of Mars: Constraining Timing, Causes, and Effects

Abstract: It has been known since the 1970s that there are landforms in the martian mid-latitudes that look like glaciers on Earth.  Recently, the wealth of new remote sensing data from Mars Reconnaissance Orbiter has given new insight into these glacial features.  In particular, SHARAD radar observations confirm the presence of clean ice in these glaciers today.  But numerous questions remain.  I will discuss constraints on the timing of mid-latitude glaciation, how long it has occurred, the patterns of ice accumulation and flow, and evidence that, in a few locations, ice has undergone melting and produced small glaciofluvial valleys.  I will also discuss how glacial and post-glacial martian landscapes might inform us about the role of ice earlier in the history of Mars.

March 1, 2016

Speaker: Seth Jacobson, Observatorie de la Cote d'Azure

Title: Growth of the terrestrial planets

Abstract: Like detectives at a crime scene, planetary scientists are presented with clues regarding the formation of the terrestrial planets in the solar system such as circular and in-plane orbits, missing mass between Earth and Jupiter, etc. I will line up the different proposed formation scenarios including the canonical models, the Grand Tack and a steep surface density model inspired by pebble accretion. I will compare these models against clues from dynamics and geochemistry. In doing so, we will recognize a relationship between the highly siderophile elements in Earth’s mantle and the date of the Moon-forming impact. I will also present evidence that compositional mixing during planet formation precludes the possibility that the Moon-forming projectile was isotopically similar enough to Earth in order to explain the current similarity between Earth and the Moon.  Recognizing the important role of giant impacts, we will examine what records they might have left on the planets and in the asteroid belt. 

Hosts:Carver Bierson

March 2, 2016

(held in E&MS A340)

Speaker: Seth Jacobson, Observatorie de la Cote d'Azure

Title: Differentiation of Earth

Abstarct: The formation of Earth’s core has historically been modeled as a unique event in its history. While these single stage models are typically admitted as approximations, results from them such as an early Moon-forming impact date and low pressure / high temperature metal-silicate equilibration contradict other data. Here, I present the results from a sophisticated multi-stage core formation model that utilizes input from planet formation simulations (like those described in the prior presentation). The model tests different proposed geochemical variations within the protoplanetary disk by assigning each initial body a composition according to oxidation and other volatile gradients. As these bodies collide and grow, each accretion event is individually modeled according to element partitioning coefficients determined by laboratory experiments and by fitting the conditions (pressure and temperature) of metal-silicate equilibration that ultimately best reproduce Earth’s bulk silicate composition in simulated Earth-like planets. From these experiments, we have determined that Earth must have grown from increasingly oxidized material, Earth’s core grew initially into stably stratified layers, and a Hadean matte is responsible for producing the near chondritic relative abundances of the highly siderophile elements. 

March 8, 2016

Speaker: Myriam Telus, Carnegie Institution of Washington

Title: Iron-60 and the Stellar Source of Extinct Radionuclides

Abstract: Short-lived (“extinct”) radionuclides are powerful tools for constraining early solar system chronology. Many are produced by both of stellar nucleosynthesis and particle irradiation. An important exception is the 60Fe-60Ni system (t1/2=2.6 Myr). Iron-60 is only produced efficiently by stellar nucleosynthesis, making it important for determining the stellar source of short-lived radionuclides. However, two outstanding problems remain regarding the 60Fe-60Ni system: (1) Ion probe data indicate that the Fe-Ni isotopic system is disturbed in the samples we have measured; and (2) There are discrepancies between results from bulk and in situ techniques. Synchrotron X-ray fluorescence analyses provide important clues to understanding and eventually resolving these dilemmas.

Hosts: Elke Teo

March 9, 2016

(held in E&MS A340)

Speaker: Myriam Telus, Carnegie Institution of Washington

Title: The formation and evolution of chondrite parent bodies

Abstract: Chondrites record the initial stages of planet formation, making them powerful tools for understanding the processes in the solar nebula that lead to the solar system as it is today.   Carbonaceous chondrites preserve evidence for extensive aqueous alteration during the early evolution of planetary bodies and there is growing evidence that ordinary chondrite parent bodies, thought to be completely dry, were also wet to some extent. I use isotopic analyses of secondary minerals to understand the source and composition of the fluid and the timing and conditions of aqueous alteration on chondrite parent bodies.