Summer 2020

Fridays at 12:00 PM

June 12, 2020

Speaker: Matthew S. Tarling, University of Otago, New Zealand

Title: Episodic tremor and slip in the geologic record: chemical reactions in serpentinite within the source region of deep episodic tremor

Abstract: Seismic and geodetic studies continue to reveal a diverse spectrum of earthquake behaviour along subduction megathrusts worldwide. This includes “slow fault slip”, which refers to a range of transient phenomena that occur over timescales longer than those of standard earthquakes. Slow slip phenomena include temporally and spatially correlated seismic tremor and slow slip events known as episodic tremor and slip (ETS). Deep ETS occurs in the transitional region between the megathrust seismogenic zone and the deeper, stably sliding zone. A number of hypotheses for the origins of ETS have been proposed. However, the geological processes controlling ETS have yet to be elucidated due to the relatively coarse resolution of geophysical datasets and the lack of direct geological observations from ETS source regions. Most hypotheses invoke contemporaneous stable and unstable frictional sliding accompanied by fluctuations in pore-fluid pressures that promote frictional failure in regions where viscous shear would otherwise occur. As geophysicists and seismologists continue to expand our understanding of slow slip and ETS, a better understanding of the chemo-mechanical processes that act in ETS source regions would provide valuable input for physical models of slow slip and megathrust behaviour. An unresolved problem is whether any of the slow earthquake phenomena documented in active subduction zones can leave a trace that can be resolved and studied in the geological record. I will summarise our current understanding of the possible geological signatures of slow slip and ETS in the rock record. Additionally, I will present field and microstructural observations from the plate boundary-scale Livingstone Fault in New Zealand, which can be used to explore how chemical reactions can promote rock hardening and generate in-situ fluid overpressures. These processes may collectively result in hydrofracturing and a transition from distributed creep to localized brittle faulting, phenomenologically analogous to episodic tremor and slip.


June 16, 2020

Speaker: James Head, Brown University

Title: Noachian Mars Climate: 'Warm and Wet' or 'Cold and Icy'?

June 26, 2020

Speaker: Brandon Johnson, Purdue University

Title: Impact basins as probes of planetary interiors

July 10, 2020

Speaker: Miki Nakajima, Rochester

Title: Consequences of Giant Impacts: Lunar Origin and Magma Ocean Formation

Abstract: The Apollo lunar samples reveal that Earth and the Moon have strikingly similar isotopic ratios, suggesting that these bodies may share the same source materials. This leads to the giant impact hypothesis, suggesting the Moon formed from a disk that was generated by an impact between Earth and a Mars-sized object. This disk would have had high temperature (~ 4000 K), and its silicate vapor mass fraction would have been ~20 wt %. More energetic impact models have been proposed and they produce higher temperatures (~ 6000 K) and vapor mass fractions (~80-90 wt%). Some or most of these models can potentially explain the isotopic similarity. However, these energetic models may face a challenge that has not been considered to date. An initially vapor-rich Moon-forming disk may have trouble forming a large Moon because growing lunar seeds can fall onto Earth in a very short timescale due to a strong headwind from the vapor. This issue could be potentially resolved if a large lunar seed form quickly by streaming instability, which is a clump formation process caused by a spontaneous concentration and gravitational collapse of the particles. Here, we investigate whether the streaming instability can operate in the Moon-forming disk based on the numerical code Athena. Our study finds that this instability can operate in the Moon-forming disk, but the seeds may not be large enough to avoid the strong gas drag. The gas drag effect is negligible for the standard model because the vapor-mass fraction of the disk is small. Thus, our study indicates that the standard giant hypothesis remains a strong candidate for the lunar origin. We also discuss its implication for exomoons. Moreover, we will also discuss a scaling law for the shape and size of an impact-induced magma ocean based on more than 100 giant impact simulations using the method called smoothed particle hydrodynamics (SPH).


July 17, 2020

Speaker: Megan Mansfield, University of Chicago

Title: Ultra-Hot Jupiters: Revealing the Atmospheres of a Novel Class of Exoplanets


Abstract: Hot Jupiters are compelling targets for thermal emission observations because their high signal-to-noise allows precise atmospheric characterization. Theory originally predicted that cooler planets would show absorption features in their secondary eclipse spectra due to having uninverted atmospheres, while warmer planets would have inverted atmospheres causing emission features in their eclipse spectra. I first discuss our group’s early observations of ultra-hot Jupiters, which led to the realization that these hottest exoplanets are a distinct class with unique high-temperature chemistry. I present new models which take into account this new high-temperature chemistry, and show how they can explain the featureless spectra we observe in many ultra-hot Jupiters. However, observed hot Jupiters still show a surprising level of diversity in their eclipse spectra. To further examine this diversity, I perform a population study of all secondary eclipse observations of hot Jupiters with the Hubble Space Telescope (HST). From this population study I propose that the spectra of hot Jupiters can be explained through compositional diversity in their atmospheres. In the coming years we will have the opportunity to study hot Jupiter atmospheres in even more detail using the James Webb Space Telescope (JWST). In particular, JWST will provide the unique capability to perform spectroscopic eclipse mapping, which will allow us to map the atmospheres of hot Jupiters in three dimensions (latitude, longitude, and altitude). I present a new method to analyze eclipse mapping observations which can be used to interpret these complicated data sets without relying on expectations from circulation models. Finally, I discuss ongoing observations of hot Jupiters which I will be leading in the coming year using both HST and ground-based high-resolution observations.

July 24, 2020

Speaker: Sergio Ruiz, University of Chile

Title: Some observations in the near field of Chilean earthquakes: From nucleation to earthquake rupture process.

July 31, 2020

Speaker: Kelsi Singer, SWRI

Title: New insights into the impact process from lunar secondary craters and the ejecta fragments that made them

Abstract: When an impact crater forms on a planetary surface, fractured pieces of the surface material are ejected and can form their own secondary craters outside of the main, or primary, crater. Secondary craters are found in large numbers on the Moon and other bodies, and they provide evidence of the impact process. The size of secondary craters is related to the size of the ejecta fragment, and the distance of the secondary crater from the primary is related to fragment ejection velocity. We mapped secondary craters around primary craters ranging in size from ~0.83–660 km in diameter using Lunar Reconnaissance Orbiter Camera (LROC) Narrow and Wide Angle Camera images. With this dataset this we were able to investigate the impact process through a new empirical lens, and found a scale-dependence that is not currently accounted for in analytical fragmentation theories. We were also able to compare this data to secondary craters on icy satellites to test for any material dependence. I will give an overview of the morphology of secondary craters and our mapping philosphy, the techniques used to investigate trends in the data, and the new results.

August 7, 2020

Speaker: Matt Siegler, PSI

Title: Planetary Heat Flow: InSight on Mars and InSights into the Moon

Bio: Dr. Matthew Siegler is a research scientist at Planetary Science Institute based in Dallas, TX. He specializes in all things thermal, including thermal modeling, infrared and microwave remote sensing, volatile stability, geothermal heat, and laboratory thermal measurements. He is a Co-I on LRO, InSight, OSIRIS-REx, 2 lunar CLPS instruments, and the upcoming Viper lunar rover. He will be talking about the struggles of measuring geothermal heat flow on Mars and how microwave instruments may aid in future missions. He is father to Jack and June and in the process of making them an awesome treehouse


August 14, 2020

Speaker: Kanani Lee, Lawrence Livermore National Laboratory

Title: Planetary diversity: From diamond planets to hot Jupiters


Abstract: With more than 3000 planets confirmed outside of our own Solar System, the realization that many are very different from those in our own Solar System lends itself easily to the discussion of plausible planetary compositions and its effect on a planet’s thermochemical evolution. In this talk I’ll bring together astronomical observations and mineral physics experiments to constrain possible exoplanetary interiors in an attempt to move from non-unique mass-radius relationships.

August 21, 2020

Speaker: Laura Wallace, GNS Science, New Zealand, and the University of Texas Insitute for Geophysics

Title: Relationships between slow slip events, megathrust locking, and seismicity at the HIkurangi subduction zone, New Zealand


August 28, 2020

Speaker: Sarah Titus, Carleton College

Title: Unraveling the history of the San Andreas fault system in central California


September 4, 2020

Speaker: Ben Weiss, MIT

Title: History of the solar nebula from meteorite paleomagnetism


Abstract: A key stage in planet formation is the evolution of a gaseous and magnetized solar nebula. However, the intensity of the nebular field, the lifetime of the nebula, and the history of mass transport in the early solar system have been poorly constrained. Here we present analyses of the remanent magnetization in several meteorite groups demonstrating that an approximately Earth-strength nebular magnetic field existed in the inner solar system (1-7 AU) during the first 1-3 My after solar system formation. Meanwhile, measurements from the Philae lander on the surface comet 67P/Churyumov-Gerasimenko suggest that the outer solar system (i.e., 14-45 AU) field was near-zero at the same time. The solar system field then declined to near-zero by ~4 My after solar system formation, indicating that the solar nebula field, and likely the nebular gas, had dispersed by this time.. Finally, our recent measurements of that two unusually volatile-rich carbonaceous meteorites find that they record weak field conditions during the lifetime of the nebula, suggesting that their materials formed at ~10-30 AU from the early Sun. This provides evidence for large-scale dynamical mixing of solids in the solar system and indicates that we may have rock samples from the proto-Kuiper belt.

September 11, 2020

Speaker: Ryan C. Hurley, Johns Hopkins University

Title: Experimental Micromechanics of 3D Granular Materials: Force Chains, Waves, and Rearrangements


Abstract: Granular materials are ubiquitous as natural and manufactured materials. However, quantifying the microscopic mechanisms governing their macroscopic properties, stress transmission, and energy dissipation has traditionally been accomplished using model materials in 2D or numerical simulations in 3D. A major challenge remains the quantitative evaluation of these microscopic mechanisms in-situ during 3D deformation. One tool for furnishing this microscopic information is in-situ X-ray computed tomography (XRCT). XRCT provides in-situ kinematics during deformation but does not provide stress states and is restricted to quasi-static strain rates. To overcome these limitations, I will discuss two approaches. First, I will discuss the simultaneous use of XRCT and 3D X-ray diffraction (3DXRD), an X-ray scattering technique that provides per-particle stress tensors in thousands of particles. I will discuss examples of using combined XRCT and 3DXRD to examine forces, energy dissipation, and wave transmission properties in compressed granular materials. Next, I will briefly discuss combining XRCT with in-situ X-ray phase contrast imaging (XPCI) or radiography, an imaging technique providing 2D projections of rapidly evolving 3D flows. I will discuss recent experiments and algorithmic developments that aim to extract 3D flow fields from this data.

September 11, 2020 - Special Planetary Section at 4:00PM

Speaker: Baoli Liu, Oxford University

Title: Exploring aeolian-fluvial interactions, and the implication on geomorphology


Abstract: On the earth, there are many areas where the action of wind and water interact. Examples of these include where sand are halted by a river flowing through them and where rivers appear to be diverted by the presence of a ‘sea of sand’; ancient processes are also indicated from stratigraphic records where windblown sands embed with river sediments. Inspiringly, similar landscapes have also been observed on Mars. This raises several fundamental questions surrounding the interactions between sediment transport by wind and water that could have major implications for climate change and how landscapes develop. To explore these questions, one field in Namibia was selected as one of the study sites. The ephemeral Huab River in Namibia marks the start border of the Skeleton Erg, at where sand dune fields have been evolved in adjacent to the channel and northward, until they meet the Kuenne River. Field and lab observations have been attempted.

September 18, 2020

Speaker: Randy Williams, University of Wisconsin-Madison

Title: Earthquake Geochemistry: Using the Chemical Composition of Fault Rocks to Decipher Fault Physics

Abstract: Fault rocks provide a fundamental record of physical processes occurring throughout the seismic cycle, including dynamic weakening during coseismic slip and subsequent recovery of fault strength during the post- and inter-seismic periods. Much previous work has focused on quantifying the physical properties of fault rocks to provide insight into the various seismic and aseismic processes occurring in faults. This talk will focus on recent research examining the geochemical signatures of earthquake processes recorded in fault rocks, with a particular emphasis on identifying the controls on variations in fault strength. I will argue that these novel approaches provide new insights into a variety of earthquake-related processes across a wide range of spatial and temporal scales - from meters to kilometers and seconds to thousands of years. Thus, the geochemical record of earthquake processes offers significant potential to re-examine earthquake processes not just as isolated phenomena associated with individual faults, but rather as an emergent property of a variety of crustal processes that transcend disciplinary boundaries.