Category: Seminars

Remote temperature measurements have increased our understanding of the physical properties of the Martian surface layer. Typical grain sizes, rock abundances, subsurface layering, soil cementation, bedrock exposures, and ice presence/compositions have been derived and mapped using temperature data in conjunction with subsurface models of heat conduction, and have helped to constrain numerous global-scale processes. However, the simplicity of these models precludes more significant advances in the characterization of the physical nature of the Martian surface. For this seminar, I will present a new model of heat conduction for planetary soils derived from a combination of finite element modeling and laboratory measurements for homogeneous particulated media accounting for the grain size, porosity, gas pressure and composition, temperature, and the effect of any cementing phase. I will show that incorporating the temperature dependence of bulk conductivity alters the predicted diurnal and seasonal temperatures as compared to temperatures predicted with a temperature-independent conductivity model. Inconsistencies between observed temperatures and those predicted using temperature-independent conductivity models have been interpreted to result from subsurface heterogeneities, but they may partially be explained by a temperature-dependency of the thermal inertia, with additional implications on the derived grain sizes. Cements are shown to significantly increase the bulk conductivity of a particulated medium, and bond fractions <5% per volume are consistent with Martian thermal inertia observations previously hypothesized to correspond to a global duricrust. I will conclude with general thoughts on the predicted thermophysical properties of particulated materials on other planetary bodies with atmospheres.

Earth’s violent accretion likely generated multiple magma oceans. In particular, the Moon-forming giant impact is often thought to have produced a whole mantle magma ocean, which would have homogenized any pre-existing chemical heterogeneity within the mantle. The ratio of primordial 3He to primordial 22Ne in the mantle preserves a record of magma oceans on the early Earth. Importantly, the 3He/22Ne ratio of the Earth’s shallow depleted mantle is significantly higher than the deep mantle. To explain this observation, I propose that at least two giant impact-induced atmospheric blow-off and magma ocean degassing episodes are required and that the last giant impact did not generate a whole mantle magma ocean. New Xe isotopic data indicate that the catastrophic mantle outgassing and atmospheric blow-off events inferred from3He/22Ne ratios were accomplished between ~30 to 55 Myrs after the start of the Solar System. Therefore, outgassing associated with giant impacts, including the Moon-forming impact, must have occurred within this time window. Previous calculations of impact-induced atmospheric erosion have, however, found that it is difficult to completely remove the atmosphere from a body as large as Earth by a giant impact. The need for atmospheric loss inferred from the noble gas data could be reconciled with the dynamics of giant impacts by considering the new high-spin Moon formation hypothesis. I will further show that the current inventory of primordial noble gases in the atmosphere must largely be derived from late accreting planetesimals, a conclusion that has implications for the composition of the early atmosphere.

The composition and spatial distribution of molecular gas in the inner few AU of young (< 10 Myr) cirmcumstellar disks are important components to our understanding of the formation and evolution of planetary systems. Following the growth of planetary systems, the energetic radiation environment plays a critical role in atmospheric heating and chemistry in these worlds. In the first part of the talk, I will present recent results on protoplanetary disks and the radiation environment in exoplanetary systems, using data from the new and refurbished spectrographs aboard the Hubble Space Telescope. We have completed the first ultraviolet spectroscopic survey of the inner molecular disks around these stars, characterizing the spatial distribution of H2 and CO at planet-forming radii (a < 10 AU), the inner molecular regions of transitional disks, and the Lyman-alpha radiation environment. I will also describe first results from a spectrocopic survey of M-dwarf exoplanet host stars. This work highlights the ubiquity of chromospheric time-variability and the importance of strong Lyman-alpha radiation to planets in the habitable zones of these systems. In the second part of the talk, I will describe current and future UV/visible space instrumentation being developed by the ultraviolet astrophysics group at the University of Colorado. We are actively involved in the development and flight-testing of next-generation optical coatings, detector systems, and grating in support of future NASA Explorer-class and flagship missions. Many of the graduate students are involved in all phases of a space-flight mission: from instrument development and mission planning to launch and data acquisition to analysis and publication of the results.

Networks of valleys with steep, amphitheater-shaped headwalls are prominent features on the surfaces of Earth and Mars.  These landforms are commonly used as diagnostic indicators of undermining and headwall retreat by groundwater-seepage erosion.  In this presentation, I question the link between seepage erosion and canyon form, and present an alternate hypothesis for canyon formation: waterfall retreat during large-scale flooding.  To support this hypothesis I will discuss three interrelated studies and some ongoing work.  First I investigated several canyons in Idaho, which have long been thought to be formed by groundwater sapping because they contain some of the largest springs in the United States.  4He and 14C dates, plunge pools, and sediment transport modeling indicate, however, that these canyons most likely formed during a catastrophic flood about 48,000 years ago. Second, Canyon Lake Gorge, Texas, was examined as a rare modern example of bedrock canyon formation during a single flood event.  Results show that the combination of well jointed rock and high flow discharges caused block plucking, waterfall formation, and a rate of erosion that was limited only by the ability of the flow to transport sediment.  Finally, theoretical modeling and flume experiments are used to show that steep headwalls can persist during canyon formation due to waterfall-induced toppling in fractured rock.

It is well established that high-speed flows in the magnetotail plasma sheet, separated from the ambient plasma by dipolarization fronts, are braked in the tail-dipole transition region of the near-Earth magnetotail. Kinetic and electromagnetic energy of the flow burst and dipolarization front is therefore converted to thermal energy of plasma and radiated by electromagnetic and plasma waves. Details of the energy conversion as yet remain unclear, largely due to the lack of multi-point observations in the transition region.

Taking advantage of THEMIS probes and geosynchronous (GEO) satellite conjunctions repeated in two events, we can study physical connections of the dipolarization front braking between X=-11 and -9 RE and magnetic and plasma oscillations observed at X=-8 RE and at GEO. It has been found that, despite different background plasma conditions in both events, slow-mode oscillations were excited in the dipole-dominated magnetotail region in response to the front braking in the transition region. No signatures of front rebounding were found. The slow-mode wave, observed at X=-8 RE, was not directly driven by dipolarized flux tube oscillations. The data analysis has shown that the slow-mode oscillations observed were triggered by the plasma pressure enhancements ahead of the front.

Electron phase-space holes (EHs) are good indicators of nonlinear activities in space plasmas and have attracted many interests in both observational and theoretical work. In a traditional theoretical picture, EHs are understood as purely electrostatic structures. However, THEMIS observed electromagnetic EHs, which cannot be fully described with traditional theory, in the plasma sheet boundary layer.

This work seeks to understand the magnetic signals of the observed electromagnetic EHs. In addition to the interpretations of the observed magnetic signals, a statistical study of the properties of the observed electromagnetic EHs reveals that those electromagnetic EHs feature fast speeds, large sizes, and strong potentials, which intrigues interests in their generation mechanism and influences on the space plasma environment.

In the standard model, Earth accreted via a series of giant impacts and the last giant impact produced the Moon and fully melted the Earth. The Moon and Earth are identical in multiple isotope systems that show significant variations between most meteorite groups and planetary bodies. Thus, the simplest explanation for the isotopic similarity is that the Moon and Earth’s mantle have a common origin. However, the canonical giant impact model predicts that the Moon is primarily composed of material from the impactor, which should have had a different isotopic signature than Earth. In addition, recent data from the deep mantle demonstrate that the early Earth was not completely mixed and preserves chemical heterogeneities established during Earth’s accretion. Previous Moon-formation studies assumed that the angular momentum after the impact was similar to present day, but N-body simulations of the growth of Earth-mass planets typically find higher spin rates at the end of accretion. I will present a new model for the origin of the Earth–Moon system. A giant impact onto a fast-spinning proto-Earth can produce a disk that is massive enough to form the Moon and composed primarily of material from Earth, but the system would have had more angular momentum than today. Subsequently, the excess angular momentum can be lost during tidal evolution of the Moon via an orbital resonance. The impact energy is primarily deposited in the impacted hemisphere, and the mantle of the post-impact Earth is stably stratified, which would inhibit immediate deep convective mixing. Hence, the Moon-forming impact need not destroy pre-existing chemical heterogeneities in the deep mantle of the proto-Earth. Finally, I will discuss implications for the volatile depletion on the Moon and the formation of Earth’s early atmosphere.

The modern wealth of data on extrasolar planets is a boon to theoretical studies of the origin and evolution of planetary systems.  However the great diversity of exoplanet systems also poses challenges to the goal of a coherent, unified theory of planet formation. This talk will present dynamical models of the most crucial stages of planet growth. First I will address the origin of the solid building blocks known as planetesimals. Aerodynamic processes, highlighted by the streaming instability, play a crucial role in the emergence of planetesimals within turbulent gas disks. Such models of early planet growth can be tested with exoplanet statistics and, more directly, with studies of the Kuiper Belt. Shifting to later phases, I will address the accretion of giant planet atmospheres onto protoplanetary cores. I will present models that challenge the conventional wisdom that a 10 Earth mass core is needed to trigger runaway gas accretion. In particular lower core masses suffice in the outer regions of the protoplanetary disk.   These models are crucial for understanding the HR 8799 multiplanet system and others that will be discovered in current and future imaging surveys. I will conclude by discussing future prospects for theoretical models to improve our understanding of planets near and far.

Despite the many hundreds of known “giant” exoplanets, theoretical models still struggle to form them.  Under the core-accretion, gas-capture model of giant planet formation, it is still challenging to explain how the required ~10 Earth mass rocky/icy cores can form within the lifetime of their host gaseous circumstellar disks.  In this talk I will present two different new lines of research to address this timescale dilemma.  I will explore whether or not a newly identified, extremely fast aerodynamic aided accretion of “pebbles” may present a mechanism to form giant planet cores extremely rapidly. Then I will discuss how volatile loss from ice-rich cores and the subsequent enrichment of the atmosphere can alter the efficiency of core growth and potentially aid the planet formation process.

A fundamental long-standing question regarding Mars history is whether the flat and low-lying northern plains ever hosted an ocean. The best opportunity to solve this problem is provided by stratigraphic observations of sedimentary deposits onlapping the crustal dichotomy. Here we use high-resolution imagery and topography to analyze a branching network of inverted channel and channel lobe deposits in the Aeolis Dorsa region, just north of the dichotomy boundary. Observations of stacked channel bodies, switches in channel direction tied to a single node, and stratal geometries indicate that these landforms represent exhumed distributary channel deposits. Observations of depositional trunk feeder channel bodies, a lack of evidence for past topographic confinement, channel avulsions at similar elevations, and the presence of a strong break in dip slope between topset and foreset beds suggest that this distributary system was most likely a delta, rather than an alluvial fan or submarine fan. The location of this delta within a thick and widespread clastic wedge abutting the crustal dichotomy boundary, unconfined by any observable craters, suggests a standing body of water potentially 105 km2 in extent or greater, and is spatially consistent with hypotheses for a northern ocean.