Month: May 2013

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.

I will address several questions related to how much life there might be in our Galaxy:
What affects whether potentially Earth-like exoplanets might be good abodes for life?
What kind of (Milankovitch-like) variations in habitability might be expected in exosolar
systems? And what does the origin of life on Earth tell us about the probability of life
elsewhere in the Galaxy? I will also discuss the longterm evolution of binary systems,
including thermal and chemical changes, and orbital evolution. In particular, Jupiter will
eventually become a hot Jupiter, and if it were somewhat closer to the Sun it would
eventually be tidally engulfed by the future-red-giant Sun.

Precise clusters offer a new set of building blocks with unique properties that can be leveraged both individually and in materials in which their coupling can be controlled by choice of linker, dimensionality, and structure. Initial measurements in both of these worlds have been made. Isolated adsorbed or tethered clusters are probed with low-temperature scanning tunneling microscopy and spectroscopy. Even closely related elements behave differently on identical substrates. Surprising spectral variations are found for repeated measurements of single isolated, tethered clusters. In periodic solids, precise clusters joined by linkers can be measured experimentally and treated theoretically with excellent agreement, in part due to the relatively weak coupling of the clusters. This coupling can be controlled and exploited to produce materials with tailored properties. Some of the rules of thumb for predicting these properties are being developed through these initial studies and the limit to which they can be applied is being explored.