Category: Seminars

Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. The dusty debris from these comets will manifest itself as a debris disk temporarily visible around newborn white dwarfs; examples of such disks may already have been seen in the Helix Nebula, and around several other young WDs. Future observations with the James Webb Space Telescope may distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. If interpreted as indeed being cometary in origin, the observation that >15% of young WDs possess such disks provides indirect evidence that low mass gas giants (thought necessary to produce an Oort cloud) are common in the outer regions of extrasolar planetary systems. Hydrogen abundances in the atmospheres of older white dwarfs can, if sufficiently low, also be used to place constraints on the joint parameter space of natal kicks and exo-Oort cloud models.

The Rosetta mission to comet 67P/Churyumov-Gerasimenko is producing fantastic but puzzling images of the comet. I will present these images and set them in context so as to trigger a discussion about what we are seeing.

The idea of mining the asteroids for their resources – long a dream – has become closer to reality with the advent of commercial companies set up to do exactly that: Planetary Resources and Deep Space Industries. Both companies emphasize sending swarms of cubesat-class probes out to prospect the resources of numbers of near-Earth asteroids (NEAs). How realistic are these companies’ goals? Careful analysis suggests that ore-bearing (i.e. potentially profitable) NEAs are rare and that the supporting astronomical data is limited and patchy in quality. The first phase of asteroid prospecting will have to be industrial in scale (characterizing 1000s of NEAs/year) and will be carried out by ground-based telescopes, primarily in the optical band. The observations demand large (2m – 8m) telescopes and high professional standards (0.1” or better astrometry; 0.1% photometry; sub-1% spectrophotometry). Just like geologists, applied astronomers will be in demand.

In this talk I will present a new bombardment model of the Hadean Earth that has been calibrated using existing lunar data and terrestrial highly siderophile elements. We find that the surface of the Hadean Earth was widely reprocessed by impacts through mixing and burial by impact-generated melt. This model may explain the absence of early terrestrial rocks. Furthermore, we will discuss an intriguing orrelation between the age distribution of Hadean zircons and the impact flux, possibly indicating impacts played an important role in Hadean zircon formation. Finally, by tracking the magnitude and timing of early collisions, we find that existing oceans would have repeatedly boiled away into steam atmospheres as a result of large collisions as late as about 4 billion years ago.

During the first million years of evolution, nascent planetary
systems are embedded in dense disk­-shaped clouds of gas. These
circumstellar disks are home to a myriad of hydrodynamical processes,
which bring about turbulence and the emergence of viscous­-like behavior,
enabling accretion of gas onto the protostar. Meanwhile, micron­size dust
grains embedded in the disk are growing through coagulation onto pebbles
and rocks. Turbulence has a positive effect on these small solids,
concentrating them into transient high pressure regions for long enough to
achieve gravitational collapse into km­-sized bodies, forming the first
planetesimals. Giant storm systems in the disk, similar to Jupiter’s Great
Red Spot, may exist in quiescent zones of the disk. These are even more
prone to collecting solid material, producing the first terrestrial
planets and cores of giant planets. In this talk I will discuss the state
of the art and recent advances in the field of planet formation, as well
as pressing problems such as the asymmetries observed in ALMA images of
circumstellar disks, and how to interpret them.

I plan to discuss two topics that I have been working on:

(1)   For Jupiter-like planets, the question of how cyclones and anticyclones (like the GRS and white ovals) are formed is not settled. I discuss computer simulations showing that both kinds of vortices can appear spontaneously in a rotating convection zone, much like the regions below the weather layers of Jupiter and Saturn.   These vortices are long-lived, and there is no need to prescribe an artificial shear to create and maintain them.

(2)   The Chinese lunar orbiters Chang’E 1 and 2 have obtained best available sets of microwave data on the moon so far. However, the two sets of data are off by 10 – 20 K, thus creating puzzle and doubts.  A correction method based on taking account of possible contamination of the low temperature calibration antennas by radiation from the lunar surface is discussed.  The disagreement can be reduced by a large factor.

One of the expanding fields of exoplanet research is the detailed characterization of exoplanets, including the properties of their atmospheres. This is currently being done for a growing sample of the so-called hot Jupiters – gas-giant planets orbiting close-in to their host star – a class of planets with no Solar System analog. I will present the results of our atmospheric study of the unique transiting exoplanet Kepler-13Ab. It is one of only two known short-period (1.76 day) transiting planets orbiting a bright hot A-type star (Teff = 7,650 K), and the host star is part of a hierarchical triple system. We have studied the planet’s emission spectrum by observing the planet’s occultation (secondary eclipse; when the planet moves behind the host star) using data from the optical to the IR, obtained with the Kepler and Spitzer space telescopes along with a ground-based observation in the near-IR. We derive a temperature of 2,750 K of the planet’s day-side hemisphere, as hot as the smallest main-sequence stars. We find evidence for a high geometric albedo (~0.3), a few times larger than that of most other hot Jupiters, and for the presence of atmospheric inversion. In addition, our revised planetary radius (1.4 Jupiter radius) is significantly smaller than previously thought, and our revised planetary mass, from measuring the beaming effect and ellipsoidal distortion in the Kepler orbital phase curve, is 5 – 8 Jupiter mass. Therefore, Kepler-13Ab is a massive high-density hot Jupiter. Time allowing, I will show how the difference between the Kepler occultation time and transit (primary eclipse) time is half a minute shorter than expected from the light travel time delay across the orbit, and discuss possible causes.

Recent observations have discovered several objects that are evidently experiencing some sort of mass shedding or breakup. The interpretations of such objects can be across this spectrum; for example, P/2013 P5 may be experiencing mass shedding, while P/2013 R3 may be breaking up into several components. P/2010 A2, on the other hand, could be some combination of these effects. Detailed observations of these bodies can be interpreted as being consistent with rotational break-up of the objects, based on the spectrum and magnitude of ejecta speeds plus some aspects of the morphology of their debris tails. An interesting hypothesis is whether all of these outcomes could have singularly arisen from the YORP effect. Specifically, could rotational instability lead to such different failure modes? The present study considers failure modes of irregularly shaped rubble-pile bodies due to a YORP-type spin up, comparing perfectly ellipsoidal shapes with actual shape models (based on radar observations and spacecraft imaging). Failure modes may be categorized into two different modes: structural failure, a process of plastic deformation finally leading to fission or global redistribution in a catastrophic way, and surface shedding at which particles sitting on the surface are lofted from there due to centrifugal accelerations exceeding gravitational accelerations prior to structural failure. We have developed dynamical and structural analysis techniques to determine the failure modes of rubble pile objects given their shape, mass, and spin rate. This talk discusses correlations between the shape and the failure mode due to a YORP-type spin up, using 21 sample asteroid models. The primary result of this study shows that irregular asteroids can be categorized into four shape classes: (i) spherical bodies undergoing structural failure, (ii, iii) ellipsoidal bodies experiencing either structural failure or surface shedding, and (iv) bifurcated bodies failing by structural failure. We discuss what the observational implications of these failure modes may be.

Much of the martian surface is covered by dunes, ripples, and other features formed by saltation, the blowing of sand by wind. In addition, Mars’ atmosphere is loaded with large quantities of mineral dust, much of which is likely emitted by saltation occurring in the ubiquitous Martian dust storms and dust devils. Yet, despite the obvious importance of sand transport to shaping the climate and surface of Mars, several enigmatic questions remain regarding the occurrence and properties of martian saltation. Foremost among these is the puzzling finding that both lander measurements and atmospheric circulation models indicate that wind speeds on Mars rarely exceed the threshold wind speed required to initiate saltation. How then does sand transport appear to be so common on Mars? Furthermore, bedforms observed by rovers contain particles so small that they should be easily suspended by turbulence and thus would be unable to form bedforms. Finally, the minimal size of martian dunes is over an order of magnitude smaller than would be expected from scaling up terrestrial analog dunes. Starting from the basic physics of saltation, this talk provides some possible solutions to these varied mysteries surrounding sand movement on Mars.

Up through the early 21^st century, the rate of dune and ripple movement on Mars, or whether activity occurred at all, was an open question. On the one hand, Viking Lander meteorology measurements and climate models indicated that winds sufficient to move sand were rare in the low density Martian atmosphere. In contrast, sand dunes, yardangs, and enigmatic mega-ripples were identified as common features, indicating that, over geologic time, sand activity had occurred. Because Mars has undergone dramatic climate shifts, causing atmospheric densities and wind speeds to change, a related question was whether any activity was current, or was relegated to the past. The previous half decade has seen a revolution in our understanding of Martian aeolian processes due to advances in spacecraft instrumentation, analytical techniques, numerical saltatation models, and studies of applicable terrestrial analogs. These investigations show that both views of Mars are true: Dunes in many regions are active, with mass fluxes comparable to some regions on Earth. In contrast, other dunes and, most notably, large megaripples, are static.

This talk summarizes this story, focusing in on two studies by the presenter, namely the quantification of dune migration rates and fluxes, and the formation of megaripples using a terrestrial analog site in Argentina. It concludes that the two views of aeolian Mars are reconcilable. Active dunes with significant sand fluxes are common where the sand supply is fresh. Megaripples form through accumulation of coarse grains through sand impact creep, commonly on pre-existing topography, and then become static through formation of a desert pavement-like surface. By considering the energetics of saltation on Mars, both processes occur today, although activity has waxed and waned under different climatic conditions. Mars is an active aeolian planet.