Category: Seminars

High-contrast imaging is a powerful tool to probe the outer architecture of planetary systems and directly study the atmospheres of extrasolar giant planets. Previous imaging surveys have primarily focused on intermediate- and high-mass stars, revealing a handful of giant planets. Yet M dwarfs, which present more favorable planet-star contrasts and make up 75% of all stars, have largely been neglected. As a result, little is known about the population of gas-giant planets at moderate separations (10-100 AU) in this stellar mass regime. For the past several years I have carried out a high-contrast adaptive optics imaging survey targeting newly identified nearby (<35 pc) young (<300 Myr) M dwarfs with Keck II/NIRC2 and Subaru/HiCIAO. With a sample size of over 120 young M dwarfs, this is the largest direct imaging planet search in this stellar mass regime. I will present the discoveries and statistical results from this survey and discuss their implications for the formation of gas-giant planets around the most common stars in our galaxy.

The past two decades have seen an explosion of discovery of extrasolar planets.  We are now assembling a picture of the variety of planetary systems in our galaxy, detecting planets in previously unexpected configurations and types.  I will describe the results from recent exoplanet surveys, including the Kepler space mission, and the ways in which our understandings of planet formation and evolution have been revolutionized.  I will describe the next steps in exoplanet discovery, including the work of low-cost small telescopes, and the future NASA missions that will move us toward the direct detection of biomarkers in the atmospheres of habitable exoplanets.

Catastrophic Evaporation of Rocky Planets

Short-period exoplanets can have dayside surface temperatures surpassing 2000 K, hot enough to vaporize rock and drive a thermal wind. I will present a radiative-hydrodynamic model of atmospheric escape from strongly irradiated, low-mass rocky planets. We find that rocky planets with masses ≤ 0.1 M_Earth and surface temperatures ≥ 2000 K disintegrate entirely in ≤ 10 Gyr. When we apply our model to Kepler planet candidate KIC 12557548b—believed to be a rocky body evaporating at a rate of dM/dt ≥ 0.1 M_Earth/Gyr—we find its present-day mass to be ≤ 0.02 M_Earth (less than twice the mass of the Moon).

Surface Layer Accretion in Protoplanetary Disks Driven by X-ray and FUV Ionization

How do protoplanetary disks accrete? Whether accretion of the disk surface layers by the magnetorotational instability (MRI) occurs at observationally significant rates, depends on how well ionized they are. Surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates. Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. I will show that ionization by stellar far-ultraviolet (FUV) radiation produces a plasma so dense that it is immune to ion recombination on condensates. MRI-turbulence in the FUV-ionized layer behaves in the ideal magnetohydrodynamic (MHD) limit and can accrete at observationally significant rates at radii ≥ 1–10 AU. At smaller radii, surface layers ionized by both X-rays and FUV radiation cannot sustain the accretion rates generated at larger distance and an additional means of transport is needed.

The semi-square-shaped South Polar Terrain (SPT) of Saturn’s small moon Enceladus (500 km diameter) hosts five regularly spaced (~35 km) and similar-length (~130-150 km) linear structural zones informally named as the tiger-stripe fractures. Although the tiger-stripe fractures are geologically active and serve as conduits of erupting water vapor, their mechanical origin remains uncertain. Existing hypotheses invoke both tidal stress and internal heating and predict the tiger-stripe fractures to be extensional cracks, propagating spreading ridges, faults with alternating opening, closing, right-slip and left-slip kinematics, and transpressional shear zones. The main issue leading to such diverse views on the mechanical origin and the kinematic development of the tiger-stripe fractures is a lack of systematic determination of the geometric and kinematic relationships among diverse geologic structures in the South Polar Terrain. To address this question, a first detailed tectonic map of the South Polar Terrain is constructed based on a detailed analysis of the most recently released CASSINI images of the SPT. The structural analysis taken in this study differs from the early work in that it does not rely on the use of fracture offsets as a mean of determining fault kinematics, as termination and overstepping of younger fractures at and across older fractures can lead to erroneous interpretations on the timing and kinematics of fracture formation. Instead, the new approach used in this study emphasizes the use of geometrically linked and kinematically related secondary and termination structures to establish the kinematics of the tiger-stripe faults. Based on this new approach and guides from terrestrial examples, we show that (1) the tiger-stripe fractures are left-slip faults and (2) the SPT is bounded by an extensional zone along its leading edge, a thrust-fold belt along its trailing edge, a right-slip shear zone along its eastern edge, and a left-slip shear zone along its western edge. A westward increase in the width of the leading-edge extensional zone requires that the SPT deformation be partitioned by translation towards the trailing-edge and clockwise rotation of the SPT. The translation of the SPT is accommodated by contraction along the trailing edge and strike-slip shear along the eastern and western edges. Meanwhile, clockwise rotation of the SPT is accommodated first by initiation of the tiger-stripe fractures as conjugate Riedel faults that was followed by bookshelf faulting. Based on the topography data we attribute the lateral translation of the SPT to gravitational sliding along an inclined brittle-ductile transition zone dipping towards the trailing-edge direction. Clockwise rotation of the SPT relative to its surrounding region could either be related to internal deformation induced by gravitational sliding or by non-synchronous rotation of the SPT above a local ocean.

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Characterizing the atmospheres on planets around other stars has become a reality in the past few years. The frontier of current and near-future efforts is observing sub-Neptune-sized planets with thick atmospheres. I will present new atmosphere scenarios for GJ 1214b, the most observed mini Neptune exoplanet to date, resulted from my self-consistent atmosphere chemistry model that can treat both H₂-dominated atmospheres and non-H₂-dominated atmospheres on low-mass exoplanets. In addition to the conventionally assumed H₂O-dominated atmosphere or hazy atmosphere, I will show that a H₂-H₂O atmosphere, a H₂-CO atmosphere, a CO-CH₄ atmosphere, and a C₂H₄-C₂H₂ atmosphere are also plausible scenarios that are consistent with the current observation of the planet. I will discuss how future observation in both transmission and thermal emission can distinguish these scenarios, and generalize the results to outline the spectral features useful for characterizing mini Neptune exoplanets. For the future when thin atmospheres on terrestrial exoplanets become observable, my atmosphere chemistry model will be pivotal as the interface between the fundamental unknowns (e.g., geological sources, biological sources, mixing rates, and escape rates) and the observables (e.g., abundances of trace gases and their spectral signatures). I will show two examples of using the atmosphere chemistry model to assess potential biosignature gas candidates: H₂S as a failed biosignature gas due to its short photochemical lifetime in virtually any types of thin atmospheres, and NH₃ as a biosignature gas on a planet with a N₂-H₂ atmosphere.

We investigated the occurrence of large (regional) dust storms during the martian dusty season—southern spring and summer (Ls 180° to 360°). We used a zonal mean climatology at 50 Pa from TES/MGS and MCS/MRO to identify the regional and global dust storms and then to characterize their extent and evolution. A 200 K contour outlines the season and latitudinal extend of the significant storms in the zonal mean climatology. We find that years without a global dust storm are quite similar. There are three regional storms each of the five years without a global dust storm covered by the two instruments (MY24, MY26, MY29, MY30 and MY31). The exact timing, duration and peak temperatures vary from year  to year, but each of the three storms has distinct characteristics and behavior and overall the years follow a very similar patern. In addition to examining the detailed characteristics of each of the three storms, this has implications for the InSight landing planned for Ls 230°. It may be an exciting and difficult to predict ride through the atmosphere.

The Korean Astronomy and Space Science Institute (KASI) is involved in a wide range of technology and astronomy projects. They operate a very long baseline interferometry network within Korea and jointly with other Asian facilities that among other things, utilizes a multi-band quasi optical input duplexer developed by Han. This allows simultaneous observations of multiple SiO masing transitions between 40 and 130 GHz. They also have a 14m millimeter telescope used for mapping molecular clouds in the 115 Ghz CO line and in other molecular emission. The institute is also involved in rocket projects with Caltech and have recently joined a consortium of Asian countries supporting and observing  at the Caltech Submillimeter Observatory in Hawaii. KASI also has numerous technology development programs.

Thermal infrared imaging of sedimentary terrestrial analogs and neutron remote sensing by the Dynamic Albedo of Neutrons (DAN) instrument on the Mars Science Laboratory (MSL) rover are discussed.  Thermal infrared data have been used on Mars spacecraft for decades, however, the broadband thermal images of various sedimentological features have not been well characterized.  Day-night aerial thermal images were acquired and ground-truth mapping performed for thermophysical characterization of sedimentary features.  Building on this work, surface images could be acquired from a rover mast at specific times of day to be used for autonomous classification of fines and cobbles, as well as unconsolidated and cemented materials.  Pulsed neutron data, like those acquired by DAN, can also be incorporated into rover autonomy.  DAN has the ability to sense anomalous hydrogen-or chlorine-rich features at or near the surface (<60cm depth) during a long duration rover traverse.  Initial results and geochemical classifications from DAN are presented.

The two main models for giant planet formation are known as core accretion, the standard model, and disk instability. There are substantial differences between these formation models, including formation timescale, favorable formation location, ideal disk properties for planetary formation, early evolution, planetary composition, etc. First, I will summarize current knowledge of the internal structures of solar- and extrasolar- giant planets, and the two formation models including their substantial differences, advantages, and disadvantages. I will then present the predicted planetary composition in each model, and discuss how theoretical models should be connected to available (and future) data.