December 6, 2013: Impact hazard assessment and deflection analysis for (101955) Bennu
TBD – please check back later for more details
November 22, 2013: The Outer Architecture of M Dwarf Planetary Systems
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.
November 15, 2013: The Golden Age of Exoplanet Discovery
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.
November 8, 2013: Part I: Catastrophic Evaporation of Rocky Planets; Part II: Surface Layer Accretion in Protoplanetary Disks Driven by X-ray and FUV Ionization
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.
November 1, 2013: A bookshelf-faulting model for the mechanical origin of the tiger-stripe fracture system in the South Polar Terrain of Saturn’s moon Enceladus
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.
October 4, 2013: Asteroid Sample Return Missions: Hayabusa and Hayabusa 2
TBA – check back for details
October 18, 2013: Atmosphere Chemistry of Low-Mass Exoplanets
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.
UCLA scientists work to forecast space weather
The Sun is a veritable force in our solar system. It emits a tremendous amount of heat and energy, called the solar wind, which constantly blows and buffets the planets at a velocity almost two thousand times faster than the average jet plane. Akin to an invisible shield, the Earth’s magnetic field deflects most of the solar wind, but it happens often that the magnetic fields of the Earth and Sun briefly and directly come into contact with one another.
When the fields connect, part of Earth’s magnetic field “peels away from the sunward side and drapes around the back of the planet,” said sixth-year graduate student, Christine Gabrielse. The backside of Earth’s magnetic field, or magnetotail, is “squeezed from the outside as a result of the peel back,” she said. Eventually, two points on the interior of the Earth’s magnetic field meet in what is called a near-Earth reconnection, releasing a great deal of energy that flows toward Earth. “These powerful phenomena, known as substorms, can create more than picturesque auroras”, Gabrielse said. “They can damage spacecraft or astronauts, or even ground-based systems.” On March 13th, 1989, one such storm caused a legendary power outage in Canada’s Quebec province that left more than three quarters of a million people without power for nearly twelve hours.
While scientists had studied substorms for years, many questions remained regarding these space weather events. Proposed by UCLA Professor Vassilis Angelopoulos, NASA’s Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission was designed to answer some of these questions.
Launched in February 2007, the mission consisted of five identical satellites deployed to critical locations around Earth. Unprecedented at the time, THEMIS allowed scientists to track the flow of energy around Earth and determine how and where substorms initiate. “The spacecraft gave us five pinholes in the magnetic curtain we are trying to see through,” said Drew Turner, an assistant researcher at UCLA working on the THEMIS mission.
From their unique orbits, engineered to simultaneously provide five key perspectives of the vast space environment, the spacecraft quickly solved the questions they’d set out to answer. “In 2008, THEMIS repeatedly showed that reconnection happens in the magnetotail first, activating a substorm,” said Gabrielse. With its primary goal accomplished, THEMIS set new objectives. Splitting the satellites into two groups, three continued to orbit Earth, while two were sent to the Moon as a ‘new’ mission called Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS). “The extension to ARTEMIS was quite miraculous,” said Turner. “The spacecraft were not equipped with the ability to maneuver out of their orbit. The THEMIS engineers and operators sent satellites to the Moon by way of tiny puffs of rocket fuel.”
Using “the most comprehensive plasma instruments we’ve sent to the Moon,” the two ARTEMIS satellites are now busy determining the rock types on its surface, said Turner. The satellites detect small variations in the Moon’s particle and electric fields allowing them to distinguish between different materials. “It’s a natural way of detecting surface composition from afar,” said Turner. In addition, the satellites are improving substorm research by studying the Earth’s space environment from their entirely new viewpoint near the Moon. “With the two spacecraft at the Moon we can test what’s happening on the other side of the reconnection,” said Shanshan Li, a fifth-year UCLA graduate student. “We can start to form a three-dimensional substorm model of Earth’s space environment.”
The THEMIS satellites that have remained in orbit around Earth are “scientific goldmines,” according to Turner. Coordinating observations with the Van Allen Probes, a pair of recently launched NASA satellites, they were able to detect a previously unknown layer of charged particles surrounding Earth. Turner said, “in a huge and complex system, my bread and butter is combining as many satellites’ data as I can to get as complete a global picture as possible.”
With the Sun approaching a period of increased activity, the media have begun to report space weather more often. “It’s good to see that society is taking an interest,” said Turner. “We’ve become increasingly dependent on space-based assets,” said Turner. “Even something as simple as using an ATM will most often result in a satellite-relayed signal at some point.” Since large space storms can have huge societal impacts, it is important to be able to see them coming. “Just like meteorologists want to be able to forecast a storm on Earth, we want to be able to predict a storm in space,” said Gabrielse. “Ultimately, our aim is to determine what’s going on in the Sun-Earth environment and try to better understand it.”
Prof. Schmitt analyzes meteorites from Mars
The LA Times and Science Daily report on UCLA Department of Earth, Planetary and Space Science Associate Professor Axel Schmitt’s recent research to determine the age of Mars’ meteorites. Read the full-length articles here: