Sept. 28, 2018

noon - 1 p.m.
Slichter 3853

Presented By: David Jewitt
UCLA / Earth, Planetary, and Space Sciences

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Two Freaks and a Conundrum

I will introduce three topics presented at the European Planetary Science Conference in Berlin this month: 1) Freak Asteroid 2) Freak Comet 3) Color Conundrum

Oct. 5, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Kynan Hughson,
UCLA / Earth, Planetary, and Space Sciences

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Ceres after Dawn: Impressions of a dwarf planet

As we approach the end of Dawn’s 11 year mission to explore the uncharted worlds of the asteroid Vesta and the dwarf planet Ceres, join me in reflection of this overachieving spacecraft’s monumental legacy at Ceres. In the past three and a half years at Ceres, Dawn has observed anomalous ammonium, vexatious volcanoes, wandering water ice, freaky flows, effervescent evaporites, capricious carbon, and many more peculiarities. As your guide to Dawn’s extraordinary observations of Ceres, I will take you on a journey to explore strange new landscapes and geology. To uncover new chemistry and new cerean paradigms. To boldly go where no spacecraft has gone before."

Oct. 12, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Man-To Hui,
UCLA / Earth, Planetary, and Space Sciences
Ariel Graykowski,
UCLA / Earth, Planetary, and Space Sciences
Dave Milewski,
UCLA / Earth, Planetary, and Space Sciences

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Student Presentations

Practice Talks for the upcoming DPS meeting. Presentations given by:
Ariel Graykowski: Fragmenting Comet 73P
Man-To Hui: Ultra-Distant Activity in Comet C/2017 K2 (PANSTARRS)
Dave Milewski: Continuous Monitoring of Active Asteroid P/2016 G1 (Pan-STARRS)
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Ariel Graykowski
73P/Schwassmann-Wachmann 3 is a Jupiter-family comet that has been observed to fragment on several occasions since 2005. Fragment C (73P-C) is believed to be the primary component of the nucleus. Knowing the nucleus rotation period is important because it can provide invaluable clues about the mechanism responsible for the breakup of this comet. Unfortunately, studies of 73P-C using a variety of techniques have reported rotational periods that range over an order of magnitude, from about 2.8 to 27.2 hours. The lower end of the reported range is compatible with rotational breakup of a low strength nucleus whereas the higher end would rule that possibility out completely. We have undertaken a systematic analysis of unpublished archival Hubble Space Telescope data from 2006 April in order to determine the rotation period and to assess other aspects of 73P-C. We find strong, cyclic photometric variations of about 0.4 magnitudes in the central light from this object. Similar variations with a smaller range are apparent in the surrounding dust coma, compatible with rotational modulation of the mass loss rate. I will discuss our measurements and inferences.
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Man-To Hui
C/2017 K2 (PANSTARRS; hereafter "K2") is an inbound Oort cloud comet exhibiting activity at least from 23.7 AU, a record heliocentric distance. We are studying the development of the activity using the Hubble Space Telescope and data scoured from the electronic archives. Our HST observations since 2017 June reveal a circularly symmetric dust coma ~10^5 km in radius, with a total effective cross-section ~10^5 {\textrm{km}^2}. The coma logarithmic surface brightness gradient is -1.01±0.01, consistent with the value expected for steady-state mass loss. The absence of a radiation-pressure caused tail suggests that the average ejected dust size is very large. Our Monte Carlo simulations indicate a mean dust diameter of ~1 mm, and an ejection speed of only a few m/s. We estimate the nucleus to be several kilometers in radius. Activity in K2, which we find is losing mass at ~10^2 kg/s, cannot be driven by the sublimation or crystallization of water ice. Instead, the sublimation of supervolatiles including CO and CO2 is suspected. Our numerical integrations show that the previous perihelion occurred >1 Myr ago, and therefore no heat from the prior orbit can be retained. Continuing observations will probe the development of activity as K2 approaches the Sun.
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Dave Milewski
We present a study of active asteroid P/2016 G1 (Pan-STARRS) (‘G1’) conducted between 2016 April and August. Our ground-based data were augmented by archival Hubble Space Telescope data as well as follow-up monitoring after G1 passed perihelion on UT 26 January 2017. The active asteroids are a subset of main belt asteroids that share the dynamical properties of asteroids but also the physical properties of comets such as the appearance of dust tails. By the use of the Jeanne Rich Centurion 28 0.7-m Telescope, the Keck-I 10-m telescope, and HST, data were obtained on 11 different epochs. We find that 1) G1’s apparent R-band magnitude faded from ∼17.8 to ∼19.6 over a period of three months and steeply dropped between UT 2016 June and UT 2016 August, 2) the absolute magnitude (assuming a phase function β = 0.02 magnitudes per degree) varied from 14.7 ± 0.09 on UT 17 April 2016 to 15.9 ± 0.02 in our last useful observation on UT 2016 August 03, 3) our photometry suggests that about 107 kg dust was ejected, and 4) the likely mechanism of activation remains impact, as suggested earlier by Moreno et al.

Oct. 19, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Vikram Dwarkadas
Univ. of Chicago/ Astronomy & Astrophysics

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Triggered Star Formation inside the Shell of a Wolf–Rayet Bubble as the Origin of the Solar System

A critical constraint on solar system formation is the high 26Al/27Al abundance ratio of 5 x 10-5 at the time of formation, which was about 17 times higher than the average Galactic ratio, while the 60Fe/56Fe value was about 2 x 10-8, lower than the Galactic value of 3 x 10-7. This challenges the assumption that a nearby supernova was responsible for the injection of these short-lived radionuclides into the early solar system. We show that this conundrum can be resolved if the Solar System was formed by triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. Aluminium-26 is produced during the evolution of the massive star, released in the wind during the W-R phase, and condenses into dust grains (that have been observed around W-R stars in IR observations). The dust grains survive passage through the reverse shock and the low density shocked wind, reach the dense shell swept-up by the bubble, detach from the decelerated wind and are injected into the shell. The dust grains will be destroyed by grain evaporation or non-thermal sputtering, releasing the 26Al into the shell. Some portions of this shell subsequently collapse due to triggering by shock and ionization fronts, as is frequently observed in wind blown bubbles. This will form the dense cores that give rise to solar-type systems. The W-R star will either collapse directly to a black hole, as in some models, or give rise to a supernova explosion. Even if the latter, the aspherical supernova does not inject appreciable amounts of 60Fe into the proto-solar-system, thus accounting for the observed low abundance of 60Fe. We discuss the details of various processes within the model using numerical simulations, as well as nucleosynthesis modelling, and analytic and semi-analytic calculations. We conclude that it is a viable model that can explain the initial abundances of 26Al and 60Fe, as well as other short-lived radionuclides. We estimate that 1%–16% of all Sun-like stars could have formed in such a setting of triggered star formation in the shell of a W–R bubble.

Oct. 26, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Mackenzie Day
UCLA / Earth, Planetary, and Space Sciences

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Aeolian sandstone preservation on Mars

Sedimentary rocks constitute a major fraction of the rock record on Mars, and recent research has highlighted deposits formed by aeolian dunes as being more common than previously thought. Modern Mars is dominated by active aeolian processes in the mid-latitude and equatorial regions, however preserving aeolian activity requires mechanisms for both sediment accumulation and cementation. On Earth these are achieved largely with subsiding basins and rising water tables, neither of which are present on Mars. Conventional wisdom therefore suggests that aeolian rocks should be rare on Mars. Nevertheless, examples of aeolian-generated sedimentary rocks are being identified with increasing frequency. On the ground, both Opportunity and Curiosity found aeolian sandstones near their respective landing sites, and from orbit many aeolian dune deposits have been found in high resolution images. In this talk we will explore the question of aeolian preservation on Mars and test whether significant aeolian deposits could be hiding in plain sight.

Nov. 2, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Dustin Schroeder
Stanford University/ Geophysics

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Beyond images: getting deeper information from terrestrial and planetary radar sounding

Radio echo sounding is a uniquely powerful geophysical technique for studying the interior of ice sheets, glaciers, and icy planetary bodies. It can provide broad coverage and deep penetration as well as interpretable ice thickness, basal topography, and englacial radio stratigraphy. However, despite the long tradition of glaciological interpretation of radar images, quantitative analyses of radar sounding data are rare and face several technical challenges. These include attenuation uncertainty from unknown ice temperature and chemistry, clutter and losses from surface and volume scattering, and a lack of problem-specific radar theory. However, there is rich, often underexploited, information in modern radar sounding data, which is being collected over terrestrial and planetary ice at an unprecedented rate. The development and application of hypothesis-driven analysis approaches for these data can place observational constraints on the morphologic, hydrologic, geologic, mechanical, thermal, and oceanographic configurations of ice sheets and glaciers. These boundary conditions – and the physical processes which they express and control – are filling a fundamental gap our ability to understand the evolution of both marine ice sheets and icy moons. These include the subglacial hydrology of marine ice sheets and the thermophysical structure of planetary ice shells.

Nov. 9, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Peter Jenniskens
SETI Institute and NASA Ames Research Center

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Earth impacting debris from comets and asteroids

The results from ongoing surveys are described that have mapped out our annual meteor showers. Data can now be accessed online in near-real time, facilitating the monitoring of unusual meteor shower activity. Comets dominate the influx at meteoroid sizes < 10 cm. Above 10 cm, meteors are mostly from debris of asteroid collisions. Those meteoroid streams manifest on Earth very differently. So far, 36 meteorite falls have been observed by cameras, providing a first glimpse into where in the asteroid belt the collisions occurred that are producing our meteorites. A recent fall over Botswana resulted in the recovery of material from asteroid 2018 LA. Since early 2017, a new instrument on the GOES-16 and GOES-17 weather satellites is providing position and light curve data for bolides that occur over the Americas.

Nov. 16, 2018

noon - 1 p.m.
Slichter 3853

Presented By: BJ Fulton
NASA Exoplanet Science Institute / IPAC / Caltech

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Exoplanet Demographics in the Era of Gaia

The Kepler and K2 missions blessed the community with a plethora of planet transit detections which enabled studies of exoplanet size demographics. Much of the recent progress in this field is driven by improved characterization of the stellar hosts. Our group recently used precise radius measurements from the California-Kepler Survey (CKS) to detect a gap in the distribution of planet radii. The paucity of planets with sizes between 1.5 and 2.0 Earth radii supports the emerging picture that close-in planets smaller than Neptune are composed of small, rocky cores enveloped by varying amounts of low-density gas that determine their total sizes. This result demonstrated the value of precise and homogeneous stellar parameters. Fortunately, Gaia DR2 has all but solved this problem for exoplanet host stars. With the combination of CKS and Gaia we updated the planet radius distribution again and found that 1) details of the distribution depend on the properties of the host stars and, 2) the region between 1.5-2.0 Earth radii is not completely devoid of planets. I will discuss implications of the radius distribution and the future prospects of exoplanets enabled by the exquisite stellar characterization from Gaia and next-generation extremely precise radial velocity instruments.

Nov. 30, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Ben Weiss
MIT/ Earth, Atmospheric, and Planetary Sciences

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History of the solar nebula and planet formation from paleomagnetic measurements of meteorites

History of the solar nebula and planet formation from paleomagnetic measurements of meteorites A key stage in the formation of planetary systems is the formation of a protoplanetary disk containing a gaseous nebula. Theoretical studies suggest that magnetic fields mediated the global evolution of protoplanetary disks by transporting angular momentum and driving disk accretion. However, the nature and history of nebular magnetic fields have been poorly constrained. Here I review recent advances in our understanding of the magnetism of the solar nebula as inferred from meteorites. I discuss the implications of these measurements for the mechanism and rate of planetary accretion, the formation the first solids, the dispersal time of the nebula, and the formation mechanisms of giant planets.

Dec. 7, 2018

noon - 1 p.m.
Slichter 3853

Presented By: Alexandra Schneider,
UCLA / Earth, Planetary, and Space Sciences
Akash Gupta,
UCLA / Earth, Planetary, and Space Sciences

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Student Presentations

Akash Gupta: Using the 'core-powered mass-loss' mechanism to understand the radius distribution of small, short-period exoplanets
NASA's Kepler mission has been a significant step in improving our understanding of planetary formation and evolution. One of its key findings is that the planets of size 1 - 4 Earth radii are the most abundant planets discovered to date. Interestingly, further observations have revealed a paucity in the occurrence of small, short-period planets around 1.5 - 2.0 Earth radii, a radius ‘valley’. Planets that fall below this radius valley are consistent with Earth-like compositions while planets above the valley have large enough radii that their cores must be engulfed in H/He atmospheres. High-energy stellar radiation is generally considered responsible for explaining these observations. However, a recent study has proposed an alternative mechanism, the ‘core-powered mass-loss’. In this talk, I will discuss the ‘core-powered mass-loss’ mechanism and will present the results from our work consisting of thermal evolution and mass-loss of small, short-period exoplanets. I will show that our results are in excellent agreement with the observations, and that they constrain the composition, mass-distribution and initial atmospheric masses of the observed exoplanet population.
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Alexandra Schneider: Exoplanetary Oxygen Fugacities From Externally-Polluted White Dwarfs
White dwarf stars polluted by accretion of rocky material from their remnant planetary systems offer unique opportunities to study the chemistry and structure of extrasolar terrestrial planets. We use HIRES observations targeting white dwarfs exhibiting an abundance of major rock-forming elements to characterize the intrinsic oxidation states of the accreted bodies. Oxidation state and core-mass fractions are key aspects of rocky, potentially Earth-like, worlds.