EPSS Colloquium - spring-2013
No magma ocean on Vesta (or elsewhere in the Asteroid Belt); volatile loss
April 2, 2013
noon - 1 p.m.
3853 Slichter
Presented By:
- John Wasson - UCLA
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There is moderately strong evidence that the Earth experienced one or more magma ocean episodes during which large-scale magmas were at or close to the surface. Because the rates of heat loss is very high, such episodes could only be powered by impacts and the periods during which magma was exposed were very brief. It has been popular to discuss magma oceans in the Asteroid Belt; in particular, several studies have inferred that the HED (howardite, eucrite, diogenite) meteorites formed in a magma ocean on Vesta. In fact, it appears to be impossible to create true magma oceans in the Asteroid Belt because impact heating liberates less heat and 26Al heats at much too low a rate. What the authors really are discussing is global magmas located beneath an insulating megaregolithic shell. All igneous models of asteroids use 26Al as the heat source even though the 26Al/27Al ratios in the chondrites in our museums are too low to produce appreciable melting. The general justification is that impact heating doesn't work so it must have been 26Al heating. In effect, 26Al is treated as a free parameter. Until recently this was justified because Hf/W model ages for iron meteorites implied that the irons are older than chondrites. And modelers are now producing large melt fractions in porous asteroids at relatively low impact velocities (~5 km s-1). The most cited paper justifying a magma ocean on the HED parent asteroid is by Greenwood et al. (2005) who showed that most HEDs formed a very tight cluster. The cluster does indeed imply a single, well mixed magma but the data only require a relatively small magma. In fact, the D17O and e54Cr data favor a model in which the HEDs form on the same asteroid as the IIIAB magmatic irons (and thus not on Vesta). Richter and Drake (1997) suggested that a magma ocean was the best way to explain the low alkali contents in eucrites. However, internal heating by 26Al is not expected to result in much volatile loss because there is no carrier phase at volatilization temperatures . Impact heat is probably much more efficient at evaporating alkalis. The isotopic links between the HEDs and IIIAB irons seem stronger than the spectroscopic links between HEDs and the surface of Vesta. These groups probably formed in one or more regional magmas produced by minor impacts on a porous asteroid.
Towards modeling the contribution of polar ice sheets to sea level rise
April 4, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Eric Larour - JPL
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Polar ice sheets are progressively becoming one of the most critical components of the Earth System. Indeed, in a warming climate, the Antarctic and Greenland ice sheets are contributing larger amounts of fresh water to the oceans, which results in increasing sea level rise. Here, we look at recent developments within the Ice Sheet System Model (ISSM) that have resulted in improved projections of sea level rise. Our focus will be on understanding what uncertainties remain in such projections, and how to improve them using a combination of modeling and observations.
Hot anoxic oceans and the evolutionary age of deep-sea chemosynthetic taxa
April 11, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Bob Vrijenhoek - Monterey Bay Aquarium
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Deep-sea chemosynthetic communities may be particularly vulnerable to large climatic changes that affect ocean temperatures and circulation patterns. Chemosynthetic animals occupy narrow redox zones, mostly at hydrothermal vents, hydrocarbon seeps, or sites of organic deposition where subsurface fluids laden with reduced gases (e.g., sulfides, methane, hydrogen) meet oxygenated seawater. Dependence on chemolithoautotrophic bacteria as primary producers renders these communities susceptible to climatic changes that alter the breadth of the oxic/anoxic interface. The fossil record reveals major transitions of chemosynthetic faunas during the middle to late Mesozoic, failing to support prior hypotheses that these environments harbor an extraordinary number of ancient relics and living fossils. The molecular phylogenetic analyses summarized herein support Cenozoic (<65 Myr old) radiations for most of the dominant invertebrate taxa now occupying these habitats. Although stem ancestors for many of the mollusks, annelids and crustaceans found at vents and seeps survived the Cretaceous/Tertiary (K/T) extinction event, their contemporary crown taxa radiated mostly after the Paleocene/Eocene thermal maximum (PETM), which led to a widespread anoxic/dysoxic event in the world’s deep-ocean basins. Perhaps these findings provide a window for viewing the future of our oceans on a warming planet.
Understanding and Predicting the Dynamic Sun and Heliosphere
April 18, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Ofer Cohen - Harvard-Smithsonian Center for Astrophysics
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Despite almost a century of modern solar observations and over 50 years of space exploration, some of the fundamental problems in solar physics are not fully understood yet. The solar corona heating, the solar wind acceleration, the initiation and propagation of Coronal Mass Ejections (CMEs), and the solar cycle do not have a complete theoretical framework to describe and predict these phenomena. The lack of a complete theory, in addition to the large range of both spatial and temporal scales involved, make it challenging to develop numerical models for solar and space physics. Despite of the challenges, a great progress has been made in the last decade to develop numerical models for the solar corona, the solar wind , and CMEs. In my talk, I will describe a state of the art model for the solar corona and the solar wind, and I will demonstrate how the model can be used to improve our understanding on solar corona phenomena and observations.
Solar Activity, Coronal Heating, and the Acceleration of the Solar Wind
April 25, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Marco Velli - California Institute of Technology, Jet Propulsion Laboratory, Pasadena, United States; Dipartimento di Fisica e Astronomia, Universita' degli Studi, Firenze, Italy
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The magnetic field is fundamental to solar activity and shapes the interplanetary environment, as clearly shown by the full three dimensional monitoring of the heliosphere provided by the measurements of the Helios, Ulysses, SOHO, ACE, Wind, STEREO, SDO and Voyager spacecraft. Magnetic fields are also the source for coronal heating and the very existence of the solar wind; produced by the sun's dynamo and emerging into the corona, magnetic fields become a conduit for waves, act to store energy and then propel plasma into the heliosphere in the form of Coronal Mass Ejections (CMEs). Transformation of magnetic energy is also the source solar energetic particle events. The way in which solar convective energy couples to magnetic fields to produce the multifaceted heliosphere is at the heart of Solar Probe Plus (SPP) exploration. In this talk I will review the role played by the magnetic field in solar and heliospheric variability and solar atmospheric dynamics, including the coronal heating and solar wind acceleration problems. I will then highlight the exciting perspectives for discovery provided by SPP and other future missions to the inner heliosphere. Tests of current theoretical models of coronal heating and wind acceleration will be described and focus areas for further numerical and theoretical efforts illustrated.
Jupiter and Saturn structure models with few layers
May 2, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Nadine Nettelmann - Department of Astronomy and Astrophysics, UC Santa Cruz
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Our understanding of Jupiter and Saturn goes hand in hand with the availability and accuracy of observational constraints as well as of the equations of state for their main constituents, hydrogen and helium. We show how the current uncertainties in the core mass of Jupiter (0-10 ME) and Saturn (0-20 ME) could be reduced with the help of measured atmospheric oxygen abundances, which is in reach for Jupiter thanks to the Juno mission, and possibly also for Saturn. Given that both planets' atmospheres are observed to be depleted in helium, we finally discuss the occurrence of H/He demixing with respect to recent ab initio data based H/He phase diagrams and argue that the assumption a layered structure with few layers remains a reasonable approximation. However, we caution that the derived total heavy element fraction is highly subject to our assumption of an adiabatic interior, which may not hold in case of He demixing.
Recent Observations of Mass Independent Compositions in Atmospheric Species
May 9, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Mark Thiemens - Scripps
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The first observation of a chemically produced mass independent process suggested application to the early solar system, which remains relevant to date. In the subsequent development of a quantum mechanical understanding of the isotopic process, numerous applications in the terrestrial atmosphere arose. At present, with the exception of water, every oxygen bearing species in the Earths atmosphere is mass independent and does not lie on the so called terrestrial fractionation line, with species both above and below the fractionation line defined by silicate rocks. In each case, new information on source strengths and transport and chemical transformation is provided that would otherwise have been unrecognized. One of the least understood components of global climate is the role of aerosol particles. The application of mass independent isotope measurements has lead to new insights into this issue. Specific aspects of the role of particles with ozone has been particularly important as the role of chemically transformation is captured. What is more important is that the record is stable and the change over time is recorded and ice core samples provide the only paleo ozone record available. Finally, measurements of Martian meteorites secondary minerals may be better interpreted from combined laboratory, atmospheric, and ice core and geologic record. Recent measurements will be discussed
ACTIVE ICE: The Importance of Ice Shelves and Streams to Ice-sheet Dynamics and Sea Level
May 16, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Todd Dupont - UC Irvine
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The potential for ice-sheet volume change to lead to significant sea-level change is of considerable scientific and societal interest. Understanding the behavior of fast-flowing outlets, a.k.a. ice streams, which drain the interior of ice sheets, is critical to the prognosis of ice volume and sea level. Of particular importance is the behavior of the floating extensions of ice streams, called ice shelves, because of their ability to buffer, or buttress, the outflow from the grounded ice. In this talk I will discuss efforts to model the dynamics of ice-stream/ice-shelf systems, focusing on two areas: i) grounding-line migration, and ii) ice-berg calving. These processes drive changes in ice-shelf volume, and ultimately affect ice volume variations.
Enceladus’ jets and Titan’s organics: latest news from Cassini
May 23, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Christophe Sotin - JPL
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The active cryovolcanism on Enceladus and the formation of heavy organic molecules in Titan’s atmosphere are two major unrelated discoveries of the Cassini mission. These two end-member icy moons of Saturn reveal important processes for our understanding of the evolution of icy moons and their habitability potential. New observations by the Visual and Infrared mapping spectrometer (VIMS) onboard the Cassini spacecraft provide additional constraints on the geological processes responsible for the formation of Enceladus’ jets. The very strong correlation between the brightness of the overall plume and the true anomaly suggests a very strong control by tidal forces as expected by previous models. On Titan, the solar occultation observations provide constraints on the composition of the organic haze by comparing their spectral properties with those of heavy organic molecules synthetized in laboratory experiments simulating Titan’s conditions. These observations also give density profiles from which one can derive the flux of organic particle falling on Titan’s surface. These observations are included in a global model describing the carbon cycle on Titan and the relationships between the different reservoirs including atmospheric methane, organic haze, lakes, seas, dune fields, subsurface clathrate hydrates and a potential deep reservoir.
New Ideas about the Origin of the Earth and Moon
May 30, 2013
4 p.m. - 5 p.m.
Geology 3656
Presented By:
- Sarah Stewart - Harvard University
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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.