Geophysics and Tectonics Seminar - winter-2015

Jan. 16, 2015
noon - 1 p.m.
3814 Geology

Presented By:

• Hilke Schlichting - MIT

Terrestrial planet formation is generally thought to have proceeded in two main stages: The first consists of the accretion of planetesimals, which leads to the formation of several dozens of roughly Mars-sized planetary embryos, and the second stage consists of a series of giant impacts between these embryos that merge to form the Earth and other terrestrial planets. Understanding how much of the planets' primordial atmosphere is retained during the giant impact phase is crucial for understanding the origin and evolution of planetary atmospheres. In addition, a planet's or protoplanet's atmosphere cannot only be lost in a giant impact, but also due to much smaller impacts by planetesimals. Therefore, in order to understand the origin and evolution of the terrestrial planets' atmospheres I will examine the contributions to atmospheric loss from both giant impacts and from planetesimal accretion and show that planetesimal impacts are likely to dominate the atmospheric mass loss during planet formation. I will discuss the implications of these findings for the formation of the terrestrial planets and their volatile budgets.

Jan. 23, 2015
noon - 1 p.m.
Geology 314

Presented By:

• Simone Marchi - U. of Colorado

n 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.

Jan. 30, 2015
noon - 1 p.m.
Geology 3814

Presented By:

• Smadar Naoz - UCLA

The search for extra-solar planets has led to the surprising discovery of many Jupiter-like planets in very close proximity to their host star, the so-called “hot Jupiters”. Even more surprising, many of these hot Jupiters have orbits that are eccentric or highly inclined with respect to the equator of the star, and some (about 25%) even orbiting counter to the spin direction of the star. This poses a unique challenge to all planet formation models. We show that secular interactions between Jupiter-like planet and another perturber in the system can easily produce retrograde HJ orbits. We show that in the frame of work of secular hierarchical triple system (the so-called Kozai mechanism) the inner orbit’s angular momentum component parallel to the total angular momentum (i.e., the z-component of the inner orbit angular momentum) need not be constant. In fact, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit allows planet- star tidal interactions to rapidly circularize that orbit, decoupling the planets and forming a retrograde hot Jupiter. We estimate the relative frequencies of retrograde orbits and counter to the stellar spin orbits using Monte Carlo simulations, and find that the they are consistent with the observations. The high observed incidence of planets orbiting counter to the stellar spin direction may suggest that three body secular interactions are an important part of their dynamical history. Interestingly, this mechanism is applicable to many other astrophysical settings.

Feb. 6, 2015
noon - 1 p.m.
Geology 3814

Presented By:

• Lauri Barge - JPL/Caltech

One of the ubiquitous properties of life on Earth, and perhaps of any life elsewhere, is that it utilizes geochemical and electrochemical disequilibria and converts free energy. Planets with a water-rock interface generate disequilibria through serpentinization reactions in the form of redox, pH, and thermal gradients at hydrothermal vents. Warm, alkaline serpentinite-hosted vents continually produce fuels which can be utilized as a source of electrons for redox metabolism, and in prebiotic systems, could have formed hydrothermal precipitates (e.g., metal sulfides or oxyhydroxides) capable of catalyzing proto-metabolic reactions and concentrating biologically significant materials. Mineral precipitation chimneys associated with vents can function as chemical flow-through reactors and geochemical fuel cells, driving reactions of carbon / nitrogen / phosphorus compounds and facilitating electron transfer to/from minerals. Some key questions for astrobiology center on understanding the geochemical energy gradients produced on wet rocky planets, including icy worlds such as Europa or Enceladus, and how these abiotic redox and pH gradients may have participated in prebiotic chemistry and the emergence of bioenergetics.

Feb. 13, 2015
noon - 1 p.m.
Geology 3814

Presented By:

• Miki Nakajima - Caltech

Seminar Description coming soon.

Feb. 13, 2015
noon - 1 p.m.
Geology 3814

Presented By:

• Aomawa Shields - UCLA

Seminar Description coming soon.

Feb. 20, 2015
noon - 1 p.m.
Geology 3814

Presented By:

• Aomawa Shields - UCLA/Harvard

The interaction between a host star and its orbiting planet can generate both radiative and gravitational effects on planetary climate. While these effects on climate are largely constrained and understood for the Earth, exoplanets fill a much more diverse range of planetary and stellar properties. To identify habitable exoplanets, it is important to understand how the climatic effects of the processes that influence the Earth’s climate might change for different host stars and planetary and orbital characteristics. I will share results from work performed using a hierarchy of models to simulate planets covered by ocean, land and water ice, with incident radiation from stars of different spectral types. Results indicate that ice extent is much greater on a planet orbiting a hotter, brighter star than on a planet orbiting a cooler, redder (M-dwarf) star at an equivalent flux distance, assuming an Earth-like atmospheric concentration of carbon dioxide. I’ll explain the reasons behind this apparent warmer planetary haven around cooler stars, address how the picture changes at the outer edge of the stellar habitable zone, and discuss the implications of our results for planetary climate and habitability. I will also explore a specific case —that of Kepler-62f, a potentially habitable planet in a five-planet system orbiting a K-dwarf star—and discuss its prospects for habitability as a function of atmospheric composition and orbital configuration, based on work performed with both N-body and global climate models. The methods presented can be used to assess the possible climates of potentially habitable planets as they are discovered.

March 6, 2015
noon - 1 p.m.
Geology 3814

Presented By:

• Moogega Cooper - JPL

Throughout the history of space exploration, we’ve been improving our understanding of life and habitability on other planets and solar bodies. It is our responsibility to preserve and protect the environments under investigation as well as our own should a sample be returned. Thus, a set of policies and guidelines have been set in place to ensure that we are good custodians of the solar system and beyond. This concept of Planetary Protection Planetary Protection will be discussed in the context of past missions and its importance will be highlighted in light of the upcoming Mars Sample Return mission.