Category: Seminars

Most planets in the solar system lose their internal heat through convection beneath a stagnant lid. However on Io, tidal heating is so intense that its mantle is partially molten. This magma migrates through Io’s mantle and erupts onto its surface. This is thought to be the main mechanism through which heat is removed from Io’s interior. Previous studies have only considered either solid-state mantle convection or magma migration, but magma generation and migration is not independent from mantle convection. Thus understanding the structure of Io’s mantle and how it loses its internal heat requires considering both mantle convection and magma migration. We use the mantle convection code StagYY, which includes the generation, segregation, and eruption of magma, to conduct two-dimensional numerical simulations of mantle convection in Io’s mantle. This allows us to constrain the distribution of melting in Io’s mantle and test the hypothesis that heat loss through volcanic eruptions dominates over heat loss through stagnant lid convection.

The New Horizons mission to the outer solar system has returned beautiful and intriguing images of Pluto and Charon which raise many new questions about the outer solar system and the formation of icy worlds. I will present background information about New Horizons and our current understanding of the Pluto-Charon system, followed by the new images and data returned by the spacecraft. The images and data will be presented so as to encourage discussion amongst the audience.

 

The rich dynamics of the Saturn ring and moon systems offer unique opportunities to study the evolution of the planet and its surrounding bodies. For instance, seismology of Saturn is made possible by the gravitational interaction between Saturn and its rings, in which density waves in the rings are excited at Lindblad resonances with Saturn’s oscillation modes. The seismic signatures in the rings suggest the existence of stable stratification in the deep interior of the planet, likely created by composition gradients between the core and envelope due to helium sedimentation and/or core erosion. These structures within the planet influence the tidal interactions which drive the outward migration of Saturn’s inner satellites. Rapid migration can occur when moons become locked in resonance with Saturn’s oscillation modes, driving the moons outward on a planetary evolution timescale.

I will present results of new calculations of the asteroidal impact flux on Mars.  Mars’ orbit is significantly eccentric and the planet orbits near the inner edge of the asteroid belt where the space density of asteroids has a large radial gradient.  The correlated secular dynamics of Mars and the asteroids plays a significant role in modulating the impact flux on this planet.  At the present epoch, this leads to a large variation — of about a factor of three — in the impact flux when Mars is near aphelion versus perihelion; significantly, the integrated annual impact flux is lower than would be expected in the absence of correlated secular dynamics.

The second part of the talk will describe some deductions about the planet mass function from the observational data of exoplanets and theoretical considerations of planetary dynamics.  I will describe analysis of the observational data from the Kepler space mission which indicates that planetary orbital separations have an approximately log-normal distribution.  Adopting some plausible ansatzs for the dynamical stability of N-planet systems to relate orbital separations to planet masses, it appears that the planet mass function is peaked in logarithm of mass, with the most probable value of log m/M⊕ ∼ (0.6 − 1.0); a modest extrapolation indicates that Earth mass planets are about ~1000 times more common than Jupiter mass planets.

Acclaimed Norwegian solar physicist Pål Brekke will lead a discussion on the fascinating phenomena of the Aurora Borealis centered on his new prize-winning documentary, The Northern Lights: A Magic Experience (duration 25 minutes).  After the screening, Brekke will talk about his experience as a longtime observer of the northern lights and his work with the documentary. In addition to Brekke’s work as a senior adviser at the Norwegian Space Center, he also worked for the European Space Agency as the deputy scientist for the Solar and Heliospheric Observatory at NASA Goddard Space Flight Center.

 

Theoretical models of tidal dissipation in Io’s interior have provided support for a global subsurface melt layer. The extremely high temperature of the lava erupting on Io’s surface also hints at an extremely hot interior consistent with an internal magma ocean. However, the only direct evidence of a subsurface magma ocean in Io is provided by the electromagnetic induction response observed by Galileo (Khurana et al. 2011, Science, 332, 1186).

Using Jupiter’s rotating magnetic field as a sounding signal, Khurana et al. (2011) analyzed the response of Io observed during four different flybys of Io by the spacecraft Galileo, and showed that the magnetic field response is global, variable and in sync with the time varying field of Jupiter. Modeling of this signature shows that the induction response from a completely solid mantle model is inadequate to explain the magnetometer observations. However, a layer of asthenosphere > 50 km in thickness with a rock melt fraction ≥ 20% is adequate to accurately model the observed magnetic field.

In this presentation, after summarizing our current knowledge of Io’s interior from Galileo’s induction measurements, I will outline a scheme to further infer properties of Io’s interior, especially its internal temperature profile, by marrying the principles of thermodynamics with those of electromagnetism. In particular, we would obtain guidance on stable mineral phases and their physical properties (such as density, melt state and electrical conductivity) from thermodynamic principles whereas how the resulting internal conductivity profile affects the magnetic environment around Io from electromagnetic theory. I will also explore how induction measurements could be obtained at multiple frequencies from a future mission and be used to constrain both the location and the thickness of the magma ocean.

Finally, I will explore the consequences of the global magma ocean of Io on its physical properties such as the current sites of tidal energy dissipation, the absence of an internal magnetic field and a lack of plate tectonics on its surface.

We are undertaking a detailed structural analysis of a targeted portion of an Aphrodite fracture zone in order to understand the architectural evolution through time and space and, ultimately, to construct thermal models in order to gain insight into possible mechanisms of heat transfer on Venus. The target area (15S-20S/110E-124E), characterized by extreme density of faults and pit chains, encompasses over 700,000 km². It is part of an extensive fracture zone that overlaps with focused coronae chains to the east,and splits into regional splays to the west,cutting highland crustal plateaus. Hybrid tectono-volcanic structures,change along strike from en echelon fractures, fractures, pit-­‐chains, graben, leaky dikes, and canali. Widths range from 1 to >5 km; lengths exceed several 100 km; structure spacing ranges from 10’s of km to lineament overlapping, intersecting, or coalescing. Hybrid structures, which play a key role in transferring material to/from depth, both predate and postdate surface deposits.The fracture zone domain is the youngest regional domain in Aphrodite Terra, and extends ~2000 km in width and over 6000 km in length. The hybrid structures likely play a significant role in cooling reflecting contemporary mechanisms/processes.

 

Co-authors: David Tovar¹, J.B. Swenson¹, and I. López², ¹ University of Minnesota Duluth, ² Universidad Rey Juan Carlos.

Fissures near the south pole of Saturn’s icy moon Enceladus are observed to be erupting jets of water, which illustrates that the small moon is presently geologically active. Farther to the north, ridges, fractures, and relaxed crater topography preserve evidence for a surprisingly complex history of recent tectonic and thermal activity on the small moon. Here we demonstrate that the regions near the south pole, and along the leading and trailing hemispheres are each morphologically distinct, suggesting unique tectonic deformation events for each area. Previous researchers have demonstrated that some terrains appear to have experienced more ductile deformation, pointing to a significant amount of heat flux generated in the interior of the moon. We show that some ridged terrains, particularly on the leading and trailing hemispheres, preserve a history of apparent brittle deformation that accommodated significant contraction. This analysis of the ridge terrains seeks to constrain the recent and long-term tectonic history of Enceladus.

The light curves of the small Kreutz sungrazing comets in different filters in the coronagraphs aboard the SOHO and STEREO satellites are distinctive and different.  Most famous is the brightness excess in the SOHO C2 and C3 orange filters, which Biesecker et al. (2002) and Knight et al. (2010) assign to the prominent yellow-orange sodium D line doublet emission at 5889 Å and 5895 Å.  I have reviewed and synthesized comet sodium emission data in the literature spanning a wide range of heliocentric distances, r.  Based on this synthesis, the temporary steep r^−8 brightness “kink” over r = 0.16 AU to 0.12 AU – superimposed upon the otherwise generally standard r^−4 brightening trend – can be explained by rapidly increasing atomic sodium resonant fluorescence emission relative to the dust continuum brightness.  At r ˂ ~0.05 AU, heat from the coma dust grains glowing at T > 1200º K should bleed into IR, red, and clear filter channels, resulting in particularly severe contamination if the albedo of coma dust is extremely low, as suggested by stellar occultation data (e.g., Lacerda & Jewitt 2012).  I present a model to extract the scattered sunlight signal from sodium and heat emission in SOHO and STEREO photometry.

The giant impact hypothesis suggests that the Moon formed out of a partially vaporized disk created by a collision between an impactor and the proto-Earth. Three major models exist for this hypothesis: (a) standard model: a Mars-sized impactor hit Earth, (b) fast-spinning Earth model: a small impactor hit the rapidly-rotating Earth, (c) sub-Earths model: two half Earth-sized objects collided. Some of these models can explain several observed features, including the nearly identical isotopic ratios between Earth and Moon. However, it has not been clear if these models can explain other geochemical constraints, such as (1) the giant impact did not mix the Earth’s mantle, (2) Moon did not lose significant amount of water (hydrogen) during its formation. In this talk, I show results from giant impact simulations and investigate whether the suggested models are consistent with these geochemical constraints. I show that the standard model is more consistent with the survival of the mantle heterogeneity than the other models. I also find that water loss from the Moon-forming disk is minor in all models. Therefore, the giant impact hypothesis is consistent with the measured lunar water abundance. Finally, I will discuss implications of our model for the formation of exomoons. Our numerical simulations indicate that whether a planet can form an impact-induced moon depends on the planetary mass and composition.