P51C-1419
The Isotope Effect to the IR Emissions of CO and CO2 produced by 15 keV H3+ irradiation of H2O (D2O)/ CO, CO2, CH4 or CH3OH icy mixtures.
Simulation of ices covered astronomical surfaces irradiated by 15 keV H3+. We have measured the particle irradiation of interstellar icy materials such as H2O mixed with CH4, CO, CO2, and CH3OH resulting the synthesis of CO and CO2. The technique used to identify and quantify the synthesized species has been infrared spectroscopy. Irradiation of pure water ice produces OH and no CO and CO2. Irradiation of pure CH4 causes the formation of CH4 and C2H6, but no CO and CO21. On the other hand, irradiation of pure CH3OH ice produces CO and CO22. Tow IR emissions of CO2 (2339 cm-1) and CO (2136 cm-1) produced by ion irradiation of all icy mixtures are compared with H2O abundance. The experimental results are discussed with a view to the relevance they could have in planetary environments such as icy moons in the external solar system.
P51C-1420
Modeled Neutral Densities of Comet 1P/Halley: a Comparison With Giotto's Neutral Mass Spectrometer
The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. To better our understanding of this environment, we compare the results from a kinetic model of the neutral cometary environment with data from the Neutral Mass Spectrometer (NMS) collected during Giotto's fly-by at comet 1P/Halley in 1986. The model solves the Boltzmann equation with a Direct Simulation Monte Carlo (DSMC) technique [Tenishev et al. (in press, Astrophys. J.)] by tracking trajectories of gas molecules and icy grains under the influence of the comet's weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus and sublimating icy grains are considered to be the source of parent species (H2O, CO, H2CO and CO2) in the coma. The densities of parent species and the corresponding dissociation products (H, H2, C, O, OH, CH, CH2) are compared to the measurements acquired by NMS along the Giotto trajectory from 40~000~km to roughly 1000~km at closest approach.
P51C-1421
Assessing the Role of Seismic Shaking on Crater Modification for 433 Eros
The surface of Eros is known to have regions of reduced small crater (0.2 - 1.0 km diameter) density that correlate well with radial distance from Shoemaker crater but not with its presumed ejecta pattern. It has been suggested that these deficits result from seismic shaking produced by the Shoemaker impact. Here we further address the role of seismic shaking on Eros by assessing its morphological effects on the largest (> 0.6 km) craters. Due to their large size, many of these craters likely predate the formation of Shoemaker. Although they are too big to be erased by seismic shaking, their dimensions (primarily their depths) should have been modified. We measure the crater depth (d) and diameter (D) using co-registered high-resolution images (MSI) and lidar data (NLR) as well as a stereophotoclinometry (SPC) shape model. The measured d/D decreases as a function of decreasing distance from Shoemaker, a trend consistent with expectations for modification by seismic shaking. Additional morphological trends as functions of proxim-ity, ejecta distribution, local slope effects, crater age, and underlying structural influences will be discussed.
P51C-1422
Effect of the Seismic Shaking Erasure (revisited model) and Ejecta Coverage Erasure on the Crater Population of Asteroid 433 Eros
To explain the crater population of asteroid Eros, scenarios including impact-induced seismic shaking and ejecta coverage are proposed. The present study upgrades the erasure models considering the seismic shaking and the ejecta coverage processes. The ejecta coverage model, based on ballistic trajectory of ejecta blocks around an ellipsoidal model of asteroid Eros suggests a non negligible effect on the crater population of asteroid Eros. The seismic shaking model is improved by performing wave propagation simulation with the spectral-element method (Komatitsch and Tromp, 1999) applied to complex models of asteroid Eros, and a model of downslope movement has been developed. These simulations allow to compute the life time of a crater, corresponding to the time it will remain on the surface of Eros without being erased. These results are applied to different series of projectiles impacting Eros during different exposure times. This study highlights the importance of the regolith layer for trapping and amplifying seismic waves, like a wave guide. Also, we find a good agreement between the observed and the modeled crater population (including our erasure models) for an exposure time of 600 Myr. Finally, these results suggest an important contribution of the ejecta coverage process, as well as the seismic shaking erasure mechanism.
P51C-1423
Surface and Near Surface Dynamics on Phobos
Phobos as a few small satellites in the solar system is orbiting around its primary inside the Roche limit. Therefore the surface material is loosely bounded and easily ejected by impactors. Whereas dynamics in the close vicinity of Phobos has been studied for both geophysical and navigation reasons, the dynamics on the surface itself has not been studied to the same extent. The gravitational field used here is the ellipsoidal model of Davis, 1981, that describes as well the past and future Phobos as it gets closer to Mars. We look at the trajectory of a test mass for any initial position and velocity. It can exhibit an unusual shape: for some initial positions a gliding test mass released with zero velocity can take off over some distance! Generally the trajectories are not 'down hill' as the motion is strongly dependent on the velocity. We discuss the consequences for material transport on or close to the surface, with in particular the possibility that some of the Phobos groves could have been dug out by rolling blocks.
P51C-1424
Is Ceres Differentiated?
The dwarf planet Ceres is the largest body in the asteroid belt, and has a density of 2.0-2.3 g/cm3, and a dark non-icy surface with signs of hydrated minerals [1-3]. As opposed to a differentiated internal structure with a nonporous rocky core and a water ice mantle [1-3], there are arguments for a slightly differentiated or undifferentiated porous interior with a mineral composition similar to CI/CM carbonaceous chondrites. Ceres' shape and dimensions reported in [3] may imply a slightly differentiated or even undifferentiated interior. Ceres' internal pressures (< ~150 MPa) are insufficient to significantly reduce porosity and microporosity of accreted chondritic materials. Thus, there is no need for abundant ice to be present to account for the density of Ceres. An existence of a rocky layer atop a water mantle is unfavorable owing to gravitational instability [1]. Observed hydrated surface materials are not consistent with the unaltered nature of the rocky layer modeled in [1]. If such a layer sunk into the water mantle, subsequent sublimation of an icy shell would have led to abundant surface salt deposits, which are not observed. Later accumulation of a layer of hydrated minerals at the surface may not be consistent with the ice-bearing surfaces of such bodies as Callisto, and with cosmic dust accumulation rates on inner solar system bodies. Therefore, the surface material could represent hydrated planetesimals from which Ceres accreted, and/or reflects in situ aqueous alteration, which is less likely. Ceres could have formed from pervasively aqueously processed low-density asteroids (largely C- and G-types) in which 26Al had largely decayed. Abundant water ice may not have been accreted. The weak heat from 40K and 232Th decay would not have caused mineral dehydration and density stratification of the interior [1]. However, evaporation of limited aqueous solutions and warm ice sublimation caused water redistribution in the porous interior, some re-condensation and escape to space. Corresponding formation of light-toned salts may account for Ceres' elevated albedo compared to carbonaceous chondrites [cf. 1]. It is also possible that Ceres and spectrally similar asteroids are rich in low-density C-H-O-N species compared to CI/CM chondrites, which are depleted in them compared to comets. Refs.: [1] McCord T.B., Sotin C. (2005) J. Geophys. Res. 110, E05009. [2] Thomas P.C. et al. (2005) Nature 437, 224-226. [3] Carry B. et al. (2008) Astron. Astrophys. 478, 235-244.
P51C-1425
Tectonics and Interior Structure of Pluto: Predictions from the Orbital Evolution of the Pluto-Charon System
The Pluto-Charon binary dwarf planet system is probably the result of an impact of similar-sized Kuiper Belt objects. The pair evolved from an unknown initial state to the dynamical end state in which we find them now, synchronously locked to each other. In this presentation, we investigate the implications of the evolution to this end state for the tidally-induced surface stresses on Pluto and Charon, and the interior state of Pluto. We predict that Pluto's tectonics are dominated by despinning stresses, and that Pluto had an internal ocean during the time that it was tectonically active. We assume that Charon rapidly despins post-impact to achieve synchronous rotation. The remaining orbital evolution is accomplished by transferring Pluto's spin angular momentum to Charon's orbit. The timescale over which the orbital evolution takes place is critically dependent on the tidal quality factor, Q, of Pluto. Previous investigators have assumed a constant Q = 100 for Pluto, but Q depends strongly on the internal structure. We have estimated the value of Q for layered viscoelastic Pluto interior structures at different tidal frequencies. The simplest (and perhaps least realistic) model is a homogeneous Pluto with uniform viscosity. In this model, the Pluto-Charon system will evolve to its current state within solar system history as long as the viscosity is less than 1019 Pa. A cold, stiff lid on Pluto's surface will lengthen the timescale of orbital evolution, and we are currently quantifying this effect. The most rapid orbital evolution occurs for an interior structure that is fully differentiated and contains a liquid water ocean decoupling the cold ice at the surface from a warm interior. If Pluto's interior ice begins relatively warm, our calculations show that tidal dissipation will add energy faster than convection can remove it, and internal melting is likely. Pluto may have had one of two fates after the Charon-forming impact: a cold interior which would not evolve to the current orbital state within solar system history, or a warmer interior which experiences a thermal runaway to produce an internal ocean. The orbital evolution of Pluto-Charon can produce hundreds of MPa of despinning stresses in Pluto's outer ice shell, and this is the largest source of surface stress. As long as the viscosity of Pluto's outer ice shell is 107 higher than the interior viscosity, these stresses will build up faster than they can viscously relax, and create fracturing on the surface that may be observed by New Horizons when it arrives at Pluto.
P51C-1426
SHOTPUT: A mission proposal to study composition and origins of small bodies in the outer solar system through fly-by and impactor science
An exploration of small bodies in the outer solar system can provide information on the origins, evolution, and composition of the solar system, including information on materials and processes that supported the origins of life. As part of NASA's Jet Propulsion Laboratory Planetary Science Summer School, we present a mission designed for the first ever in-situ measurements of the Trojan and Centaur asteroids. The mission has two major scientific goals. The first, to characterize the chemical and physical composition of Trojan and Centaur bodies, will be accomplished by measuring fundamental properties of both bodies as well as characterizing their organic components. The second goal is to characterize the origins and possible evolution of these bodies. In pursuit of this second goal we seek to determine where in the solar system these bodies originated, whether they have undergone dynamical migration, and what evolutionary processes they have experienced. The proposed mission incorporates fly-by imaging and imaging spectroscopy, measurements of the chemical composition of dust, and a set of impactors intended to expose subsurface material for analysis. The proposed seven year mission includes a flyby of main belt asteroid (108144) 2001 HM1, a flyby and impactor release at the Trojan (624) Hector (a suspected contact binary) with companion P/2006, and a flyby with impactor release at the Centaur 39P/Oterma. Along with science goals and targets, we will present a full mission design including instrument package, launch dates and mission timeline, encounter strategy, spacecraft design, and cost estimates.