First Principles Based Reactive Atomistic Simulations to Understand the Effects of Molecular Hypervelocity Impact on Cassini’s Ion and Neutral Mass

Introduction: NASA's Cassini Cassini's Ion and Neutral Mass Spectrometer (INMS) has returned a wealth of data on the composition of Titan's upper atmosphere and the plumes of Enceladus. However the encounter velocity (up to 17.73 km/sec) of the spacecraft with the plume or atmosphere has profound effects on the fragmentation of the observed molecules. Measured compositional changes of H 2 O, H 2 , CO 2 and CO appear to be correlated with velocity (see Figure 1). To understand this phenomenon , we have applied first principles based atomistic methods to simulate the physics of hy-pervelocity impacts of organic and inorganic molecules on metal surfaces. These methods include non-adiabatic and adiabatic reactive molecular dynamics calculations designed to elucidate the relationship between impact energy, fragmentation efficiency and fragmentation pathways. We report here on the predicted impact of species such as ice-water, CO 2 , CH 4 , and NH 3 , on oxidized titanium, as well as HC species on diamond surfaces. These simulations provide the dynamics of product distributions during and after a hypervelocity impact event, ionization fractions, and dissociation probabilities for the various species of interest as a function of impact velocity (energy). We are using these results to determine the relevance of the fragmentation process to Cassini INMS results, and to quantify its effects on the observed spectra. The Ion and Neutral Mass Spectrometer (INMS): The INMS is a quadrupole mass spectrometer equipped with two separate ion sources, the closed source and the open source. Data on the composition of the Enceladus plumes and the upper atmosphere of Titan has primarily been obtained with the closed source, whose operation we will briefly describe. The closed source is made up of a spherical titanium antechamber which is connected to a hot filament electron impact ionizer by means of a transfer tube[1]. The ionizer fragments and ionizes the incident molecules before they are focused in the quadrupole for mass analysis. The fragmentation patterns are an important means of identifying the parent molecules, and an accurate knowledge of these is necessary to deconvolute the overlapping peaks that result from the mixture of gases entering the mass spectrometer. Gas enters the antechamber through the entrance aperture, and is thermalized through collision with the antechamber walls before entering the ionizer. This arrangement achieves a ram enhancement of the gas pressure in the antechamber above that of the ambient gas, due to the high …