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2012 Annual Science Report

Rensselaer Polytechnic Institute Reporting  |  SEP 2011 – AUG 2012

Project 2: Processing of Precometary Ices in the Early Solar System

Project Summary

The discovery of numerous planetary systems still in the process of formation gives us a unique opportunity to glimpse how our own solar system may have formed 4.6 billion years ago. Our goal is to test the hypothesis that the building blocks of life were synthesized in space and delivered to the early Earth by comets and asteroids. We use computers to simulate shock waves and other processes that energize the gas and dust in proto-planetary disks and drive physical and chemical processes that would not otherwise occur. Our work seeks specifically to determine (i) whether asteroids and comets were heated to temperatures that favor prebiotic chemistry; and (ii) whether the requisite heating mechanisms operate in other planetary systems forming today.

4 Institutions
3 Teams
3 Publications
0 Field Sites
Field Sites

Project Progress

A. Studies of Multifluid Magnetohydodynamic Shock Waves

The goal of this project is to simulate molecular line emission from shocked interstellar gas including shock waves produced by the bipolar outflows emitted by protostars and the shocks associated with colliding clouds and interacting supernova remnants. We aim to provide tools which can be used to interpret the wealth of molecular line observations of prebiotic molecules to be expected over the next several years from SOFIA, ALMA, and other infrared and radio observatories. For example, the SOFIA Cycle 1 observing program includes observations of molecular line emission from interacting supernova remnants using the GREAT spectrometer (PI J. Hewitt, NASA Goddard). Realistic simulations of shock-excited molecular line emission from weakly-ionized plasmas require time-dependent, magnetohydodynamic calculations, which treat the plasma as three interacting fluids (the neutral gas, ion/electron plasma, and charged dust grains). There is no active research group in the US, to our knowledge, with the capacity to carry out such calculations.

In 2011-12 we made two significant advances:

(i) Ciolek and Roberge (2012a,b) developed an “operator splitting” algorithm for simulating multifluid, MHD shock waves. The algorithm allows for mass, momentum, and energy exchange between different fluids by ion-electron recombination, elastic ion-neutral scattering, or any other process (Fig. 1). The algorithm was implemented on Rensselaer’s 30,000 CPU supercomputer and an extensive suite of benchmark tests was carried out. The tests show that our code can achieve accuracies of at least 10 parts per million. It also has the dynamic range to resolve structures over at least two orders of magnitude in spatial scale (Fig. 2) and many orders of magnitude in time.

(ii) Roberge, Ciolek and Katz (2012) completed a study on the effects of dust on multifluid shock waves. They found that, when charged dust grains are partially coupled to the magnetic field, the speed at which a dusty plasma can communicate compressive disturbances can be modified significantly. When dust decouples from the magnetic field it still exerts a ``passive’’ effect by producing an electric field; this electric field alters the dynamics of the ions and electrons, which modifies the overall shock structure. Roberge et al. carried out numerical simulations of shock formation including dust dynamics and found quasi-self similar behavior at large times.

B. Thermal Environments of Primitive Solar System Bodies

The diverse inventory of amino acids and other organic molecules found in meteorites implies that asteroids once provided environments very conducive to prebiotic chemistry. Menzel and Roberge (2012a,b,c) continued their studies on the thermal environments of asteroids, comets, and other primitive bodies in the solar nebula and protoplanetary disks. Their objective is to understand how the interaction of a primitive body with the ambient disk material may alter its thermal environment. Possible mechanisms include (i) heating of a body’s outer regions by energy dissipation in shear flows near the body surface; and (ii) heating by a new “electrodynamic heating” mechanism. Menzel and Roberge described the physical processes which determine the structure of shear flows in weakly-ionized protoplanetary disks and derived the governing equations which describe the flow for bodies of arbitrary shape. Illustrative solutions were obtained for two very simple body shapes.

C. Collaborative Work with Other NYCA Investigators

Roberge collaborated with the team (Baldwin, Watson, Delano, Swindle et al.) studying argon diffusion in lunar glasses (Gombosi et al. 2012a,b). Roberge devised an algorithm which speeded up numerical simulations of argon diffusion by a factor of ~1 trillion. Roberge and Watson devised numerical and analytical techniques for simulating “multi-event” diffusion, wherein diffusion is driven by discrete heating events. Applications to impact heating of lunar glasses and other extraterrestrial materials are being explored.

Counter-propagating shock waves caused by the collision of two interstellar clouds with relative velocity 20 km/s, The initial conditions are indicated as black dashed lines; the other curves describe the solution 632 yr after the collision. Top: number density of the neutral gas (solid black curve). The density jumps are in order from left to right: the front of a reverse shock, a contact discontinuity, and the front of a forward shock. Contact discontinuities in the neutral fluid (blue dash) and ion-electron fluid (green dash) are indicated. Middle: ion number density for two models where mass transfer by ion-electron recombination is included (solid black) and neglected (solid red). Bottom: velocity of the ion-electron plasma with mass transfer included (black with circles) and neglected (solid red). The solid black curve is the velocity of the neutral fluid.

Collision of two clouds with relative velocity 20 km/s, shown at the very early time t=0.629 yr. Top: velocities of the neutral fluid (solid black curve) and ions (circles). The dash-dot curve is an approximate analytic solution for the ion velocity solution. Inset: the velocity solution on a spatial scale expanded by ~1000X, showing that our code resolves structure over a large dynamic range. The neutral fluid contains counter-propagating shock waves which are driving the disturbance visible in the ion-electron fluid on the larger scale. Bottom: increase in the magnetic field relative to its value B_0 in the undisturbed clouds. The dash-dot curve is an analytic approximation for the magnetic field solution. Inset: magnetic field solution on an expanded scale.

    Wayne Roberge
    Project Investigator
    Glenn Ciolek
    Research Staff

    Max Katz
    Graduate Student

    Raymond Menzel
    Graduate Student

    Will Cunningham
    Undergraduate Student

    Allycia Gariepy
    Undergraduate Student

    Nick Senno
    Undergraduate Student

    Objective 1.1
    Formation and evolution of habitable planets.

    Objective 3.1
    Sources of prebiotic materials and catalysts

    Objective 3.2
    Origins and evolution of functional biomolecules