Hypervelocity Impacts and the Stability of Organic Material, 15-9044

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 Principal Investigators
Deborah S. Bass
Daniel C. Boice
G. Randall Gladstone
Gregory P. Miller
Scott A. Mullin
William M. Murphy
J. Hunter Waite
James D. Walker

Inclusive Dates: 10/01/97 - 11/01/99

Background - As part of a multidivisional, interdisciplinary project, researchers are investigating the role of very high speed impacts in the origin of life. It is speculated that organic-rich planetesimals played a part in the origin of life on Earth. However, the mechanism by which organics could have been delivered from space to a planetary surface is difficult to determine. Particularly problematic is the question of the stability of organic material under hypervelocity impact conditions. Although some evidence suggests organic molecules cannot survive impacts from projectile velocities greater than about 10 kilometers/second, other investigators have found that impacts create a favorable environment for post-shock recombination of organic molecules in the plume phase. Understanding the mechanisms involved in delivering organics to a planetary surface remains difficult to assess due to the lack of experimental results of hypervelocity impacts, particularly in the velocity range of tens of kilometers per second. It is possible that prebiotic molecules may have existed or still exist on planets other than Earth. The discovery of potential fossil life in the martian meteorite ALH84001 then stimulates the following questions: Did the organic molecules ALH84001 originate on Mars? Could the organics have originated on Earth, been subsequently transported to Mars, and then returned to Earth where they were discovered? However, there has been no direct detection of organic material on Mars. The structures that have been identified as possible prebiotic markers, the polycyclic aromatic hydrocarbons (PAHs), were likely incorporated into ALH84001 some four billion years ago, during the late heavy bombardment. Determining the stability of such organics during the hypervelocity impacts regime of the late heavy bombardment is an obvious problem to investigate; was the environment on Mars conducive to the formation or preservation of organic material delivered from space?

Approach - During this project, the team completed the three major planned objectives: 1) use CTH impact physics code to explore the pressure and temperature range of hypervelocity impacts; 2) demonstrate the feasibility of using SwRI's ballistic facilities to experimentally study hypervelocity impacts of organic material on planetary bodies; and 3) demonstrate the use of diagnostic tools previously developed at SwRI to understand the physics and chemistry of hypervelocity impacts and the stability of organic materials.

Accomplishments - Organic material preservation and destruction from impact shocks, the synthesis of organics in the post-impact plume environment, and implications of these processes for Earth and Mars can be investigated by launching an inorganic projectile into an analog planetesimal-and-impactor organic-rich target. Initial work focused on saturating well-characterized zeolitic tuff with an aqueous solution containing dissolved naphthalene, a common PAH. Porosity measurements, thin-section, and X-ray diffraction analyses were performed to determine that the tuff is primarily fine-grained clinoptilolite. To distinguish between contaminants and compounds generated or destroyed in the impact, scientists from the Chemical and Chemical Engineering Division tagged the aqueous component of the target with deuterium. Mechanical and Materials Engineering Division scientists explored the pressure and temperature ranges of hypervelocity impacts (11.2 kilometers/second) through simulations with CTH impact physics computer code. Using an inhibited shaped-charge launcher, scientists from the Mechanical and Materials Engineering Division also impacted the aluminum- and rock organic-rich target with an aluminum projectile. Preliminary analysis revealed that naphthalene survived the shock wave of an impact velocity of 11.2 kilometers/second. Currently, investigators are modifying the target design for future impacts as well as correlating the post-impact compounds with the CTH-simulated pressures and temperatures as a function of radial distance from the impact center. This work resulted in proposals currently under review with the National Science Foundation and NASA and a publication ready for submission.

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