Chief of Astrochemistry and Project Scientist for OSIRIS-REx
NASA Goddard Space Flight Center
“Chirality of Extraterrestrial Amino Acids”
A little over 4.5 billion years ago, our solar system was a disk of gas and dust, newly collapsed from a molecular cloud, surrounding a young and growing protostar. Sometime around 4 billion years ago, life emerged on Earth, and possibly other planets and moons. The chemistry that led to life has largely been consumed by the geology of Earth and the organisms that inhabit it. To understand life’s origin(s) there are a number of different approaches. Here we focus on constraining the unknowns of the ancient Earth and proto-solar nebula to understand the organic chemistry available for the origin of life. Meteorites are the remnants of planet formation. They contain primordial matter from the accretion of the solar system and record the chemistry and processes prior to the domination of biology and plate tectonics on our planet. For the first time, we have surveyed amino acid abundances, chirality, and stable isotopic ratios across all eight classes of carbonaceous chondrites. We have used amino acids in these rocks to probe the early solar system. Furthermore, one of the greatest mysteries of life is how and when biology became homochiral. The only abiotically generated organic compounds with chiral excess are found in some amino acids in some meteorites. The terrestrially rare amino acid isovaline is always found to be racemic or show an L-excess of up to 20% and in one case extraterrestrial L-aspartic acid showed a 59% L-excess. These excesses appear to be correlated with either the metal content of the meteorite or the amount of hydrothermal processing the samples experienced on the asteroid. What is the origin of such enantiomeric excesses? Are they responsible for biological homochirality? August 2018 NASA’s OSIRIS-REx mission will rendezvous with near-Earth asteroid 101955 Bennu. It will return pristine samples of the surface Bennu to Earth in 2023 for worldwide analysis to even better constrain organic chemistry of the early solar system and attempt to answer these questions.