Exobiology and Evolutionary Biology

Tracing Prebiotic Organic Compounds from Interstellar Clouds to the Solar System

PI: DeWayne Halfen

The source of the organic molecules on the early Earth involved in the origin of life is unknown. Some theories propose that lightning in the atmosphere reacted with simple carbon-containing species, such as methane and carbon dioxide, to produce prebiotic compounds. However, a more plausible scenario suggests that the carbon-bearing species came from interstellar space via comets and meteorites. Over 150 molecules have been discovered thus far in interstellar gas, and the majority of these compounds are organic in nature. Most of the organic species are present in giant molecular clouds, which are also the sites of star and solar planet formation. Our own Solar System was created from a molecular cloud, and comets and meteorites are remnants of this formation process. Recent NASA missions, such as Stardust, have detected organic species in a comet by directly sampling the gas and dust from this object. Radio and infrared astronomical observations have also detected simple organic molecules in cometary gases. In addition, meteorites found on Earth, including Murchison, have been found to be rich in organic compounds. Some of these compounds have isotopic signatures that are characteristic of interstellar space. Hence, there could be a connection between terrestrial prebiotic chemistry and the interstellar medium.

To help establish this connection, we have been investigating the chemical complexity of a giant molecular cloud located in the center of the Milky Way galaxy, Sagittarius B2(N). We have been conducting a continuous spectral-line survey of the gas-phase molecules in this object using the Arizona Radio Observatory Kitt Peak 12m and Submillimeter Telescope (SMT) on Mt. Graham. The survey covers the 1, 2, and 3mm atmospheric windows from 65-280 GHz. To determine an accurate picture of the organic gas-phase content of this cloud, multiple rotational transitions of a given chemical species must be observed. Organic molecules typically have very complicated rotational spectra. This complexity means that chance coincidences with other interstellar features are extremely likely. The wideband nature of this survey insures that these molecules can be definitely detected and a very accurate abundance determined. Several potential prebiotic species, such as glycolaldehyde, acetamide, and methyl amine have already been shown to exhibit prominent spectra in this cloud from this survey. The observational part of this survey is approximately 87% completed. We propose to complete the observations with the Arizona facilities towards Sgr B2(N). In addition, we plan to start a new survey towards the giant molecular cloud Orion-KL. We will analyze the data from these surveys and obtain an accurate assessment of the chemical composition of these clouds. Additional laboratory spectroscopy will be carried out as necessary to assign molecular transitions. We will then compare the chemical content of the molecular clouds to that found in comets and meteorites to establish a possible synthetic connection between these objects. This part of the study will include an expert in chemical analysis of carbonaceous chondrites, as well as a synthetic chemist. Once this initial study is complete, the data will be made available to the astrobiological community.

This proposal seeks to address Goal 3, Subgoal 3C of the NASA Strategic Plan and Goal 3 of NASA’s Astrobiology Roadmap that seeks to determine the “Sources of organic compounds required for the origin of life.”