Scientific Justification: Background
A primary focus of current astrobiology research is to investigate our
prebiotic molecular origins. As such, over the last several years
there have been significant advances in this field primarily due to
dedicated searches for large molecules that are possible precursors to
molecules of biological importance (biomolecules) in the interstellar
medium and in the atmospheres of comets. Successful dedicated
searches with the Green Bank Telescope (GBT) in the last two years
have led to the detections of interstellar aldehydes namely propenal
(CH2CHCHO) and propanal (CH3CH2CHO) (Hollis et al. 2004a), simple
aldehyde sugars like glycolaldehyde (CH2OHCHO) (Hollis et al. 2004b),
the first keto ring molecule to be found in an interstellar cloud,
cyclopropenone (c-H2C3O) (Hollis et al. 2006a), the third organic
imine to be detected in an interstellar cloud, ketenimine (CH2CNH)
(Lovas et al. 2006b), and acetamide (CH3CONH2) (Hollis et al. 2006b),
the largest interstellar molecule detected with a peptide bond. These
searches and detections all took place in the direction of the
Galactic center towards Sgr B2(N) in a region that has been named the
``Large Molecule Heimat'' and have greatly enhanced our understanding
of the distribution and abundance of these large organic molecules in
interstellar clouds. However, dedicated searches are very limited in
the identification for new interstellar species and transitions
because they often cover only a restricted range of frequencies and
bandwidth. What is needed is a complete survey over a large frequency
range and bandwidth to obtain as much information as possible.
There are many specific reasons why a deep spectral line survey is
important and timely and could significantly advance the field of
astrobiology. First, although we have made significant progress in
understanding the chemical content and formation mechanisms in
interstellar clouds, we still cannot predict with certainty which
species will be present. A number of species such as the 3-carbon
sugar glyceraldehyde and the simplest amino acid, glycine, have eluded
detection so far although models and the presence of precursor species
suggest that these molecules should be present. A survey offers the
possibility of ensemble averages of transitions, which may be the only
way to make detections of the largest, most complex species for which
the population is distributed over many energy states (large partition
function). This technique was not feasible in the past since weak
signatures of complex molecular species emitting or absorbing in the
300 MHz to 50 GHz region were not amenable to detection until the
advent of the GBT.
Second, in practical terms, a sensitive and methodical survey is one of the most efficient ways to detect new species. Very often, targeted lines prove unobservable because of unexpected radio frequency interference (RFI) or confusion from interloping spectral lines. To achieve the detection, one must then observe another spectral line in the ensemble, but this often requires another observing proposal and thus delays the scientific result.
Third, large databases of laboratory spectroscopic transition frequencies exist already. A thorough, sensitive survey would allow astrochemists to utilize these databases to make firm, multi-transition identifications of species. This would greatly advance the field of astrochemistry by creating a comprehensive inventory of species. Furthermore, this legacy survey will contain unidentified spectral features that can be assigned to new interstellar species in a spectral region that contains low-energy transitions of complex prebiotic molecules and as such, would spur laboratory spectroscopists to make additional measurements of likely species.
Fourth, this survey will produce a complete inventory of known interstellar molecules and their transitions that can be evaluated and synergistically used as multiple probes of physical conditions. This is potentially one of the most important outcomes of this survey. To fully understand the role of interstellar clouds in our molecular origins, an inventory of molecular species is necessary in prototypical sources. For example, the trends in molecular abundances and distributions provide the keys to how they are formed.
Finally, this is an important and advantageous time to make this survey. The results will guide further instrumentation development on the GBT (specifically including the specifications for focal plane arrays at key frequencies), and can guide the scientific development of ALMA and the EVLA, both of which expect astrochemistry to be an important component of their scientific programs. The results may also be valuable to other projects such as the space-based Hershel/HIFI mission, which will be coming on line in just a few years.
Overall, inventories of molecules are important to understand astrochemistry and chemical evolution. This legacy survey will produce a complete inventory of known interstellar species that can be used as probes of physical conditions. The survey will also discover new interstellar species that will provide constraints that aid in our understanding of astrochemistry and chemical evolution.