mountain profile Institute for Astronomy University of Hawaii



Maintained by LG

Could Comets with Asteroidlike Orbits Be a Significant Source of Earth's Water?

The distribution of volatiles, and in particular water, in our solar system is a primary determinant of solar system habitability and planetary formation. In particular, the origin of terrestrial water is a fundamental unresolved planetary science issue.

There are three leading scenarios for its origin:

Comets provide one of the mechanisms for large-scale transport and delivery of water within our solar system, and asteroids provide another source of volatiles. However, neither comets nor asteroids can explain both Earth's water and its noble gas inventory.

A recently discovered new class of icy bodies in the outer asteroid belt, the main-belt comets (MBCs), are comets in near-circular orbits within the asteroid belt that are dynamically decoupled from Jupiter. Dynamics suggest they formed in situ, beyond the primordial snow line, and as such represent a class of icy bodies that formed at a distance from the Sun that has not yet been studied in detail and that could potentially hold the key to understanding the origin of water on terrestrial habitable worlds.

The University of Hawaiii NASA Astrobiology Institute (UHNAI) team, led by Karen Meech, has been very active in searching for additional MBCs and characterizing both previously known and new ones. Project work includes automated searches for MBCs using the Pan-STARRS1 (PS1) telescope. PS1's first discovery of an MBC occurred in 2010, when it imaged the activity of P/2010 La Sagra. MBC 2006 VW139 was discovered on 2011 November 5, and prediscovery images were then observed in data taken during August. Their work also includes impacts in the asteroid belt that masquerade as MBCs, including imaging, and dust dynamical and thermal models.



Composite image of P/2010 A2 LINEAR obtained using the Gemini North telescope


Space missions to comets

The 0.58 km radius nucleus of comet 103P/Hartley 2 as seen by the
Deep Impact spacecraft on November 4, 2010

The team led by Karen Meech has been active in coordinating and executing Earth-based observations in support of three extended Discovery missions to comets: Deeo Impact, EPOXI and StardustNExT. The ground-based and Earth-orbital data plays a critical role in the interpretation and understanding of the in-situ data obtained by the spacecraft.

The EPOXI flyby of comet 103P/Hartley 2 on 4 November 2010 revealed a small, highly active comet with CO2-driven jets and a swarm of icy chunks surrounding the nucleus, and the StardustNExT flyby of comet 9P/Tempel 1 on 14 February 2011 allowed us to visit a comet nucleus for the second time to look for changes on the surface after it had made one orbit around the Sun.

From the ground-based data for the EPOXI mission, the group showed that carbon dioxide played a very important role—one even more important than water in the activity of the comet near perihelion.

The same team is part of the SEPPCoN large Spitzer program, which determined radii for 100 comets, and the NEOWISE IR-sky survey mission, which will result in radii and carbon dioxide production estimates for 155 comets. They have a 25-year database of heliocentric light curves for many of these comets and are working on getting a complete set of albedo and gas production rates for all of these. Soon they will be able to merge studies of solar system chemistry, as seen from early leftovers from 4.5 Gyr ago, with chemical models of protoplanetary disks, in order to make resolved disk chemistry observations with ALMA. A large data set with radii, albedos, water and carbon dioxide production rates will allow them to look for correlations between chemistry in comets, mechanisms of activity, and formation location and dynamics all of which feed into disk models and predictions for disk-resolved observations.

Toby Owen is associated with the 2015 Rosetta mission to comet Churyumov-Gerasi­menko which will make first cometary measurement of noble gases and nitrogen isotopes in ammonia . He is also planing to supplement the mission results with ground-based observations of comet Pan-STARRS 2011/C4.