Exploring the Interior of a Comet

Note: While I'm traveling this month with only occasional short access to the internet, I'm reading through some of the back log of proposed mission concepts.  I'll post short summaries of the more interesting ideas.  Unfortunately, I'm unlikely to have time to search down internet sites to provide hot links to the abstracts and presentations I'm reading.  I'll try to provide sufficient information on each that you hopefully can easily do a search to find the original documents.

In this post, I'll a mission concept for the Discovery program (~$425M PI cost, ~$800M fully burdened cost) that would explore the interior of a comet.

The proposal was described in an extended abstract (2 pages) for this year's 41st Lunar and Planetary Science Conference.

Deep Interior Radar Imaging of Comets
LPSC abstract 2670.pdf

One surprise of our exploration of asteroids and the tiny moons of Mars is that many small bodies appear to be rubble piles loosely held together by gravity.  We know little about the interior of comets (fast, distant flybys produce poor gravity measurements).  However, as the authors of the abstract state, the early reconnaissance of comets "has so far yielded the discovery of an unanticipated range of diversity in geomorphic forms: multiplicities of pits, craters with vertical overhangs, global scale layering, mesas and plains.  It has also revealed new geological processes that are revolutionizing our concepts of the cometary interior -- the discovery of repetitive mini-outburts, of patches of enhanced H2O ice, and of caldera-like depressions and smooth flows... It is time to capitalize on these discoveries by moving into a new, detailed exploratory phase where we learn how comets work."

The authors propose to use ice penetrating radar to produce images of the interior of a comet with 10 m resolution that would be comparable to a "medical ultrasonographic brightness scan."  Ice and ground penetrating radars have a long heritage both from airplanes for Earth studies and at Mars where they have successfully explored the upper few kilometers of the Martian crust.  For a small comet -- the proposed target is the 3 km diameter 79P/duToit-Hartley comet -- the radar could study the entire interior as the spacecraft orbits the spinning comet.

The authors note that the Rosetta mission will conduct an inital probe of the interior of a comet as it tracks radio waves from its Philae lander through the body of a comet.  They imply that radar would offer superior interior imaging, but acknowledge the contribution the Rosetta mission will make.

The authors do not state whether or not they have proposed this mission for the current Discovery mission selection.

Editorial thoughts: I don't know if the radar system required for this mission is so costly that a dedicated mission would be needed.  Ice penetrating radars come in different flavors.  The radar proposed for NASA's Jupiter Europa Mission, for example, is a more capable and massive (and mass tends to be strongly correlated with instrument cost) than the radar proposed for ESA's Jupiter Ganymede Mission.  It would seem to me that a spacecraft capable of additional studies such as surface imaging and compositional studies of emitted gases and dust in addition to the radar instrument would be a very attractive mission.  However, the radar unit may be too costly for a Discovery-class mission to fly additional instruments.  The abstract does not provide any information on this.

I have also been struck by the diversity of comet surfaces we have seen from the few flyby spacecraft to date.  Their interiors may be equally varied.  It may be that rather than one mission providing a definitive answer, ice penetrating radar may be needed on a number of comet missions before we begin to understand that variety and the processes that created it.