Disruption of asteroids & comets
Spontaneous fragmentation of comets is one of the least understood, but most important aspects of their nature. Attempts to explain the mysterious phenomenon date back to 1846, when astronomers were surprised to see periodic comet Biela had split into two pieces and later disappeared. In the course of time many similar events were observed, but their trigger remains unclear to date. Equally bizarre are disrupting asteroids, identified for the first time in 2010.
Disrupted asteroid P/2012 F5 imaged by Keck II in mid-2014. Top panel shows a wide-angle view of the main nucleus and smaller fragments (indicated by arrows when you move a mouse cursor over the image) embedded in a dust trail. Bottom panel shows a close-up view with the trail numerically removed.
(c) M. Drahus, W. Waniak (OAUJ) / W. M. Keck Observatory
Among possible explanations, there are two widely accepted theories. One says the strange behavior is a result of a hypervelocity collision with another minor object. The other one sees in it a consequence of rotational disruption - a hypothetical process of launching dust and fragments by large centrifugal forces (generated by fast rotation) surpassing object's self-gravity. A small comet or asteroid can spin up beyond the limit of rotational stability as a result of non-gravitational forces, generated by mass loss or heat radiation.
I'm interested in measuring how fast such disrupted objects rotate and in this way verify the rotational beak-up scenario. The study enabled me & my team to identify the first asteroid, P/2012 F5 (Gibbs), whose rapid rotation and close-by fragments are clear signs of a recent rotational disruption. I'm also interested in modeling the consequences of rotational disruption for the entire population of comets, and seek confirmation in the statistical data.
Drahus et al. 2015, ApJL 802, L8
Drahus 2014, DPS meeting 46, 200.04
Jewitt et al. 2010, Nature 467, 817
Rotational dynamics of comet nuclei
A prerequisite for rotational disruption is evolution of the rotation rate. Despite several tentative detections of this process in comets, it was robustly measured only in 2007. The study led by Mike Belton showed that the nucleus of comet 9P/Tempel 1 was slowly spinning up. I contributed to this work by catching this effect "in the act" upon applying to the data dynamical periodicity-search algorithms created earlier with Waclaw Waniak.
A subsequent project, supported by a group of collaborators, allowed us to measure very rapid spin changes of comet 103P/Hartley 2, visited by NASA's EPOXI spacecraft. We measured a rotational deceleration of roughly 1 minute per day and the spin period at the epoch of observations of about 18 hours. This time, our original dynamical approach was applied, for the first time, to a time series of molecular spectra, taken at very high resolution in the millimeter-wavelength domain.
The results obtained so far confirm that the spin-period evolution can be rapid and possibly lead to rotational disruption. This process is fastest for the smallest and most active cometary nuclei. With my colleagues we believe that many such objects may be rotationally disrupted before being discovered, becoming in this way the main source of the Zodiacal dust in our solar system. It would be very interesting to identify one day a fast rotator rapidly spinning up, and check if it breaks up when it should. Another challenge is to start measuring spin changes in Oort-cloud comets.
Drahus et al. 2011, ApJL 734, L4
Drahus & Waniak 2006, Icarus 185, 544
Belton et al. 2011, Icarus 213, 345
Belton & Drahus 2007, BAAS 39, 498
Compositional structure of cometary ices
While it's been well established that comets are primordial remnants of the solar system formation, it is not clear how they actually formed. This can be investigated through the chemical structure of the nucleus. A homogeneous composition would suggest rapid creation in one place. However, cometary nuclei may in fact originate from smaller cometesimals, which accumulated into comets in the early solar system. Because of the expected radial migration of cometesimals, these hypothetical building blocks of comets could originate at different heliocentric distances in the proto-solar disk and have different chemical compositions. In this case, the resulting cometary nucleus would be chemically heterogeneous.
Examples of the line profiles of CH3
OH (left panel) and HCN (right panel), illustrating the temporal variation of comet 103P/Hartley 2 as the nucleus rotates (from top to bottom).
With my colleagues we addressed this problem by observing spectroscopically two characteristic cometary volatiles: HCN and CH3OH. A long time series obtained for comet 103P/Hartley 2 revealed dramatic changes in the spectral profiles as the nucleus rotated. We concluded that the object was chemically heterogeneous. Whether it's a primordial or evolutionary property is an open issue, but our reasoning suggests the latter.
The HCN spectra of 103P/Hartley 2 were also used by Waclaw Waniak to investigate the photochemical relation of this molecule with CN. Stay tuned as the results of his state-of-the-art modeling are coming up soon.
Drahus et al. 2012, ApJ 756, 80
Waniak et al. 2012, A&A 543, A32
Waniak et al. 2009, EM&P 105, 327