Posted on 09/11/2015 by Karen
A fall of comet dust and a field of boulders
From the earliest clear views of comet 67P/Churyumov-Gerasimenko it was obvious that the surface is a collection of contrasts: smooth plains, imposing craggy cliffs and scatterings of boulders. Rosetta scientists are now digging into the detail to explain how some of these features may have arisen and what this means for our understanding of comets.
The study of comet 67P/C-G reveals a dramatic surface environment where considerable amounts of material – up to 1000 kg per second – are ejected from the comet. Not all of this makes it into space, instead some falls back to coat the nucleus.
These small, solid particles – typically with sizes ranging from micrometres to tens of centimetres – are ejected when icy material sublimes. As the ice turns from solid to gas, it escapes into space, propelling the solid particles with it. The smallest of these expelled dust grains – millimeter-sized or smaller – obtain sufficient velocity to escape the influence of the comet and become part of the comet’s tail, which can stretch for millions of kilometres through space.
But some of the larger particles (centimetre-sized or greater) fall back, meaning that particles from one part of the comet can descend to the surface on another part of the comet’s double-lobed nucleus. This ‘airfall’ creates smooth plains that can be as much as a few metres thick.
Nicolas Thomas from Universität Bern, Switzerland, and collaborators have used data from OSIRIS, the science camera on Rosetta, to study these dusty plains. They then used computer models to investigate the mechanisms at work.
The neck, where the two lobes join together and which has been named the Hapi region, has been notably active and so provided an obvious site for investigation. Computer models show that particles ejected from Hapi with speeds below 0.8 m/s will become airfall.
Within the neck, there are ripples in the dust that resemble wind-blown features on Earth and other planets. To understand how these are produced, the team modeled the gas dynamics near the surface of the comet. They found that escaping gas could quickly reach 500 m/s.
Although this speed would partly compensate for the extremely low gas density around the comet, the escaping gas would need to be located very close to the dust deposits to allow the ripples to form. This led the team to investigate other possibilities.
On Earth, the major force to overcome when sculpting wind-blown ripples is gravity holding the grains in place. On the comet, where gravity is minuscule, the major hurdle is the cohesive forces between the dust grains holding them together. This realisation opened another possible formation scenario.
A second computer model showed that as the airfall hits the surface, it dislodges the blanket of particles already there. This makes them more susceptible to weaker gas forces, and also allows the comet’s feeble gravitational field to pull them into the linear features seen in the images.
Although there are many details that remain to be calculated, it is now clear that the airfall phenomenon is an important process in defining the surface properties of a significant fraction of 67P’s nucleus.
Dusty plains are not the only thing covering the surface of the comet. There are also boulders scattered across the dramatic landscape. Maurizio Pajola, University of Padova, Italy, and colleagues have identified 3546 boulders larger than 7 metres in size.
They studied OSIRIS images taken on 5 and 6 August 2014, when Rosetta was close enough to resolve 2.44-2.03 metres per pixel. On an imaged surface of 36.4 km^2 – this is 75% of the total surface of 67P – the number of boulders larger than 7 metres averages to about 100/km^2. Knowing what the boulder distribution was on most of the comet’s surface was a crucial input, provided by Maurizio and by Jean-Baptiste Vincent, to the landing site selection process last year. Since then, Maurizio and his colleagues have been able to study the boulders and their distribution in more detail.
These boulders are not uniformly distributed across the comet. On the smaller lobe, often dubbed the head, there are more smaller boulders than on the larger lobe, called the body. In particular, the size distributions are related to how fractured the formation area is. Thermal stress causes fractures on the surface, which can dislodge blocks – forming new boulders – and there is also a continuous fragmentation of boulders that have already been formed. By examining the frequency with which boulders greater than 7 metres are found on the head, the neck and the body of the comet, the team shows that the head is more fractured than the body: the size-distribution for the head is steeper than that of the body.
In the neck region, there is more uniformity between the number of large and small boulders. One particularly intriguing boulder field (see anaglyph above) occurs here and the team suggests that it was created by blocks falling from the cliffs of Hathor, which are located on the body of the comet.
The team has identified three possible reasons why there are fewer small boulders in this field. It could be firstly, because small icy boulders have sublimed away; secondly, small icy boulders could have been ejected from the comet’s surface by outgassing events; thirdly, airfall could have covered the smallest boulders removing them from view. It could be any one or all three mechanisms at work simultaneously.
In all of this work, planetary scientists are starting to build a picture of the processes at work on 67P. It is clear that the comet is a place of sudden change and gradual evolution, making it all the more fascinating to watch as Rosetta’s mission continues.
This blog post is based on the papers “Redistribution of particles across the nucleus of comet 67P/Churyumov-Gerasimenko” by N. Thomas et al., and “Size-frequency distribution of boulders ≥ 7 m on comet 67P/Churyumov-Gerasimenko” by M. Pajola et al., both published in the Astronomy and Astrophysics special issue on Rosetta mission results pre-perihelion.
Image credit for all images on this page:
Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA