Predicting comets: a matter of perspective

Contrast stretched NAVCAM image of the nucleus of comet 67P/Churyumov-Gerasimenko to highlight the “jets” of dust emitted from all over the surface. CC BY-SA IGO 3.0

In general, any cometary activity is difficult to predict and many comets are known for sudden changes in brightness, break ups and simple disappearances. Fortunately, the Rosetta target comet 67P/Churyumov-Gerasiminko (67P/C-G) is much more amendable to theoretical predictions. The OSIRIS and NAVCAM images show light reflected from a highly structured dust coma within the space probe orbit (ca 20-150 km).

Is is possible to predict the dust coma and tail of comets?

Starting in 2014 we have been working on a dust forecast for 67P/C-G, see the previous blog entries. We had now the chance to check how well our predictions hold by comparing the model outcome to a image sequence from the OSIRIS camera during one rotation period of 67P/C-G on April 12, 2015, published by Vincent et al in A&A 587, A14 (2016) (arxiv version, there Fig. 13).

Comparison of Rosetta observations by Vincent et al A&A 2016 (left panels) with the homogeneous model (right panels). Taken from Kramer&Noack (ApJL 2016) Credit for (a, c): ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Our results appeared in Kramer & Noack, Astrophysical Journal Letters, 823, L11 (preprint, images). We obtain a surprisingly high correlation coefficient (average 80%, max 90%) between theory and observation, if we stick to the following minimal assumption model:

  1. dust is emitted from the entire sunlit nucleus, not only from localized active areas. We refer to this as the “homogeneous activity model”
  2. dust is entering space with a finite velocity (on average) along the surface normal. This implies that close to the surface a rapid acceleration takes place.
  3. photographed “jets” are highly depending on the observing geometry:
    rotateif multiple concave areas align along the line of sight, a high imaged intensity results, but is not necessarily the result of a single main emission source. As an exemplary case, we analysed the brightest points in the Rosetta image taken on April 12, 2015, 12:12 and look at all contributing factors along the line of sight (yellow line) from the camera to the comet. The observed jet is actually resulting from multiple sources and in addition from contributions from all sunlit surface areas.

What are the implications of the theoretical model?

If dust is emitted from all sunlit areas of 67P/C-G, this implies a more homogeneous surface erosion of the illuminated nucleus and leaves less room for compositional heterogeneities. And finally: it makes the dust coma much more predictable, but still allows for additional (but unpredictable) spontaneous, 20-40 min outbreak events. Interestingly, a re-analysis of the comet Halley flyby by Crifo et al (Earth, Moon, and Planets 90 227-238 (2002)) also points to a more homogeneous emission pattern as compared to localized sources.

Weathering the dust around comet 67P/Churyumov–Gerasimenko

Bradford robotic telescope image of comet 67P (30th Oct 2015)
Bradford robotic telescope image of comet 67P/Churyumov–Gerasimenko (180s exposure time, 5:43 UTC, 30-10-2015). © 2015 University of Bradford

Comet 67P/Churyumov–Gerasimenko is past its perihelion and is currently visible in telescopes in the morning hours. The picture is taken from Tenerife by the Bradford robotic telescope, where I submitted the request. The tail is extending hundred thousands kilometers into space and consists of dust particles emitted from the cometary nucleus, which measures just a few kilometers. In a recent work just published in the Astrophysical Journal Letters (arxiv version), we have explored how dust, which does not make it into space, is whirling around the cometary nucleus. The model assumes that dust particles are emitted from the porous mantle and hover over the cometary surface for some time (<6h) and then fall back on the surface, delayed by the gas drag of gas molecules moving away from the nucleus. As in the predictions for the cometary coma discussed previously, we are sticking to a minimal-assumption model with a homogeneous surface activity of gas and dust emission.

Dust trajectories reaching the Philae descent area computed from a homogeneous dust emission model. Figure from Kramer/Noack Prevailing dust-transport directions on comet 67P/Churyumov-Gerasimenko, Astrophysical Journal Letters, 813, L33 (2015)
Dust trajectories reaching the Philae descent area computed from a homogeneous dust emission model. From Kramer/Noack “Prevailing dust-transport directions on comet 67P/Churyumov-Gerasimenko”, Astrophysical Journal Letters, 813, L33 (2015).

The movements of 40,000 dust particles are tracked and the average dust transport within a volumetric grid with 300 m sized boxes is computed. Besides the gas-dust interaction, we do also incorporate the rotation of the comet, which leads to a directional transport.
The Rosetta mission dropped Philae over the small lobe of 67P/C-G and Philae took a sequence of approach images which reveal structures resembling wind-tails behind boulders on the comet. This allowed Mottola et al (Science 349.6247 (2015): aab0232) to derive information about the direction of impinging particles which hit the surface unless sheltered by the boulder. Our model predicts a dust-transport inline with the observed directions in the descent region, it will be interesting to see how wind-tails at other locations match with the prediction. We put an interactive 3d dust-stream model online to visualize the dust-flux predicted from the homogeneous surface model.

Day and night at comet 67P/Churyumov–Gerasimenko

Comet 67P/Churyumov–Gerasimenko has passed its nearest distance to the sun and its tail has been observed from earth. The comet emits dust and displays spectacular but short-lived outbreaks of localized jet activity. Very detailed OSIRIS pictures of the near-surface dust emission ready for stereo viewing have been posted by Brian May. The pictures also allow one to have a look at the prediction from the homogeneous dust emission model discussed previously. When you direct your attention in Brian May’s pictures to the background activity, you find very similar patterns as expected from the homogenous emission model. This activity is dimmer but steadily blowing off dust from the nucleus. Matthias Noack and I have generated and uploaded a visualization of the dust data obtained from the homogeneous activity model. In contrast to a localized activity models, collimated jets arise from a bundle of co-propagating dust trajectories emanating from concave surface areas. The underlying topographical shape model is a uniform triangle remesh of Mattias Malmer’s excellent work based on the release of Rosetta’s NAVCAM images via the Rosetta blog. The following video takes you on a flight around 67P/C-G, with 16 hours condensed into 90 sec.

The video is a side-by-side stereoscopic 3d rendering of 67P/Churyumov–Gerasimenko and the dust cloud, which can be viewed in 3d with  a simple cardboard viewer. While the observer is encircling the nucleus, day and night passes and different parts of the comet are illuminated.

Gas flow around 67P/C-G computed from a homogeneous activity model.
Gas flow around 67P/C-G computed from a homogeneous activity model. arxiv:1505.08041

In the homogeneous activity model each sunlit triangle emits dust with an initial velocity component along the surface normal. Then dust is additionally dragged along within the outwards streaming gas, which is also incorporated in the model. In contrast to compact dust particles, the gas molecules are diffusing also in lateral directions and thus gas is not helping to collimate jets by itself. The Rosetta mission with its long term observation program offers fascinating ways to perform a reality check on various models of cometary activity, which differ considerably in the underlying physics and assumptions about the original distribution and lift-off conditions of the dust eventually forming the beautiful tails of comets.

Homogeneous dust emission and jet structure near active cometary nuclei: the case of 67P/Churyumov-Gerasimenko by Tobias Kramer, Matthias Noack, Daniel Baum, Hans-Christian Hege, Eric J. Heller.

For red-cyan glasses try our 3d video on youtube (flash player required, watch out for the settings and 3d options, 1080p HD recommended).

Dusting off cometary surfaces: collimated jets despite a homogeneous emission pattern.

Effective Gravitational potential of the comet (including the centrifugal contribution), the maximal value of the potential (red) is about 0.46 N/m, the minimal value (blue) 0.31 N/m computed with the methods described in this post.
Effective Gravitational potential of the comet (including the centrifugal contribution), the maximal value of the potential (red) is about 0.46 N/m, the minimal value (blue) 0.31 N/m computed with the methods described in this post. The rotation period is taken to be 12.4043 h. Image computed with the OpenCL cosim code. Image (C) Tobias Kramer (CC-BY SA 3.0 IGO).

Knowledge of GPGPU techniques is helpful for rapid model building and testing of scientific ideas. For example, the beautiful pictures taken by the ESA/Rosetta spacecraft of comet 67P/Churyumov–Gerasimenko reveal jets of dust particles emitted from the comet. Wouldn’t it be nice to have a fast method to simulate thousands of dust particles around the comet and to find out if already the peculiar shape of this space-potato influences the dust-trajectories by its gravitational potential? At the Zuse-Institut in Berlin we joined forces between the distributed algorithm and visual data analysis groups to test this idea. But first an accurate shape model of the comet 67P C-G is required. As published in his blog, Mattias Malmer has done amazing work to extract a shape-model from the published navigation camera images.

  1. Starting from the shape model by Mattias Malmer, we obtain a re-meshed model with fewer triangles on the surface (we use about 20,000 triangles). The key-property of the new mesh is a homogeneous coverage of the cometary surface with almost equally sized triangle meshes. We don’t want better resolution and adaptive mesh sizes at areas with more complex features. Rather we are considering a homogeneous emission pattern without isolated activity regions. This is best modeled by mesh cells of equal area. Will this prescription yield nevertheless collimated dust jets? We’ll see…
  2. To compute the gravitational potential of such a surface we follow this nice article by JT Conway. The calculation later on stays in the rotating frame anchored to the comet, thus in addition the centrifugal and Coriolis forces need to be included.
  3. To accelerate the method, OpenCL comes to the rescue and lets one compute many trajectories in parallel. What is required are physical conditions for the starting positions of the dust as it flies off the surface. We put one dust-particle on the center of each triangle on the surface and set the initial velocity along the normal direction to typically 2 or 4 m/s. This ensures that most particles are able to escape and not fall back on the comet.
  4. To visualize the resulting point clouds of dust particles we have programmed an OpenGL visualization tool. We compute the rotation and sunlight direction on the comet to cast shadows and add activity profiles to the comet surface to mask out dust originating from the dark side of the comet.

This is what we get for May 3, 2015. The ESA/NAVCAM image is taken verbatim from the Rosetta/blog.

Comparison of homogeneous dust model with ESA/NAVCAM Rosetta images.
Comparison of homogeneous dust mode (left panel)l with ESA/NAVCAM Rosetta images. (C) Left panel: Tobias Kramer and Matthias Noack 2015. Right panel: (C) ESA/NAVCAM team CC BY-SA 3.0 IGO, link see text.

Read more about the physics and results in our arxiv article T. Kramer et al.: Homogeneous Dust Emission and Jet Structure near Active Cometary Nuclei: The Case of 67P/Churyumov-Gerasimenko (submitted for publication) and grab the code to compute your own dust trajectories with OpenCL at