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Rosetta blog: The ups and downs of a comet’s surface

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Posted November 11, 2015

This time last year little was known about Agilkia, the area chosen as Philae's landing site. One year on and the situation has changed, thanks to remote and in-situ measurements that have contributed to building up a picture of this iconic area on the comet.

As can be seen from the many results reported in this blog, Rosetta is providing unrivalled information about the surface of comet 67P/Churyumov-Gerasimenko. One team has been looking in particular at the region around where Philae touched down in order to put the lander's results in their correct global context.

The Agilkia area on comet 67P/Churyumov-Gerasimenko. The circle indicates where Philae touched down for the first time on 12 November 2014. The dashed line marks the comet’s equator. The large depression is the Hatmehit region. This image is a composite of five frames from the OSIRIS Narrow Angle Camera.

The Agilkia area on comet 67P/Churyumov-Gerasimenko. The circle indicates where Philae touched down for the first time on 12 November 2014. The dashed line marks the comet’s equator. The large depression is the Hatmehit region. This image is a composite of five frames from the OSIRIS Narrow Angle Camera, acquired on 14-15 September 2014. (An unannotated version of this image is available here.)Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Agilkia was the name chosen for the 1 km^2 landing ellipse at which Philae was targeted. Last year, as Rosetta drew closer to the comet, smaller and more distinct terrains within this ellipse became apparent. Fiorangela La Forgia, of the University of Padova, Italy, and colleagues have studied images from OSIRIS, the science camera on Rosetta, to define these areas according to their geological appearance.

On 12 November 2014, Philae touched down as planned in Agilkia, within a region named Ma'at. (Comet regions, defined according to their geomorphology, have been named after ancient Egyptian deities.) It then rebounded, and after two hours drifting about 100m above the comet surface, it settled into its final position, named Abydos and thought to be located on the other side of the Hatmehit region, close to the border with Bastet.

From OSIRIS images, we see that Ma’at, and the neighbouring region Nut, are mainly covered by smooth deposits of fine-grained material. The size of these grains was revealed by the ROLIS images from Philae to be of the order of a few centimetres. Although the depth of this layer is unknown it is probably highly variable across the planet: a 35 m-wide impact crater in the Ash region shows that the depth there may reach several metres, whereas in other places, the view across walls of more compacted material suggests that the layer is thin.

This layer may be the result of 'airfall', which is produced when dust is ejected from the surface but lacks the necessary velocity to escape from the comet. Instead it falls back, dusting the surface. There are a number of longitudinal dune-like structures visible in the Agilkia area, which seem to indicate the drift-paths for this airfall. This is unusual on comets and may be the result of a prevailing 'wind' direction in the comet's gas activity.

Fiorangela and her colleagues mapped the local gravity across the Agilkia area and conclude that the dunes are not caused by dust piling up under the effect of gravity because the regions concerned are almost flat.

The Agilkia area also contains some pits. These are smaller than another population of pits already observed on the surface of the comet, which may arise from the surface collapsing to form sinkholes, and which sometimes generate jets of material that spray from the comet.

One suggestion is that the smaller pits are related to ice. Several bright spots have been seen on the smooth dusty plains that run across 67P/C-G’s surface. The most obvious interpretation of these is that they are icy chunks of comet material that have been partially buried by the airfall. Subsequently, as the comet approached the Sun, the added energy has caused them to sublime. This ejects the fine dust particles too, leading to the excavation of the pits.

Geomorphological map of the area referred to as Agilkia. The main geological units have been identified on the basis of their shape and structure. Credit: Image courtesy of F. La Forgia et al., 2015

Geomorphological map of the area referred to as Agilkia. The main geological units have been identified on the basis of their shape and structure. Image courtesy of F. La Forgia et al., 2015

There are also widespread outcrops, such as cuestas (hills with a steep slope on one side and a shallow slope on the other side), terraces and steep walls, around the region where Philae touched down, as well as many boulders strewn across the surface.

Philae is thought to have finally come to rest in the Hatmehit region, close to the border with Bastet. Although the exact location is still a matter for ongoing investigation, Hatmehit itself is a circular depression that is covered with dune-like features and boulders of various sizes. The nature of this depression is still unknown. From their study of the region, Fiorangela and colleagues conclude that it is not an impact crater but could be an area that sank following the sublimation of sub-surface ices.

As well as looking at the form of the landscape in Agilkia, the team also looked at the photometric properties, to gauge how much light the surface reflects at different wavelengths. The simple answer is: not a lot. The landing site shows an average reflectance of just 0.96% in the orange band (649.2 nm; one of five filters on the OSIRIS camera).

The team reports that there is a marked similarity between the geological units and the reflectance. In particular, smooth deposits have the highest reflectance compared to the outcropping material. These readings are compatible with the surface of the comet being composed of organic material (as reported by the VIRTIS team earlier this year) that shows only small local variation. Nevertheless, Fiorangela and her colleagues suggest that this variation must be the result of small differences in the organic compounds that are present because the combined reflectance and colour variations cannot be explained by surface texture and grain size alone.

Put together, these results present a growing picture of the overall geological and compositional properties around the Philae landing site. As such, they will allow the science teams to put the specific details of the lander's measurements into a global comet context.

This blog post is based on the paper Geomorphology and Spectrophotometry of Philae's Landing Site on Comet 67P Churyumov-Gerasimenko by F. La Forgia et al., published in the Astronomy and Astrophysics special issue on Rosetta mission results pre-perihelion. The images used in the study were acquired between 1 August 2014 and 12 November 2014 when Rosetta was between 735km and 10 km from the comet surface.

Source: Rosetta blog

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