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

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This time final year tiny was famous about Agilkia, a area selected as Philaes alighting site. One year on and a conditions has changed, interjection to remote and in-situ measurements that have contributed to building adult a design of this iconic area on a comet.

As can be seen from a many formula reported in this blog, Rosetta is providing peerless information about a aspect of comet 67P/Churyumov-Gerasimenko. One group has been looking in sold during a segment around where Philae overwhelmed down in sequence to put a lander’s formula in their scold tellurian context.

The Agilkia area on comet 67P/Churyumov-Gerasimenko. The round indicates where Philae overwhelmed down for a initial time on 12 Nov 2014. The dashed line outlines a comet’s equator. The vast basin is a Hatmehit region. This design is a combination of 5 frames from a OSIRIS Narrow Angle Camera.

The Agilkia area on comet 67P/Churyumov-Gerasimenko. The round indicates where Philae overwhelmed down for a initial time on 12 Nov 2014. The dashed line outlines a comet’s equator. The vast basin is a Hatmehit region. This design is a combination of 5 frames from a OSIRIS Narrow Angle Camera, acquired on 14-15 Sep 2014. (An unannotated chronicle of this design is accessible here.)Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Agilkia was a name selected for a 1 km^2 alighting ellipse during that Philae was targeted. Last year, as Rosetta drew closer to a comet, smaller and some-more graphic terrains within this ellipse became apparent. Fiorangela La Forgia, of a University of Padova, Italy, and colleagues have complicated images from OSIRIS, a scholarship camera on Rosetta, to interpretation these areas according to their geological appearance.

On 12 Nov 2014, Philae overwhelmed down as designed in Agilkia, within a segment named Ma’at. (Comet regions, tangible according to their geomorphology, have been named after ancient Egyptian deities.) It afterwards rebounded, and after dual hours flapping about 100m above a comet surface, it staid into a final position, named Abydos and suspicion to be located on a other side of a Hatmehit region, tighten to a limit with Bastet.

From OSIRIS images, we see that Ma’at, and a beside segment Nut, are especially lonesome by well-spoken deposits of fine-grained material. The distance of these grains was suggested by a ROLIS images from Philae to be of a sequence of a few centimetres. Although a abyss of this covering is opposite it is substantially rarely non-static opposite a planet: a 35 m-wide impact void in a Ash segment shows that a abyss there might strech several metres, since in other places, a perspective opposite walls of some-more compressed element suggests that a covering is thin.

This covering might be a outcome of ‘airfall’, that is constructed when dirt is ejected from a aspect though lacks a required quickness to shun from a comet. Instead it falls back, powdering a surface. There are a series of longitudinal dune-like structures manifest in a Agilkia area, that seem to prove a drift-paths for this airfall. This is surprising on comets and might be a outcome of a prevalent ‘wind’ instruction in a comet’s gas activity.

Fiorangela and her colleagues mapped a internal sobriety opposite a Agilkia area and interpretation that a dunes are not caused by dirt pier adult underneath a outcome of sobriety since a regions endangered are roughly flat.

The Agilkia area also contains some pits. These are smaller than another race of pits already celebrated on a aspect of a comet, that might arise from a aspect collapsing to form sinkholes, and that infrequently beget jets of element that mist from a comet.

One idea is that a smaller pits are associated to ice. Several splendid spots have been seen on a well-spoken dry plains that run opposite 67P/C-G’s surface. The many apparent interpretation of these is that they are icy chunks of comet element that have been partially buried by a airfall. Subsequently, as a comet approached a Sun, a total appetite has caused them to sublime. This ejects a excellent dirt particles too, heading to a mine of a pits.

Geomorphological map of a area referred to as Agilkia. The categorical geological units have been identified on a basement of their figure and structure. Credit: Image pleasantness of F. La Forgia et al., 2015

Geomorphological map of a area referred to as Agilkia. The categorical geological units have been identified on a basement of their figure and structure. Image pleasantness of F. La Forgia et al., 2015

There are also widespread outcrops, such as cuestas (hills with a high slope on one side and a shoal slope on a other side), terraces and high walls, around a segment where Philae overwhelmed down, as good as many boulders strewn opposite a surface.

Philae is suspicion to have finally come to rest in a Hatmehit region, tighten to a limit with Bastet. Although a accurate plcae is still a matter for ongoing investigation, Hatmehit itself is a round basin that is lonesome with dune-like facilities and boulders of several sizes. The inlet of this basin is still unknown. From their investigate of a region, Fiorangela and colleagues interpretation that it is not an impact void though could be an area that sank following a sublimation of sub-surface ices.

As good as looking during a form of a landscape in Agilkia, a group also looked during a photometric properties, to sign how most light a aspect reflects during opposite wavelengths. The elementary answer is: not a lot. The alighting site shows an normal reflectance of usually 0.96% in a orange rope (649.2 nm; one of 5 filters on a OSIRIS camera).

The group reports that there is a noted likeness between a geological units and a reflectance. In particular, well-spoken deposits have a top reflectance compared to a outcropping material. These readings are concordant with a aspect of a comet being stoical of organic element (as reported by a VIRTIS group progressing this year) that shows usually tiny internal variation. Nevertheless, Fiorangela and her colleagues advise that this movement contingency be a outcome of tiny differences in a organic compounds that are benefaction since a total reflectance and colour variations can't be explained by aspect hardness and pellet distance alone.

Put together, these formula benefaction a flourishing design of a altogether geological and compositional properties around a Philae alighting site. As such, they will concede a scholarship teams to put a specific sum of a lander’s measurements into a tellurian comet context.

This blog post is formed on a paper Geomorphology and Spectrophotometry of Philaes Landing Site on Comet 67P Churyumov-Gerasimenko by F. La Forgia et al., published in a Astronomy and Astrophysics special emanate on Rosetta goal formula pre-perihelion. The images used in a investigate were acquired between 1 Aug 2014 and 12 Nov 2014 when Rosetta was between 735km and 10 km from a comet surface.

Source: Rosetta blog