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Science on the surface of a comet

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Posted August 4, 2015

Complex molecules that could be key building blocks of life, the daily rise and fall of temperature, and an assessment of the surface properties and internal structure of the comet are just some of the highlights of the first scientific analysis of the data returned by Rosetta’s lander Philae last November.

This well-lit image was acquired by Philae’s CIVA camera 4 at the final landing site Abydos, on the small lobe of Comet 67P/Churyumov–Gerasimenko, on 13 November 2014. The image shows one of the CONSERT antennas in the foreground, which seems to be in contact with the nucleus. The dimensions of the antenna, 5 mm in diameter and 693 mm long, help to provide a scale to the image. The pebble-like features, blocks and cliffs observed in the CIVA images corresponds to what has been seen at larger scales from orbit. A large range of brightness is also seen, perhaps associated with different mineral compositions. Copyright ESA/Rosetta/Philae/CIVA

This well-lit image was acquired by Philae’s CIVA camera 4 at the final landing site Abydos, on the small lobe of Comet 67P/Churyumov–Gerasimenko, on 13 November 2014. The image shows one of the CONSERT antennas in the foreground, which seems to be in contact with the nucleus. The dimensions of the antenna, 5 mm in diameter and 693 mm long, help to provide a scale to the image. The pebble-like features, blocks and cliffs observed in the CIVA images corresponds to what has been seen at larger scales from orbit. A large range of brightness is also seen, perhaps associated with different mineral compositions. Copyright ESA/Rosetta/Philae/CIVA

Early results from Philae’s first suite of scientific observations of Comet 67P/Churyumov­-Gerasimenko were published today in a special edition of the journal Science.

Data were obtained during the lander’s seven-hour descent to its first touchdown at the Agilkia landing site, which then triggered the start of a sequence of predefined experiments. But shortly after touchdown, it became apparent that Philae had rebounded and so a number of measurements were carried out as the lander took flight for an additional two hours some 100 m above the comet, before finally landing at Abydos.

Images taken by Philae’s ROsetta Lander Imaging System, ROLIS, trace the lander’s descent to the first landing site, Agilkia, on Comet 67P/Churyumov–Gerasimenko on 12 November 2014. The first image was taken just over 3 km from the comet, and indicates the position of Agilkia and the area covered by the next image in the sequence, taken just 67 m away. The six images that follow were taken at approximately 10 second intervals prior to landing, with the final image of the sequence acquired 9 m above the touchdown site. The time the images were acquired, along with distance from the surface and image resolution, are marked on each image. The final slide is annotated with the estimated touchdown position and orientation of Philae, which has been calculated to within ±20 cm. Copyright ESA/Rosetta/Philae/ROLIS/DLR

Images taken by Philae’s ROsetta Lander Imaging System, ROLIS, trace the lander’s descent to the first landing site, Agilkia, on Comet 67P/Churyumov–Gerasimenko on 12 November 2014. The first image was taken just over 3 km from the comet, and indicates the position of Agilkia and the area covered by the next image in the sequence, taken just 67 m away. The six images that follow were taken at approximately 10 second intervals prior to landing, with the final image of the sequence acquired 9 m above the touchdown site. The time the images were acquired, along with distance from the surface and image resolution, are marked on each image. The final slide is annotated with the estimated touchdown position and orientation of Philae, which has been calculated to within ±20 cm. Copyright ESA/Rosetta/Philae/ROLIS/DLR

Some 80% of the first science sequence was completed in the 64 hours following separation before Philae fell into hibernation, with the unexpected bonus that data were ultimately collected at more than one location, allowing comparisons between the touchdown sites.

Inflight science

After the first touchdown at Agilkia, the gas-sniffing instruments Ptolemy and COSAC analysed samples entering the lander and determined the chemical composition of the comet’s gas and dust, important tracers of the raw materials present in the early Solar System.

COSAC analysed samples entering tubes at the bottom of the lander kicked up during the first touchdown, dominated by the volatile ingredients of ice-poor dust grains. This revealed a suite of 16 organic compounds comprising numerous carbon and nitrogen-rich compounds, including four compounds – methyl isocyanate, acetone, propionaldehyde and acetamide – that have never before been detected in comets.

Meanwhile, Ptolemy sampled ambient gas entering tubes at the top of the lander and detected the main components of coma gases – water vapour, carbon monoxide and carbon dioxide, along with smaller amounts of carbon-bearing organic compounds, including formaldehyde.

Importantly, some of these compounds detected by Ptolemy and COSAC play a key role in the prebiotic synthesis of amino acids, sugars and nucleobases: the ingredients for life. For example, formaldehyde is implicated in the formation of ribose, which ultimately features in molecules like DNA.

This image, created from Philae’s ROLIS descent camera, focuses on the largest boulder seen in the image captured at 67.4 m above Comet 67P/Churyumov–Gerasimenko. It is best viewed with red/blue–green glasses. The 3D view highlights the fractures in the 5 m-high boulder, along with the tapered ‘tail’ of debris and excavated ‘moat’ around it. This ‘tail’ feature has also been identified around boulders elsewhere on the comet in OSIRIS images taken from orbit. The ROLIS team thinks the tails appear as a result of the region ‘behind’ an obstacle being shielded from erosion via the impact of falling particles arriving in a prevailing direction, perhaps from activity elsewhere on the comet. Copyright ESA/Rosetta/Philae/ROLIS/DLR

This image, created from Philae’s ROLIS descent camera, focuses on the largest boulder seen in the image captured at 67.4 m above Comet 67P/Churyumov–Gerasimenko. It is best viewed with red/blue–green glasses. The 3D view highlights the fractures in the 5 m-high boulder, along with the tapered ‘tail’ of debris and excavated ‘moat’ around it.
This ‘tail’ feature has also been identified around boulders elsewhere on the comet in OSIRIS images taken from orbit. The ROLIS team thinks the tails appear as a result of the region ‘behind’ an obstacle being shielded from erosion via the impact of falling particles arriving in a prevailing direction, perhaps from activity elsewhere on the comet. Copyright ESA/Rosetta/Philae/ROLIS/DLR

The existence of such complex molecules in a comet, a relic of the early Solar System, imply that chemical processes at work during that time could have played a key role in fostering the formation of prebiotic material.

Comparing touchdown sites

Thanks to the images taken by ROLIS on the descent to Agilkia, and the CIVA images taken at Abydos, a visual comparison of the topography at these two locations could be made.

ROLIS images taken shortly before the first touchdown revealed a surface comprising metre-size blocks of diverse shapes, coarse regolith with grain sizes of 10–50 cm, and granules less than 10 cm across.

The regolith at Agilkia is thought to extend to a depth of 2 m in places, but seems to be free from fine-grained dust deposits at the resolution of the images.

The largest boulder in the ROLIS field-of-view measures about 5 m high, with a peculiar bumpy structure and fracture lines running through it that suggest erosional forces are working to fragment the comet’s boulders into smaller pieces.

The boulder also has a tapered ‘tail’ of debris behind it, similar to others seen in images taken by Rosetta from orbit, yielding clues as to how particles lifted up from one part of the eroding comet are deposited elsewhere.

Zooming in to a portion of the fractured cliff face imaged by CIVA camera 4 reveals brightness variations in the comet’s surface properties down to centimetre and millimetre scales. The dominant constituents are very dark conglomerates, likely made of organics. The brighter spots could represent mineral grains, perhaps even pointing to ice-rich materials. The left hand image shows one of the CONSERT antennas in the foreground, which seems to be in contact with the nucleus. The dimensions of the antenna, 5 mm in diameter and 693 mm long, help to provide a scale to the image. Copyright ESA/Rosetta/Philae/CIVA

Zooming in to a portion of the fractured cliff face imaged by CIVA camera 4 reveals brightness variations in the comet’s surface properties down to centimetre and millimetre scales. The dominant constituents are very dark conglomerates, likely made of organics. The brighter spots could represent mineral grains, perhaps even pointing to ice-rich materials. The left hand image shows one of the CONSERT antennas in the foreground, which seems to be in contact with the nucleus. The dimensions of the antenna, 5 mm in diameter and 693 mm long, help to provide a scale to the image. Copyright ESA/Rosetta/Philae/CIVA

Over a kilometre away at Abydos, not only did the images taken by CIVA’s seven microcameras reveal details in the surrounding terrain down to the millimetre scale, but also helped decipher Philae’s orientation.

The lander is angled up against a cliff face that is roughly 1 m from the open ‘balcony’ side of Philae, with stereo imagery showing further topography up to 7 m away, and one camera with open sky above.

The images reveal fractures in the comet’s cliff walls that are ubiquitous at all scales. Importantly, the material surrounding Philae is dominated by dark agglomerates, perhaps comprising organic-rich grains. Brighter spots likely represent differences in mineral composition, and may even point to ice-rich materials.

From the surface to the interior
The MUPUS suite of instruments provided insight into the physical properties of Abydos. Its penetrating ‘hammer’ showed the surface and subsurface material sampled to be substantially harder than that at Agilkia, as inferred from the mechanical analysis of the first landing.

The results point to a thin layer of dust less than 3 cm thick overlying a much harder compacted mixture of dust and ice at Abydos. At Agilkia, this harder layer may well exist at a greater depth than that encountered by Philae.

Summary of Philae’s MUPUS measurements at Abydos, its final landing site on Comet 67P/Churyumov–Gerasimenko. The graph at top shows the average surface temperature profile measured by the MUPUS thermal mapper, situated on the lander’s ‘balcony’. Gaps correspond to times when the instrument was not recording data. The profile shows a clear rise and fall in temperature, corresponding to lows of about –180ºC and ‘highs’ of about –145ºC in sync with the comet’s 12.4 hour day. The peaks are interpreted as infrared radiation from the directly insolated surface, with the more gentle variations outside of the peaks attributed to indirect lighting. The thermal inertia implied by the measured rapid rise and fall in the temperature suggest that the surface consists of a thin layer of dust atop a compacted dust-ice crust. The graph at the bottom shows the hammering profile of the MUPUS penetrator. The displacement is expressed as the position of the depth sensor with respect to its starting position above the surface. An initial displacement of about 27 mm, perhaps through a thin layer of dust, is observed, followed by oscillations of 10–15 mm and smaller displacements. The reason for the lower amplitude after 80 minutes is unclear, but could indicate that the tip of the penetrator had locked to the ground. The data suggest that the instrument was hammering more or less on the spot, although not necessarily at exactly the same spot each time, with indentations of a few millimetres and recoils of up to 10 mm. The red lines indicate the power levels of MUPUS, which correspond to 0.49, 1.59, 2.17 and 4.23 joules, respectively. Discussion is ongoing as to whether the data reflect the full use of energy level 4. In any case, the results provide an estimate of the strength of the surface beneath the thin dust layer as at least 2 MPa. Copyright Spacecraft graphic: ESA/ATG medialab; data from Spohn et al (2015)

Summary of Philae’s MUPUS measurements at Abydos, its final landing site on Comet 67P/Churyumov–Gerasimenko. The graph at top shows the average surface temperature profile measured by the MUPUS thermal mapper, situated on the lander’s ‘balcony’. Gaps correspond to times when the instrument was not recording data. The profile shows a clear rise and fall in temperature, corresponding to lows of about –180ºC and ‘highs’ of about –145ºC in sync with the comet’s 12.4 hour day. The peaks are interpreted as infrared radiation from the directly insolated surface, with the more gentle variations outside of the peaks attributed to indirect lighting. The thermal inertia implied by the measured rapid rise and fall in the temperature suggest that the surface consists of a thin layer of dust atop a compacted dust-ice crust.
The graph at the bottom shows the hammering profile of the MUPUS penetrator. The displacement is expressed as the position of the depth sensor with respect to its starting position above the surface. An initial displacement of about 27 mm, perhaps through a thin layer of dust, is observed, followed by oscillations of 10–15 mm and smaller displacements. The reason for the lower amplitude after 80 minutes is unclear, but could indicate that the tip of the penetrator had locked to the ground. The data suggest that the instrument was hammering more or less on the spot, although not necessarily at exactly the same spot each time, with indentations of a few millimetres and recoils of up to 10 mm. The red lines indicate the power levels of MUPUS, which correspond to 0.49, 1.59, 2.17 and 4.23 joules, respectively. Discussion is ongoing as to whether the data reflect the full use of energy level 4. In any case, the results provide an estimate of the strength of the surface beneath the thin dust layer as at least 2 MPa. Copyright Spacecraft graphic: ESA/ATG medialab; data from Spohn et al (2015)

The MUPUS thermal sensor, on Philae’s balcony, revealed a variation in the local temperature between about –180ºC and –145ºC in sync with the comet’s 12.4 hour day. The thermal inertia implied by the measured rapid rise and fall in the temperature also indicates a thin layer of dust atop a compacted dust-ice crust.

Moving below the surface, unique information concerning the global interior structure of the comet was provided by CONSERT, which passed radio waves through the nucleus between the lander and the orbiter.

Based on the most recent calculations using CONSERT data and detailed comet shape models, Philae’s location has been revised to an area covering 34 x 21 m. The best fit area is marked in red, a good fit is marked in yellow, with areas on the white strip corresponding to previous estimates now discounted. One lander candidate proposed previously in the vicinity lies 62 m from the red marked area of the new CONSERT region, suggesting this is no longer a viable candidate. Copyright ESA/Rosetta/Philae/CONSERT

Based on the most recent calculations using CONSERT data and detailed comet shape models, Philae’s location has been revised to an area covering 34 x 21 m. The best fit area is marked in red, a good fit is marked in yellow, with areas on the white strip corresponding to previous estimates now discounted. One lander candidate proposed previously in the vicinity lies 62 m from the red marked area of the new CONSERT region, suggesting this is no longer a viable candidate. Copyright ESA/Rosetta/Philae/CONSERT

The results show that the small lobe of the comet is consistent with a very loosely compacted (porosity 75–85%) mixture of dust and ice (dust-to-ice ratio 0.4–2.6 by volume) that is fairly homogeneous on the scale of tens of metres.

In addition, CONSERT was used to help triangulate Philae’s location on the surface, with the best fit solution currently pointing to a 21 x 34 m area.

“Taken together, these first pioneering measurements performed on the surface of a comet are profoundly changing our view of these worlds and continuing to shape our impression of the history of the Solar System,” says Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument at the IAS in Orsay, France.

“The reactivation would allow us to complete the characterisation of the elemental, isotopic and molecular composition of the cometary material, in particular of its refractory phases, by APXS, CIVA-M, Ptolemy and COSAC.”

“With Philae making contact again in mid-June, we still hope that it can be reactivated to continue this exciting adventure, with the chance for more scientific measurements and new images which could show us surface changes or shifts in Philae’s position since landing over eight months ago,” says DLR’s Lander Manager Stephan Ulamec.

“These ground-truth observations at a couple of locations anchor the extensive remote measurements performed by Rosetta covering the whole comet from above over the last year,” says Nicolas Altobelli, ESA’s acting Rosetta project scientist.

“With perihelion fast approaching, we are busy monitoring the comet’s activity from a safe distance and looking for any changes in the surface features, and we hope that Philae will be able to send us complementary reports from its location on the surface.”

Source: ESA

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