26. April 2019
Article by the VIRTIS scientists in Nature Astronomy

Roset­ta 'post-mis­sion' – new find­ings re­lat­ing to the tem­per­a­ture and na­ture of the comet's sur­face

Measured and modelled surface temperatures of Comet 67P
Mea­sured and mod­elled sur­face tem­per­a­tures of Comet 67P
Image 1/4, Credit: VIRTIS Team.

Measured and modelled surface temperatures of Comet 67P

(a) Ther­mal im­age de­rived from the VIR­TIS da­ta of 22 Au­gust 2014, ac­quired at a dis­tance of 60 kilo­me­tres above the comet sur­face (spa­tial res­o­lu­tion 15 me­tres per pix­el). The cold­est tem­per­a­tures are shown in dark red tones, while the warmest tem­per­a­tures are whitish.

(b) Re­sult of ther­mo­phys­i­cal mod­elling (see orig­i­nal pa­per) for the same pe­ri­od of time as the VIR­TIS ob­ser­va­tions shown in (a). The grey (shad­ed) colour­ing in­di­cates points with mod­elled tem­per­a­ture val­ues low­er than mi­nus 113 de­grees Cel­sius, for which nei­ther the VIR­TIS da­ta nor the mod­el can pro­vide ac­cu­rate val­ues. Oth­er spu­ri­ous grey dots are due to the pro­jec­tion of the da­ta on­to the shape mod­el.

(c) Dif­fer­ence be­tween the tem­per­a­ture val­ues mea­sured by VIR­TIS and rep­re­sent­ed in (a), and the the­o­ret­i­cal tem­per­a­ture val­ues shown in (b). In this im­age, the blue and blue-green colours mark ar­eas of the nu­cle­us where the mod­elled tem­per­a­ture val­ues are sig­nif­i­cant­ly greater than the mea­sured ones, while the in­verse re­la­tion is in­di­cat­ed by the colours from yel­low to red. The green colour shows ar­eas where there is sub­stan­tial agree­ment be­tween the ob­served and cal­cu­lat­ed da­ta.

(d) Re­sult of ther­mo­phys­i­cal mod­elling with no ice and 10 per­cent sur­face rough­ness. Colour cod­ing is as de­scribed above for (a). (e) Dif­fer­ence be­tween the tem­per­a­ture val­ues mea­sured by VIR­TIS and rep­re­sent­ed in (a) and the the­o­ret­i­cal tem­per­a­ture val­ues shown in (d). Colour cod­ing is as de­scribed above for (c).
False-colour image of the Hapi region on 67P
False-colour im­age of the Hapi re­gion on 67P

False-colour image of the Hapi region on 67P

False colour im­age of the Hapi re­gion on Comet 67P / Churyu­mov-Gerasi­menko. This re­gion con­nects the ‘head’ and ‘body’ of the comet. In this im­age, the dif­fer­ences in re­flec­tiv­i­ty have been in­creased to em­pha­sise the bluish col­oration of this re­gion. Knowl­edge about the re­flec­tiv­i­ty is key to de­ter­min­ing the sur­face com­po­si­tion. The bluish col­oration in­di­cates the pres­ence of wa­ter ice on or di­rect­ly be­neath the dusty sur­face. The da­ta used to cre­ate this im­age were ac­quired on 21 Au­gust 2014 with the Op­ti­cal, Spec­tro­scop­ic and In­frared Re­mote Imag­ing Sys­tem (OSIRIS) on board Roset­ta. The space­craft was 70 kilo­me­tres away from the comet dur­ing da­ta ac­qui­si­tion.
Comet 67P/Churyumov-Gerasimenko on 3 August 2014
Comet 67P/Churyu­mov-Gerasi­menko on 3 Au­gust 2014

Comet 67P/Churyumov-Gerasimenko on 3 August 2014

Im­age of Comet 67P/Churyu­mov-Gerasi­menko ac­quired by Roset­ta’s OSIRIS cam­era on 3 Au­gust 2014 at a dis­tance of 285 kilo­me­tres. The res­o­lu­tion is 5.3 me­tres per pix­el.
VIRTIS – Visible and Infrared Thermal Imaging Spectromete
VIR­TIS – Vis­i­ble and In­frared Ther­mal Imag­ing Spec­trom­e­ter
Image 4/4, Credit: ESA – J. HUART

VIRTIS – Visible and Infrared Thermal Imaging Spectrometer

The VIR­TIS in­stru­ment on board the Roset­ta or­biter mea­sures the tem­per­a­ture, the chem­i­cal and min­er­alog­i­cal com­po­si­tion and the dis­tri­bu­tion of el­e­ments on the comet’s nu­cle­us and in its co­ma by ob­serv­ing in the vis­i­ble and near-in­frared parts of the spec­trum.
  • Some years after the end of the Rosetta mission, scientists are continuing to analyse the data acquired by Rosetta to obtain important insights for comet research and the study of the early Solar System.
  • Focus: Spaceflight, exploration

Two-and-a-half years have passed since the operational phase of the Rosetta mission came to an end in September 2016. However, scientific evaluation of the enormous amounts of data from the instruments on the spacecraft and the Philae lander is still ongoing. The team of scientists working on the VIRTIS instrument have now published new findings relating to the surface temperature and thermal effects on the 'duck-shaped' Comet 67P / Churyumov-Gerasimenko in the 22 April 2019 issue of Nature Astronomy. Germany's scientific contributions to VIRTIS are led by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).

The Visible InfraRed and Thermal Imaging Spectrometer (VIRTIS) acquired infrared images of the comet from on board the Rosetta orbiter during August and September 2014, approximately one year before the comet reached perihelion – the point in its orbit closest to the Sun. During the period under consideration, the comet was still distant from the Sun, and its level of activity was low. The researchers converted the images into thermal maps.

Temperature is the most important parameter for deriving the gas and dust activity typical of comets. First, the VIRTIS team measured the average temperature of the comet's nucleus on its daytime side. While the average surface temperature over the two months was approximately minus 60 degrees Celsius, the scientists also identified places that were significantly warmer, at around minus 43 degrees Celsius. These included a cavity in the surface, where the inner walls reflected the thermal radiation and thus led to stronger warming, referred to as self-heating.

Self-heating also occurs at the 'duck's neck' connecting the two lobes of the comet. Temperatures were higher here than the laws of black-body radiation would imply. Assuming a dust-dominated surface a few millimetres thick and minimal sublimation of volatile substances, self-heating is attributable to surface roughness. The self-heating effect is enhanced by the striking concave shape of the 'neck'.

Another significant finding concerns the thermal gradients caused by sudden shadows cast alternately onto the 'neck' by the two lobes of the comet during solar illumination. These localised shadows on the 'neck' created extreme temperature differences within the space of just a few minutes, which might be 10 times greater than normal daily variations in temperature on other areas of the surface. "To better investigate seasonal temperature effects on the nucleus, we concentrated on a region named Imhotep, which is relatively flat and far from the 'neck', and where the self-heating effect is significantly lower," says Gabriele Arnold of the DLR Institute of Planetary Research. "For this area, we compared the observations performed by VIRTIS with those of the Microwave Instrument for the Rosetta Orbiter (MIRO), another instrument on board Rosetta. MIRO made it possible to measure the temperature in larger depressions on the comet. The findings of the two instruments can be explained by the theory that the Imhotep region has a thin surface layer consisting mainly of loose dust."

Imhotep was also observed a few months later, when the comet was much closer to the Sun. The temperature values obtained by VIRTIS were much higher than before, but lower than expected, given that the scientists were working on the assumption that the surface layer consisted only of loose dust. This led the researchers to conclude that the composition of the uppermost layers must have changed over time. The quantity of volatiles within it must have increased, resulting in a higher degree of sublimation and more intense comet activity. This in turn can cause surface temperatures to be lower than would be reached by a layer consisting solely of dust.

All the observational evidence suggests a comet nucleus with thermal behaviour that is dominated by phenomena associated with the morphology and chemical and physical state of the thin uppermost surface layer, which is only a few centimetres thick. In the subsurface, the nucleus is thought to remain essentially unchanged and has only been weakly influenced by previous approaches to the Sun.

Gabriele Arnold sums up: "The work that has just been published shows that the ongoing evaluation of the large quantity of data acquired will continue to provide unique findings for comet research and the study of the early Solar System, even years after the end of the Rosetta mission."

  • The Rosetta comet mission

    After more than 20 years of scientists and engineers working on the Rosetta mission, the 10-year journey to Comet 67P / Churyumov-Gerasimenko and nearly two years of scientific data acquisition from orbit and on the surface by the Philae lander, the operational part of the mission came to an end in September 2016; in other words, #GoodbyePhilae.

    Rosetta was the first space mission to closely accompany a comet on its journey around the Sun. Among the many discoveries made about Comet 67P, Rosetta performed direct and repeated measurements of the surface temperature of a comet nucleus with an unprecedented spatial resolution. From this, the thermal properties and activity patterns of the nucleus could be derived.

  • About VIRTIS

    The VIRTIS instrument on board the Rosetta orbiter acquired infrared images of the comet nucleus, which were converted into thermal maps. This allowed changes in the temperature of the nucleus to be continuously studied for almost two months in late summer 2014, around a year before the comet reached perihelion.

    VIRTIS has provided information on the composition of the comet's nucleus and the distribution of material on its surface, and the gases and molecules within the coma. VIRTIS was built by a consortium of teams under the scientific leadership of the Istituto di Astrofisica e Planetologia Spaziali (IAPS) at the Italian National Institute for Astrophysics (Istituto Nazionale di Astrofisica; INAF) in Rome, which was also in charge of scientific operations. The consortium included the Laboratoire d'Études Spatiales et d'Instrumentation en Astrophysique (LESIA) at the Observatoire de Paris in France, and the DLR Institute of Planetary Research in Germany.

    The development of the instrument was funded and coordinated by the various national space agencies: Agenzia Spaziale Italiana (ASI, Italy), Centre National d’Études Spatiales (CNES, France) and DLR (Germany). The mission was supported by the Rosetta Science Ground Segment and the Rosetta Mission Operations Centre.
  • Philipp Burtscheidt
    Ger­man Aerospace Cen­ter (DLR)
    Pub­lic Af­fairs and Com­mu­ni­ca­tions
    Telephone: +49 2203 601-2323
    Fax: +49 2203 601-3249
  • Dr Gabriele Arnold
    Ger­man Aerospace Cen­ter (DLR)

    DLR In­sti­tute of Plan­e­tary Re­search
    Telephone: +49 30 67055-370
    Linder Höhe
    51147 Köln
  • Dr.rer.nat. Ekkehard Kührt
    Ger­man Aerospace Cen­ter (DLR)

    In­sti­tute of Plan­e­tary Re­search, As­ter­oids and Comets
    Telephone: +49 30 67055-514
    Fax: +49 30 67055-340
    Linder Höhe
    51147 Köln

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