Copyright: Spacecraft: ESA/ATG medialab;
Data: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR; G. Filacchione et al (2016)
Rosetta’s comet has been seen changing colour and brightness in front of
the ESA orbiter’s eyes, as the Sun’s heat strips away the older surface
to reveal fresher material.
Rosetta’s Visible and InfraRed Thermal Imaging Spectrometer, VIRTIS,
began to detect these changes in the sunlit parts of Comet
67P/Churyumov–Gerasimenko – mostly the northern hemisphere and
equatorial regions – in the months immediately following the
spacecraft’s arrival in August 2014.
A new paper, published in the journal Icarus, reports on the
early findings of this study, up to November 2014, during which time
Rosetta was operating between 100 km to within 10 km of the comet
nucleus. At the same time, the comet itself moved along its orbit closer
to the Sun, from about 542 million km to 438 million km.
VIRTIS monitored the changes in light reflected from the surface over a
wide range of visible and infrared wavelengths, as an indicator of
subtle changes in the composition of the comet’s outermost layer.
Copyright: ESA/Rosetta/NAVCAM
When it arrived, Rosetta found an extremely dark body, reflecting about
6% of the visible light falling on it. This is because the majority of
the surface is covered with a layer of dark, dry, dust made out of
mixture of minerals and organics.
Some surfaces are slightly brighter, some slightly darker, indicating
differences in composition. Most of the surface is slightly reddened by
organic-rich material, while the occasional ice-rich material shows up
as somewhat bluer.
Even when Rosetta first rendezvoused with the comet far from the Sun,
ices hidden below the surface were being gently warmed, sublimating into
gas, and escaping, lifting some of the surface dust away and
contributing to the comet’s coma and tail.
VIRTIS shows that as the ‘old’ dust layers were slowly ejected, fresher
material was gradually exposed.
This new surface was both more
reflective, making the comet brighter, and richer in ice, resulting in
bluer measurements.
Copyright: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
On average, the comet’s brightness changed by about 34%. In the Imhotep
region, it increased from 6.4% to 9.7% over the three months of
observations.
“The overall trend seems to be that there is an increasing water-ice
abundance in the comet’s surface layers that results in a change in the
observed spectral signatures. In that respect, it’s like the comet is
changing colour in front of our eyes,” says Gianrico Filacchione, lead
author of the study.
“This evolution is a direct consequence of the activity occurring on and
immediately beneath the comet’s surface. The partial removal of the
dust layer caused by the start of gaseous activity is the probable cause
of the increasing abundance of water ice at the surface.”
“The surface properties are really dynamic, changing with the distance
from the Sun and with the levels of comet activity,” adds Fabrizio
Capaccioni, VIRTIS principal investigator.
“We’ve started analysing the subsequent datasets and can already see
that the trend continues in the observations made beyond November 2014.”
“The evolution of surface properties with activity has never been
observed by a cometary mission before and is a major science objective
of the Rosetta mission,” says Matt Taylor, ESA’s Rosetta Project
Scientist.
“It is great to see science papers being published directly addressing
this topic and we’re looking forward to seeing how things have changed
over the entire mission.”
Notes for Editors
“The global surface composition of 67P/CG nucleus by Rosetta/VIRTIS. 1)
Pre-landing phase,” by G. Filacchione et al. is published in Icarus. doi:10.1016/j.icarus.2016.02.055
A follow-up paper is in preparation covering the period November 2014 to May 2015.
For further information, please contact:
Gianrico Filacchione
VIRTIS deputy principal investigator
INAF-IAPS, Rome, Italy
Email: gianrico.filacchione@iaps.inaf.it
Fabrizio Capaccioni
VIRTIS principal investigator
INAF-IAPS, Rome, Italy
Email: fabrizio.capaccioni@iaps.inaf.it
Matt Taylor
ESA Rosetta Project Scientist
Email: matt.taylor@esa.int
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Source: ESA/Rosetta
