[meteorite-list] Asteroid Lutetia: Postcard From The Past (Rosetta)
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 27 Oct 2011 14:51:49 -0700 (PDT)
Asteroid Lutetia: postcard from the past
European Space Agency
27 October 2011
ESA's Rosetta spacecraft has revealed asteroid Lutetia to be a primitive
body, left over as the planets were forming in our Solar System. Results
from Rosetta's fleeting flyby also suggest that this mini-world tried to
grow a metal heart.
Rosetta flew past Lutetia on 10 July 2010 at a speed of 54 000 km/hr and
a closest distance of 3170 km. At the time, the 130 km-long asteroid was
the largest encountered by a spacecraft. Since then, scientists have
been analysing the data taken during the brief encounter.
All previous flybys went past objects, which were fragments of
once-larger bodies. However, during the encounter, scientists speculated
that Lutetia might be an older, primitive 'mini-world'.
Now they are much more certain. Images from the OSIRIS camera reveal
that parts of Lutetia's surface are around 3.6 billion years old. Other
parts are young by astronomical standards, at 50-80 million years old.
Astronomers estimate the age of airless planets, moons, and asteroids by
counting craters. Each bowl-shaped depression on the surface is made by
an impact. The older the surface, the more impacts it will have
accumulated. Some parts of Lutetia are heavily cratered, implying that
it is very old.
On the other hand, the youngest areas of Lutetia are landslides,
probably triggered by the vibrations from particularly jarring nearby
Debris resulting from these many impacts now lies across the surface as
a 1 km-thick layer of pulverised rock.
There are also boulders strewn across the surface: some are 300-400 m
across, or about half the size of Ayers Rock, in Australia.
Some impacts must have been so large that they broke off whole chunks of
Lutetia, gradually sculpting it into the battered wreck we see today.
"We don't think Lutetia was born looking like this," says Holger Sierks,
Max-Planck-Institut f??r Sonnensystemforschung, Lindau, Germany. "It was
probably round when it formed."
Rosetta's VIRTIS spectrometer found that Lutetia's composition is
remarkably uniform across all the observed regions.
"It is striking that an object of this size can bear scars of events so
different in age across its surface while not showing any sign of
surface compositional variation," says Fabrizio Capaccioni, INAF, Rome,
This is just the start of the mystery.
Rosetta also let scientists investigate beneath the asteroid's surface.
It appears that Lutetia tried to grow an iron core like a bona-fide
planet when it formed.
During the encounter, Lutetia's weak gravity tugged on Rosetta. The
slight change in Rosetta's path was reflected in radio signals received
back at Earth, indicating a mass of 1.7 million billion tonnes.
This was a surprise.
"The mass was lower than expected. Ground-based observations had
suggested much higher values," says Martin P??tzold, Universit??t zu K??ln,
Germany, leader of the radio science team.
Nevertheless, when combined with its volume, Lutetia still turns out to
have one of the highest densities of any known asteroid: 3400 kg per
cubic metre. The density implies that Lutetia contains significant
quantities of iron, but not necessarily in a fully formed core.
To form an iron core, Lutetia would have had to melt as a result of heat
released by radioactive isotopes in its rocks. The dense iron would then
sink to the centre and the rocky material would float to the top.
However, VIRTIS indicates that Lutetia's surface composition remains
entirely primordial, displaying none of the rocky material expected to
form during such a molten phase.
The only explanation appears to be that Lutetia was subjected to some
internal heating early in its history but did not melt completely and so
did not end up with a well-defined iron core.
These results, all gathered during just a short flyby, make Lutetia a
unique asteroid and an invaluable postcard from the past, at a time when
Earth was forming.
"We picked a most important member of the asteroid belt," said Rita
Schulz, ESA's Rosetta Project Scientist.
"All the asteroids encountered so far were different from each other,
but Lutetia is the only one in which both primordial and differentiation
features have been found.
"These unexpected results clearly show that there is still much more to
investigate before we understand the belt fully."
Having now left Lutetia far behind, Rosetta is in hibernation and en
route to its 2014 rendezvous with comet Churyumov???Gerasimenko.
Contact for further information
Notes to editors
OSIRIS is the Optical, Spectroscopic & Infrared Remote Imaging System.
The principal investigator is Holger Sierks, Max-Planck-Institut f??r
Sonnensystemforschung, Lindau, Germany.
VIRTIS is the Visible, InfraRed and Thermal Imaging Spectrometer. The
principal investigator is Fabrizio Capaccioni, INAF, Rome, Italy.
The Radio Science Investigation principal investigator is Martin
P??tzold, Universit??t zu K??ln, Germany.
'The Surface Composition and Temperature of Asteroid 21 Lutetia as
observed by ROSETTA/VIRTIS' by A. Coradini, F. Capaccioni et al. is
published in this week's issue of Science.
'Asteroid (21) Lutetia ??? Low Mass, High Density' by M. P??tzold et al. is
published in this week's issue of Science.
'Images of Asteroid 21 Lutetia: A Remnant Planetesimal from the Early
Solar System' by H. Sierks et al. is published in this week's issue of
For further information, please contact
ESA Science and Robotic Exploration Communication Officer
Email: markus.bauer at esa.int <mailto:markus.bauer at esa.int>
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
INAF, Rome, Italy
Tel: +39 331 6865567
Email: fabrizio.capaccioni at iasf-roma.inaf.it
<mailto:fabrizio.capaccioni at iasf-roma.inaf.it>
Universit??t zu K??ln, Germany
Email: mpaetzol at uni-koeln.de <mailto:mpaetzol at uni-koeln.de>
Max-Planck-Institut f??r Sonnensystemforschung, Lindau, Germany
Tel: +49 5556 979 242
Email: sierks at mps.mpg.de <mailto:sierks at mps.mpg.de>
ESA Rosetta Project Scientist
Email: rschulz at rssd.esa.int <mailto:rschulz at rssd.esa.int>
Received on Thu 27 Oct 2011 05:51:49 PM PDT