[meteorite-list] Rosetta Flyby Uncovers the Complex History of Asteroid Lutetia

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 30 May 2012 14:37:13 -0700 (PDT)
Message-ID: <201205302137.q4ULbED1000631_at_zagami.jpl.nasa.gov>

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50394

Rosetta flyby uncovers the complex history of asteroid Lutetia
European Space Agency
29 May 2012

The long and tumultuous history of asteroid (21) Lutetia is revealed by
a comprehensive analysis of the data gathered by ESA's Rosetta
spacecraft when it flew past this large main-belt asteroid on 10 July
2010. New studies have revealed the asteroid's surface morphology,
composition and other properties in unprecedented detail. In particular,
extensive studies of Lutetia's geological features have opened a unique
window into the complex history of this peculiar object.

On its way to rendezvous with comet 67P/Churyumov-Gerasimenko, ESA's
Rosetta spacecraft flew by the main-belt asteroid (21) Lutetia, reaching
the closest approach, at a distance of about 3170 km, on 10 July 2010.
>From this unique vantage point, Rosetta gathered high-resolution images,
spectra, and other data, providing scientists with a valuable dataset
with which to probe this peculiar asteroid in great detail.

The first results from the flyby, published in late 2011, revealed the
mass and volume of Lutetia, leading to an estimate of the asteroid's
density, which turned out to be surprisingly high. Data from the flyby
also suggested that Lutetia is a primordial planetesimal formed during
the very early phases of the Solar System. These and other findings
called for further investigations about the nature and history of Lutetia.

"The images collected by Rosetta during the flyby have uncovered, for
the first time, the wide variety of craters and other geological
features that scar the surface of Lutetia," notes Rita Schulz, Rosetta
Project Scientist at ESA. "Scientists have explored this rich pool of
data thoroughly in order to characterise many of Lutetia's properties,
from its surface morphology and composition to its shape and internal
structure, revealing its underlying geological history," she adds. The
results of these studies are reported in a series of 21 papers published
in a special issue of the journal Planetary and Space Science.

The OSIRIS camera on Rosetta has surveyed the part of Lutetia that was
visible during the flyby ??? about half of its entire surface, mostly
coinciding with the asteroid's northern hemisphere. These unique,
close-up images have allowed scientists to identify regions
characterised by very distinct geological properties with an accuracy of
a few hundred metres.

Counting craters is a powerful tool that is used to compare the regions
and to uncover their past history. By recording the number, spatial
distribution, shapes and sizes of the hundreds of craters that mark the
surface of each region, it is possible to date the epoch when these
craters were produced by collisions with smaller bodies. In the case of
the largest craters, it is even possible to reconstruct the details of
the impact that created them.

By tracing craters and other features on Lutetia's surface, scientists
have put together a geological map for the asteroid. Their studies have
shown that Lutetia's surface comprises regions spanning a wide range of
ages: each of them reveals a chapter in the long and tumultuous history
of this asteroid.

At one end of this age spectrum, the two heavily cratered Achaia and
Noricum regions represent the most ancient portions on the surface of
Lutetia: with ages between 3.4 and 3.7 billion years or more, they are
almost as old as the asteroid itself. Some of the craters that densely
populate these two regions date back to an early epoch in the Solar
System's history, right after the so-called Late Heavy Bombardment, when
the flux of bodies impacting asteroids, planets and their satellites was
significantly larger than it is at present.

Massilia, the largest crater identified on the asteroid, is located in a
younger region named Narbonensis. With a diameter of 57 km, this crater
provides evidence of the most dramatic event in the history of Lutetia:
numerical simulations suggest that the 'projectile' responsible for
producing this very wide crater was quite large, with a diameter of
about 7.5 km. However, the probability of such a large body colliding
with the asteroid is quite low, and so this must have occurred when
Lutetia was relatively young.

The youngest patch on the surface of Lutetia is the Baetica region,
located in the vicinity of the asteroid's North Pole. This region hosts
a number of superimposed craters, named the North Polar Crater Cluster
(NPCC), which include three large ones with sizes exceeding 10 km. These
craters represent the signature left by a series of subsequent impacts
that took place quite recently on geological timescales ??? namely, in the
last few hundred million years.

The smooth appearance of the craters in Baetica, which have not been
dotted yet with many smaller craters, indicate that its surface is much
younger than the heavily battered areas of Lutetia. Furthermore, this
region still bears signs of the events that created the NPCC, as
indicated by the ejecta that were released during the impacts and then
spread on the surrounding area, rather than leaving the asteroid's
surface, as a result of its relatively strong gravitational pull. The
presence of these 'fresh' deposits, which include many large boulders
with sizes up to 300 metres, is another hint at this region's young age.

In addition to craters, other geological markers, such as lineaments and
faults, represent an important window into the turbulent past of
asteroids and other Solar System bodies. The remarkable images collected
by OSIRIS during the flyby have revealed an intricate network of linear
features covering long distances across Lutetia's surface, up to 80 km
in some cases. Many of these features are the results of seismic
phenomena that also caused deformations on pre-existing craters.
Lineaments and faults have been mostly detected in the oldest portions
of Lutetia's surface. In contrast, a lack of such features in the young
region near the North Pole suggests that the recent impacts that gave
rise to the NPCC did not cause significant fractures on the surface of
the asteroid.

Prior to the flyby, one of the most puzzling aspects of Lutetia was its
surface composition: different datasets have hinted at either a metallic
or a chondritic composition, thus making the classification of this
asteroid particularly problematic. Scientists have now addressed the
issue by combining data gathered with four remote-sensing instruments on
Rosetta ??? OSIRIS, VIRTIS, MIRO, and ALICE ??? which cover visible,
infrared, microwave and ultraviolet wavelengths. The new data show that
Lutetia has an unusual surface composition that does not fit into the
schemes established before the flyby and may result from the complex
collision history of the asteroid.

The peculiar composition of Lutetia, when considered along with its high
density, raises the possibility that this asteroid might have a
partially differentiated structure, with a metallic core overlain by a
primitive chondritic crust. The only other differentiated asteroid that
has been visited by a spacecraft is Vesta, one of the largest asteroids
in the Solar System and significantly larger than Lutetia. Whereas it is
reasonable to expect such an internal structure in asteroids as large as
Vesta, it is still unclear whether this should be the case also for
objects of Lutetia's size. Therefore the possible evidence of a
differentiated structure suggested by the new data is particularly
intriguing.

The flyby of Lutetia also provided a rare opportunity to obtain 'in
situ' measurements of the surrounding environment of the asteroid,
allowing scientists to search for an exosphere, an internal magnetic
field or satellites. All three searches did not find significant
evidence for any of these possibilities, and could only set upper limits.

"The data collected during Rosetta's flyby of Lutetia have provided us
with a brand new view on this intriguing object," comments Schulz. "I
expect that scientists will continue to investigate these unique and
extraordinary data for years to come, pushing forward our knowledge
about this asteroid and its origin, and revealing new details about the
Solar System's past history," she adds.

In the meantime, Rosetta proceeds towards its final destination, comet
67P/Churyumov-Gerasimenko, which will be reached in 2014. "We are
eagerly awaiting the next and probably most exciting phase of the
mission," concludes Schulz.

Related publication
Rosetta Fly-by at Asteroid (21) Lutetia. Special issue of Planetary
and Space Science, Volume 66, Issue 1, Pages 1-212 (June 2012)

Notes for editors

On its 10-year journey towards comet 67P/Churyumov-Gerasimenko, ESA's
Rosetta spacecraft has flown past two main-belt asteroids: (2867) Steins
in 2008, and (21) Lutetia in 2010. The flyby of Lutetia took place on 10
July 2010, when Rosetta flew past the asteroid at a distance of 3168.2
km and at a relative speed of 15 km/s.

Most of the scientific instruments on Rosetta were switched on as the
spacecraft approached the rotating asteroid, resulting in imaging and
spectral observations covering a spectral range from ultraviolet to
microwave radiation, and a number of in-situ measurements of the
asteroid's environment.

The Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS)
returned 462 pictures of the illuminated northern hemisphere of Lutetia
through both its narrow-angle camera (NAC) and wide-angle camera (WAC).
These images cover more than 50 per cent of the asteroid's surface and
were instrumental in revealing its surface in unprecedented detail.

The ALICE, VIRTIS and MIRO instruments were used to gather spectra at
ultraviolet, infrared and microwave wavelengths, respectively, in order
to probe the chemical composition of Lutetia's surface.

In-situ searches for an exosphere were conducted with the ROSINA
instrument; spectra from the COSAC/Philae, Ptolemy/Philae and ALICE
instruments were also used to search for the asteroid's exosphere.
Magnetic field measurements were performed with the ROMAP, RPC-MAG/OB
and RPC-MAG/IB sensors.


Contacts

Rita Schulz
ESA Rosetta Project Scientist
Research and Scientific Support Department
Science and Robotic Exploration Directorate
ESA, The Netherlands
Email: rschulz at rssd.esa.int
Phone: +31-71-565-4821
Received on Wed 30 May 2012 05:37:13 PM PDT


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