The long and tumultuous history of asteroid (21) Lutetia isrevealed by a comprehensive analysis of the data gathered by ESA'sRosetta spacecraft when it flew past this large main-belt asteroidon 10 July 2010. New studies have revealed the asteroid's surfacemorphology, composition and other properties in unprecedenteddetail. In particular, extensive studies of Lutetia's geological featureshave opened a unique window into the complex history of thispeculiar 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 3170km, on 10 July 2010. |
From this unique vantage point, Rosettagathered high-resolution images, spectra, and other data, providingscientists with a valuable dataset with which to probe thispeculiar asteroid in great detail. The first results from the flyby, published in late 2011, revealedthe mass and volume of Lutetia, leading to an estimate of theasteroid's density, which turned out to be surprisingly high. Data from the flyby also suggested that Lutetia is a primordialplanetesimal formed during the very early phases of the SolarSystem. These and other findings called for further investigationsabout 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 othergeological features that scar the surface of Lutetia," notes RitaSchulz, Rosetta Project Scientist at ESA.
"Scientists have explored this rich pool of data thoroughly inorder to characterise many of Lutetia's properties, from itssurface morphology and composition to its shape and internalstructure, revealing its underlying geological history," she adds.The results of these studies are reported in a series of 21 paperspublished in a special issue of the journal Planetary and SpaceScience. The OSIRIS camera on Rosetta has surveyed the part of Lutetia thatwas 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 identifyregions characterised by very distinct geological properties withan accuracy of a few hundred metres. Counting craters is a powerful tool that is used to compare theregions and to uncover their past history. By recording the number,spatial distribution, shapes and sizes of the hundreds of cratersthat mark the surface of each region, it is possible to date theepoch when these craters were produced by collisions with smallerbodies.
In the case of the largest craters, it is even possible toreconstruct 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 regionsspanning a wide range of ages: each of them reveals a chapter inthe long and tumultuous history of this asteroid. At one end of this age spectrum, the two heavily cratered Achaiaand Noricum regions represent the most ancient portions on thesurface of Lutetia: with ages between 3.4 and 3.7 billion years ormore, they are almost as old as the asteroid itself. Some of the craters that densely populate these two regions dateback to an early epoch in the Solar System's history, right afterthe so-called Late Heavy Bombardment, when the flux of bodiesimpacting asteroids, planets and their satellites was significantlylarger than it is at present.
Massilia, the largest crater identified on the asteroid, is locatedin a younger region named Narbonensis. With a diameter of 57 km,this crater provides evidence of the most dramatic event in thehistory of Lutetia: numerical simulations suggest that the'projectile' responsible for producing this very wide crater wasquite large, with a diameter of about 7.5 km. However, the probability of such a large body colliding with theasteroid is quite low, and so this must have occurred when Lutetiawas 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 regionhosts a number of superimposed craters, named the North PolarCrater Cluster (NPCC), which include three large ones with sizesexceeding 10 km. These craters represent the signature left by a series ofsubsequent impacts that took place quite recently on geologicaltimescales - namely, in the last few hundred million years. The smooth appearance of the craters in Baetica, which have notbeen dotted yet with many smaller craters, indicate that itssurface is much younger than the heavily battered areas of Lutetia. Furthermore, this region still bears signs of the events thatcreated the NPCC, as indicated by the ejecta that were releasedduring the impacts and then spread on the surrounding area, ratherthan leaving the asteroid's surface, as a result of its relativelystrong gravitational pull. The presence of these 'fresh' deposits,which include many large boulders with sizes up to 300 metres, isanother hint at this region's young age.
In addition to craters, other geological markers, such aslineaments and faults, represent an important window into theturbulent past of asteroids and other Solar System bodies. The remarkable images collected by OSIRIS during the flyby haverevealed an intricate network of linear features covering longdistances across Lutetia's surface, up to 80 km in some cases. Many of these features are the results of seismic phenomena thatalso caused deformations on pre-existing craters. Lineaments andfaults have been mostly detected in the oldest portions ofLutetia's surface.
In contrast, a lack of such features in theyoung region near the North Pole suggests that the recent impactsthat gave rise to the NPCC did not cause significant fractures onthe surface of the asteroid. Prior to the flyby, one of the most puzzling aspects of Lutetia wasits surface composition: different datasets have hinted at either ametallic or a chondritic composition, thus making theclassification of this asteroid particularly problematic. Scientists have now addressed the issue by combining data gatheredwith four remote-sensing instruments on Rosetta - OSIRIS, VIRTIS,MIRO, and ALICE - which cover visible, infrared, microwave andultraviolet wavelengths. The new data show that Lutetia has an unusual surface compositionthat does not fit into the schemes established before the flyby andmay result from the complex collision history of the asteroid.
The peculiar composition of Lutetia, when considered along with itshigh density, raises the possibility that this asteroid might havea partially differentiated structure, with a metallic core overlainby a primitive chondritic crust. The only other differentiated asteroid that has been visited by aspacecraft is Vesta, one of the largest asteroids in the SolarSystem and significantly larger than Lutetia. Whereas it is reasonable to expect such an internal structure inasteroids as large as Vesta, it is still unclear whether thisshould be the case also for objects of Lutetia's size. Thereforethe possible evidence of a differentiated structure suggested bythe new data is particularly intriguing. The flyby of Lutetia also provided a rare opportunity to obtain 'insitu' measurements of the surrounding environment of the asteroid,allowing scientists to search for an exosphere, an internalmagnetic field or satellites.
All three searches did not findsignificant evidence for any of these possibilities, and could onlyset upper limits. "The data collected during Rosetta's flyby of Lutetia have providedus with a brand new view on this intriguing object," commentsSchulz. "I expect that scientists will continue to investigate these uniqueand extraordinary data for years to come, pushing forward ourknowledge about this asteroid and its origin, and revealing newdetails 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.
"Weare eagerly awaiting the next and probably most exciting phase ofthe mission," concludes Schulz. Rosetta Fly-by at Asteroid (21) Lutetia. Special issue of Planetaryand Space Science, Volume 66, Issue 1, Pages 1-212 (June 2012).
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