Lutetia has an irregular shape and is heavily cratered, with the largestimpact crater reaching 45 km in diameter. Thesurface is geologically heterogeneous and is intersected by a system of grooves and scarps, which are thought to be fractures. It has a high overallbulk density, suggesting that it is made of metal-richrock.
TheRosetta probe passed within 3,162 km (1,965 mi) of Lutetia in July 2010.[8] It was the largest asteroidvisited by a spacecraft untilDawn arrived atVesta in July 2011, and it is currently the 3rd largest asteroid to have ever been imaged in high quality by a spacecraft (after Vesta andCeres).
Lutetia was discovered on 15 November 1852, byHermann Goldschmidt from the balcony of his apartment inParis.[9][10] A preliminary orbit for the asteroid was computed in November–December 1852 by German astronomerGeorg Rümker and others.[11] In 1903, it was photographed at opposition byEdward Pickering atHarvard College Observatory. He computed an oppositionmagnitude of 10.8.[12]
On 10 July 2010, theEuropeanRosettaspace probe flew by Lutetia at a minimum distance of3168 ± 7.5 km at a velocity of 15 kilometres per second on its way to thecomet67P/Churyumov-Gerasimenko.[4] The flyby provided images of up to 60 meters per pixel resolution and covered about 50% of the surface, mostly in the northern hemisphere.[3][8] The 462 images were obtained in 21 narrow- and broad-band filters extending from 0.24 to 1 μm.[8] Lutetia was also observed by the visible–near-infrared imaging spectrometer VIRTIS, and measurements of the magnetic field and plasma environment were taken as well.[3][8]
Lutetia orbits the Sun at the distance of approximately 2.4 AU in the inner asteroid belt. Its orbit lies almost in theplane of ecliptic and is moderately eccentric. The orbital period of Lutetia is 3.8 years.[13]
TheRosetta flyby demonstrated that the mass of Lutetia is (1.700 ± 0.017)×1018 kg,[4] smaller than the pre-flyby estimate of 2.57×1018 kg.[14] It has one of the highest densities seen in asteroids at 3.4 ± 0.3 g/cm3.[3] Taking into account possibleporosity of 10–15%, the bulk density of Lutetia exceeds that of a typical stony meteorite.[4]
TheRosetta probe found that the asteroid has a moderately red spectrum in visible light and an essentially flat spectrum in thenear infrared. No absorption features were detected in the range covered by observations, 0.4–3.5 μm, which is at odds with previous ground-based reports of hydrated minerals and carbon-rich compounds. There was also no evidence ofolivine. However, the spacecraft only observed half of Lutetia, so the existence of these phases cannot be completely ruled out. Together with the high bulk density reported for Lutetia, these results suggest that Lutetia is either made ofenstatite chondrite material, or may be related to metal-rich and water-poorcarbonaceous chondrite of classes like CB, CH, or CR.[5][21]
Rosetta observations revealed that the surface of Lutetia is covered with aregolith made of loosely aggregated dust particles 50–100 μm in size. It is estimated to be 3 km thick and may be responsible for the softened outlines of many of the larger craters.[3][8]
21 Lutetia's orbit, and its position on 1 January 2009 (NASA Orbit Viewer applet)
TheRosetta probe's photographs confirmed the results of a 2003lightcurve analysis that described Lutetia as a rough sphere with "sharp and irregular shape features".[22] A study from 2004 to 2009 proposed that Lutetia has a non-convex shape, likely because of a large crater,Suspicio Crater.[23] It is not yet clear whetherRosetta's findings support this claim.
The analysis ofRosetta images in combination with photometric light curves yielded the position of the north rotational pole of Lutetia:RA =51.8°±0.4°,Dec =+10.8°±0.4°. This gives anaxial tilt of 96° (retrograde rotator), meaning that the axis of rotation is approximatelyparallel to theecliptic, similar to the planetUranus.[3]
The surface of Lutetia is covered by numerousimpact craters and intersected by fractures,scarps and grooves thought to be surface manifestations of internal fractures. On the imaged hemisphere of the asteroid there are a total of 350 craters with diameters ranging from 600 m to 55 km. The most heavily cratered surfaces (in Achaia region) have a crater retention age of about 3.6 ± 0.1 billion years.[3]
The surface of Lutetia has been divided into seven regions based on their geology. They are Baetica (Bt), Achaia (AC), Etruria (Et), Narbonensis (Nb), Noricum (Nr), Pannonia (Pa), and Raetia (Ra). The Baetica region is situated around the north pole (in the center of the image) and includes a cluster of impact craters 21 km in diameter as well as their impact deposits. It is the youngest surface unit on Lutetia. Baetica is covered by a smooth ejecta blanket approximately 600 m thick that has partially buried older craters. Other surface features include landslides, gravitationaltaluses and ejecta blocks up to 300 m in size. The landslides and corresponding rockoutcrops are correlated with variations of albedo, being generally brighter.[3]
The two oldest regions are Achaia and Noricum. The former is a remarkably flat area with a lot of impact craters. The Narbonensis region coincides with the largest impact crater on Lutetia—Massilia. It includes a number of smaller units and is modified by pit chains and grooves formed at a later epoch. Other two regions—Pannonia and Raetia are also likely to be large impact craters. The last Noricum region is intersected by a prominent groove 10 km in length and about 100 m deep.[3]
The numerical simulations showed that even the impact that produced the largest crater on Lutetia, which is 45 km in diameter, seriously fractured but did not shatter the asteroid. So, Lutetia has likely survived intact from the beginning of the Solar System. The existence of linear fractures and the impact crater morphology also indicate that the interior of this asteroid has a considerable strength and is not arubble pile like many smaller asteroids. Taken together, these facts suggest that Lutetia should be classified as a primordialplanetesimal.[3]
Studies of patterns of fractures on Lutetia lead astronomers to think that there is a ~45 kilometer impact crater on the southern side of Lutetia, named Suspicio Crater, but becauseRosetta only observed Lutetia's northern part, it is not known for certain what it looks like, or if it exists at all.[24]
This animation is an artist's impression of a possible scenario to explain how Lutetia came to now be located in the asteroid belt.
In March, 2011, the Working Group for Planetary Nomenclature at theInternational Astronomical Union agreed on a naming scheme for geographical features on Lutetia. Since Lutetia was a Roman city, the asteroid's craters are named aftercities of the Roman Empire and the adjacent parts of Europe during the time of Lutetia's existence. Its regions are named after the discoverer of Lutetia (Goldschmidt) and afterprovinces of the Roman Empire at the time of Lutetia. Other features are named after rivers of the Roman Empire and the adjacent parts of Europe at the time of the city.[25]
The composition of Lutetia suggests that it formed in the inner Solar System, among the terrestrial planets, and was ejected into the asteroid belt through an interaction with one of them.[26]
^abCoradini, A.; Capaccioni, F.; Erard, S.; Arnold, G.; De Sanctis, M. C.; Filacchione, G.; Tosi, F.; Barucci, M. A.; Capria, M. T.; Ammannito, E.; Grassi, D.; Piccioni, G.; Giuppi, S.; Bellucci, G.; Benkhoff, J.; Bibring, J. P.; Blanco, A.; Blecka, M.; Bockelee-Morvan, D.; Carraro, F.; Carlson, R.; Carsenty, U.; Cerroni, P.; Colangeli, L.; Combes, M.; Combi, M.; Crovisier, J.; Drossart, P.; Encrenaz, E. T.; Federico, C. (2011). "The Surface Composition and Temperature of Asteroid 21 Lutetia As Observed by Rosetta/VIRTIS".Science.334 (6055):492–494.Bibcode:2011Sci...334..492C.doi:10.1126/science.1204062.PMID22034430.S2CID19439721.
^Leuschner, A. O. (1935). "Research surveys of the orbits and perturbations of minor planets 1 to 1091 from 1801.0 to 1929.5".Publications of Lick Observatory.19: 29.Bibcode:1935PLicO..19....1L.
^Pickering, Edward C. (January 1903). "Missing Asteroids".Harvard College Observatory Circular.69:7–8.Bibcode:1903HarCi..69....7P.
^Bell, J.F.; et al. (2015). Richard P. Binzel; Tom Gehrels; Mildred Shapley Matthews (eds.).Asteroids: The Big Picture in Asteroids II. University of Arizona Press. pp. 921–948.ISBN978-0-8165-1123-5.
^Feierberg, M; Witteborn, Fred C.; Lebofsky, Larry A. (1983). "Detection of silicate emission features in the 8- to 13 micrometre spectra of main belt asteroids".Icarus.56 (3): 393.Bibcode:1983Icar...56..393F.doi:10.1016/0019-1035(83)90160-4.
^Dollfus, A.; Geake, J. E. (1975). "Polarimetric properties of the lunar surface and its interpretation. VII – Other solar system objects".Proceedings of the 6th Lunar Science Conference, Houston, Texas, 17–21 March.3: 2749.Bibcode:1975LPSC....6.2749D.
^Belskaya, I. N.; Fornasier, S.; Krugly, Y. N.; Shevchenko, V. G.; Gaftonyuk, N. M.; Barucci, M. A.; Fulchignoni, M.; Gil-Hutton, R. (2010). "Puzzling asteroid 21 Lutetia: Our knowledge prior to the Rosetta fly-by".Astronomy and Astrophysics.515: A29.arXiv:1003.1845.Bibcode:2010A&A...515A..29B.doi:10.1051/0004-6361/201013994.S2CID16296351.
