) , mp_category=

Main belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...

, mpc_name=(21) Lutetia , orbit_ref = , epoch=May 31, 2020 ( JD 2459000.5) , semimajor=2.435 AU , perihelion=2.037 AU , aphelion=2.833 AU , eccentricity=0.16339 , period=3.80 yr (1388.1 d) , inclination=3.064° , asc_node=80.867° , arg_peri=249.997° , mean_anomaly=87.976° , dimensions=c/a = P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56
, mean_radius = , volume= , mass= , density=
, rotation=0.3402 d (8.1655 h) , axial_tilt = 96° , right_asc_north_pole = 51.8 ± 0.4° , declination = +10.8 ± 0.4° , spectral_type= M ( Tholen) , magnitude = 9.25 to 13.17 , abs_magnitude=7.29 , albedo=0.19 ± 0.01 (geometrical)
0.073 ± 0.002 (bond) , single_temperature=170–245 K Lutetia ( minor planet designation: 21 Lutetia) is a large M-type asteroid in the main

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, c ...

. It measures about 100 kilometers in

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...

(120 km along its major axis). It was discovered in 1852 by

Hermann Goldschmidt Hermann Mayer Salomon Goldschmidt (June 17, 1802 – August 30 or September 10 1866) was a German-French astronomer and painter who spent much of his life in France. He started out as a painter, but after attending a lecture by the famous Fren ...

, and is named after

Lutetia The Gallo-Roman town of ''Lutetia'' (''Lutetia Parisiorum'' in Latin, in French ''Lutèce'') was the predecessor of the modern-day city of Paris. It was founded in about the middle of the 3rd century BCE by the Parisii, a Gallic tribe. Trac ...

, the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

name of

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), ma ...

. Lutetia has an irregular shape and is heavily cratered, with the largest

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

reaching 45 km in diameter. The

surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...

is geologically heterogeneous and is intersected by a system of grooves and scarps, which are thought to be fractures. It has a high overall

bulk density Bulk density, also called apparent density or volumetric density, is a property of powders, granules, and other "divided" solids, especially used in reference to mineral components (soil, gravel), chemical substances, (pharmaceutical) ingredient ...

, suggesting that it is made of metal-rich rock. The ''

Rosetta Rosetta or Rashid (; ar, رشيد ' ; french: Rosette ; cop, ϯⲣⲁϣⲓⲧ ''ti-Rashit'', Ancient Greek: Βολβιτίνη ''Bolbitinē'') is a port city of the Nile Delta, east of Alexandria, in Egypt's Beheira governorate. The Ro ...

'' probe passed within of Lutetia in July 2010. It was the largest asteroid visited by a spacecraft until ''Dawn'' arrived at Vesta in July 2011.

Discovery and exploration

Lutetia was discovered on 15 November 1852, by

from the balcony of his apartment in

. A preliminary orbit for the asteroid was computed in November–December 1852 by German astronomer Georg Rümker and others. In 1903, it was photographed at opposition by Edward Pickering at

Harvard College Observatory The Harvard College Observatory (HCO) is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, United St ...

. He computed an opposition magnitude of 10.8. There have been two reported stellar

occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...

s by Lutetia, observed from

Malta Malta ( , , ), officially the Republic of Malta ( mt, Repubblika ta' Malta ), is an island country in the Mediterranean Sea. It consists of an archipelago, between Italy and Libya, and is often considered a part of Southern Europe. It lies ...

in 1997 and Australia in 2003, with only one chord each, roughly agreeing with

IRAS The Infrared Astronomical Satellite (Dutch: ''Infrarood Astronomische Satelliet'') (IRAS) was the first space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten mo ...

measurements. On 10 July 2010, the European ''

space probe A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; or ...

flew by Lutetia at a minimum distance of km at a velocity of 15 kilometres per second on its way to the

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...

67P/Churyumov-Gerasimenko. The flyby provided images of up to 60 meters per pixel resolution and covered about 50% of the surface, mostly in the northern hemisphere. The 462 images were obtained in 21 narrow- and broad-band filters extending from 0.24 to 1 μm. 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.

Characteristics

Orbit

Lutetia orbits the Sun at the distance of approximately 2.4 AU in the inner asteroid belt. Its orbit lies almost in the plane of ecliptic and is moderately eccentric. The orbital period of Lutetia is 3.8 years.

Mass and density

The ''

'' flyby demonstrated that the mass of Lutetia is (1.700 ± 0.017) kg, smaller than the pre-flyby estimate of 2.57 kg. It has one of the highest densities seen in asteroids at 3.4 ± 0.3 g/cm³. Taking into account possible

porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...

of 10–15%, the bulk density of Lutetia exceeds that of a typical stony meteorite.

Composition

Lutetia is classified among the enigmatic

M-type asteroid M-type (aka M-class) asteroids are a spectral class of asteroids which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes, and are widely thought to be the source of iron meteorites. Definition ...

s, most of which were historically believed to be nearly purely

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

lic. However, radar observations of the M-types suggest that two-thirds of them, including Lutetia, may instead consist of metal-enriched silicates. Indeed, telescopic spectra of Lutetia have shown a flat, low frequency

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

similar to that of carbonaceous chondrites and

C-type asteroid C-type (carbonaceous) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...

s and unlike metallic meteorites, evidence of

hydrated Drinking is the act of ingesting water or other liquids into the body through the mouth, proboscis, or elsewhere. Humans drink by swallowing, completed by peristalsis in the esophagus. The physiological processes of drinking vary widely among o ...

minerals, abundant

silicates In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name i ...

, and a thicker

regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestr ...

than most asteroids. The ''Rosetta'' probe found that the asteroid has a moderately red spectrum in visible light and an essentially flat spectrum in the

near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...

. 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 of

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

. 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 of

enstatite chondrite Enstatite chondrites (E-type chondrites) are a rare form of meteorite, rich in the mineral enstatite. Only about 200 E-Type chondrites are currently known, comprising about 2% of the chondrites that fall on Earth.Norton, O.R. and Chitwood, L.A. F ...

material, or may be related to metal-rich and water-poor carbonaceous chondrite of classes like CB, CH, or CR. ''Rosetta'' observations revealed that the surface of Lutetia is covered with a

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.

Shape and axial tilt

The ''Rosetta'' probe's photographs confirmed the results of a 2003

lightcurve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular freq ...

analysis that described Lutetia as a rough sphere with "sharp and irregular shape features". A study from 2004–2009 proposed that Lutetia has a non-convex shape, likely because of a large crater, Suspicio Crater. It is not yet clear whether ''Rosettas findings support this claim. The analysis of ''Rosetta'' images in combination with photometric light curves yielded the position of the north rotational pole of Lutetia: , . This gives an

axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orb ...

of 96° (retrograde rotator), meaning that the axis of rotation is approximately parallel to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

, similar to the planet

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus ( Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars), grandfather of Zeus (Jupiter) and father of ...

Surface features and nomenclature

Surface features

The surface of Lutetia is covered by numerous

s 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. 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, gravitational taluses and ejecta blocks up to 300 m in size. The landslides and corresponding rock

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...

s are correlated with variations of albedo, being generally brighter. 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. 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 a

rubble pile In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...

like many smaller asteroids. Taken together, these facts suggest that Lutetia should be classified as a primordial

planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...

Suspicio Crater

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 because ''Rosetta'' only observed Lutetia's northern part, it is not known for certain what it looks like, or if it exists at all.

Nomenclature

In March, 2011, the Working Group for Planetary Nomenclature at the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

agreed on a naming scheme for geographical features on Lutetia. Since Lutetia was a Roman city, the asteroid's craters are named after cities 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 after

provinces of the Roman Empire The Roman provinces (Latin: ''provincia'', pl. ''provinciae'') were the administrative regions of Ancient Rome outside Roman Italy that were controlled by the Romans under the Roman Republic and later the Roman Empire. Each province was ruled ...

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.

Origin

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.Battered asteroid Lutetia a rare relic of Earth's birth
Space.com

References

External links

Rosetta snaps views of asteroid Lutetia

Parent article of image by Planetary Society
* * {{DEFAULTSORT:000021 Background asteroids

20100710 Articles containing video clips M-type asteroids (Tholen) Xk-type asteroids (SMASS) 18521115