21 Lutetia
Encyclopedia
21 Lutetia is a large main-belt
asteroid
of an unusual spectral type
. It measures about 100 kilometers in diameter
(120 km along its major axis). It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia
, the Latin
name for Paris
.
Lutetia has irregular shape and is heavily cratered with the largest impact crater reaching 45 km in diameter. The surface is geologically heterogeneous and is intersected by system of groves and scarps, which are thought to be fractures. It has a high average density meaning that it is made of metal rich rock.
The Rosetta
probe passed within 3162 km (1,964.8 mi) of Lutetia in July 2010. It was the largest asteroid to be visited by a spacecraft until the Dawn mission arrived at 4 Vesta
in July 2011.
. 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
. He computed an opposition
magnitude of 10.8.
There have been two reported stellar
occultation
s by Lutetia, observed from Malta
in 1997 and Australia
in 2003, with only one chord
each, roughly agreeing with IRAS
measurements.
On July 10, 2010, the European
Rosetta
space probe
flew by Lutetia at a minimum distance of and a velocity of 15 kilometres per second on its way to the comet
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.
of 10–15%, the bulk density of Lutetia exceeds that of a typical stony meteorite.
s, most of which are metallic, Lutetia is one of the anomalous members that do not display much evidence of metal on their surface. Indeed, there were various indications of a non-metallic surface: a flat, low frequency spectrum
similar to that of carbonaceous chondrite
s and C-type asteroid
s and not at all like that of metallic meteorites
, a low radar
albedo
unlike the high albedos of strongly metallic asteroids like 16 Psyche
, evidence of hydrated materials on its surface, abundant silicates, and a thicker regolith
than most asteroids.
The Rosetta probe actually found that the asteroid has a moderately red spectrum in the visible light and essentially flat spectrum in the near infrared. No absorption features were detected in the whole wavelength range covered by observations—0.4–3.5 μm. Thus previous reports of hydrated minerals and organic compounds on the surface of Lutetia has been disproven. The surface also does not contain any olivine
. Together with the high density of Lutetia these results indicate that it is either made of the enstatite chondrite
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 regolith
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.
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
of 96° (retrograde rotator), meaning that the axis of rotation is approximately parallel
to the ecliptic
, similar to the planet Uranus
.
s and intersected by fractures, scarp
s 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 talus
es and ejecta blocks up to 300 m in size. The landslides and corresponding rock outcrop
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 having 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 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 not shattered 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
like many smaller asteroids. Taken together, these facts suggest that Lutetia should be classified as a primordial planetesimal
.
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 (Goldschmitt) and after provinces 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 Lutetia.
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
of an unusual spectral type
Asteroid spectral types
Asteroids are assigned a type based on spectral shape, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition...
. It measures about 100 kilometers in diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
(120 km along its major axis). It was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia
Lutetia
Lutetia was a town in pre-Roman and Roman Gaul. The Gallo-Roman city was a forerunner of the re-established Merovingian town that is the ancestor of present-day Paris...
, the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
name for Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
.
Lutetia has irregular shape and is heavily cratered with the largest impact crater reaching 45 km in diameter. The surface is geologically heterogeneous and is intersected by system of groves and scarps, which are thought to be fractures. It has a high average density meaning that it is made of metal rich rock.
The Rosetta
Rosetta (spacecraft)
Rosetta is a robotic spacecraft of the European Space Agency on a mission to study the comet 67P/Churyumov–Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The spacecraft was launched on 2 March 2004 on an Ariane 5 rocket and will reach the comet by...
probe passed within 3162 km (1,964.8 mi) of Lutetia in July 2010. It was the largest asteroid to be visited by a spacecraft until the Dawn mission arrived at 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....
in July 2011.
Discovery and exploration
Lutetia was discovered on November 15, 1852, by Hermann Goldschmidt from the balcony of his apartment in ParisParis
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
. 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
Edward Charles Pickering
Edward Charles Pickering was an American astronomer and physicist, brother of William Henry Pickering.Along with Carl Vogel, Pickering discovered the first spectroscopic binary stars. He wrote Elements of Physical Manipulations .Pickering attended Boston Latin School, and received his B.S. from...
at Harvard College Observatory
Harvard College Observatory
The Harvard College Observatory 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, USA, and was founded in 1839...
. He computed an opposition
magnitude of 10.8.
There have been two reported stellar
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
s by Lutetia, observed from Malta
Malta
Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...
in 1997 and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
in 2003, with only one chord
Chord (astronomy)
In the field of astronomy the term chord typically refers to a line crossing an object which is formed during an occultation event. By taking accurate measurements of the start and end times of the event, in conjunction with the known location of the observer and the object's orbit, the length of...
each, roughly agreeing with IRAS
IRAS
The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....
measurements.
On July 10, 2010, the European
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
Rosetta
Rosetta (spacecraft)
Rosetta is a robotic spacecraft of the European Space Agency on a mission to study the comet 67P/Churyumov–Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The spacecraft was launched on 2 March 2004 on an Ariane 5 rocket and will reach the comet by...
space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
flew by Lutetia at a minimum distance of and a velocity of 15 kilometres per second on its way to the comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
67P/Churyumov-Gerasimenko
67P/Churyumov-Gerasimenko
Comet Churyumov–Gerasimenko, officially designated 67P/Churyumov–Gerasimenko, is a comet with a current orbital period of 6.6 years. It is the destination of the European Space Agency's Rosetta spacecraft mission, launched on March 2, 2004....
. 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.
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 orbit period of Lutetia is 3.8 years.Mass and density
The Rosetta 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 before in asteroids at 3.4 ± 0.3 g/cm3. Taking into account possible porosityPorosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of 10–15%, the bulk density of Lutetia exceeds that of a typical stony meteorite.
Composition
The composition of Lutetia has puzzled astronomers for some time. While classified among the M-type asteroidM-type asteroid
M-type asteroids are asteroids of partially known composition; they are moderately bright . Some, but not all, are made of nickel-iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts,...
s, most of which are metallic, Lutetia is one of the anomalous members that do not display much evidence of metal on their surface. Indeed, there were various indications of a non-metallic surface: a flat, low frequency spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
similar to that of carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
s and C-type asteroid
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
s and not at all like that of metallic meteorites
Iron meteorite
Iron meteorites are meteorites that consist overwhelmingly of nickel-iron alloys. The metal taken from these meteorites is known as meteoric iron and was one of the earliest sources of usable iron available to humans.-Occurrence:...
, a low radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
unlike the high albedos of strongly metallic asteroids like 16 Psyche
16 Psyche
16 Psyche )is one of the ten most massive main-belt asteroids. It is over 200 kilometers in diameter and contains a little less than 1% of the mass of the entire asteroid belt. It is the most massive of the metallic M-type asteroids....
, evidence of hydrated materials on its surface, abundant silicates, and a thicker regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
than most asteroids.
The Rosetta probe actually found that the asteroid has a moderately red spectrum in the visible light and essentially flat spectrum in the near infrared. No absorption features were detected in the whole wavelength range covered by observations—0.4–3.5 μm. Thus previous reports of hydrated minerals and organic compounds on the surface of Lutetia has been disproven. The surface also does not contain any olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
. Together with the high density of Lutetia these results indicate that it is either made of the enstatite chondrite
Enstatite chondrite
Enstatite chondrites are a rare form of meteorite thought to comprise only about 2% of the chondrites that fall on Earth. Only about 200 E-Type chondrites are currently known....
material or may be related to metal-rich and water-poor carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
of classes like CB, CH, or CR.
Rosetta observations revealed that the surface of Lutetia is covered with a regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
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 analysis that described Lutetia as a rough sphere with "sharp and irregular shape features". A study carried out by I.N. Belskaya et al in 2004–2009 had proposed that "Lutetia has a non-convex shape, probably due to a large 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
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
of 96° (retrograde rotator), meaning that the axis of rotation is approximately parallel
Parallel (geometry)
Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...
to the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
, similar to the planet Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
.
Surface features
The surface of Lutetia is covered by numerous impact craterImpact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s and intersected by fractures, scarp
Scarp
-Landforms and geology:* Cliff, a significant vertical, or near vertical, rock exposure* Escarpment, a steep slope or long cliff that occurs from erosion or faulting and separates two relatively level areas of differing elevations...
s 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 talus
Talus
Talus may refer to:* Talus , a sloped portion of a fortified wall* Talus, California, a community in Inyo County* Talus bone, an ankle bone* Talus, fictional planet in Star Wars...
es and ejecta blocks up to 300 m in size. The landslides and corresponding rock outcrop
Outcrop
An 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 superficial deposits are covered by a mantle of soil and vegetation and cannot be...
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 having 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 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 not shattered 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
Rubble pile
In astronomy, rubble pile is the informal name for an object that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity...
like many smaller asteroids. Taken together, these facts suggest that Lutetia should be classified as a primordial planetesimal
Planetesimal
Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger...
.
Nomenclature
In March, 2011, the Working Group for Planetary Nomenclature at the International Astronomical UnionInternational Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
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 (Goldschmitt) and after provinces 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 Lutetia.
