HED meteorite
Encyclopedia
HED meteorites are a subgroup of achondrite
meteorites. HED stands for "Howardite
–Eucrite
–Diogenite
".
These achondrites came from a differentiated parent body and experienced extensive igneous processing not much different from the magmatic rocks found on Earth and for this reason they closely resemble terrestrial igneous rocks.
HED meteorites are broadly divided into:
Several subgroups of both eucrites and diogenites have been found.
The HED meteorites account for about 5% of all falls, which is about 60% of all achondrite
s.
, their differences being due to different geologic histories of the parent rock. Their crystallization ages have been determined to be between 4.43 and 4.55 billion years from radioisotope ratios. HED meteorites are differentiated meteorites
, which were created by igneous processes in the crust of their parent asteroid.
It is thought that the method of transport from 4 Vesta to Earth
is as follows:
Achondrite
An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies...
meteorites. HED stands for "Howardite
Howardite
Howardites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta, and as such are part of the HED meteorite group. There are about 200 distinct members known.-Characteristics:...
–Eucrite
Eucrite
Eucrites are achondritic stony meteorites, many of which originate from the surface of the asteroid 4 Vesta and as such are part of the HED meteorite group. They are the most common achondrite group with well over 100 distinct finds at present....
–Diogenite
Digenite
Digenite is a copper sulfide mineral with formula: Cu9S5. Digenite is a black to dark blue opaque mineral that crystallizes with a trigonal - hexagonal scalenohedral structure. In habit it is usually massive, but does often show pseudo-cubic forms. It has poor to indistinct cleavage and a brittle...
".
These achondrites came from a differentiated parent body and experienced extensive igneous processing not much different from the magmatic rocks found on Earth and for this reason they closely resemble terrestrial igneous rocks.
Classification
 |
 |
HED meteorites are broadly divided into:
- HowarditeHowarditeHowardites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta, and as such are part of the HED meteorite group. There are about 200 distinct members known.-Characteristics:...
s - EucriteEucriteEucrites are achondritic stony meteorites, many of which originate from the surface of the asteroid 4 Vesta and as such are part of the HED meteorite group. They are the most common achondrite group with well over 100 distinct finds at present....
s - DiogeniteDiogeniteDiogenites are a subgroup of the HED meteorite group, a type of achondritic stony meteorites.-Origin and composition:Diogenites originates from deep within the crust of the asteroid 4 Vesta, and as such are part of the HED meteorite group...
s
Several subgroups of both eucrites and diogenites have been found.
The HED meteorites account for about 5% of all falls, which is about 60% of all achondrite
Achondrite
An achondrite is a stony meteorite that does not contain chondrules. It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies...
s.
Origin
These are all thought to have originated from the crust of the asteroid 4 Vesta4 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....
, their differences being due to different geologic histories of the parent rock. Their crystallization ages have been determined to be between 4.43 and 4.55 billion years from radioisotope ratios. HED meteorites are differentiated meteorites
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
, which were created by igneous processes in the crust of their parent asteroid.
It is thought that the method of transport from 4 Vesta to Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
is as follows:
- An impact on 4 Vesta ejected debris, creating small (10 km diameterDiameterIn 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...
or less) V-type asteroidV-type asteroidThe spectra of the V-type asteroids or Vestoids are similar to that of 4 Vesta, by far the largest asteroid in this class ....
s. Either the asteroidal chunks were ejected as such, or were formed from smaller debris. Some of these small asteroids formed the Vesta familyVesta familyThe Vesta or Vestian family of asteroids is a large and prominent grouping of mostly V-type asteroids in the inner asteroid belt in the vicinity of 4 Vesta...
, while others were scattered somewhat further. This event is thought to have happened less than 1 billion years ago. There is an enormous impact crater on 4 Vesta covering much of the southern hemisphere which is the best candidate for the site of this impact. The amount of rock that was excavated there is many times more than enough to account for all known V-type asteroids. - Some of the more far-flung asteroid debris ended up in the 3:1 Kirkwood gapKirkwood gapA Kirkwood gap is a gap or dip in the distribution of main-belt asteroids with semi-major axis , as seen in the histogram below...
. This is an unstable region due to strong perturbations by JupiterJupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, and asteroids which end up here get ejected onto far different orbits on a timescale of about 100 million years. Some of these bodies are perturbed into near-Earth orbits forming the small V-type near-Earth asteroids such as e.g. 3551 Verenia3551 Verenia3551 Verenia is an Amor asteroid and a Mars crosser discovered in 1983 by R. Scott Dunbar. Although Verenia passed within 40 Gm of the Earth in the 20th century, it will never do so in the 21st...
, 3908 Nyx3908 Nyx3908 Nyx is an Amor and Mars-crosser asteroid. It was discovered by Hans-Emil Schuster on August 6, 1980, and is named after Nyx, the Greek goddess of the night, after which Pluto's moon Nix is also named...
, or 4055 Magellan4055 Magellan4055 Magellan is an Amor asteroid discovered on February 24, 1985, by E. Helin at Palomar.-See also:* V-type asteroid* HED meteorite* 4 Vesta* 3908 Nyx* 3551 Verenia- External links :*...
. - Later smaller impacts on these near-earth objects dislodged rock-sized meteorites, some of which later struck Earth. On the basis of cosmic rayCosmic rayCosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
exposure measurements, it is thought that most HED meteorites arose from several distinct impact events of this kind, and spent from about 6 to 73 million years in space before striking the Earth.