Extraterrestrial materials are of value to science as they preserve the primitive composition of thegas and dust from which theSun and the Solar System formed.
Extraterrestrial material for study on earth can be classified into a few broad categories, namely:
Meteorites too large to vaporize on atmospheric entry but small enough to leave fragments lying on the ground, among which are included likely specimens from the asteroid and Kuiper belts as well as from the moon and from Mars.
Moon rocks brought to Earth by robotic and crewed lunar missions.
Cosmic dust collected on Earth, in the Earth's stratosphere, and inlow Earth orbit which likely include particles from the present day interplanetary dust cloud, as well as from comets.
Examples of extraterrestrial material collected on Earth includecosmic dust andmeteorites. Some of the meteorites found on Earth had their origin in another Solar System object such as theMoon,[1]Martian meteorites,[2][3] and theHED meteorite fromVesta.[4][5] Another example is the JapaneseTanpopo mission that collected dust from low Earth orbit.[6]In 2019, researchers found interstellar dust in Antarctica which they relate to theLocal Interstellar Cloud. The detection of interstellar dust in Antarctica was done by the measurement of the radionuclides Fe-60 and Mn-53 by highly sensitiveAccelerator mass spectrometry, where Fe-60 is the clear signature for a recent-supernova origin.[7]
To date, samples ofMoon rock have been collected by robotic and crewed missions. The cometWild 2 (Genesis mission) and the asteroidItokawa (Hayabusa mission) have each been visited by robotic spacecraft that returned samples to Earth, and samples of thesolar wind were also returned by the roboticGenesis mission.[8][9]
Material obtained from sample-return missions are considered pristine and uncontaminated, and theircuration and study must take place at specialized facilities where the samples are protected from Earthly contamination and from contact with the atmosphere.[13][14][15][16] These facilities are specially designed to preserve both the sample integrity and protect the Earth from potential biological contamination. Restricted bodies include planets or moons suspected to have either past or presenthabitable environments to microscopic life, and therefore must be treated as extremelybiohazardous.[17][18]
Present dayelemental abundances are superimposed on an (evolving) galactic-average set of elemental abundances that was inherited by theSolar System, along with some atoms from localnucleosynthesis sources, at the time of the Sun's formation.[19][20][21] Knowledge of these average planetary systemelemental abundances is serving as a tool for tracking chemical and physical processes involved in theformation of planets, and the evolution of their surfaces.[20]
Isotopic abundances provide important clues to the origin, transformation and geologic age of the material being analyzed.[22]
Extraterrestrial materials also carry information on a wide range of nuclear processes. These include for example: (i) the decay of now-extinctradionuclides fromsupernova byproducts introduced into Solar System materials shortly before the collapse of oursolar nebula,[23] and (ii) the products ofstellar andexplosive nucleosynthesis found in almost undiluted form inpresolar grains.[24] The latter are providing astronomers with information on exotic environments from the earlyMilky Way galaxy.
Noble gases are particularly useful because they avoid chemical reactions, secondly because many of them have more than one isotope on which to carry the signature of nuclear processes, and because they are relatively easy to extract from solid materials by simple heating. As a result, they play a pivotal role in the study of extraterrestrial materials.[25]
Particles subject to bombardment by sufficiently energetic particles, like those found incosmic rays, also experience the transmutation of atoms of one kind into another. Thesespallation effects can alter the trace element isotopic composition of specimens in ways which allow researchers to deduct the nature of their exposure in space.[citation needed]
These techniques have been used, for example, to look for (and determine the date of) events in the pre-Earth history of a meteorite's parent body (like a major collision) that drastically altered the space exposure of the material in that meteorite. For example, theMurchison meteorite landed in Australia in 1967, but its parent body apparently underwent a collision event about 800,000 years ago[26] which broke it into meter-sized pieces.
Astrobiology is an interdisciplinary scientific field concerned with theorigins,early evolution, distribution, and future oflife in theuniverse. It involves investigations on the presence of theorganic compounds on comets, asteroids, Mars or the moons of thegiant planets. Several sample-return missions to asteroids and comets are currently in the works with a key interest in astrobiology. More samples from asteroids, comets and moons could help determine whether life formed in other astronomical bodies, and if it could have been carried to Earth by meteorites or comets — a process termedpanspermia.[27][28][29]
The abundantorganic compounds in primitive meteorites and interplanetarydust particles are thought to originate largely in theinterstellar medium. However, this material may have been modified in the protoplanetary disk and has been modified to varying extents in the asteroidal parent bodies.[30]
Cosmic dust contains complex organic compounds (amorphous organic solids with a mixedaromatic-aliphatic structure) that can be created naturally bystars and radiation.[31][32][33] These compounds, in the presence of water and otherhabitable factors, are thought to have produced and spontaneously assembled the building blocks of life.[34][35]
The origin of water on Earth is the subject of a significant body of research in the fields ofplanetary science,astronomy, andastrobiology. Isotopic ratios provide a unique "chemical fingerprint" that is used to compare Earth's water with reservoirs elsewhere in the Solar System. One such isotopic ratio, that ofdeuterium to hydrogen (D/H), is particularly useful in the search for the origin of water on Earth. However, when and how that water was delivered to Earth is the subject of ongoing research.[36][37]
^M. W. Caffee, J. N. Goswami, C. M. Hohenberg, K. Marti and R. C. Reedy (1988) inMeteorites and the early solar system (ed. J. F. Kerridge and M. S. Matthews, U Ariz. Press, Tucson AZ) 205-245.
^[Project 2. Extraterrestrial Materials: Origin and Evolution of Organic Matter and Water in the Solar System.]NASA Astrobiology Institute, 2007 Annual Report.
