Extraterrestrial materials
Encyclopedia
Most atoms on Earth came from the interstellar
dust and gas from which the Sun
and Solar System
formed. However, in the space science
community, "extraterrestrial materials" generally refers to objects now on Earth that were solidified prior to arriving on earth. In October 2011, scientists reported that one form of extraterrestrial material, cosmic dust
, contains complex organic
matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.
A minor but notable subset of the materials that make up the foregoing collections were also solidified outside of our solar system. These are often referred to as interstellar materials, some of which are also presolar
i.e. they predate the formation of our solar system.
Each of these types of material is treated elsewhere. This entry, therefore, is here primarily to consider the relationship between types of extraterrestrial materials on Earth, as well as the types of extraterrestrial material we'd like to have a closer look at.
and oxygen
atoms, which form gases when they combine, the solid material in a given planetary system
probably depends on whether carbon abundances were greater, or less, than those of oxygen. What's left over goes into solids. There was more oxygen than carbon in our Solar System, so solid planetary surfaces (as well as primitive meteorites) are largely made up of oxides like SiO2 (silicate
s, resulting in silicate planets). If the converse was true, carbide
s might be the dominant form of "rock" (resulting in carbon planet
s).
Larger planetary bodies in our early Solar System were hot enough to experience melting and differentiation
, with heavier elements
(like nickel
) finding their way into the core
. The lighter silicate rocks presumably float to the surface. Thus this might explain why iron
on Earth's surface is depleted in nickel, while iron meteorite
s (from the center of a differentiated planetoid) are rich in nickel. Since it's not a very active rock-forming element, most of our nickel is in the Earth's core. A key feature of extraterrestrial materials, in comparison to naturally occurring terrestrial materials, is therefore Ni/Fe ratios above several percent.
Another consequence of the leftover oxygen in our early Solar System is the fact that presolar carbides from star system
s with higher ratios of C/O are easier to recognize in primitive meteorites, than are presolar silicates. Hence presolar carbon and carbide grains were discovered first.
sources, at the time of the Sun's formation. Knowledge of these average planetary system elemental abundances is serving as a powerful tool for tracking chemical and physical processes involved in the formation of planets, and the evolution of their surfaces. Awaiting more on this here cf. cosmic abundance.
.
Nuclear particle tracks subsequently found many uses in extraterrestrial materials, thanks both to the exposure of those materials to radiation in space, and to their sometimes ancient origins. These applications included (i) determining the exposure of mineral surfaces to solar flare
particles from the sun, (ii) determining the spectrum of solar flare particle energies from the early Sun, and (iii) the discovery of fission tracks from extinct isotopes of Plutonium 244 in primitive meteorites.
effects can alter the trace element isotopic composition of specimens in ways which allow researchers in the laboratory to fingerprint the nature of their exposure in space.
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 the Murchison meteorite
landed in Australia in 1967, but its parent body apparently underwent a collision event about 800,000 years ago which broke it into meter-sized pieces.
and our Solar System provides a blank slate on which to detect the effect of nuclear processes on earth, in our solar system, and in objects coming to us from outside the solar system. Awaiting more here, see natural abundance
.
es are particularly interesting from an isotopic perspective, first because they avoid chemical interactions, secondly because many of them have more than one isotope on which to carry the signature of nuclear processes (xenon
has many), and finally because they are relatively easy to extract from solid materials by simple heating. As a result, they play a pivotal role in the unfolding drama of extraterrestrial materials study.
Some clues to moving further might be found in this book and this article.
Until more perspective is added here on the importance of these tools to extraterrestrial materials research, you can find clues to some strategies not mentioned here under radiometric dating
.
s, like those supernova byproducts introduced into Solar System materials shortly before the collapse of our solar nebula
, and (ii) the products of stellar
and explosive
nucleosynthesis found in almost undiluted form in presolar grains. The latter are providing astronomers with up-close information on the state of the whole periodic table in exotic environments scattered all across the early Milky Way.
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
dust and gas from which the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
formed. However, in the space science
Space science
The term space science may mean:* The study of issues specifically related to space travel and space exploration, including space medicine.* Science performed in outer space ....
community, "extraterrestrial materials" generally refers to objects now on Earth that were solidified prior to arriving on earth. In October 2011, scientists reported that one form of extraterrestrial material, cosmic dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
, contains complex organic
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.
Categories
To date these fall into a small number of broad categories, namely:- Meteorites too large to vaporize on atmospheric entry but small enough to leave fragments lying on the ground
- moon rockMoon rockMoon rock describes rock that formed on the Earth's moon. The term is also loosely applied to other lunar materials collected during the course of human exploration of the Moon.The rocks collected from the Moon are measured by radiometric dating techniques...
s brought back by the Apollo missionsProject ApolloThe Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F... - unmelted micrometeorites typically less than 100 micrometres in diameter collected on Earth and in the Earth's stratosphere
- specimens returned from space-borne collection missions like NASA's Long Duration Exposure Facility (LDEF) and the StardustStardust (spacecraft)Stardust is a 300-kilogram robotic space probe launched by NASA on February 7, 1999 to study the asteroid 5535 Annefrank and collect samples from the coma of comet Wild 2. The primary mission was completed January 15, 2006, when the sample return capsule returned to Earth...
sample return mission.
A minor but notable subset of the materials that make up the foregoing collections were also solidified outside of our solar system. These are often referred to as interstellar materials, some of which are also presolar
Presolar grains
Presolar grains are isotopically-distinct clusters of material found in the fine-grained matrix of primitive meteorites, such as chondrites, whose differences from the surrounding meteorite suggest that they are older than the solar system...
i.e. they predate the formation of our solar system.
Each of these types of material is treated elsewhere. This entry, therefore, is here primarily to consider the relationship between types of extraterrestrial materials on Earth, as well as the types of extraterrestrial material we'd like to have a closer look at.
Oxidation State
Thanks to the "romance" between carbonCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atoms, which form gases when they combine, the solid material in a given planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
probably depends on whether carbon abundances were greater, or less, than those of oxygen. What's left over goes into solids. There was more oxygen than carbon in our Solar System, so solid planetary surfaces (as well as primitive meteorites) are largely made up of oxides like SiO2 (silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s, resulting in silicate planets). If the converse was true, carbide
Carbide
In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...
s might be the dominant form of "rock" (resulting in carbon planet
Carbon planet
A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form if protoplanetary discs are carbon-rich and oxygen-poor. According to planetary science, it would develop differently from Earth, Mars and Venus, planets...
s).
Larger planetary bodies in our early Solar System were hot enough to experience melting and differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...
, with heavier elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
(like nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
) finding their way into the core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
. The lighter silicate rocks presumably float to the surface. Thus this might explain why iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
on Earth's surface is depleted in nickel, while iron meteorite
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:...
s (from the center of a differentiated planetoid) are rich in nickel. Since it's not a very active rock-forming element, most of our nickel is in the Earth's core. A key feature of extraterrestrial materials, in comparison to naturally occurring terrestrial materials, is therefore Ni/Fe ratios above several percent.
Another consequence of the leftover oxygen in our early Solar System is the fact that presolar carbides from star system
Star system
A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems.-Binary star systems:A stellar...
s with higher ratios of C/O are easier to recognize in primitive meteorites, than are presolar silicates. Hence presolar carbon and carbide grains were discovered first.
Elemental Abundances
Present day elemental abundances are superimposed on an (evolving) galactic-average set of elemental abundances that was inherited by our Solar System, along with some atoms from local nucleosynthesisNucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
sources, at the time of the Sun's formation. Knowledge of these average planetary system elemental abundances is serving as a powerful tool for tracking chemical and physical processes involved in the formation of planets, and the evolution of their surfaces. Awaiting more on this here cf. cosmic abundance.
Impact and irradiation effects
The atmosphere of planet earth, and the Earth's magnetic field, protect us from impact and irradiation by a wide range of objects that fly around in space. Those who know about these effects can often learn from them a lot about the history of extraterrestrial materials.Microcraters etc.
Microcraters, "pancakes", and "splashes" were first seen on the surface of lunar rocks and soil grains. They tell us about the direct exposure of an object's surface to space in the absence of a vacuum. These structures are quite unlike features found on the surface of terrestrial rocks and soil. They have also been identified on grains found in meteorites, and on man-made objects exposed to the micrometeorite flux in space.Nuclear particle tracks
Nuclear track damage trails from the passage of heavy ions in non-conducting solids were first reported by E. C. H. Silk and R. S. Barnes in 1959. Their etchability along with many subsequent applications were established by Fleischer, Price and Walker, starting with their work at General Electric Laboratories in Schenectady, New York. See also solid state nuclear track detectorSolid state nuclear track detector
A solid-state nuclear track detector or SSNTD is a section of a solid material uncovered to nuclear radiation , etched, and inspected microscopically...
.
Nuclear particle tracks subsequently found many uses in extraterrestrial materials, thanks both to the exposure of those materials to radiation in space, and to their sometimes ancient origins. These applications included (i) determining the exposure of mineral surfaces to solar flare
Solar flare
A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...
particles from the sun, (ii) determining the spectrum of solar flare particle energies from the early Sun, and (iii) the discovery of fission tracks from extinct isotopes of Plutonium 244 in primitive meteorites.
Nuclear spallation effects
Particles subject to bombardment by sufficiently energetic particles, like those found in cosmic rays, also experience the transmutation of atoms of one kind into another. These spallationSpallation
In general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection or vaporization of material from a target during impact by a projectile...
effects can alter the trace element isotopic composition of specimens in ways which allow researchers in the laboratory to fingerprint the nature of their exposure in space.
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 the Murchison meteorite
Murchison meteorite
The Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass , the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds....
landed in Australia in 1967, but its parent body apparently underwent a collision event about 800,000 years ago which broke it into meter-sized pieces.
Isotopic abundances
The isotopic homogeneity of our planetPlanet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
and our Solar System provides a blank slate on which to detect the effect of nuclear processes on earth, in our solar system, and in objects coming to us from outside the solar system. Awaiting more here, see natural abundance
Natural abundance
In chemistry, natural abundance refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass of these isotopes is the atomic weight listed for the element in the periodic table...
.
Noble gases
Noble gasNoble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
es are particularly interesting from an isotopic perspective, first because they avoid chemical interactions, secondly because many of them have more than one isotope on which to carry the signature of nuclear processes (xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
has many), and finally because they are relatively easy to extract from solid materials by simple heating. As a result, they play a pivotal role in the unfolding drama of extraterrestrial materials study.
Some clues to moving further might be found in this book and this article.
Radiometric dating in general
Isotopic abundances provide important clues to the date of events that allowed a material to begin accumulating gaseous decay byproducts, or that allowed incident radiation to begin transmuting elements. Because of the harsh radiations, unusual starting compositions, and long delays between events sometimes experienced, isotopic dating techniques are of special importance in the study of extraterrestrial materials.Until more perspective is added here on the importance of these tools to extraterrestrial materials research, you can find clues to some strategies not mentioned here under radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
.
Other isotopic studies
The categories of study mentioned before this are classic isotopic applications. However, extraterrestrial materials also carry information on a wide range of other nuclear processes. These include for example: (i) the decay of now-extinct radionuclideRadionuclide
A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...
s, like those supernova byproducts introduced into Solar System materials shortly before the collapse of our solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
, and (ii) the products of stellar
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...
and explosive
Supernova nucleosynthesis
Supernova nucleosynthesis is the production of new chemical elements inside supernovae. It occurs primarily due to explosive nucleosynthesis during explosive oxygen burning and silicon burning...
nucleosynthesis found in almost undiluted form in presolar grains. The latter are providing astronomers with up-close information on the state of the whole periodic table in exotic environments scattered all across the early Milky Way.
See also
- meteorites
- moon rockMoon rockMoon rock describes rock that formed on the Earth's moon. The term is also loosely applied to other lunar materials collected during the course of human exploration of the Moon.The rocks collected from the Moon are measured by radiometric dating techniques...
- cosmic dustCosmic dustCosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
- interplanetary dust cloudInterplanetary dust cloudThe interplanetary dust cloud is cosmic dust which pervades the space between planets in the Solar System and in other planetary systems...
- presolar grainsPresolar grainsPresolar grains are isotopically-distinct clusters of material found in the fine-grained matrix of primitive meteorites, such as chondrites, whose differences from the surrounding meteorite suggest that they are older than the solar system...
- cosmochemistryCosmochemistryCosmochemistry or chemical cosmology is the study of the chemical composition of matter in the universe and the processes that led to those compositions. This is done primarily through the study of the chemical composition of meteorites and other physical samples...
External links
- Planetary Science Research Discoveries Educational journal with articles about extraterrestrial materials.