Asteroidal water is water or water precursor deposits such as hydroxide (OH−) that exist in asteroids (i.e., small Solar System bodies (SSSBs) not explicitly in the subcategory of comets). The "snow line" of the Solar System lies outside of the main asteroid belt, and the majority of water is expected in minor planets (e.g. Kuiper belt objects (KBOs) and Centaurs). Nevertheless, a significant amount of water is also found inside the snow line, including in near-Earth objects (NEOs).
The formation of asteroidal water mirrors that of water formation in the Solar System, either from transfer via bombardment, migration, ejection, or other means. Asteroidal water has recently been pursued as a resource to support deep space exploration activities, for example, for use as a rocket propellant, human consumption, or for agricultural production.
History
Meteorites
Since the early 1800s, meteorites have been assumed to be "space rocks", not terrestrial or atmospheric phenomena. At this time, asteroids were first discovered, then in increasing numbers and categories.
Many meteorites show signs of previous water. The petrological scale, numbered 1 through 7, indicates increasing aqueous alteration from type 2 to 1. Signs of water include phyllosilicates ("clay" and serpentinites), sulfides and sulfates, and carbonates, as well as structural signs: veins, and alteration or total erasure of individual chondrules.
Some meteorites, particularly the CI class, currently contain water. As these include both finds (with their Earth entry and impact unobserved) and falls (meteorites from a known, recent meteor event), that water cannot be entirely terrestrial contamination. As the precision of isotopic abundance analyses grew, they confirmed that meteorite water differs from Earth water. As water at Earth (especially its atmosphere) is well-mixed, significantly different isotope levels would indicate a separate water source.
Water content of the CI and CM types are often in double-digit percentages.
Much telescopic observation and hypothesizing attempted to link meteorite classes to asteroid types. The Galileo and NEAR missions then established S-type asteroids as the parent bodies of ordinary chondrites; the Dawn mission confirmed hypotheses that 4 Vesta was the HED parent. Ongoing projects are sending spacecraft to C-, "thus we expect to clarify the origin of life by analyzing samples acquired from a primordial celestial body such as a C-type asteroid to study organic matter and water in the solar system..." M-, D-, and P-type bodies.
Versus comets
The planets, and to an extent the asteroid belt, were previously held to be static and unchanging; the belt was a former or stalled planet.
In the late 1860s, Hubert Newton and Giovanni Schiaparelli simultaneously showed that meteor showers (and by implication, meteorites) were comet debris.
After the discovery of many near-Earth asteroids, not in the belt, it was apparent they had planet-crossing, unstable orbits. Their number could not have survived from the Solar System's formation, and required replenishment from some other population. Some, such as Opik and Wetherill, hypothesized that most or all NEOs were actually extinct or dormant comets, requiring no ejection process from the main belt. The comets' orbits had become more circular after encounters with planets, possibly augmented by comet jetting. Centaurs, too, required some similar model.
A growing understanding of Solar System dynamics, including more observations, of more bodies, replicated by faster computer models, eliminated this requirement. Kirkwood gaps were evidence of loss from the main belt, via resonances with the planets. Later, the Yarkovsky effect, insignificant to a planet, could augment mechanisms.
Empirically, meteor cameras began tracing meteor trajectories, which led back to the asteroid belt. The Příbram (1959), Lost City (1970), and Innisfree (1977) meteorites had arrived via Apollo-like, belt-tangent orbits. Even afterward, some maintained that comets best explained carbonaceous chondrite meteorites or even ordinary chondrites.
As comets
The issue of asteroids versus comets reemerged with observations of active asteroids- that is, emission from small bodies in what were considered asteroidal orbits, not comet-like orbits (high eccentricity and inclination). This includes both Centaurs, past the snow line, and main belt objects, inside the line and previously assumed dry. Activity could, in some cases, be explained by ejecta, escaping from an impact. However, some asteroids showed activity at perihelion, then at subsequent perihelia. The probability of impacts with this timed pattern was considered unlikely versus a model of comet-like volatile emissions.
Observations of the Geminid meteor shower linked it to (3200) Phaeton, a body in a cometary orbit but with no visible coma or tail, and thus defined as an asteroid. Phaeton was a rock comet, whose emissions are largely discrete particles and not visible.
Observations of (1) Ceres emitting hydroxide (OH), the product of water after exposure to the Sun's ultraviolet levels, were further evidence. Ceres is well within the snow line, exposed to ultraviolet, and Cererean water was considered speculative, at least on its surface.
The IAU General Assembly of 2006 addressed this issue. Overshadowed by Pluto was the creation of small Solar System body (SSSB), a category needing no comet-asteroid distinction, nor establishment/disestablishment of volatile emission.
Hydrology and morphology
Micro- and nanoscale water occurs as fluid inclusions in both carbonaceous and ordinary chondrites. However, as "bubble" diameters decrease, search costs increase geometrically. Their characterization is at the state of the art for most analytical techniques, and the method had seen slow progress to this point. Independently-confirmed fluid inclusions are, at minimum, Peetz and Jilin, with many other reports.
Minerals which appear waterless to the eye or hand may nevertheless be hydrated. Unfrozen water consists of molecular layers (one to possibly fifteen molecules thick 4.3.4 Silicas) bound to, and kept from crystallizing by the equal or stronger attraction of the mineral of adsorption.
Water can persist at higher temperatures than normal in the form of hydrated minerals: those minerals which can bind water molecules at the crystalline level. Salts, including halite (table salt, NaCl) are ionic and attract individual, polar water molecules with electrostatic forces. Alternately, the parent mineral may be e. g., sulfate, and that mineral may retain hydroxide (OH). When freed from the crystal structure, hydroxide reverts to water and oxygen. These are considered water, in the usage of geochemistry and Solar System science.Chapter: Hydrogen (1) "...minerals of particular interest, such as those that contain water"
Short of this binding, a surface may retain a monolayer or bilayer of water molecules or hydroxide. Phyllosilicate minerals assemble into microscopic plates, sheets, or fibers, rather than bulk crystals. The layers trap water between them; the large surface area created can hold much water. This is also considered water, in the geotechnical, geochemical, and astronomical usages.Chapter: Meteoritic constraints on conditions in the solar nebula "Water in chondrites is contained within clay minerals"
On an even finer level, most rocks are silicates, or in some cases metal oxides, containing an oxygen fraction. Hydrogen content, as substitutions or interstitials, can react with oxygen (displacing its existing cation) to form hydroxide or water. The solar wind is a reducing environment, containing hydrogen atoms and protons (effectively hydrogen, in the form of hydrogen nuclei). Either may be implanted into exposed surfaces, as the small hydrogen atom is highly soluble. A lesser contribution may come from the proton component of cosmic rays. Both pyroxene and olivine, common asteroid minerals, can hydrate in this manner. This, too, is considered water within the geochemistry and geophysics fields.
Solar System science and asteroid mining ascribe hydrated minerals as containing water, in a similar sense as ice giant.
On a macroscopic scale, some thickness of crust may shelter water from evaporation, photolysis and radiolysis, meteoric bombardment, etc. Even where a crust does not originally exist, impurities in ice may form a crust after its parent ice escapes: a lag deposit.
On a geologic scale, the larger asteroids can shield water, phyllosilicate, ice, etc. contents in their interiors via a high thermal mass. Below some depth, the diurnal temperature variation becomes negligible, and the effect of solar insolation- a daytime temperature peak- does not boil out water. A low obliquity helps; while the tropics take solar insolation, two polar regions see little sunlight and can help maintain a low average temperature.
Water parent materials
Phyllosilicates
CI meteorites are mostly phyllosilicates. The phyllosilicates serpentinite, montmorillonite and saponite (clay), tochilinite, chamosite, cronstedtite, and mica have been identified in meteorites.
Sulfates and sulfides
Sulfur is found in meteorites; it has a fairly high cosmic abundance. The abundance in common (chondrite) meteorites is greater than that in Earth's crust; as a differentiated body, our crust has lost some sulfur to an iron core, and some to space as hydrogen sulfide gas. The element is present in all meteorites; carbonaceous chondrites and enstatite chondrites in particular have higher sulfur contents than the ordinary chondrites. In C1 and C2 chondrites, sulfur is found predominantly as free sulfur, sulfate minerals, and in organic compounds at a net 2–5 percent. A slight enrichment is due to cosmic-ray produced S36 and S33.
Sulfur-bearing, hydrated minerals identified via meteorites include epsomite, bloedite, gypsum/bassanite, and jarosite.
Carbonate
As the name implies, carbonaceous chondrites formed with chondrules and carbon. The carbonates whewellite/vaterite, hydromagnesite, calcite/dolomite, aragonite, and breunnerite have been found in meteorites.
By meteorite classification
Type 1 2 3 4 5 6 Overall Texture No chondrites Very sharply defined chondrites Very sharply defined chondrites Well-defined chondrites Chondrites readily delineated Poorly defined chondrites Texture of matrix All fine-grained, opaque Much opaque matrix Opaque matrix Transparent, micro-crystalline matrix Recrystallized matrix Recrystallized matrix Bulk carbon content ~2.8% ~0.6–2.8% ~0.2–1.0% <0.2% <0.2% <0.2% Bulk water content ~20% ~4-18% <0.2% <0.2% <0.2% <0.2%
-Petrological Scale (Van Schmus, Wood 1967). Since this time, a type seven has been added.
This taxonomy was preceded (Wiik 1956: Type I 20.08% water, Type II 13.35% water) and followed (Keil 1969, Mason 1971chapter: Introduction), with all in general agreement on these levels.
Meteorites are valuable ground truth. Studies, such as neutron activation analysis, can be performed without the mass and volume constraints of space flight. Meteorites also sample multiple depths of their parent bodies, not just dehydrated crusts or space-weathered rinds.
Yet meteorites are not sufficient. The body of meteoritics is dominated by durable examples, and deficient in classes and subclasses; one or more types may be missing entirely. Earth entry and exposure may then alter or remove some materials, while contaminating others. Such meteorites have speculative or unknown parent bodies, and no wider context of the sample versus the rest of that parent body.
Carbonaceous chondrites