Water asteroids

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

As evidenced by their low abundances, carbon compounds, water, and other volatiles such as nitrogen compounds were probably not significantly abundant constituents of the bulk of the solids that formed near the Earth. Many of the carriers of these volatiles condensed in cooler, more distant regions and were then scattered into the region where the Earth was forming. Eragments of comets and asteroids formed in the outer solar system still fall to Earth at a rate of 1 x 10 kg/yr and early in the... 

The second important source for the hydrosphere and the oceans are asteroids and comets. Estimating the amount of water which was brought to Earth from outer space is not easy. Until 20 years ago, it was believed that the only source of water for the hydrosphere was gas emission from volcanoes. The amount of water involved was, however, unknown (Rubey, 1964). First estimates of the enormous magnitude of the bombardment to which the Earth and the other planets were subjected caused researchers to look more closely at the comets and asteroids. New hypotheses on the possible sources of water in the hydrosphere now exist the astronomer A. H. Delsemme from the University of Toledo, Ohio, considers it likely that the primeval Earth was formed from material in a dust cloud containing anhydrous silicate. If this is correct, all the water in today s oceans must be of exogenic origin (Delsemme, 1992). 

New computer simulations of the accretion process of the protoearth indicate that only a few large bodies with a high water concentration collided with the Earth during the later bombardment. They apparently came from the same region of the asteroid belt as the carbonaceous chondrites. 

However, the origin of the water on Mars is still unknown. Since the Earth and Mars have some common features in their history, the water on Mars could have come both from its interior and from comets and asteroids. The huge size of the Martian shield volcanoes, one class of which resembles the shield volcanoes Kilauea and Mauna Kea on Hawaii, suggests that a large proportion of the water was of volcanic origin. 

The element was discovered by Klaproth in 1803 and also in the same year by Berzelius and Hisinger. It is named after the asteroid Ceres. Cerium is found in several minerals often associated with thorium and lanthanum. Some important minerals are monazite, aUanite, cerite, bastnasite, and samarskite. It is the most abundant element among aU rare-earth metals. Its abundance in the earth s crust is estimated to be 66 mg/kg, while its concentration in sea water is approximately 0.0012 microgram/L. 

Water is a unique substance that plays a major role in geochemistry and cosmochemistry and is a critical component of life. The physical properties of water control the environment on the Earth s surface and have played significant roles in the history of other planets, comets, and asteroids. Most plants and animals are about 60% water by volume, and most biological reactions involve water. It is no exaggeration to say that water is the key to our existence. 

When water, the universal solvent, is present on a planet, an asteroid, or in a meteorite, a wide variety of chemical reactions take place that can completely alter the mineralogy and chemistry of an object. Some meteorites show extensive evidence of aqueous alteration. To understand the conditions under which the alteration occurred, one must be able to infer the amount, composition, and temperature of the fluids from the minerals that they produced. 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

Minerals containing bound water (OH) or water molecules (H20) adsorbed onto mineral surfaces give rise to prominent spectral absorptions near 3 pm that are observed in some asteroid classes (Rivkin et al., 2002). These absorptions have different shapes (Fig. 12.14) -the hydroxl feature is sharp and the H20 feature is more subdued. Infrared spectra for asteroids with high albedos are generally characterized by the water feature, whereas spectra of low-albedo asteroids tend to have the sharp OH feature. Although the hydroxl band is partly obscured when viewed through the Earth s atmosphere (the dashed lines in Fig. 12.14), it is still an easily recognizable feature in the spectra of some asteroid classes. 

Different shapes of the -3 pm absorption feature in asteroid spectra. Low-albedo asteroid spectra show a sharp feature attributed to hydroxl-bearing minerals, whereas high-albedo asteroid spectra have a more subdued feature arising from adsorbed water molecules. Dashed lines in the middle of the spectra are regions obscured by the Earth s atmosphere. 

Gravitational stirring of icy planetesimals by the giant planets could have sent many comets careening into the inner solar system, providing a mechanism for late addition of water to the terrestrial planets. Comets impacting the Earth and the other terrestrial planets would have delivered water as ice (Owen and Bar-Nun, 1995 Delsemme, 1999), whereas the accretion of already altered carbonaceous chondrite asteroids would have delivered water in the form of hydroxl-bearing minerals (Morbidelli el al., 2000 Dauphas et al., 2000). 

It is also possible that neither of these mechanisms for providing water to the inner planets is correct. Another hypothesis is that absorption of water onto dust particles in the accretion disk might account for the Earth s oceans (Drake, 2005). As already mentioned, the amount of water required to explain Earth s water is not large on a per-gram basis. Regardless of whether comets, asteroids, or nebular particles were the source of our planet s oceans, the water likely came from more distant regions of the nebular disk. 

Dauphas, N., Robert, F. and Marty, B. (2000) The late asteroidal and cometary bombardment in water deuterium to protium ratio. Icarus, 148, 508-512. 

The phylum Echinodermata comprises about 7000 living species [177]. Echinoderm means spiny-skinned and these organisms are characterised by the tube feet, which they use to move about. These have suction discs on the ends, which operate by an internal bulb pumping water in and out of the foot, causing expansion and contraction. The phylum is sub-divided into five classes the asteroids (sea stars), the holothurians (sea cucumbers), the crinoids (sea lilies), the echinoids (sea urchins) and the ophiuroids (brittle stars) [178]. As stated in the introduction to this review, sulfated sterols and saponins, which comprise the majority of echinoderm metabolites containing sulfur, are not included here. 

The chemistry and mineralogy of meteorites suggest that many of them are remnants of condensates from the Solar Nebula or fragments of asteroids and planetesimals that once inhabited the early solar system (Faure, 1998, 105 Wasson and Kallemeyn, 1988, 536). The mineralogy of meteorites also indicates that some planetesimals were once large and hot enough to differentiate metallic cores and other internal layers (Faure, 1998, 105). A number of meteorites even reveal the existence of liquid water in the interiors of some planetesimals (Chapman, 1999, 341). 

Miss Muxdroozol slaps her hands together. Hey, shouldn t we be trying to figure out how to get off this asteroid and looking for food and water Are we nuts talking about stars at a time like this ... 

Meteorites provide perhaps the best record of the chemical evolution of small bodies in the Solar System, and this record is supplemented by asteroidal spectroscopy. Meteorites show progressive degrees of thermal processing on their parent asteroids, from primitive carbonaceous chondrites that contain percent-level quantities of water, through ordinary chondrites that show a wide range of degree of thermal metamorphism, to the achondrites that have been melted and differentiated. 

Earth and Mars clearly contain H2O. Venus s atmosphere is very dry, and composed mainly of CO2, but the high D/H ratio of the small amount of water present suggests Venus was once much wetter than today (Zahnle 1998). Mercury is perhaps too small and too close to the Sun to have acquired and retained water. Water may have been present in much of the material that accreted to form the Earth. Small amounts of water may have been adsorbed onto dust grains at 1 AU by physisorp-tion or chemisorption (Drake 2005). Once Jupiter formed, substantial amounts of water could have been delivered to the growing Earth in the form of planetesimals and planetary embryos from the Asteroid Belt (Morbidelli et al. 2000). It is also possible that Earth lay beyond the snowline at some point during the evolution of the solar nebula (Chiang et al. 2001) so that local planetesimals contained ice. 

Water and other volatiles could have been supplied to Earth by comets and asteroids as part of the late veneer. The arguments for and against this hypothesis have recently been reviewed by Drake (2005). The D/H ratio measured in three comets to date is 2 x higher than on Earth, suggesting that comets could not have supplied more than 50% of Earth s water (Robert 2001). However, these comets may not be representative of objects colliding with the early Earth. If the Ar/H20 ratio measured in comet Hale-Bopp is typical, comets would have delivered 2 x 104 times more Ar than is presently found in Earth s atmosphere if they were the main source of Earth s water (Swindle Kring 2001). Consideration of the abundances of noble metals and noble gases led Dauphas Marty (2002) to estimate that comets contributed <1% of the Earth s water. It is unlikely that carbonaceous chondrites supplied most of the late veneer since these objects have different Os isotope ratios than Earth s mantle,... 

Energy released during impacts causes large embryos and planets to melt and differentiate, forming iron-rich cores and silicate mantles. Earth acquired most of its water before its core finished forming, possibly from the Asteroid Belt. It gained the last 1 % of its mass in the form of non-fractionated material after core formation was complete. 

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