Water carbonaceous chondrites

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... 

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. 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

EUer and Kitchen (2004) have re-evaluated the hydrogen isotope composition of water-rich carbonaceous chondrites by stepped-heating analysis of very small amounts of separated water-rich materials. Their special aim has been to deduce the origin of the water with which the meteorites have reacted. They observed a decrease in 5D with increasing extent of aqueous alteration from 0%c (least altered, most volatile rich) to —200%c (most altered, least volatile rich). 

Bulk hydrogen and 36Ar contents versus mineral alteration index for CM carbonaceous chondrites. These trends reflect progressive incorporation of water and destruction of the noble gas carrier as alteration advances. After Browning et al. (1996). 

Grimm, R. E. andMcSween, H. Y. (1989) Water and the thermal evolution of carbonaceous chondrite parent bodies. Icarus, 82, 244-280. 

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). 

The presence of organic molecules in samples of extraterrestrial matter has been known for more than a century. Some of the greatest chemists of the nineteenth century were involved in the analysis of samples of meteoritic material. They were able to show that carbonaceous chondrites (as they are now named) contain organic molecules. The first to detect carbon in a meteoritic sample was Thenard, in 1806, by analysis of a sample of the Alais meteorite. This result was confirmed in 1834 by Berzelius, who was also the first to detect the presence of water of crystallisation. Working on a sample of the Kaba meteorite, Wohler (1858) confirmed the presence of organic matter, and in a paper dated 1859 said, I am still convinced that besides free carbon this meteorite contains a low-melting point, carbon containing substance which seems to be similar to certain fossil hydrocarbon-like substances... . 

Amino acids - and more generally organic substances - are mainly detected in carbonaceous chondrites, a minor, carbon-rich class of meteorites. These are believed to have originated from parent bodies having underwent alteration by liquid water at some stages of their existence, as attested by geochemical studies [63]. 

Murchison (meteorite) A carbonaceous chondrite, type II (CM2), suspected to be of cometary origin due to its high water content (12 percent)... 

Carbonaceous chondrites are rare stony meteorites that contain complex carbon compounds from which they get their name. Some contain water and amino acids, the building blocks of life. Carbonaceous chondrites are believed to be samples of our solar system s earliest rocks, unchanged after nearly 4.6 billion years. 

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. 

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,... 

A second interpretation of carbonaceous chondrites is as primary condensates of the solar nebula. By this view, their hydrolytic alteration is due to melting in cometary nuclei during close passes with the Sun, or due to transient heating events by shock waves or collisions (McSween, 1999). Other carbonaceous chondrites show metamorphic alteration with minerals similar to those in Earth formed during deep burial under elevated temperatures and pressures (Brearley, 1999). Like soils and paleosols on Earth and Mars, carbonaceous chondrites demonstrate the great antiquity of hydrolytic weathering in dilute acidic solutions, presumably of carbonic acid derived from water vapor and CO2. These remain the principal gases released from volcanoes, and soils remain important buffers for this environmental acid. 

Water cycling in the Archaean More difficult to predict is how water was distributed in the Archaean mantle. It will be shown later (see Section 5.2) that the Earth was initially volatile-rich, when it accreted and it subsequently lost water and other volatiles. Indicative here is the comparison between the water content of carbonaceous chondrites, the likely primitive material of Earth accretion (up to ca. 10 wt%), and the estimated water content of the present-day silicate Earth and hydrosphere (0.19-0.24 wt%, see Table 5.2). 

Based on information from these sources, scientists estimate that the nucleus of a comet consists of about 42 percent volatile compounds (about 80 percent of which is water) and 58 percent solid particles, generally described as "dust." About 45 percent of this nuclear dust is similar in chemical composition to the carbonaceous chondrites found in meteorites that is, they are primarily silicate in composition. (Carbonaceous chondrites are discussed at greater length in the next section.) Another 40 percent of the nuclear dust is organic in nature, that is, composed of carbon compounds of varying degrees of complexity. The final 15 percent of nuclear dust is composed of very small particles with masses of only a few atto-grams (10 l8g). 

---