Diamond Showers

If we exclude the noble gas He since it does not react with any other element, the three most abundant elements H, C, and O will form the most common molecules of water (H2O), methane (CH4), and carbon dioxide (CO2) (Fig. 1.9). Thus, these three compounds are the common gases in planets that harbor an atmosphere. Both methane and carbon dioxide can decompose under high pressure, and the segregated carbon will form diamond.

The carbon atom in methane is surrounded by four hydrogen atoms that form a tetrahedron. The carbon's atom forms four bonds that extend to each of the four hydrogen atoms. This bonding structure (sp3) is exactly that of diamond except that each of the bond is slightly ionic due to the different bonding strength of electrons between the carbon and hydrogen atoms. Hence, we may view c

Figure 1.9. Common atmospheric gases are derived from the three most abundant cosmic elements.

H H20 O

Figure 1.9. Common atmospheric gases are derived from the three most abundant cosmic elements.

Figure 1.10. Methane is a gas composed of floating single atom diamonds. Each atom is confined in a tetrahedron cage of hydrogen atoms.

methane as a caged single atom of diamond (Fig. 1.10). Methane is the common ingredient in natural gas. It may be of interest to note that cooking is often done by burning floating diamonds.

When methane is heated in the presence of hydrogen atoms, single atoms of diamond can be coerced to link together to form multiple atom diamonds. With time, micron diamonds may grow

Figure 1.11. Diamond micron powder formed by CVD (left diagram) and the linking of micron sized diamond to form a diamond film (right diagram).

to microscopic size (Fig. 1.11), and eventually, they may link to form a continuous layer. This is a common CVD practice to produce diamond films.

Methane can also decompose under high pressures to form diamond. Physicists have discovered that diamond can be formed by pressing the solid form of methane to 50 GPa, or by shooting it with a bullet traveling at a speed of 30 mach (10 km/sec). It is, therefore, conceivable that diamond snowflakes may shower from the sky in these giant planets. If the temperature is high enough (e.g. 4000°C), these snowflakes may melt to form a rain of diamonds. At a much higher pressure, this diamond rain may condense to form diamond ice or hail stones. At a great depth, diamond rain or diamond ice may accumulate to become a diamond ocean or diamond continent, depending on the temperature.

Of all the gas giants in the solar system, Uranus and Neptune contain the most methane in their atmosphere. Moreover, if the methane is under a high enough pressure without overheating as in the case of the atmosphere on Saturn and Jupiter (Fig. 1.12), such an environment is conducive to form diamond. In 1981, the physicist Marvin Ross pointed out that Uranus and Neptune may have accumulated a thick layer of diamond ice around its rocky core where the pressure may be as high as 60 GPa and the temperature at 7000°C.

8000

Satuni and j up iter

Figure 1.12. The pressure-temperature profile for Saturn, Jupiter, Uranus, Neptune and Earth. Note that it is possible for the methane on Uranus and Neptune to decompose to form diamond.

8000

Satuni and j up iter

Pressure (GPa)

Figure 1.12. The pressure-temperature profile for Saturn, Jupiter, Uranus, Neptune and Earth. Note that it is possible for the methane on Uranus and Neptune to decompose to form diamond.

One place to look for a spectacular diamond storm is deep within Neptune. Neptune has a blue appearance because the presence of methane in its atmosphere, which absorbs yellow light. Astronomers have found that this blue gaseous planet could form its own energy as it radiates 2.6 times more heat than it absorbs from the Sun. However, the source of such energy is unknown.

In 1999, Robin Benedetti from University of California at Berkeley squeezed methane in a pair of diamond anvils and heated it up with a laser to simulate the pressure and temperature conditions of about one third of that found in the center of Neptune. He found that methane decomposed to form carbon atoms and these atoms then bonded to one another to become diamond flakes. Hence, diamond snow may shower from the skies on Neptune (Fig. 1.13). The condensation of carbon to form diamond on Neptune could account for the mysterious heat source that has baffled astronomers.

Not only can methane form diamond under high pressure, so can carbon dioxide. In 1995, metallurgist Pravin Mis-try accidentally discovered that diamond could be formed by bombarding carbon dioxide with laser beams. Apparently, the

Figure 1.13. The blue color of Neptune reflects the abundance of methane in its atmosphere. Inside this interior methane atmosphere, showers of diamond flakes may rain down onto its rocky core.

decomposed carbon atoms banged against one another and linked together to form a diamond lattice. Carbon dioxide is also common on gaseous giant planets. It is the major constituent in the atmosphere of Venus and is 100 times thicker than the atmosphere on Earth. Although the pressure (20MPa) and temperature (400°C) there may not high enough to form diamond rapidly, given the long period of geological time, a thin layer of diamond may have veneered to the surface of Venus.

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