Diamond Oceans on the Outer Planets?

by Paul Gilster on January 18, 2010

What goes on under the clouds of Neptune and Uranus? A new paper reinforces the possibility that there are oceans of liquid diamond in such places, diamond seas with solid diamond icebergs. It’s a notion with a pedigree. I’m looking, for example, at a paper by David Stevenson (Caltech) from the Journal de Physique from November of 1984, where I find this:

There is clear evidence that many hydrocarbons decompose (or collapse) upon shock compression, probably into graphite and hydrogen. It is very important to establish the range of temperature and C:H ratios for which this decomposition can occur. It is equally important to establish whether an actual phase separation occurs (implying possible formation of a diamond or liquid metallic (?) carbon layer in Uranus and Neptune) or whether a collapsed but intimately mixed C-H structure results.

Stevenson’s work built on that of Marvin Ross, who suggested the possibility of diamonds in this environment in 1981. Researchers at the University of California at Berkeley demonstrated about ten years ago that the high temperatures and pressures found inside planets like these can turn methane into diamond, and in fact that the diamonds settling into Neptune’s core could account for the excess heat radiated by the planet. Now a team led by Jon Eggert (Lawrence Livermore National Laboratory) has explored the conditions under which diamonds melt, and has shown that, like water, liquid diamond when freezing and melting produces solid forms atop liquid ones.

To melt diamond requires high pressures, which has made measuring its melting point extremely difficult. But the pressures found inside gas giants, and the high temperatures that go with them, should be enough to do the job. This Discovery News story notes that Eggert’s team subjected a small diamond (about a tenth of a carat) to a laser beam, liquifying the diamond at pressures some 40 million times greater than at sea level on Earth. As they then reduced the temperature and pressure, solid chunks of diamond began to appear at about 11 million times sea level pressure with a temperature of some 50,000 Kelvin.

Image: Voyager’s view of Uranus. Does an ocean of diamond lurk beneath these clouds? Credit: NASA/JPL.

The diamond chunks, like tiny icebergs, did not sink but floated. Eggert believes an ocean of liquid diamond could explain the fact that, unlike the relatively close match we find on Earth, the magnetic and geographical poles on Uranus and Neptune do not align, and can be offset by as much as 60 degrees from the north-south axis. Put an ocean of liquid diamond in just the right place and the magnetic field displacement makes more sense. We won’t know for sure without further study, but Eggert’s work makes the prospects of diamond oceans more plausible, reinvigorating what may be the most exotic scenario in the Solar System, one that could extend to other gas giants and even to the interiors of brown dwarfs.

The paper is Eggert et al., “Melting temperature of diamond at ultrahigh pressure,” Nature Physics 6 (1 January 2010), pp. 40-43 (abstract). The Ross paper is “The ice layer in Uranus and Neptune — diamonds in the sky?” Nature 292 (30 July, 1981), 435 – 436 (abstract). Stevenson’s paper is “High pressure physics and chemistry in giant planets and their satellites,” Journal de Physique Vol. 45 (November, 1984), pp. C8-97 to C8-103 (abstract).

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{ 17 comments }

MattC January 18, 2010 at 11:22

Must be how the diamond asteroids in V.V’s “deepness in the sky” formed.

kurt9 January 18, 2010 at 13:05

Lucy in the sky with diamonds.

Athena Andreadis January 18, 2010 at 13:16

Given the chemical and physical properties of the various carbon polymers and the size and pressure of the two sub-giants, hydrocarbon or graphite oceans are far likelier.

Tulse January 18, 2010 at 15:51

In what sense would “liquid diamond” still be diamond? Isn’t diamond by definition a crystalline solid?

Gary Allen January 18, 2010 at 17:03

Tulse said:
“In what sense would “liquid diamond” still be diamond? Isn’t diamond by definition a crystalline solid?”

Tulse beat me to the punch. The “liquid diamond” description is baloney. Diamond is a specific solid crystalline state of carbon. By definition diamond can not be liquid. The following link shows the phase diagram for carbon:

http://dao.mit.edu/8.231/carbon_phase_diagram.jpg

The triple point pressure for carbon is 108 bars. Consequently liquid carbon only occurs where there is high pressure such as within the crust of a planet or asteroid. It is not uncommon to find graphite dust and small diamonds in meteorites. Also the core of Jupiter is not a big diamond (Arthur C. Clarke got that one wrong). The pressure and temperature of Jupiter’s core is not correct for diamond and there are probably no diamonds within Jupiter’s atmosphere. However it is likely that Jupiter’s core is metallic hydrogen which is why Jupiter has such an intense magnetic field.

AlfaCentavra January 18, 2010 at 20:07

… temperature of some 50,000 Kelvin.
???

Zephir January 19, 2010 at 13:58

Diamond lattice is well known by fact, it clearly violates the rule, the highest strength, the more dense lattice. It even violates the rule, the highest strength, the higher number of the closest neighborhoods (i.e. coordination number). The correct rule is, the higher strength, the closer atom distances in the lattice together with least number of closest atoms possible.

Therefore the atomic packing factor of diamond cubic lattice is far from most compact arrangement of spheres with face-centered cubic (FCC) unit cell of 74% packing efficiency, so it could really behave like ice at the water during melting. The most dense matter could appear like nested dodecahedron foam or streaks of dark matte

Jeff Milam January 19, 2010 at 14:43

What happens to a diamond that has been liquefied when it cools? Will it take different forms under different pressures?

Gary Allen January 19, 2010 at 17:52

Jeff Milam asked:
“What happens to a diamond that has been liquefied when it cools? Will it take different forms under different pressures?”

Diamond is a strange material phase because it is “metastable”. If you look at the phase diagram that I linked above, diamond at less than 4500 K should turn into graphite after pressure has been reduced. However diamonds retain their phase at temperatures less than 4500 K because they are metastable. There is probably(?) a specific temperature/pressure corridor that allows this. Somewhere I read that diamonds are actually changing into graphite but they’re doing it very slowly. Liquid carbon at pressures greater than 10 GPa will transition into diamond as temperature is reduced. Somewhere I read that metallic hydrogen might be metastable. Does anyone know anything about that?

Adam January 19, 2010 at 20:12

Diamond is any configuration of carbon akin to the tetrahedral molecules we’re used to at STP. Under the extreme conditions produced by the experiment a high temperature liquid of tetrahedral carbon formed – it most definitely isn’t liquid ‘carbon’ as we know it at low pressure, which has no ordering of the interatomic bonds, unlike liquid diamond which is so ordered. It’s a technically correct description to call it diamond.

Holy Cow January 21, 2010 at 17:32

I’m sure the use of “liquid diamond” was only to appeal to a broader, maybe non-scientific, community. Publicity is publicity…good or bad.

kelvin January 26, 2010 at 13:56

Dear All, on a related matter I would recommend the following paper:

The ice layer in Uranus and Neptune – diamonds in the sky?, Marvin Ross. Nature Vol292, 30 July 1981, p435-436.

This was the paper that gave Arthur C Clarke ideas for 2001 A Space Odyssey and he discusses it at the back of one of the Odyssey books. For those of you that havn’t read all the Odyssey books yet, I won’t spoil it any further.
Cheers. Kelvin

Ron S January 26, 2010 at 19:57

Clarke got the idea for a work he produced in the 1960s from a 1981 paper? Hmm.

Eniac January 27, 2010 at 14:52

Where do you think Clarke got all his ideas from?

Satellites? Carbon fibers? Space elevators? You don’t just make this stuff up. He was obviously cheating, most likely with a time machine in his basement.

Ron S January 27, 2010 at 17:30

Yes, of course. He probably got hold of the time machine blueprint when he was visited by Leonardo da Vinci.

I suggest a program of searching the basements of all futurists.

Sean G January 31, 2010 at 10:26

If the pressure is not high enough, then yes, the diamond would degenerate back into another form of carbon, like graphite or hydrocarbons. But what makes a diamonds a diamonds is not the way the molecules area arranged, but the way the atoms within the molecules are arranged. As the molecules start to lose their grip on each other a liquid forms. Assuming the pressure is high enough, the molecule maintains it layout.

ljk June 29, 2011 at 23:30

Clocking Neptune’s Spin

Neptune as seen by the Voyager 2 spacecraft in 1989. (Photo: NASA)

(Click image to enlarge) In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Image: Erich Karkoschka)

By Daniel Stolte, University Communications, June 29, 2011

By tracking atmospheric features on Neptune, a UA planetary scientist has accurately determined the planet’s rotation, a feat that had not been previously achieved for any of the gas planets in our solar system except Jupiter.

A day on Neptune lasts precisely 15 hours, 57 minutes and 59 seconds, according to the first accurate measurement of its rotational period made by University of Arizona planetary scientist Erich Karkoschka.

His result is one of the largest improvements in determining the rotational period of a gas planet in almost 350 years since Italian astronomer Giovanni Cassini made the first observations of Jupiter’s Red Spot.

Full article here:

http://uanews.org/node/40494

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