Theoretical research in the Trieste SISSA/ICTP Condensed Matter Group is shedding new light on the behaviour of molecular matter under extreme conditions. It is all part of a larger effort to better understand the forces that shape our Universe.

Extreme Measures

We all know what water does under 'normal' conditions--that is, normal for us. Heat water to 100°C and it begins to boil into a gaseous state. Chill water to 0°C and it freezes into ice.

But how would water behave under 'extreme' conditions--for example, the very high temperatures and intense pressures found deep within the Earth's crust or at the core of large, remote planets?

Such conditions thankfully don't exist on the Earth's surface. (If they did we wouldn't be here.) But they are commonplace elsewhere.

In fact, conditions found at the cores of the Earth and the giant planets of our solar system are so extreme that they cannot be replicated in laboratories except under 'shock-wave' experiments that take place under difficult-to-create explosive conditions. As a result, scientists have turned to an alternative, more sedate, strategy based on a combination of theoretical physics and modelling. The latter benignly simulates the conditions created by shock-wave experiments at researchers' desks and computer terminals.

These simulations have advanced our basic understanding not only of Earthly matter but of matter on other planets. What would the 'solar' brew of water, ammonia and methane found in the deep, hot 'ice layers' of Neptune and Uranus look like if we could see it? Would the brew be solid or liquid? Would it mix or separate, insulate or conduct? Would the molecules hold together or fall apart? Would they be transformed into something else?

The only certainty we can count on is that this brew would behave differently there than it does in our everyday world under 'normal' pressures and temperatures. That's because the water, ammonia and methane in these remote places experience physical conditions radically different than conditions on or near the Earth's surface.

For example, higher pressures found inside the planets of our solar system would push the nuclei of these materials closer together, accelerating the quantum movement of electrons into a frenzy of kinetic energy. Changes in crystal structure would follow as the materials sought to relieve this pent-up energy.

High temperature, on the other hand, would ultimately cause the newly created crystal structures to melt. Inside the giant planets, the protons of water and ammonia may melt on their own and run loose in a 'superionic state' before the heavyweight constituents--oxygen and nitrogen--melt too. High-temperature molecular dissociation could cause the formation of plasmas with high conductivity, which is a precondition for the creation of planetary magnetic fields.

Research and modelling to date have helped scientists foreshadow some of the likely responses to such conditions. But much remains to be learned because as the saying goes "the devil is in the details." Recent desktop explorations into condensed matter theory, relying on sophisticated calculations of electronic structures and computer simulations of matter at the atomic and molecular level, are now beginning to fill some of the gaps in our knowledge.

Such explorations are based on what physicists call density functional theory, whose principles were first articulated by Walter Kohn, Pierre Hohenberg and Lu Sham in the 1960s. Some 30 years later, in 1998, Kohn received the Nobel Prize in Chemistry for his efforts.

Under this theory, the properties of most solids can be accurately calculated when the material's nuclear positions are known. The problem is how to pinpoint the nuclear positions, especially under conditions of extraordinarily high temperatures and pressures.

In the mid 1980s, Roberto Car and Michele Parrinello, researchers with the SISSA/ICTP Condensed Matter Group in Trieste, illustrated how accurate theoretical simulations could be used to help overcome this problem. Their technique--so-called "first-principles molecular dynamics simulations"--subsequently came to bear their names and is now known as the Car-Parrinello Molecular Dynamics Method.

Applications of this method to ultra-high pressures began in earnest about five years ago, again largely conducted by a Trieste research team comprised of Marco Bernasconi, Guido Chiarotti, Paolo Focher, Parrinello and myself. Sandro Scandolo and Jorge Kohanoff joined our efforts early on, while Carlo Cavazzoni and Alessandro Laio became part of the team somewhat later.

Initially, the theory helped to expand our understanding of high pressure transformations in such elements as silicon and carbon (the latter involving the transformation of graphite into diamond); such molecular crystals as hydrogen and oxygen; and such carbon-based systems as acetylene and carbon monoxide.

With these early successes in hand, researchers embarked on more difficult problems, including examinations into the theoretical underpinnings driving the molecular transformations of matter in the interior environments of planets.

Under the extreme conditions found there, we determined that methane would first associate into hydrocarbons and only at higher pressures dissociate into hydrogen plus diamond. We speculated that the initial association may explain Neptune's excess ethane--a hydrocarbon--first detected in that remote planet's atmosphere by the spaceship Voyager 2 during its 1989 'close-approach' mission. Our reasoning went like this: Under high pressure, the ethane is synthesized from the methane and diffused into the atmosphere where it becomes visible.

Research efforts in Trieste may also help us better understand Neptune's planetary magnetic field observed by Voyager 2 during the same flight. The high-pressure, high-temperature water found deep inside the planet is predicted by Car-Parrinello computer simulations to form a conducting plasma capable of sustaining the dynamo currents necessary to generate the field.

Most recently, the planetary aims of condensed matter theorists in Trieste have moved closer to home to calculate the state of the Earth's solid iron core--and, more specifically, the temperature at which the iron would melt deep inside the Earth. Such calculations could help determine the temperature of the inner boundary of the Earth's core, and thus the temperature of the centre of the Earth (our calculations indicated that it is a 'cool' 5100°C).

All in all, the molecular dynamics simulation methods pioneered by the Trieste researchers promise to shed light and reason on the behaviour of matter subject to extreme conditions. In the process, we may be able to anticipate the behaviour of water on Neptune or iron at the Earth's core with the same confidence that we now enjoy each time we light the gas under a pot of water to prepare a spaghetti dinner.

For more detailed information about this research, see the technical reports written by the SISSA/ICTP condensed matter team in Science (28 February 1997 and 1 January 1999).

Erio Tosatti is a professor of physics at the International School for Advanced Studies (SISSA) and a long-time consultant with ICTP's Condensed Matter Group.

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