ICTP scientists are engaged in a fascinating journey of discovery to determine what lies beneath the surface of the planets. Sandro Scandolo explains what they've been up to down there.

Core of the Matter

The successful touchdown and subsequent exploration of the surface of Mars by the US National Aeronautics and Space Administration's (NASA) space rovers Spirit and Opportunity this past January is a breathtaking victory for both science and society--worthy of the world's attention and awe. Photos of Mars' sand-duned, crater-faced surface beamed through 200 million kilometres of space and onto television and computer screens across the globe represent a stirring triumph of human ingenuity and imagination.
The photos revealed a desert terrain pockmarked, creased and wrinkled by meteorites and long-dormant volcanoes. Yet, scientists believe that beneath Mars' sometimes rocky--sometimes sandy--crust lies a core of soupy, white-hot metallised liquid. It's a core not much different than that lying within our own planet Earth.
No space voyage, regardless of its scope, can delve beneath the surface of planets to tell us what lies within. Indeed the deepest explorations beneath the Earth's surface have probed no farther than 12 kilometres--half the distance from downtown London to Heathrow airport. Better to reach for the stars, so it seems, than to grovel in the bowels of the planets, including our own.
Such efforts--whether taking place here on Earth or through distant excursions to Mars--literally just scratch the surface, failing to shed light on the hidden world that comprises more than 99 percent of a planet's mass.
The good news is that we don't need complex and costly missions to 'explore' the Earth's or--for that matter--other planets' internal environments.
Indeed scientists have relied on such conventional scientific tools as the recording of seismic waves to learn a great deal about the Earth's internal structure. Such studies have shown, for example, that the Earth's core is substantially denser than the other segments of the planet. Similarly, measurements of mass, gravitational forces and magnetic fields, observed through standard methods of remote sensing, have allowed scientists to develop both comprehensive density profiles and a deeper understanding of the internal dynamics not only of the Earth but also of the other planets in the solar system from Mercury to Saturn and beyond.
But the most recent advances in our understanding of the interior environments of planets have come not from observations but from laboratory 'recreations' that enable scientists to imitate the torrid interiors of planets here on Earth. A decade-old revolution in compression techniques, made possible by diamond-tipped pincers, have allowed scientists to literally 'squeeze' micron-sized samples of planetary material into ever-smaller volumes. The study of these compressed samples, in turn, has shed revealing light on the large-scale composition, dynamics and indeed historical evolution of planets.
These 'diamond-squeezed' samples weigh just a millionth of a gramme, less than the weight of the ink-stained dot found at the end of this sentence. Yet, because the laws of nature remain constant to the atomic scale, there is no reason to believe that such experiments cannot be miniaturised even farther, until they are tapered to the size of a few atoms or molecules.
This would be the scale at which current theory and today's computing facilities would enable physicists and chemists to numerically determine the behaviour patterns of atoms and electrons in matter.
In other words, our minuscule laboratory simulations are on the threshold of uncovering the workings of the vast underbelly of our planets--a world that will long remain unseen and untouched.
Call it the computer exploration of inner space--scientific immaculate conception, if you will--devised through simulations where atoms can vibrate, collide and collapse under extraordinary 'programmed' replications of pressure and temperature.
For the past five years, a largely Trieste-based group of scientists, which includes Erio Tosatti who has been associated with the Centre for more than three decades and recently served as ICTP's acting director, has been at the forefront of 'bottom-up' simulated laboratory explorations that seek to understand how matter behaves in the forbidding environments found in the interior of planets.
Their analytical framework of choice has been the study of phase transitions in molecular dynamics. Their tool of choice has been high-speed parallel computers.
In 1997, the group, which in addition to Tosatti and myself included Francesco Ancilotto (University of Padua) and Guido Chiarotti (International School for Advanced Studies, Trieste), tried to predict the behaviour of methane (CH4) in the interior of the giant planet Neptune.
About 15 years earlier, scientists at Lawrence Livermore National Laboratory in the United States had concluded that extreme pressure inside Neptune, the solar system's fourth largest planet (Jupiter, Saturn and Uranus are larger) causes methane molecules to completely dissociate, enabling carbon atoms to reassemble into carbon-only diamond clusters. Their analysis created this tantalising hypothesis: Could a giant diamond mine be hiding in the core of Neptune?

Computer simulation of methane

Simulations carried out at ICTP over the past seven years confirm that Neptune's central core could indeed be loaded with diamonds. But the vast majority of the planet's mass likely consists of hydrocarbon chains since less intense pressures found throughout most of the region would mean that the methane molecules only partially dissociate to create an endless series of carbon atom chains surrounded by hydrogen atoms.
Transforming methane into diamonds provides a dramatic story line. But like so many things in life (and perhaps beyond), the truth is often much more mundane.
Our studies at ICTP won't bring riches but they do suggest why methane has been eliminated from Neptune's current list of chemical constituents despite the fact that it had once been counted--along with water (H2O) and ammonia (NH3)--as one of the planet's three most abundant constituents. Similarly, the wafting of hydrocarbons from the interior into the atmosphere may also help explain why Neptune's life-denying atmosphere is laden with hydrocarbons.

View of Neptune from spacecraft Voyager 1

Jupiter and Saturn are believed to be compositionally much simpler than Neptune. A single atomic species, hydrogen, makes up most of their mass, with traces of helium and other light elements. No experimental apparatus is presently able to recreate in the laboratory the extreme conditions found in the interiors of these two planets. Simulations are the only method available. And the picture that has emerged from our simulations, which we began in spring 2002, have proven quite interesting.
Extreme pressures and temperatures cause hydrogen to dissociate inside Jupiter and Saturn, much like methane in Neptune. But even more surprising is the observation that the pressure-induced transition from a molecular fluid to a dissociated fluid has been accompanied by a large and sudden increase in the density of hydrogen.

Computer simulation of dissociated hydrogen

In light of this finding, the current picture of Jupiter and Saturn as homogeneous fluid spheres will need to be dramatically modified to account for the sharp transition between a molecular fluid envelope and a dissociated fluid core. Is there any chance we will ever be able to verify this hypothesis? And, more generally speaking, how much trust should we place in the outcomes of numerical simulations?
For those who are impatient, the answer may be soon forthcoming. The spacecraft Cassini, launched by NASA in October 1997, will enter Saturn's orbit during the first week in July to begin a four-year tour of the 'ringed' planet. By monitoring Saturn's magnetic field and gravitational impulses, Cassini will provide a more detailed density profile of the planet that should help determine whether the atom clusters that my colleagues and I have 'virtually' squeezed in our computers are conveying the truth.
Like theoretical and experimental physicists of a quarter century ago who played their theories and experiments in a wonderfully synchronised intellectual duet (think of Abdus Salam and Carlo Rubbia in revealing and then confirming the existence of the W and Z particles), space scientists and computer modellers may often work in tandem in the future to advance the frontiers of the last frontier. Their efforts, it is hoped, will help us see the solar system's planets both in their outer and inner glory.

Sandro Scandolo
ICTP Condensed Matter Physics Group

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