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News from ICTP 107 - Features - Core of the Matter
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