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IT IS A PLANET where sunrise to sunset is an entire year. The Sun can be 11
times brighter than it is in Earth’s sky. With almost no atmosphere, the
planet’s equatorial surface bakes at a sizzling 450 °C near noon, but cools
to below –170 °C late at night. The landscape, after billions of years
of explosive impacts by comets and asteroids, is scarred and lifeless. The
floors of the craters nearest the planet’s poles, in permanent shadow, are deep
freezes that may hold ice deposited by aeons of cometary collisions. And every
116 days a prominent blue dot appears at its brightest in the ink-black sky.

This alien world is Mercury, the innermost planet of our Solar System, and
one known to the astronomers of ancient civilisations as the morning and evening
star that darted most rapidly across the heavens. Mercury is the Roman
counterpart to Greek Hermes, the messenger of the gods of Olympus.

Despite the fact that Mercury is sometimes the closest planet to Earth, we
know less about it than we do about any of the others except Pluto. It has been
visited by only one spacecraft, Mariner 10, which made three flybys in 1974-75.
But in July last year, NASA approved a mission called Mercury Surface, Space
Environment, Geochemistry and Ranging—or Messenger. The projected cost is
$286 million. And in September 2009, it should become the first
spacecraft ever to orbit Mercury.

Messenger will reveal the hidden geology of the half of Mercury never before
imaged at close range, probe the details of its mysterious magnetic field, and
analyse its tenuous atmosphere. But above all, Messenger will try to discover
what manner of heat or violence smelted the iron planet from the crude ore of
the early Solar System.

Any spacecraft launched headlong towards Mercury would be accelerated by the
Sun’s gravity and fly past so fast—at around 10 kilometres a second or
more — that no existing propulsion system could stop it. So Messenger will
follow a more roundabout route, using the gravity of the planets to adjust its
trajectory and slow down. It will take off in 2004 and then fly by the Earth
once, Venus twice and Mercury twice before going into Mercury orbit. Even then,
Messenger will have to decelerate by 1.6 kilometres per second. At launch, more
than 60 per cent of its mass will be fuel, primarily for this orbit-insertion
burn.

So every other component must be trimmed to its lowest possible mass. Two
small phased-array antennas replace the usual big dish for communicating with
Earth, and Messenger’s seven miniaturised instruments and their electronics will
weigh only 32 kilograms. Instead of heavy, complicated refrigerators, a thermal
shade made of ceramic cloth will shield these instruments from the heat of the
Sun. And to protect them from the bright sunlit face of Mercury, the designers
chose an elliptical orbit that takes the spacecraft far away from the planet,
allowing it to cool down.

What do we already know about this enigmatic planet? The most crucial single
fact is that Mercury is remarkably dense. At 5430 kilograms per cubic metre, it
is denser than Venus and nearly as dense as the Earth. That is all the more
remarkable because, whereas the interiors of Venus and Earth are highly
compressed by the weight of overlying material, Mercury is too small for that to
happen. The most likely explanation is that about two-thirds of the mass of the
planet is iron—a ratio of metal to rock more than twice that of Earth. If
most of the metal has sunk into the centre, the iron core has a radius 75 per
cent as large as the planet’s.

Why is Mercury made mostly of iron? The inner planets, we believe, formed as
a cloud of gas and dust surrounding the Sun coalesced and condensed. Rocky
silicate particles and metals stuck together, gradually forming larger lumps of
matter called planetesimals, which in turn collided and merged to form the
planets. Most simulations of this process fail to produce an iron-rich planet
like Mercury. Somehow, Mercury must have lost most of its silicates. There are
three possible ways for this to have happened. It could be that in the inner
part of the solar nebula, where Mercury formed, thick gas dragged the lighter
silicate particles into the Sun, leaving dense iron-rich particles behind.
Another idea is that the young Sun was so dazzlingly bright that its radiation
vaporised most of Mercury’s outer silicate layers. Or did some huge body hit the
embryonic planet, blasting most of the rock into space? In short, the silicates
could have been sifted, seared or smashed away.

Whatever happened to Mercury must also have affected the other inner planets,
because they all formed at about the same time and by similar processes. We know
that all four inner planets have different compositions. Some of these
differences may be due to the random nature of accretion, particularly the final
phases when Mercury-sized and even Mars-sized objects were hitting the larger
planets. Others may be a function of distance from the Sun. If Mercury’s
silicates were sifted away, for instance, then such sorting should have had a
smaller but perhaps important effect on the ratio of metal to rock in the other
planets.

Fortunately, each hypothesis for Mercury’s high iron content predicts a
different composition for the planet’s rock. If the impact model is right, there
should be low levels of elements such as aluminium and calcium that tend to
concentrate in planetary crusts, as these outer layers of the planet would have
been blasted away. The sorting model implies that the rock-forming elements
should be in proportions expected for solar material in the planet-forming
nebula at Mercury’s distance from the Sun. The vaporisation model predicts that
there should be lower concentrations of volatile elements, which have low
boiling points.

Messenger will aim a battery of instruments at this problem. Three of them
will analyse gamma rays, neutrons and X-rays emitted by different elements on
the planet’s surface. Another spectrometer will look at visible and
near-infrared wavelengths to map the distribution of different minerals.

We already have one tantalising clue to this puzzle. In 1985, Andrew Potter
of NASA’s Johnson Space Center in Houston and Thomas Morgan, then of
Southwestern University in Georgetown, Texas, detected the characteristic
emission lines of sodium in Mercury’s atmosphere, using Earth-based telescopes.
A year later, they found potassium, too. This result might seem to kill off the
vaporisation theory—such volatile elements would have been eradicated if
the Sun was bright enough to boil off most of the planet’s rock. But it has a
lifeline: the sodium and potassium we see might have been brought in by
meteorites over the past few billion years. Messenger will be able to
concentrate its gaze on recent craters, where deeper crust is newly exposed, in
order to settle whether the vaporisation theory can work.

Mercury’s huge iron core is involved in another mystery. When Mariner 10 flew
by Mercury, the biggest surprise was finding a magnetic field—one that is
strong enough to generate an Earth-like magnetosphere that largely shields the
planet from the solar wind. Such strong fields are not the norm in the inner
Solar System: Venus doesn’t have one, and neither does our Moon. Mars did have a
magnetic field, but it turned off less than half a billion years after the
planet formed. So why does Mercury retain a strong field?

Earth’s magnetism is produced by dynamo action in a liquid metal outer core.
And indeed, Mercury’s core must have been molten once. Otherwise it is hard to
explain the planet’s peculiar spin rate. In 1965, astronomers discovered with
ground-based radar that the planet rotates in 59 Earth days, precisely two
thirds of its year—and it is because of this peculiar resonance that the
solar day on Mercury, the time between successive appearances of the Sun at a
given position in the planet’s sky, lasts for two Mercury years. Mercury’s spin
was probably slowed to this rate by tides raised by the Sun, a process that is
much easier to explain if Mercury had a fluid core at the time.

Frozen core

But being smaller than Earth, Mercury must have cooled faster, which leads
many planetary geologists to think that its core should have frozen solid by
now. Perhaps a high concentration of a light element such as sulphur lowers the
melting point of the iron, permitting a thin outer shell of metal to remain
molten. Dynamo generation in such a thin shell might produce a magnetic field,
but that process is not well understood.

To find out the origin of Mercury’s field, Messenger will measure the field’s
shape. If Mercury’s magnetism is caused by a dynamo in a mostly liquid core, as
on Earth, then the field should be roughly a simple dipole. If the field
geometry is messier, then the liquid fraction of the core may be much smaller
than on Earth, or another mechanism may be at work—perhaps thermoelectric
effects at the core-mantle boundary. It is even possible that the outer rocky
shell of the planet is a giant permanent magnet that retains an ancient field,
set up when the core was liquid.

Messenger has a second way of testing for a fluid outer core. The Sun’s tides
still affect the planet’s spin rate, making it vary slightly throughout the
year. If there is a fluid outer core, this variation will be about twice as
large as it will be if the core is entirely solid. By measuring the shape of the
planet with an altimeter over several Mercury years, the spin rate and its
variation can be determined precisely enough to distinguish between these
possibilities.

A hot planetary interior might also have produced volcanic activity.
Mariner’s images covered only about 45 per cent of the surface at a resolution
of about 1 kilometre, which is too coarse to see volcanic flow fronts. So
Messenger’s third task is to determine Mercury’s geological history in much
greater detail—and not just for the portion seen by Mariner.

What Mariner did see was a landscape dominated by craters—so many that
the imaging team, led by Bruce Murray of Caltech, argued that the surface must
date from the first third of Solar System history. In other words, no major
geological activity has occurred for the past 3 billion years. The surface of
Mercury viewed by Mariner is a dead landscape like the Moon, not an active one
like the Earth.

Shrinking surface

Cooling of a planet can influence the geological history in other ways.
Cutting across every type of terrain, Mariner 10 found strange linear features
called lobate scarps. To Robert Strom of the University of Arizona’s Lunar and
Planetary Laboratory, and others on the imaging team, these scarps looked like
compressive faults, formed by shrinking and cracking of the surface as the
interior of the planet cooled. Messenger’s high-resolution mapping should
confirm or refute this idea.

More importantly, Messenger will map the 55 per cent of the surface never
before seen by a spacecraft—one of the last uncharted surfaces in the
Solar System. There could be big surprises from this unseen hemisphere. Radar
images obtained by John Harmon of Arecibo Observatory in Puerto Rico, although
coarse compared with spacecraft images, show features that resemble the youngest
volcanoes on Mars and Venus. Mercury could be like Mars, with one ancient,
cratered hemisphere and one hemisphere of volcanoes and lava plains shaped by
volcanic eruptions or disruption of the crust, perhaps as recently as a few
hundred million years ago.

A by-product of imaging the planet’s unseen face will be the discovery of
hundreds of geological features, which will all have to be named. The
International Astronomical Union has decreed that Mercury’s features should
honour figures from literature and the fine arts. There are already craters
named after Monet, Mozart and Shakespeare, but Messenger will provide
opportunities for many others. After 2009, Mercury maps might bear the names
Picasso, Prokofiev and Faulkner.

However romantic the names, Mercury’s landscape will remain hostile to humans
and their machinery. But perhaps not all of it. In 1991, two groups of
astronomers detected radar-bright deposits at Mercury’s poles, concentrated in
the floors of large impact craters. The deposits polarise radar in the same way
that water ice does on Mars and the Galilean satellites of Jupiter. If water ice
really is trapped in the permanently shadowed crater floors, it could provide a
vital resource to future explorers. But some astronomers think that the signals
could come from sulphur, not water. Messenger will adjudicate, using neutron and
gamma-ray spectrometers to look for emission from hydrogen and sulphur.

The new mission will deepen our understanding not only of the solar family’s
innermost outpost, but of all Earth-like planets. And there may be many other
Mercurys out there, beyond our Solar System. We cannot yet detect a planet as
small as Mercury, or even the Earth, around another star, but more than half of
the giant planets found so far are at least as close to their parent star as
Mercury is to the Sun. Although a hellish extreme from the perspective of our
otherwise chilly Solar System, Mercury is in good extra-solar company.