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Ice-rinks in Hell
HENRY GEE

The temperature on Mercury, the closest of the planets to the Sun, can
get as high as 700 K (427 °C): yet there are persistent reports of ice
at its poles. But where did this ice come from? This difficult
question is tackled by Julianne I. Moses of the Lunar and Planetary
Institute, Houston, Texas and colleagues in a report in the journal
Icarus. Their conclusion is that the water on Mercury was probably
delivered by the impact of water-rich asteroids or comets: however,
their study, which used model simulations, was hampered by our
ignorance both of Mercury and the population of 'Sun-grazing'
asteroids and comets. It could be that the only way to solve the
problem definitively is to travel to Mercury and see.

The prize could be worth the trip. Mercury could have a thousand times
as much ice in its polar regions than our Moon, whose deposits of ice
were revealed by the recent Lunar Prospector mission. Ice on the Moon
is likely to be well-mixed with dust, but the ice on Mercury is
probably much cleaner -- true ice-rinks in Hell. Despite the presence
of sunshine bright enough to melt the most obdurate ice-cream cone in
seconds, there are places on Mercury in permanent shadow, where
temperatures could plummet below the temperatures of around 110 K
(minus 163 °C) required to ensure the stability of ice for the
duration of the history of the Solar System.

The planet itself is an unlikely source of water. Mercury is very
dense, proportionately richer in metals such as iron than in the
silicate rocks that could be a source of water. It is possible that
the original rocky crust of Mercury was removed by impacts in the
violent early history of the Solar System -- impacts which could have
expunged Mercury's original inventory of water and other 'volatiles'.

Another source could be the constant rain of 'interplanetary dust
particles' (IDPs) that showers the planets (including the Earth) from
space. Over time, IDPs could have delivered sizeable amounts of water
to Mercury, but the resulting ice would have been far dirtier than
indicated by the bright spots of clear, reflective ice implied by
radar observations.

The most likely prospects are impacts by asteroids and comets. Comets
contain more ice than asteroids: the impact of a single, large comet
could deposit enough water on Mercury to account for the observations.
But cometary impacts are so energetic that most of the impactor would
be jetted back into space, and the gravity of small Mercury would not
be powerful enough to capture the débris.

This leaves the asteroids, and a subset of comets whose orbits are
dominated by the gravitational field of the giant planet Jupiter.
Sadly, we do not know enough about Sun-grazing asteroids and comets to
make good estimates of the rate at which such things would have struck
Mercury. The situation is complicated even further by the possibility
that some extinct and icy remnants of comets travel incognito,
disguised as asteroids -- it is often impossible to tell the
difference between the two. Whatever the source of the ice on Mercury,
though, Moses and colleagues claim that there is plenty of it.

© Macmillan Magazines Ltd 1999 - NATURE NEWS SERVICE
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