getting wider and the f loor of the Pacific Ocean is
being pushed below South America. That kind of
thing doesn’t go on with any other rocky planet.
Each rocky planet has some features in common,
but each has a very different history. We need to
compare them. If we wanted to understand trees
and how they work, you wouldn’t just look at an
oak tree. You need to look at a fir tree and a palm
tree as well so you can understand how they all
work. It’s the same with the planets. You’re not
going to understand Earth if you don’t understand
how and why each of the three comparison bodies
is different.
Would it be too simplistic to say that some
features on Mercury were formed in a similar
way to plate tectonics?
It’s not that similar because in plate tectonics, when
India crashed into Asia and started to go under
it, it kept going under indefinitely, throwing up
the Himalayas. Here the displacement is only five
kilometres [three miles] or so. The faults on Mercury
move a little way and then they grind to a halt,
because Mercury’s surface is not mobile. On Earth,
plates are sliding around continuously. The features
on Earth [move around a lot] because the planet is
cooling down, and therefore contracting thermally.
It’s the effects of thermal contraction that’s driving
the thrust movement at the surface.
You’re not going to get 1,000 kilometres [621
miles] worth of displacement [on Mercury], one slab
over another, because the planet is not shrinking
now. The shrinkage of Earth doesn’t really get
manifested because it’s completely outpaced,
but the whole idea of plates moving around and
some sliding underneath each other, it’s because
of tectonic processes. It’s the same kind of fault
displacement [on Mercury], except the total amount
of movement on it is much less.
Why have there only been two missions to
Mercury so far?
I guess it didn’t look that exciting. When Mariner 10
went there in the 1970s, it looked a bit Moon-like
because it’s covered in craters, but there are very
important differences between Mercury and the
Moon. However, it’s less exciting than Mars, where
it’s had water f lowing on the surface and there
might be life, and less exciting than Venus, which is
the Earth’s twin except it evolved very differently.
Although you can f ly past Mercury relatively
easily, if you wanted to send a spacecraft and whiz
by it, once you’ve gone by you’ve gone by. The
tricky thing is to get captured into orbit. That’s
very difficult at Mercury, because the changing
velocity you need to be captured into orbit is
very, very great. This is because if you send a
spacecraft towards Mercury, you’re basically falling
towards the Sun, and the Sun’s gravity will grab a
hold of you and make you go faster and faster and
faster, so when you get to Mercury, you’re going far
too fast to stop. It would just swing by the planet
and disappear.
The trick that MESSENGER did, and the trick that
BepiColombo will do, is to find a way to slow down
so you get to Mercury going slowly enough to be
captured into orbit. Both space probes had Venus
f lybys and then a series of Mercury f lybys, and
each f lyby slows you down. We’ve got six or seven
f lybys for BepiColombo. On the seventh or eighth
approach to Mercury, the spacecraft is going slowly
enough that it can just use a small amount of fuel
and get captured into orbit.How have these previous missions shaped the
science goals of BepiColombo?
I think Mercury has turned out to be far more
interesting than we suspected before MESSENGER
got there. For example, the explosive volcanism
we didn’t expect, or the recent hollows, places
where the surface is patchy and stuff has been
dissipating away into space. These regions dissipate
at a very slow rate, millimetres per thousand years
or something, but that’s still young in terms of the
Solar System. These are actively growing patches,
but we don’t know why. We don’t know what’s
being lost, except it’s got to be volatiles somehow.
But is it sublimation like when you warm up dry
ice and it just turns to vapour? Or is it breaking
chemical bonds? Photons can break chemical
bonds, micrometeorites can break chemical bonds
and charged particles from solar wind can break
chemical bonds. Any of those three could be
attacking the surface and just helping it strip away
to space, and that could be the source of some of
the sodium in Mercury’s exosphere.
The science goals of BepiColombo have been
rewritten. We still want to understand how Mercury
formed and so on. How it got so much iron, but
has so little rock. And now we want to know how
you couple that with being rich in volatiles. What
are the volatiles that are being lost, and how has it
hung on to them? Especially as these volatiles seem
to have lost so much rock to give it a small amount
of rock on a large core. It’s a totally puzzling planet
and I’m glad we’re going!Professor David Rothery
©NASARight: The
light blue,
rounded
depressions
on the surface
of Mercuryare
the ‘hollows’Above:
The MIXS
instrument is
packed into
the Mercury
Planetary
Orbiter (MPO)© ESA