sulphur and low calcium.” We can understand our
observations because we’ve already done the best
geological mapping we can.How does the presence of craters on Mercury
relate to the planet’s age or past volcanic activity?
The craters we’re dealing with are usually impact
craters, and you can recognise them as they’re
almost exactly circular, produced by the shock of an
impact. This is because impacts occur at Mercury
at tens of kilometres per second. That shockwave
radiates out from the point of impact, which
is almost always circular with circular ejected
patterns. The surface that’s been there for 4 billion
years is going to be absolutely covered in craters.
You can see that a surface that has a small
number of craters superimposed is younger. But on
some of these larger plains you can see the older
craters that were f looded by the lava still visible,
because they were f looded but not buried at such
depths that all trace has been lost. You can see
these ghost craters where there are traces of
circular features.
And then volcanically, there was also a series
of explosive volcanic eruptions where holes
were ripped in the ground. For example, there’s
a 30-kilometre (19-mile) wide, three-kilometre
(1.9-mile) deep hole that was ripped out. Probably
not in a single eruption, but a series of eruptions.
And each time it went boom, it f lung out a plume
of material which just fell back to the surface.
There’s no atmosphere on Mercury, so it didn’t start
convecting like a volcanic eruption on Earth and
just fell back down, and that’s given us this 200- orInterview Professor David Rothery
© NASA
300-kilometre (120- or 190-mile) wide explosive
eruption deposit on the surface.
You can work out the age of events, because the
hole was ripped through material of a certain age,
and the deposit is sitting on top of some of these
young lava f lows. To work out the age of events,
it’s a detective story. You see what’s on top of what,
what has ripped a hole through something else.
There are geological faults on Mercury where parts
of the surface have been thrust over features.How does understanding Mercury help us
understand Earth?
We’ve got four rocky planets in the Solar System:
Mercury, Venus, Earth and Mars. They’ve all got
factors in common. I mean, they’ve all got cores,
which is where the dense, iron-rich material has
gone, and they’re surrounded by rock. But only
Mercury and Earth have cores that are molten, and
we know that because the Earth and Mercury have
magnetic fields. You’ve got motion churning around,
this electrically conductive f luid, which is what you
need to generate a magnetic field. But it doesn’t
happen inside Mars or Venus. Why? We’re not sure.
Venus is almost the same size as Earth, and its
core, as far as we can tell, is about the same size as
Earth’s core. It’s completely solid; the core is not in
motion. There is something strange going on there,
but we don’t understand why some planets generate
their own magnetic fields and some don’t.
Then there is how the surface behaves. Earth
is the only rocky planet to have a rigid outer shell
that can slide around on a very weak interior, hence
plate tectonics. For example, the Atlantic Ocean isMars
3,389.5km
(radius)Mercury
2,439.7km
(radius)Ve nu s
6,051.8km
(radius)Earth
6,371km
(radius)Right: Rupes
are long,
steep cliffs
that extend
over several
hundreds of
kilometres.
They can tell
astronomers
a lot about
the planet’s
history