if Mercury was boring if we hadn’t had
Messenger there to prove it wasn’t.”
One thing Messenger experienced directly
was just how hot the planet gets. The orbiter
had to periodically back away from Mercury
just to keep its instruments cool – on the day
side, the surface reaches temperatures of up
to 430°C. And because there is barely any
atmosphere to hold heat in and spread it
around the planet, the temperature drops
precipitously on the night side when the
sun sets – down to about -175°C. Day and night
are drawn out affairs, too: with an average of
176 Earth days elapsing between sunrises, one
day is longer than the planet’s year.
These temperature extremes mean life as
we know it is almost impossible on Mercury.
There are a few sunless craters near the poles
that may have a small amount of water ice, but
they are probably too cold for liquid water, with
little or no oxygen. “Pretty much every place
gets very hot and very cold,” says Byrne. “It’s
not great if you’re a bug, let alone anything
more complicated.” The low surface pressure
and lack of air make it even less hospitable.
More problems come from within. The
planet is intensely volcanic, says Lillian Ostrach
at the US Geological Survey’s Astrogeology
Science Center in Arizona. “The whole
northern plains area, which makes up more
than 10 per cent of the surface, is solidified
lava.” Mercury has signs of what is known
as explosive volcanism, a more dramatic
type of eruption. Elements that boil at
relatively low temperatures, known as
volatiles, form gas bubbles in lava, which pop
at the surface to spew hot liquid rock in all
directions. Most of these eruptions seem to
have happened about 3.8 billion years ago,
when much of Mercury’s interior was still
molten from the heat of formation.Volatile nature
In theory, most volatile elements on Mercury’s
surface should have already boiled away due to
its proximity to the sun. But they are still there.
“Before we got there, we had this idea that it
should be depleted in volatile elements,” says
Kathleen Vander Kaaden at the Lunar and
Planetary Institute in Texas. “And then we get
there and we realise that there are lots of these
volatiles on the surface.”
On a planet with no liquid water and little
weather, these volatiles play a key role in
shaping Mercury’s surface features. In
addition to powering its volcanoes, they are
also responsible for Mercury’s most un-Earth-
like attribute. Instead of an atmosphere, thetiny planet has an exosphere, a thin and
tenuous layer of particles floating above the
surface. “It’s not so much an atmosphere as a
slow evaporation of the planet,” says Valeria
Mangano at the National Institute of
Astrophysics in Rome, Italy.
The exosphere is mostly hydrogen and
oxygen, with some helium, sodium and
potassium as well. It probably forms when
charged particles from the sun sneak through
the planet’s magnetic field and hit the surface,
sending tiny particles of dust and gas flying.
“It is continuously created and continuously
fading away into space,” says Mangano.
You read that right, Mercury is slowly
shrinking. The process isn’t only powered by
evaporation, either. As its molten core began
to cool, the planet started to solidify, causing
its surface to contract. Cliffs on the surface
that can be up to about 1000 kilometres long
and 3 kilometres high show that its diameter
has shrunk by several kilometres since its crust
formed, crumpling the landscape. As Mercury’ssurface is all one shell rather than several
tectonic plates, these cliffs probably couldn’t
have formed any other way.
But despite millions of years of cooling,
some of the core is still liquid. Part of the
reason it has taken so long to freeze may be
simply that the core is enormous. It takes up
85 per cent of Mercury’s radius – far more than
any other planet we know of. Dig just a few
hundred kilometres through the crust and
you will hit molten iron-rich core. On Earth,
you would have to dig almost 3000 kilometres
to reach the outer edge of the core.
This thin veneer of rock over a huge core,
combined with the unexpected volatiles, has
led some researchers to conclude that Mercury
may have formed further from the sun and
then smashed into another, larger protoplanet
as it migrated inward, stripping off its rocky
outer layer. “It could come in and hit the young
Earth or the young Venus and lose its rock but
not its volatiles in the process,” says Rothery.
Without this hit-and-run, Mercury might be a
much bigger planet, and one more structurally
similar to Earth.
Planetary scientists are still scratching their
heads over the mysteries Messenger revealed.
Which is where BepiColombo comes in. The
hope is that it will answer some of those
questions. BepiColombo consists of two
orbiters that will separate when they reachMariner 10 gave us our first close-up
glimpse of Mercury in 1973“ Mercury’s surface
is continuously
fading away
into space”
SS
PL/GE
TT
Y44 | New Scientist | 22 June 2019