You might think it will forever be
impossible to live on Mercur y. But
nature, uncharacteristically, may
have given future explorers a break.
The planet has no axial tilt, so there
are no seasons. This means that at the
planet’s poles, there may be craters
where the Sun never shines. And
there lies a miracle, discovered by
Messenger: water ice, delivered byMercury, the innermost planet in
our Solar System, was once an
enigma. For years, its proximity to
the Sun made it difficult for
astronomers to observe – but the
space age changed all that. NASA’s
Messenger, the second space probe to
Mercury and the first orbiter, was
launched in 2004 and orbited the
planet from 2011 to 2015. The data it
returned gave us real knowledge
about Mercury for the first time.PECULIAR PLANET
Mercury takes 88 Earth days to orbit
the Sun, and rotates once on its axis
every 59 Earth days. It was not until
the 1960s that even these basic facts
were established. But standing in any
one place on Mercury, you don’t see
the Sun rise every 59 Earth days
because of that short year. Mercury is
so close to the Sun that tidal forces
have locked in its rotation periods:
three Mercury ‘days’ are the same as
two Mercury ‘years’. The net effect
is that at any point on Mercury’s
surface you will only see a sunrise
every 176 Earth days.
If you were to stand on Mercury,
superficially it might seem like the
Moon: a small, airless world with a
rocky surface distorted by huge,
ancient craters. But the details vary,
because of the planet’s different
location and composition. On
Mercury, there are peculiar linearfeatures called rupes (Latin for
‘clif fs’ ) that resemble wr ink les on a
shrivelled apple – and it’s thought
that is pretty much how the rupes
formed, with the planet shrinking by
a kilometre or so as it cooled.
And Mercury’s gravity might
surprise you. Though the planet is
only a little larger than the Moon, its
gravity is about twice as high.
Mercury was once a more massive
world with, like Earth, an iron
core and a rocky mantle. An
immense collision with another
young world may have stripped off
much of that mantle, leaving the
planet with an outsize core and a
higher density to match.
If you could stand on Mercury, the
Sun would look twice the size in the
sky as it does from Earth – and there
would be barely a scrap of
atmosphere to shelter you, or to
retain the heat in the night. At
‘noon’ the ground is hot enough for
lead to melt; at ‘midnight’ the
temperature plummets to nearly
-200°C. Even the Messenger craft
had to be designed to take these
chal lenges into account. “The front
side of the sunshade routinely
experienced temperatures in excess
of 300°C,” explains Helene Winters,
mission project manager. “Whereas
the majority of components in its
shadow routinely operated near
room temperature”.STAGE 1
SCIENCE