Mercury and provide the most comprehensive
global view we have ever had of the strange
little world. It launched in 2018, but because of
the convoluted path necessary to skim safely
into orbit, it won’t arrive until the end of 2025.
Meanwhile, Venus has been unloved of late.
“Venus is sort of the middle child,” says Tracy
Gregg of the University at Buffalo in New York,
less loved than the favoured siblings of Mars
and the gas giants. Of the 27 successful visits
since 1962, only five occurred after 1990. That
means our knowledge is even less up-to-date
than it is for Mercury. It isn’t that Venus is
particularly hard to get to – it is just hard
to learn about once you get there.
“The Soviet landers lasted between an hour
and an hour and a half on the surface before,
essentially, they cooked,” says Gregg. Surface
temperatures are around 470°C, with crushing
pressure from the heavy atmosphere 90 times
that at sea level on Earth.
Venus’s inhospitable conditions have very
different origins to Mercury’s. “Usually people
think that Venus is so warm because it’s nearer
to the sun than Earth, but it’s not true,” says
Pedro Machado at the Institute of Astrophysics
and Space Sciences in Portugal. “Its clouds
reflect around 70 per cent of incoming
radiation from the sun.” Instead, Venus is hot
because of a runaway greenhouse effect – its
thick carbon dioxide atmosphere traps heat
near the surface. That thick haze, which
inexplicably rotates 60 times faster than
the solid planet, means that the heat gets
distributed all around the world instead of
radiating away into the chill of space at night.
Unlike Mercury, there is no cold side.
The atmosphere also hides the surface from
orbiters, so we have very limited data on what
lies below the Venusian clouds. “We don’t
know what Venus is made of,” says Byrne. “We
don’t know if it’s still tectonically active, we
don’t know what the cratering rate is, we don’t
know why its atmosphere is moving so fast.”
Venus is a place of mysteries. From radar
data, we can see that it has what appear to be
volcanic landforms: channels carved by lava,
plains of volcanic rock and in excess of
1600 major volcanoes – more than anywhere
else in the solar system, even though there is
no evidence that they are active now. It is also
home to the longest channel in the solar
system, which once carried lava nearly
7000 kilometres. But nobody knows where
the lava came from or where it went after
creating the channel. “We don’t see a big pile
of lava at the end of these channels. We don’t
see a volcanic mountain or a volcanic crater at
the beginning of these channels,” says Gregg.
“These channels on Venus have no source, no
sink, and yet there they are.”
There are also strange bright areas of terrain
called tesserae, which tend to be full of long
ridges and troughs that form when the crust
shifts due to tectonic activity. There have been
hints from landers that these areas may be rich
in silica, like continental crust on Earth. “If we
were to determine that the tesserae were
continental crust, it would be a big finding,
because it would mean there was much more
complicated geology and chemistry than is
happening now,” says Byrne.
Crucially, this would also be one of many
areas of similarity to Earth. “Venus and Earth
should be twins,” says Gregg. “They are made
up of the same stuff, they are about the same
distance from the sun, they are about the same
size. And yet...”If Venus and Earth formed close to where
they are now, they should have about the same
amount of water, whether they formed with it
or got it from meteorites later on. There is no
obvious reason for Venus to be so different,
aside from maybe that a lack of plate tectonics
made it difficult to sequester carbon dioxide in
rocks. Early in its history, Venus may even have
been pleasant for life, with surface water and a
less dense atmosphere. Not now. “Being on the
surface is like diving in the ocean at a depth
of around 1 kilometre, and it’s also like an
incinerator,” says Machado. “There are better
places to go for a holiday.”
Perhaps the implausibility of life on Venus
and Mercury is why it has been hard to build
interest in visiting them. “We are probably not
going to find life there,” says Machado. “And we
can’t go to the surface with an astronaut one
day and put a flag in the soil.”
Nevertheless, they may be important for us.
“The goal is not just to understand Mercury or
Venus,” says Byrne. “It’s to understand our own
world. It’s to understand other worlds around
other stars.” And to fathom those other worlds,
it is important to figure out why the four rocky
planets in our own solar system are so different
from one another – and why only one seems
to be right for life. “To understand our own
planet – and even exoplanets – we need to
understand the three others as well,” says
Rothery. Yet for only one of them, Mars, do we
even know what the surface is predominantly
made of or where all the water went.
Comprehending the differences that seem
so monumental here could help us parse the
tiny dissimilarities we see between worlds
circling other, distant stars. “A seemingly Earth-
like exoplanet could be a paradise planet
like Earth or a hell planet like Venus,” says
Machado. To tell which is which, we are going
to have to return to the two sizzling coals
nearest the sun. ❚“ Venus and Earth
should be twins.
They are made up
of the same stuff.
And yet...”
MERCURY VENUS MARSPlanetary popularity contest
Exploration of our solar system used to be mainly
about Venus, but in recent years, Mars has been
prioritised. Of the four rocky planets, Mercury
remains relatively unlovedEARTH1962197219821992200220122022NASAMariner 10MessengerBepiColombomissions that visited multiple planetsUSSR/Russia Europe Japan IndiaLeah Crane is a reporter at
New Scientist. Follow her on
Twitter @DownHereOnEarth22 June 2019 | New Scientist | 45