What Sky Looks Like on Other Planets
When we look up from the surface of Earth, we’re met with roughly the same image every
day: blue skies, clouds, and a bright, yellowish sun.
But all of those colors are determined by the Earth's atmosphere, and how light behaves
when passing through it…
And the solar system’s other planets have very different atmospheres, with light reflecting
across them in very different ways.
This is Unveiled, and today we’re answering the extraordinary question; What does the
sky look like on other planets?
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So, why is the sky blue on Earth?
It’s a question that took scientists some time to answer... and it wasn’t until 1911,
when Einstein worked out his equations for the scattering of light, that we finally began
to understand.
Fundamentally, it’s about the specific molecules that make up the Earth’s atmosphere - which
are mainly nitrogen and oxygen - and how they react to light from the sun.
Visible light travels in waves, and as those waves hit the molecules of our atmosphere
they’re diffracted and spread.
While it appears a white-ish light to look at, light from the sun is actually all colors
of the rainbow - and blue light has a shorter wavelength and higher frequency, which means
it’s diffracted and scattered much more than the other colors.
And so, the sky appears blue.
In contrast, red light is the opposite, with a longer wavelength and shorter frequency,
so it’s much less likely to diffract.
That’s why we only tend to see a red sky at sunset or sunrise, when the sun is furthest
away and sunlight has passed through much more of the atmosphere, effectively losing
its blue-ness before it reaches your position.
So, that’s Earth, but what about elsewhere?
Mercury’s atmosphere contains hydrogen, helium, and oxygen - however, Mercury is so
small and close to the sun that it has very little atmosphere left.
Because of that, incoming sunlight is barely diffracted by anything, so the sky (as seen
from the surface, when not directly looking at the sun) would look almost as though viewed
from space: vast expanses of grey, black, apparent darkness with thousands of bright
stars scattered about.
And that’s despite the sun itself appearing more than twice as large as it does here on
Earth, and more than six times as bright.
Earth itself would be quite clearly visible in the Mercurian sky as a bright and twinkling
orb.
And, as one day on Mercury also lasts 59 Earth days, you’d have plenty of time to enjoy
the view.
Onto Venus, which is a strange planet by anyone’s standards.
The atmosphere is 96% carbon dioxide, making it extremely lethal for humans.
And because of the structure of that atmosphere, with 90x as much atmospheric pressure as here
on Earth, the sun isn’t actually visible from the surface.
Nevertheless, if you were able to see the sun, you’d watch it rise in the west and
set in the east.
That’s because Venus rotates in a retrograde motion, the opposite direction to that of
most other solar system planets.
It’s thought that the sky would appear orange and cloudy, providing a foreboding backdrop
for a pretty unforgiving place.
Surface temperatures reach 480 degrees Celsius, enough to destroy most of anything humans
could send there, and the Venusian winds whip violently across the plains, too, churning
those orange clouds to form a famously hellish landscape.
On Earth’s other side sits Mars, which has an atmosphere 100x thinner than our own.
However, unlike with Mercury, scientists aren’t exactly sure what happened to Mars’ atmosphere…
but what little remains is 95% carbon dioxide.
And, despite our sending various rovers to its surface, finding out what the Martian
sky looks like is still a challenge because of the way our eyes respond to light.
So far, scientists have loosely determined that during the day it would appear a butterscotch
color and a rosy red at sunset, while the area immediately around the sun would at times
look recognisably blue.
During a typical and violent Martian dust storm, though, all would appear a reddish
haze - adding further fuel to Mars’ “Red Planet” reputation.
Other stand out features of the Martian sky are its two moons - Phobos would appear around
one third the size of the sun, while Deimos would look much smaller.
Jupiter is the first of the gas giants and the largest planet in the solar system.
It would be hard to actually stand on the surface, because there isn’t really one,
with large amounts of hydrogen and helium accounting for most of its mass.
But, regardless, the Jovian sky is believed to be a very dim blue color, as the sun is
27 times fainter there than it is on Earth.
The deeper into the planet you go, the darker and more varied the clouds become; from blue,
to brown, to red, and - once you’re close to Jupiter’s centre - total darkness.
While sky-gazing on Jupiter you could also view some of the massive planet’s multiple
moons, amidst otherwise probably violent and stormy conditions.
Io would appear larger than our own moon does; Europa would be smaller but still noticeable;
and Ganymede would also be smaller than Io, but just as bright.
All three moons would eclipse the sun fairly frequently, though… so there’s that, too!
And, if you’re really lucky, you might even glimpse one of Jupiter’s most famous phenomena
- diamond rain!
Next, to Saturn - where hydrogen accounts for the vast majority of the atmosphere, then
helium.
Again, the sky would appear blue, but there’s also a noticeable smog around Saturn… so
it’d be blue with a murky, yellowish tinge.
Unlike Jupiter, Saturn’s moons wouldn’t be much to look at, as even the largest of
them, Titan, would seem only half the size of Earth’s own from the Saturnian surface.
But of course, the rings of Saturn would be the main event on this particular skyline!
The massive bands of glittering dust and ice would stretch across the entire sky, though
they would look different depending on where upon the planet you were viewing them from…
If you’re right below them, they’d look quite thin; if you’re further away, they’d
seem to expand.
Uranus and Neptune are so far from the sun that their skies are thought to be reasonably
similar.
Neptune is so far out, in fact, that it’s only completed one full orbit around our central
star since it was discovered in 1846! Uranus has rings a little like Saturn, but they wouldn’t
be as visible from the surface.
Its sky would take on a very faint, light blue tone (with so little sunlight actually
reaching it), though its five largest moons would be just about visible - in amongst the
freezing ice storms that rampage across the surface.
Finally, Neptune also offers an exceptionally dim, azure blue sky, while its relatively
thin rings would also be invisible from the planet’s surface.
Perhaps surprisingly, the moons Triton and Proteus would be fairly easy to pick out,
and even Neptune’s smaller satellites would be at least be identifiable, if you knew where
to look!
Though our solar system’s other skies offer a mysterious point of interest, Earth’s
is the only one that naturally allows for our survival… so it could yet be quite a
long time before we see any of the others first hand.
But that doesn’t mean that we shouldn’t be curious.
Fundamentally, the basic laws on the scattering of light explain why most planets would host
an in some way bluish sky regardless of their position in the solar system, because that’s
just how we’re programmed to view them.
The notable exception is the notably dusty Mars - which, in the future, could well prove
the first alien sky we actually encounter.
And that’s what sky looks like on other planets.
What do you think?
Is there anything we missed?
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