Alien skies: The Universe seen from Mars and Jupiter
If it’s true that, as Shakespeare said, "There are more things in heaven than we can imagine",
then it might be justified to ask whether in the skies of the other planets of the Solar System
celestial phenomena could occur quite different from those we are accustomed
to observe on our planet. Different colors, speeds,
masses, shapes and volumes... environments mostly hostile to life, and very dark skies
where large and small moons move, with nights that last months or a few hours, or where the
Sun can suddenly reverse its apparent motion... In this journey we will try to put together
science and imagination, visiting the completely alien skies of Mars, Jupiter and their moons.
We are on Mars. The atmosphere of Red Planet has a composition similar to that of Venus:
it is composed mostly of carbon dioxide, a small percentage of nitrogen, argon and traces of oxygen
and water. However, the density is minimal and the pressure at the surface is less than one-hundredth
of the Earth's pressure. There is water vapor in the air, but too little and the density is
too low for it to rain. However, you can see mist and clouds in the sky of Mars.
The main characteristic of its atmosphere, which significantly influences the landscape
and the appearance of the sky, is the large amount of dust, mostly iron oxides, suspended in the air.
A visitor to Mars could witness, especially when the planet is closer to the Sun
and the temperature is higher, the most impressive dust storms of the solar system: sometimes only
local phenomena, but in other cases so gigantic as to affect the entire planet, as happened in 2001.
And it is just to the ferrous dust in suspension that we owe the particular brown-yellowish color
of the Martian skies. And to the same powders we owe the fact that the daylight spreads on Mars
long before the rising of the Sun, and that it still illuminates the sky long after the Sun
has set. The numerous probes that we have sent to Mars in recent decades have given us many
suggestive images. We have been able to observe sunsets in which the Sun is bathed in a blue light
instead of red as on Earth. And violet skies, due to the presence of ice crystals in the air.
However, we must remember that these images are only an approximation of reality.
All the probes that capture the sky and the surface of Mars, in fact,
do so through filters that produce images in shades of gray that are then processed
into something that resembles as much as possible what human eyes could see, if they were on Mars.
Since Mars is about 1.5 astronomical units away from the Sun, the amount of light that
arrives on the surface is less than half of the amount that arrives on our planet;
and in low light conditions our eyes shift their sensitivity to blue because we move from using
"conical" color-sensitive cells to "stick-like" cells, which are practically colorblind. So, the
first astronaut who landed on Mars would probably describe his sky much bluer than you might expect.
The light actually perceived on the ground always depends on local weather conditions.
But there is no doubt that the general effect, for a human visitor, would be that of a world
with little light, with equally little contrast. Certainly, the disk of the Sun would appear to us
smaller than from the Earth. Its angular diameter, seen from Mars, is 19 arcminutes at aphelion
and 23 at perihelion, respectively 60% and 70% of the diameter observed from Earth.
Despite the great similarity between Mars and Earth in the initial observational conditions
(the duration of the day and night virtually identical, as well as the respective
inclinations of the polar axis), a hypothetical Martian astronomer could have fun observing
shows forbidden to us Earthlings. He could see for example with a good telescope the
details of the hidden face of the Moon. Or, even better, he could observe the transits of Mercury,
Venus and Earth on the solar disk... As we have already seen for Mercury,
also on the Red Planet the Earth-Moon system would shine in the sky like a jewel formed
by two points of light separated at most by about 23 arcminutes. And even an observer on Mars,
therefore, would be able to see with the naked eye the Moon orbiting around the Earth.
Many Martian probes, orbiters and even rovers, have improvised astronomers, taking meteorites,
constellations, transits, eclipses and even more or less total eclipses of the Sun.
Even the moons of Mars, in fact, can project their shadow on the occasion of their
passages in front of the Sun. That of Phobos, in particular, a dark spot of about 50 km,
"walks" three times a day on the face of the planet illuminated by the Sun.
There are many photographs, for example those taken by the Mars Global Surveyor,
that take this incredible phenomenon of the shadow that slides fast among the Martian deserts; as
well as iconic have become the photos taken by the rover Curiosity that depict the dark silhouette of
Phobos partially eclipse the disk of the Sun. And hinting at the two satellites,
we must admit that these two very small moons animate the sky of Mars. Phobos, the largest,
is a sort of asteroid of quite irregular shape, porous and pitted, with an average
diameter of about 22 km and brightness that can reach 50 times that of Venus seen from Earth.
Its appearance also varies very quickly due to the phenomenon of phases, the cycle of which is
completed overnight. It orbits at a distance of only 6000 km from the Martian surface,
which allows it to make a complete tour around the planet in 7.6 hours,
a period much shorter than the rotation of Mars. For this reason, Phobos rises in the west,
on average twice in the same day, crosses the sky in just over four
hours, and sets in the east. Also because of its peculiar proximity to the planet,
Phobos is eclipsed by the shadow of Mars, immediately after rising until just before sunset,
remaining a wandering dark shadow for much of its apparent path of the Martian sky.
Its average angular diameter is about 10 arcminutes, half that of the Sun. Consequently,
total eclipses cannot occur on the planet because the two moons are both too small to
cover the solar disk in its entirety. On the other hand, from the equator it is possible to observe
transits of Phobos almost every day; they are very fast and end in less than half a minute or so.
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Deimos, observed from Mars, is instead so small - just 3 arcminutes - that it appears almost
punctiform, even if it shines as much as Jupiter seen from Earth. When it passes in
front of the Sun's disk it produces more the effect of a transit than an eclipse. It orbits
around Mars at 20,000 km from the surface, with a period of 30 hours. Consequently, unlike Phobos,
it rises in the east and sets in the west, but takes 2.7 days to complete the entire route.
Certainly interesting would be to contemplate Mars from the surface of Phobos.
The planet would appear to us 2500 times brighter than the full Moon seen from Earth
and 6400 times larger, so much so that it would cover a quarter of the visible horizon.
However, we must hurry to go to Phobos, if we want to make time to enjoy such a spectacle.
The satellite, in fact, is on an unstable path:
the tidal forces slow it down and narrow the orbit radius of about twenty meters every century.
In about eleven million years it will be so close to the surface of Mars
that it will be shattered, creating a beautiful ring of debris around the planet.
But let's reluctantly leave the Red planet and head towards Jupiter.
Not having the giant of the Solar System a solid surface we must necessarily imagine a situation
that allows us to observe its sky as if we were solidly anchored to the surface. Let's
imagine then to navigate hanging from a balloon probe just beyond the highest gaseous layers and
still follow the fast motion of rotation of the planet as if we were located above its equator.
The sky, looking outwards, would obviously be completely black,
maybe just bluish, and we would see below us a great spectacle of clouds of different colors,
red, bluish, white, placed at different heights and subjected to a continuous movement, due to the
powerful horizontal winds that, depending on the latitude, blow from west to east or east to west,
creating endless vortexes and storms. The colors of the different gases that the wind
disrupts leave the blackness of space upwards, and in the sky there are truly formidable lights: they
are the so-called Medicean satellites (Io, Europa, Ganymede and Callisto)... similar or superior in
size and angular dimensions to our Moon. I
Apart from the Sun, the four Medicean Satellites are by far the most striking objects in the
sky of Jupiter, while the numerous small moons orbiting at various distances from the gas giant
would be virtually invisible to the naked eye; all minus the small but very close
Amalthea (165 km in diameter), which shines as bright as Venus seen from Earth and is visible
as an irregular disk of 5 arcminutes. About the latter, there is also to say
that being its period of revolution a little longer than Jupiter's period of rotation,
its movement in the Jovian sky is very slow and retrograde, so that the small moon takes
about 29 hours from sunrise to sunset. The Medicean satellites all move with
direct motion, always turning the same face to Jupiter, and obviously show themselves with even
greater brightness, and with average dimensions ranging from 36 arcminutes of Io to 9 arcminutes
of Callisto. Due, however, to the smaller amount of light coming from the Sun at these distances,
the satellites would appear darker than how we see our Moon. Even less, of course, would appear to us
the brightness of the more distant satellites. On the whole, the Medicean (or Galilean)
satellites would seem to move all from east to west, left behind by Jupiter's fast rotation,
but Jupiter's diameter is so important compared to the distances of the satellites, and the
rotation period so fast, that due to a very high diurnal parallax, the two outer moons, Ganymede
and Callisto, during their apparent direct motion would show rapid reversals, respectively of 22 and
44 arcminutes of amplitude. For Io and Europa, with the apparent faster motion, there would be
instead only a slowdown. A bit what, as we saw in the video "Alien Skies. The Universe
Seen by Mercury, Venus and Moon", happens for the motion of the Sun observed in the sky of Mercury.
Io, the Medicean Moon closer to the planet, we would see it slightly larger than the full Moon
in the Earth's sky, although less bright. And in fact, Io is the Moon of apparent diameter
larger than the Solar System if seen from the surface of its primary planet.
From Jupiter, the Sun would appear to us about 27 times less bright than if observed from Earth,
and with an average angular diameter of 6 arcminutes: more or less a fifth of the
size of the solar disk seen from our planet. Due to their relative proximity to Jupiter
and also to the fact that the orbits are completely coplanar with each other and slightly
inclined on the ecliptic, the moons discovered by Galileo (but also Amalthea) would all appear
large enough to completely cover the Sun with their disks. This would cause continuous
eclipses of the Sun; as well as numerous periodic eclipses between the moon and moon.
All this happens on Jupiter... but what would the sky look like when
seen from the surfaces of the satellites? None of Jupiter's moons possess a substantial
atmosphere, so their skies would appear almost completely black. And for the hypothetical
observer, by far the most prominent feature of the sky would be... Jupiter! If we were in
fact on the strange surface of Io, rich in highly colored volcanic deposits, we would see in the sky
a Jupiter of about 20° angular diameter (40 times that of our Moon), while an observer on Amalthea,
would see Jupiter looming with a diameter of 68°. Can you even imagine what an extraordinary and
frightening vision this would be? .
But we are only halfway through our journey... in the next video,
we will visit the complicated skies of Saturn and those of very remote planets like Uranus,
Neptune and Pluto. Look for it on Insane Curiosity Channel!