Alien Skies. The Universe Seen by Saturn, Uranus, Neptune and Pluto
Dawns and sunsets of all colors… Celestial bodies with ever-changing 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... All this is our Solar System.
Also in this third and last part of our journey we will try to put together science and imagination,
visiting skies completely alien to our experiences as inhabitants of planet Earth.
Just as we did for Jupiter, we will have to imagine Saturn
somehow floating above its clouds to be able to see what is happening in its sky.
And if we could really do that, we would probably see it slowly fade from the blue of the high
atmosphere to the black of the surrounding space. The composition of Saturn's atmosphere
is in fact similar to that of Jupiter, with a prevalence of molecular hydrogen and helium.
Seen from there, the Sun is about 100 times less bright than it appears from the Earth,
and its disc, with an angular diameter of just 3.4 arcminutes, from those distances begins to
be appreciable with difficulty. As well as you can see with difficulty the Earth, which
still shines like a star of second magnitude, but is separated from the Sun a little more than 6°...
The larger moons of Saturn, although less conspicuous than those of Jupiter,
form an overall necklace of remarkably luminous objects. In order of magnitude: Titan, Rhea,
Dione, Tethys, Enceladus and Mimas. The apparent dimensions speak to us of satellites
at most half the size of our Moon, but it is also true that six of them, those we have listed,
have an apparent brightness around magnitude -7, or on average five to ten times brighter
than Venus seen from Earth! Remarkable are the cases of Enceladus and Tethys,
which thanks to a very high albedo (their surfaces are absolutely white) exceed in
brightness even the great Titan (5150 km in diameter). Can you imagine the spectacle we could
enjoy at the sight of these six moons, bright as headlights, turning in the night sky of Saturn
accompanied by the overflowing beauty of the rings that form the background?
Yes, because by far the most spectacular element of Saturn's sky is obviously that of the rings...
An element that if fascinates seen from the "outside" must be even shocking if observed
from somewhere on the planet, from where you would see the entire sky crossed by an iridescent arc.
The rings become more and more conspicuous as the latitude increases and, consequently,
the angle under which they are observed. Seen from the equator they are instead practically
invisible because of their incredible subtlety. During the equinoxes, which happen about every
14 years, the Sun hits the cutting rings and, consequently, the shadow they cast on the planet
is minimal. But far from the equinoxes, the rings cast complex and very wide shadows on Saturn.
To be able to observe from the high atmosphere of the planet or from some
of its many moons the alternation of shadows and lights on the underlying cloud systems
must be an extraordinarily suggestive view; a view that we can say we know well
thanks to the interplanetary probes that have visited the planet. Especially thanks to Cassini,
who for years wandered around the planet's satellite system, photographing incredible
cosmic landscapes, where moons and rings gave life to geometric shapes of unique beauty.
The great Titan, which has even planetary dimensions, could be in the future an
excellent observation point for the first human explorers... but unfortunately, Titan is also the
only satellite in the Solar System to have a dense atmosphere; so dense that it probably obstructs
any kind of astronomical observation. Saturn would be invisible behind the orange smog curtain,
and even the Sun would be just a lighter spot in the mist, barely illuminating the surface
of the ice and methane lakes. The images sent to Earth by the Huygens probe show that Titan's sky
is a light mandarin color. However, an astronaut on the surface would perceive a brown or dark
orange color. As a result of its greater distance from the Sun and the opacity of its atmosphere,
the surface of Titan receives only about one-thousandth of the Earth's
sunlight. The day on Titan is therefore only as bright as twilight on Earth.
However, in the upper atmosphere, the sky would have a blue color and Saturn would be visible.
Ten astronomical units beyond Saturn we come across Uranus, another gas giant, but 2.4 times
smaller in size than the Lord of the rings. The Sun seen by Uranus really begins to look
like a point of light without dimension: only 1.6 arcminutes angular diameter and a brightness that
is just 1/400 of the one that reaches the Earth. From a distance of almost 20 AU, the inner planets
of the solar system from Mercury to Mars are now lost in the radiance of the star and it makes no
sense to consider them for observational purposes. Uranus is not a hospitable place: also without a
solid surface, with winds blowing up to 900 km per hour, it is the coldest planet of the solar
system, with an average temperature of -220°C. Yet Uranus also has its fascinating peculiarities.
First of all, the rotation axis of the planet is inclined by 98° on the orbital plane. The rotation
appears therefore retrograde like that of Venus, but, unlike Venus, which rotates "upside-down",
Uranus is lying on its side: as if it rolled along its orbit. Because of this peculiarity,
the climate and landscape change in a completely different way compared to other planets. At the
poles of Uranus, the day and night are very long: they last 42 Earth years,
although with different shades of darkness and light. The only periods in which sunlight is
directed mainly towards the equatorial zones is during the equinoxes, which fall every 42 years.
And it is only on horseback of the equinoxes that Uranus sees an alternation of day and night
marked by its period of rotation, of 17 hours. “BE sure to join the Insanity Curiosity Channel...
Click on the bell, you will help us to make products of ever higher quality!”
Uranus has also a complex system of rings, which is however extremely soft
and not very bright, so it is not certain that from above the atmosphere of the planet,
against a sky probably aquamarine-colored for the presence of methane, it is possible to see it.
Also for Uranus, the show comes from its moons, the largest of which, thanks to their reduced
distance from the planet, would appear to us in the form of discs not much smaller than
that of our Moon, and with brightness such as to transform even the sky of Uranus into a racetrack
of lights that chase each other relentlessly. Apart from the classic satellites - Ariel,
Titania, Oberon, Umbriel, Miranda - which range from 500 to 1500 km in diameter,
what is surprising is to see that even small but very close satellites,
such as Portia, Juliet or Puck (all on the hundred kilometers) sometimes show themselves with an
angular diameter and brightness really important. All of them are however locked in 1:1 resonance
and always show the same face to the planet. From the sky of Uranus, all the other planets
seem to have disappeared, even the biggest ones. From Uranus,
Jupiter shines like an opaque second magnitude star and Saturn even as a third magnitude star.
Barely visible to the naked eye is Neptune, the planet we are about to visit.
Neptune... The planet discovered in 1846 by mathematical calculation travels in
the depths of the Solar System, covering its orbit in about 165 Earth years, at a
distance of 30 astronomical units from the Sun. At such a distance the sunlight is a thousand
times less intense than the one that hits the Earth, while the angular diameter of our star
is reduced to a little more than 1 arcminute. Yet, this giant of gas and ice, due to an unknown
source of internal heat is "hotter" than Uranus although it is much further away from the Sun.
And this internal heat unleashes on Neptune the most powerful winds of the Solar System.
If you were in the high atmosphere of the planet, you could observe spectacular
whirlpool systems that travel at supersonic speed, sometimes exceeding 2000 km per hour!
The planet is surrounded by at least 27 moons, most of them small and asymmetrical,
with dimensions of 100-200 km; but almost all of them so close to the planet to appear
to a hypothetical observer with angular dimensions just under half of our lunar disc. Even their
brightness, despite the reduced illumination coming from the Sun, is maintained at such
a level (the brightness of Venus observed from the Earth) that even on that icy and
gloomy planet there is a very varied and busy sky. Obviously, the superstar of that satellite system
is Triton, the seventh-largest of the entire solar system (after Titan, our Moon and the four
Mediceans), and the only one among the greats to move along a retrograde orbit. He always
shows the same face to Neptune, who therefore occupies a fixed position (or almost) in his sky,
showing himself with an apparent diameter of 8°! On the contrary, in the sky of Neptune,
Triton would instead show itself to our eyes with the same angular dimensions as our Moon,
and at the same distance. The only difference would be in the brightness, 250 times lower.
Without forgetting that even Neptune, like the other gas giants, has a system of rings, probably
of recent formation and certainly inconspicuous because of the scarce light they receive from the
Sun and the reddish color, due to the charcoal materials mixed with ice from which they are
composed. No possibility to appreciate them with the naked eye, therefore. Not even sharpening our
gaze as we move away in the direction of Pluto, the last destination of this long journey.
Pluto? Yes... although almost 15 years have already passed since its downgrading to "Dwarf
Planet", we don't feel like excluding the old good Pluto from our tour among the planets of the Solar
System. Also because the visit made by the New Horizon probe has shown beyond any reasonable
doubt that Pluto has all the characteristics of a real planet. A double planet, even! Yes,
because 20,000 km away from Pluto there is also Charon, a giant moon compared to the
size of the primary planet. As we know, the two bodies rotate around a common center of mass and
always turn to each other the same hemisphere. Possible how much improbable inhabitants situated
on the hidden faces of the two objects would not know therefore never of the reciprocal existence.
The inhabitants of the visible face of Charon would see Pluto, which has a diameter of about
2300 km, looming in their sky with an angular diameter of 7.3 degrees, equal to 15 times
that of the full Moon seen from Earth. Charon, instead, which has a diameter of 1200 km, would
appear in Pluto's sky with an angular diameter of 3.7 degrees: about 7 times that of the full Moon.
Surely it would be possible to see distinctly with the naked eye from Pluto the superficial
characteristics of Charon, and vice versa, even if the whole landscape would appear
immersed in a light more than twilight because of the great distance from the Sun.
Pluto and Charon, together with the other very small moons of the system,
run a very elliptical orbit, which lasts 248 Earth years. The distance from the Sun varies
between 4.4 billion kilometers to the perihelion and the abyss of more than 7.3 billion kilometers
to the aphelion. At such enormous distances, the Sun appears very small:
on average a little more than one arcminute at perihelion, a thirtieth of the apparent
size that it has in the terrestrial sky, and 35 arcseconds at aphelion. The brightness of our star
in the sky of Pluto and Charon is, consequently, equally reduced: 1/876 of what we see from Earth,
when the twin planets are at the minimum distance; just 1/2430 when they are at the maximum distance.
We must not think, however, that such a tiny and distant Sun implies a perennial
night... the Sun in fact illuminates the two icy bodies with a light that is still almost
500 times greater than that of the full Moon in the night sky of the Earth!
In 2015 New Horizons gave us extraordinary images of these two remote worlds,
revealing the unthinkable existence of truly alien skies right on the edge of our Solar System!
"The first two episodes of Alien Skies: "The Universe Seen by Mercury, Venus and Moon" and
"The Universe Seen by Mars an Jupiter", you can always find them on Insane curiosity Channel!