Mohawk Valley Astronomical Society

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Alien Skies, Part 2 of 2: The Outer Planets & Titan

by Perry Pezzolanella

The skies from the inner planets and Pluto are different from anything seen from Earth, but the skies from the giant planets are even more unusual, strange even, unlike anything possible from Earth. The giants present a realm of rings and multiple moons along with dim sunlight and brutal cold.

Jupiter has no solid surface, but within the upper atmosphere the sky should appear blue, but dimmer as sunlight is about 27 times fainter and the Sun only as large as 5 arcminutes across. The rings may be faintly visible at night. Since sunlight is only 4% as bright as compared to Earth, the four largest moons would not appear as bright as one would think. Io would shine at magnitude -11.2, Europa -9.7, Ganymede -9.4, and Callisto -7.0. These four moons would not show much detail from Jupiter. Io, being the closer moon, would show bright and dark patches, Europa would be a featureless white disc, Ganymede would show vague dark and light patches, and Callisto would be too far away to see anything. Ganymede is the largest moon, but due to its distance would be about half as large as Io as seen from Jupiter. Io would appear about as large as the Moon is seen from Earth and would be the largest of any of the moons as seen from its parent planet. Little Almathea would appear as large as Callisto and shine as bright as magnitude -4.9. The other moons would be too small to appear anything more than stars.

Saturn has no solid surface so the sky is probably also blue. The most dramatic sight would be the rings. Arching high overhead near the equator they would appear razor thin, but elsewhere they would be awesome, especially closer to the poles, but they would appear closer to the horizon there. The moons are not especially impressive as most are fairly small and the largest ones are far away from Saturn. Titan is the largest moon but would appear only half as large as the Earth’s Moon and only as bright as magnitude -7.0. The thick orange haze would be visible but this actually makes Titan the dimmest moon seen from Saturn. Mimas shines at magnitude -7.8, Enceladus -7.9, Tethys -8.8, Dione -8.0, and Rhea -7.8 due to their icy surfaces. Compared with our Moon in our sky at an average diameter of 31’ arcminutes, Saturn’s moons all appear smaller in its sky with Mimas at 7’-11’, Enceladus 7’-9’, Tethys 12’-15’, Dione 10’-12’, Rhea 8’-11’, Titan 14’-15’, and Iapetus star-like at 1 arcminute. Hyperion, Phoebe and the other smaller moons would appear star-like. Since most of the moons orbit in the same plane as Saturn’s rings, the rings would appear as a thin line across the sky, except from Mimas, Iapetus, and Phoebe. From distant Iapetus whose orbit is tipped 14.7 degrees, the rings would appear to open up as much as 20º, almost as wide as seen from Earth. The rings are open as much as 3º from close Mimas. Tipped Phoebe also has a modest view of the rings, but orbits so far away from Saturn at nearly 13 million miles that they would appear very small.

Uranus also has no surface but the sky is probably blue. None of the moons would appear as large or as bright as the Full Moon on Earth but would still appear as discs. Miranda would be 11’-15’ (arcminutes) across, Ariel 20’-23’, Umbriel 15’-17’, Titania 11’-13’, and Oberon 8’-9’. Portia and Juliet would appear as large as Miranda at times and several other inner moons would appear as large as Oberon. The smaller outer moons would not be visible and the rings would be too dark to be seen or barely detectable as they reflect only 4% of the sunlight, as dark as coal and very thin. Ariel would shine brightest at magnitude -7.4 and Oberon only as bright as magnitude -4.9, a little brighter than Venus in our sky. Visible as discs these moons are dim because Uranus orbits an average of 1.7 billion miles from the Sun where daylight is more like twilight on Earth about 15 minutes after sunset.

Neptune should also have a blue sky but the rings are not visible as they are too thin and dark. Triton appears slightly smaller than the Full Moon from Earth. The smaller moon, Proteus, would appear about half as large as the Full Moon. As small as the inner moons are, they orbit Neptune very closely and all would appear surprisingly large, showing discs and making for a bizarre sky from Neptune’s cloud tops. The angular diameters compared to the Earth’s Moon of the 31’ (arcminute) average are: Naiad 7’-13’, Thalassa 8’-14’, Despina 14’-22’, Galatia 13’-18’, Larissa 10’-14’, Proteus 12’-16’, and Triton 26’-28’. An alignment of these moons would provide a spectacular sight. Imagine a sky with up to seven full moons or crescent moons or a mixture of phases shining at once! Nereid is not large enough to show a disc and would not be readily noticed as it shines like any other star at magnitude +2.2 to +6.4 depending on where it is on its highly elongated orbit. All the moons would appear dim because Neptune orbits about 2.7 billion miles from the Sun and daylight is no brighter than twilight on Earth about 30 minutes after sunset. Triton is icy and would shine at magnitude -7.1 at full phase. Neptune would span 8º in Triton’s starlit sky, about 16 times as large as the Full Moon, but only 1/256th as bright. Triton has an atmosphere but it is thin enough for unobstructed views of the sky. There may be a pale haze along the horizon possibly due to sublimation of the polar ice cap and also geyser activity.

Saturn’s largest moon, Titan, is a unique world all its own for it is almost earthlike with clouds, haze, wind, rain, flowing liquid, and huge seas. The only difference is that the surface is a frigid -290ºF and the liquid is methane. Titan is completely covered in a thick haze and smog of organic compounds, which gives it an orange color. The view of the heavens from the surface of Titan is impossible due to the thick haze and smog; the scenery is bathed in an eerie, dim orange light due to this thick atmosphere. The Sun would appear nothing more than a brighter area in the murky sky barely illuminating the surface. Methane rain falls from the clouds and it is possible for rainbows to form but it is too dim with 1/3000 less sunlight and the scenery no brighter than a bright moonlit night on Earth, so the rainbows could only be seen in the infrared.

We take for granted our skies here on Earth that never seem to change unless there is a storm, bright comet, or a lunar or solar eclipse. In our own little part of the vast universe there are skies of broiling sulfuric acid mist, frigid nitrogen hazes, and icy rings, along with multiple moons that light up the night. As alien as these skies may seem, they are real and have already been visited by our robotic ambassadors. Someday humans will venture forth to the more amiable worlds, and experience the beauty and wonder around us firsthand.