December 3 – Celebrating the Discovery of Himalia

nPostednon December 3, 2015

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nLikenme, you probably do not know the names of all the moons of all thenplanets in the solar system…Well, for one thing, there are just toonmany moons! And scientists frequently discover new ones!

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nHownmany is “too many”? There are at least 173 moons known to bencircling the eight planets, and at least 8 moons circling dwarfnplanets (including Pluto’s 5 moons). A few of the smaller ones don’tneven have names.

nHere are some moons that DO have names:

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nWenmight think that we know something about moons because we’re allnpretty familiar with Earth’s one moon – called, appropriatelynenough, “The Moon.”

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nWhatndo we know about our moon? It’s a spherical (round) body that is much smaller than the planet it circles. It is airless and rockynand covered with craters.

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nDonthose things describe the moons circling the other planets (and dwarfnplanets)?

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nNotnso much. n

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nOnlyn 19 of the 181 moons are spherical bodies; the others are too smalln for gravity to have pulled them into a round shape. n

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Mars’s moon Deimos.

nMostn moons are much, much smaller than their parent planet / dwarfn planet, but a few are not. Pluto’s Charon is half the diameter ofn Pluto! n

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Both Pluto and Charon are pretty small.
Here they are compared to the size of the U.S.

nTenn of the moons have at least some measurable atmosphere, and Tritonn (which circles Neptune) and Io (which circles Jupiter) have moren substantial atmospheres. Saturn’s Titan has a fully developedn atmosphere.

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Titan’s atmosphere

nSomen of the moons that circle the outer planets are considered icy moonsn that have surfaces composed mostly of ice. Some of them may alson have oceans between the surface ice and their presumed rocky cores.n Examples of icy moons include Europa and Enceladus.

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Here are two possible models
for what is inside Europa.

nManyn of the moons are indeed covered with craters, but some of them haven processes that erase whatever craters form when meteorites impactn their surface. For example, icy Europa seems to be covered withn cracks and faults, and Uranus’s moon Miranda seems to be jumbledn together, with ridges and cliffs and fissures. Io seems to have non craters because lava flows so constantly there that whatever cratersn form from impacting bodies are quickly covered!

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Europa, above.
Miranda, just below,
and Io below that.

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nJustna moment….

AsnI wrote all of the above, I learned something new: Earth’s moon DOESnhave a little bit of atmosphere, after all.

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nInwrote above that the Moon is “airless,” because I’d heard thatnadjective used for the Moon a bajillion times. But, according tonNASA’s recent experiments, our moon does have an extremely thinnatmosphere made up of helium, argon, and possibly neon, ammonia,nmethane, and carbon dioxide. It also contains some gases that arenunusual in planetary atmospheres, including sodium and potassium. n

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nApparentlynthis incredibly thin atmosphere (which would be considered a veryngood vacuum on Earth) comes from photons (“packets” of light) andnparticles in the solar wind knocking atoms off of the Moon’s surfacenor reacting with rocks and dust on the surface. Also, part of the airnmay come from evaporation, out-gassing, or materials kicked up bynmeteorites.

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nWhynam I telling you this today?

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nInwas inspired to talk about all the varied moons in our solar systemnbecause today is the anniversary of the 1904 discovery of Jupiter’snmoon Himalia. Actually, Himalia is the largest of Jupiter’snnon-spherical moons. n

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nRemember,nbeing round is something that happens when a body in space is massivenenough that gravity pulls all the particles together in the smallestnpossible shape. Spherical bodies are, therefore, larger thannirregularly shaped bodies.

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nHimalianis the fifth largest Jovian moon. (We’re comparing size by mass, innthis case, not by dimension.)

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nNow,nwouldn’t you think that we would have some great photos of Himalia?nRemember, it’s the largest of the 63 smallest Jovian moons!

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Himalia.

Yep, this is as good a photo
as we’ve got!

nBut,nit turns out, Himalia is one of the largest moons in the solar systemnthat is NOT imaged in detail!

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nI’dnlove to have those photos, because there is a possibility that one ofnJupiter’s other small moons, Dia, crashed into Himalia – and it isneven possible that Himalia has a ring that was created from thencrash!

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