December 3 – Celebrating the Discovery of Himalia

Posted on December 3, 2015

Like me, you probably do not know the names of all the moons of all the planets in the solar system...Well, for one thing, there are just too many moons! And scientists frequently discover new ones!

How many is “too many”? There are at least 173 moons known to be circling the eight planets, and at least 8 moons circling dwarf planets (including Pluto's 5 moons). A few of the smaller ones don't even have names.

Here are some moons that DO have names:

We might think that we know something about moons because we're all pretty familiar with Earth's one moon – called, appropriately enough, “The Moon.”

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

Do those things describe the moons circling the other planets (and dwarf planets)?

Not so much.

  • Only 19 of the 181 moons are spherical bodies; the others are too small for gravity to have pulled them into a round shape.
Mars's moon Deimos.
  • Most moons are much, much smaller than their parent planet / dwarf planet, but a few are not. Pluto's Charon is half the diameter of Pluto!
Both Pluto and Charon are pretty small.
Here they are compared to the size of the U.S.
  • Ten of the moons have at least some measurable atmosphere, and Triton (which circles Neptune) and Io (which circles Jupiter) have more substantial atmospheres. Saturn's Titan has a fully developed atmosphere.
Titan's atmosphere
  • Some of the moons that circle the outer planets are considered icy moons that have surfaces composed mostly of ice. Some of them may also have oceans between the surface ice and their presumed rocky cores. Examples of icy moons include Europa and Enceladus.
Here are two possible models
for what is inside Europa.
  • Many of the moons are indeed covered with craters, but some of them have processes that erase whatever craters form when meteorites impact their surface. For example, icy Europa seems to be covered with cracks and faults, and Uranus's moon Miranda seems to be jumbled together, with ridges and cliffs and fissures. Io seems to have no craters because lava flows so constantly there that whatever craters form from impacting bodies are quickly covered!
Europa, above.
Miranda, just below,
and Io below that.

Just a moment....

As I wrote all of the above, I learned something new: Earth's moon DOES have a little bit of atmosphere, after all.

I wrote above that the Moon is “airless,” because I'd heard that adjective used for the Moon a bajillion times. But, according to NASA's recent experiments, our moon does have an extremely thin atmosphere made up of helium, argon, and possibly neon, ammonia, methane, and carbon dioxide. It also contains some gases that are unusual in planetary atmospheres, including sodium and potassium.

Apparently this incredibly thin atmosphere (which would be considered a very good vacuum on Earth) comes from photons (“packets” of light) and particles in the solar wind knocking atoms off of the Moon's surface or reacting with rocks and dust on the surface. Also, part of the air may come from evaporation, out-gassing, or materials kicked up by meteorites.

Why am I telling you this today?

I was inspired to talk about all the varied moons in our solar system because today is the anniversary of the 1904 discovery of Jupiter's moon Himalia. Actually, Himalia is the largest of Jupiter's non-spherical moons.

Remember, being round is something that happens when a body in space is massive enough that gravity pulls all the particles together in the smallest possible shape. Spherical bodies are, therefore, larger than irregularly shaped bodies.

Himalia is the fifth largest Jovian moon. (We're comparing size by mass, in this case, not by dimension.)

Now, wouldn't you think that we would have some great photos of Himalia? Remember, it's the largest of the 63 smallest Jovian moons!


Yep, this is as good a photo
as we've got!
But, it turns out, Himalia is one of the largest moons in the solar system that is NOT imaged in detail!

I'd love to have those photos, because there is a possibility that one of Jupiter's other small moons, Dia, crashed into Himalia – and it is even possible that Himalia has a ring that was created from the crash!

We will have to settle for space art.
This picture of Himalia is by artist
Damien Perrotin.

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