Daughter of a rocket engineer, granddaughter of a planetarium director, I've been a huge fan of astronomy and space exploration all my life.
Earth and Various Moons to Scale
Our Moon Is Not Alone
Our Earth's moon is big, beautiful, and special: it tickles us with tides, helps hold the planet steady to prevent climate-wrecking wobbles, and gave us the 24-hour-day by slowing down young Earth's hyperactive 6-hour rotation.
However, our moon is less unique than we thought. Ever since Galileo Galilee first gazed at Jupiter with his telescope in the 17th century and spotted the four big Jovian moons — Europa, Io, Ganymede and Callisto — we've known that other planets have moons, too.
When we started sending out space probes, we discovered many more moons. I remember being excited when the 1970s Voyager flybys pushed Jupiter's moon-count up into the 20s. Now, depending on what you count as a moon, Jupiter may have about seventy. Even little Pluto is now known to have at least five moons, and some asteroids have moons, too.
In fact, moonless Venus and Mercury and one-moon Earth appear to be the exception. So let's check out some of these bizarre mini-worlds that have been discovered in our own lifetimes!
It Ain't Pretty, but It's Colorful
Io: Last One Is a Rotten Egg
Io, an inner moon of Jupiter, has been everybody's darling since the Voyager spacecraft first discovered active volcanoes on it in 1979. So what? Well, volcanoes should only pop up on planet-sized bodies with cores of molten rock. A little moon far out in the cold of space should've cooled to solid rock eons ago.
Before the Voyager probes, we thought that moons were boring, dead worlds like ours, an eerie airless desert of dust and old lavas where nothing ever happens unless a meteor plunks down. Boy were we wrong.
Io is like a piece of taffy caught in a three-way tug-of-war between the massive gravitational pull of Jupiter, Europa and the giant moon Ganymede. Just like kneaded pie dough, Io has heated up from the inside. Unlike pie dough, the inside is molten rock which explodes through the crust in enormous volcanic eruptions. Most of the material spewed out is sulfur, the same chemical that makes eggs yellow. Mixed with various compounds, the sulfur can turn rust red or blanch to white.
Io's volcanoes are almost always erupting plumes over a hundred miles into space. Some of this material surrounds Jupiter in a vast nebula.
Europa the Water World
Europa: Jupiter's Icy Ocean Moon
The next moon out is Europa. It's so precious that NASA actually ordered the Galileo space probe on a kamikaze plunge into Jupiter at the end of its mission to make sure the craft never crashed into Europa. What's so special about Europa, that we didn't want to risk contaminating it with any microbes clinging to an aging robot?
It's an ocean world covered by a shell of ice. Just like Io, the tidal forces of Jupiter and the other moons keep Europa warm inside. Cracks covering Europa's surface show where the ice has broken. Slushy water squeezes out of these fissures onto the surface, where it refreezes.
Biologists have found that amino acids, the building blocks of life, are carried around our solar system in comets and asteroids. Their impact scars peck Europa's surface. We also know that liquid water is vital for life, at least on our planet: it readily dissolves and transports chemicals more efficiently than almost any other substance.
That's why scientists have rested their hopes on Europa, the most likely place in our solar system to harbor life outside Earth. Unlike Mars, whose surface is battered by solar radiation and too cold and dry for water to remain in liquid form, Europa's global ice cap protects the watery cradle within.
Below are snapshots of just a few of Jupiter's moons. It has dozens more, perhaps over a hundred, but most of them are just blobby asteroids.
Jovian Moons (Moons of Jupiter) Photo Gallery
Dactyl, Orbiting Asteroid Ida
That Galileo spacecraft really gets around. On its way out to snoop on Jupiter for several years, it had to pass through the Asteroid Belt, so it took some snapshots of the asteroid Ida (the peanut-shaped rock at right) as it zoomed past.
To the surprise and delight of astronomers, they saw that Galileo had discovered the first moon orbiting an asteroid, just as the astronomer it was named for had discovered the first moons orbiting another planet.
Since little Dactyl was discovered, a few other asteroids have been found with moons — or more than a few, if you count "dwarf planets" like Pluto and Eris.
I think Dactyl is adorable, but there's just one problem. Dactyl is Greek for "finger" (compare pterodactyl, "wing-fingered"). Ida looks more dactyl-shaped to me than its little soccer ball companion.
[See a close-up photo of Dactyl on NASA's Ida & Dactyl page.]
A White Jewel of a Moon
The Surface of Another World
Enceladus, the Ice Geyser Moon
Enceladus, one of the innermost moons of Saturn, is like a cross between Jupiter's Europa and Io, but has a special beauty all its own. It's a tiny 310-mile-across ice world whose surface reflects almost 100% of sunlight that strikes it, making it one of the brightest objects in the solar system for its size.
Heating and squeezing from Saturn's gravity and the tug of other moons have resurfaced Enceladus so that it has only a few recent impact craters. Its southern hemisphere is striated with huge "tiger stripe" cracks that release plumes of ice from its interior.
These enormous geysers release vast quantities of water onto Saturn's cloud tops and into the area around Saturn, much in the way Io's volcanoes surround Jupiter with a faint sulfurous nebula. Scientists are having a field day analyzing what they describe as a dusty plasma, a very unusual state of matter, coming from Enceladus' plumes.
Titan, the Methane Moon
Massive Titan is the second largest moon in the solar system, orbiting Saturn. It's the only moon we know of with a substantial atmosphere, and more importantly, it's the only other moon where we've landed a spacecraft! As you'll see in the slideshow below, Titan's dense methane smog gives it the external appearance of a bland, featureless tan sphere. However, dig below the clouds with radar or infrared, and you're treated to an almost terrestrial landscape.
When the Cassini probe reached Saturn in 2004 for a four-year mission — which in fact is still ongoing! — it dropped off a mini Huygens lander through Titan's clouds to land on the surface. This was a daring thing to do, since some theories guessed that Titan had methane oceans. It doesn't have oceans, but it does have methane lakes, rivers, monsoon rains — in fact, everything that water does on planet Earth, liquid methane does on Titan. Unfortunately, it's so expensive to send equipment out to Saturn and back that it's not economically feasible to use Titan for all our natural gas needs.
Despite the flammable atmosphere and methane everywhere, Titan is very, very cold: water there is solid rock, and the moon's surface temperature is -290° F.
The Moons of Saturn
Triton, the Frozen Cantelope Moon
Voyager 2 reached Neptune in 1989 and discovered wacky terrains on the surface of Triton, a moon 22% the size of ours.
This far out, most of the moons (and even the so-called gas giants) are mostly ice. Triton's surface is unique in the solar system: it's mostly nitrogen ice with methane ice caps. It also has ice volcanoes!
Surface of the Moon Triton (Orbiting Neptune)
What's With the Warpaint?
Miranda, Moon of Uranus
Uranus' moon Miranda has a name that means "wonderful," thanks to the bizarre giant chevron shape chipping its surface. This 300-mile-wide satellite doesn't seem large enough to have geological faulting, however, the alternate theory, that it was shattered multiple times by collisions and reassembled itself, seems even more fantastic. Scientists are still debating Miranda's complicated geology. (See that link for more photos of and information about this marvelous little moon.)
I remember staying up late to watch the Voyager 2 flyby of Uranus in 1986. When this picture slooooooowly arrived in mission control, there was a lot of head-scratching! (Yes, back then, we actually watched live coverage of space missions on late-night TV.)
By the way, if you pronounce "Uranus" the way classics majors are taught to pronounce Latin, you avoid various embarrassing pronunciations that cause students to titter. Class, try saying it this way: Oo-raaah-nus, with that middle syllable matching the "ah" sound of British English "father." Teachers, you're welcome.
Phobos and Deimos: The Doomed Moons of Mars
The Moons of Pluto
The above photo is grainy, but the smallest two moons are 6 to 15 miles across, and it was taken by the long-past-its-expiration-date Hubble Space Telescope from how many gazillion miles away?
Hubble needs to stop making cool discoveries, or there will be nothing for the New Horizions spacecraft to discover when it reaches Pluto in 2015!
Actually, the fact that Pluto has been demoted to Dwarf Planet makes the New Horizons mission even more important, because it turns out that Pluto is one of several rocky mini-worlds ("dwarf planets" or "trans-Neptune Objects") orbiting out beyond Neptune in the Kuiper Belt, all of them pretty much unchanged since the solar system coalesced. Studying Pluto may give us insights into how the inner planets formed. I can't wait for the first pictures to come back. It's amazing that we've got a space probe that far out!
Overview of All Moons in the Solar System
- Solar System Exploration: Planets: Our Solar System: Moons
NASA's Solar System Exploration website is a fascinating site with great photos of all the planets and other objects in the solar system. Here's a fairly up-to-date list of planetary moons; see the text for moons orbiting a few dwarf planets.
Kristen Howe from Northeast Ohio on September 21, 2015:
Ellen, this was another wonderful hub from you on astronomy and our planets. This was so interesting to read about, right now.
fordie on June 24, 2012:
Fascinating. A lot of interesting facts supporting incredible photos. Will be looking up with different eyes tonight
Ellen_Friedman on June 24, 2012:
Excellent topic, beautiful collection of images. You've put poetry into the moons of our neighbors! Well done
Sandra Busby from Tuscaloosa, Alabama, USA on June 24, 2012:
I always read your hub, Greekgeek, to see what I can learn. Thanks for the lovely images, too.
CrawfsPlace from South Australia on June 24, 2012:
Really interesting hub! You have a great writing style which keeps technical topics interesting to the reader. I love space and everything associated with it so you have a fan in me!
DragonBallSuper on June 24, 2012:
hi! I love your hub. this is very fascinating, very interesting to read.. i also kinda love their names.