The weirdest moons in the solar system, and what it would look like to stand on themS

It would sometimes seem as though there are two solar systems: One including planets, and another including moons. Until recently, most books about the solar system focused on the planets, stopping only to acknowledge the existence of their satellites with a kind of half-embarrassed nod. They would be discussed as a kind of afterthought.

But they absolutely have some of the weirdest landscapes in the entire solar system. Which, needless to say, makes them of immense interest to me as a science artist.

The Voyager fly-bys of Jupiter, Saturn, Uranus and Neptune in the 1970s gave us our first detailed images of many moons. Places like Io, with its raging volcanoes, were certainly eye-openers. Others followed in pretty quick order, like Enceladus with its icy volcanoes and Europa with its massive seas beneath a thick layer of ice, proved that the solar system's moons were capable of being every bit as peculiar and interesting as the planets themselves. Titan, whose thick atmosphere enrobes a moon with ice dunes and vast methane lakes, generate headlines on a fairly regular basis. Still, few people have looked at moons as a body of objects in their own right, rather than colorful hangers-on, like courtiers among royalty.

It's hard to not blame astronomers and science writers for having taken such a patronizing view of the satellites. After all, they are, compared to the worlds they orbit, pretty small punkins. Jupiter's largest moon is twenty-seven times smaller than the giant planet. Even our relatively huge moon is still four times smaller than the earth.

But if you separate them from the worlds they orbit and look at them individually, they become much more impressive. Ganymede turns out to be larger than the planet Mercury and only 1000 km smaller than Mars. And looked at objectively, the earth's moon may already be a planet in its own right (but that's another story).

Allow me to present you with illustrations of a few of my favorite moons...

Io, pictured at the top, is literally seething with the most—-and most powerful—-volcanoes in the solar system. Its original surface is buried under thousands of feet of material blasted from the interior of the moon. Indeed, it would not be too much to say that Io is busy turning itself wrong-side-out. Io is a large moon, about 2% bigger than our own. At first glance, it resembles a cheese pizza, with its blotchy, swirling patterns of red, yellow, orange and white, or perhaps something that had been left sitting in the refrigerator too long.

Unlike most of the other moons in the solar system, which are covered with craters, Io's surface features hundreds of volcanic calderas, or large flat-floored depressions, many of which are violently active. In fact, there are more volcanoes erupting at any one time on Io than on Earth, making this moon the most volcanically active body in the solar system, with new eruptions breaking out all the time. Some of the most powerful eruptions can throw bright plumes of material up to 300 km (186 miles) above the surface. But these eruptions look nothing like those we see on our planet. Since Io has very little atmosphere the dust and gas cannot create the billowing clouds we see spewing from volcanoes on Earth. Instead, Io's volcanoes look like vast garden sprinklers, the gas and dust curving in huge arcs, forming umbrella-shaped plumes over the vents. My illustration depicts what one of these volcanoes might look like from close quarters, with rivers of red, yellow and black molten sulfur pouring from its caldera.

The weirdest moons in the solar system, and what it would look like to stand on themS

Titan may be my hands-down favorite. This giant moon—-it is 4/10ths the size of the earth—- bears an uncanny resemblance to our own planet. The first photos taken by the Huygens probe as it drifted toward the surface revealed a landscape that looked for all the world like the coastline of California or Scandinavia. Titan has an atmosphere (albeit of frigid nitrogen and methane) denser than the earth's, and it possesses perfectly familiar rivers, lakes and seas...filled with liquid methane.

But what makes Titan of special interest to me as an artist are its thermal features: cryogeysers and cryovolcanoes spewing ice and snow instead of steam, boiling water, red-hot rocks and molten lava. Such things were only speculative, however, until images taken in 2004 by the Cassini orbiter revealed a cryovolcano volcano on the surface. Instead of lava, the volcano probably erupts water, ammonia and methane. On Earth these are normally liquid, like water, or a gas, like methane. But Titan is very cold. Its surface is a frigid -290°F (-179°C). At that temperature, water freezes to ice as hard as steel and methane is liquid. The illustration depicts one of these huge vents erupting near a Titanian lake.

The weirdest moons in the solar system, and what it would look like to stand on themS

Enceladus is certainly a runner-up to Titan. Most of the landscape of this little (500 km/310 mi) moon is a wilderness of icy craters. But near its south pole is a region of long, sinuous channels called "tiger stripes" that resemble the claw marks of some gigantic animal more than the stripes on a tiger. From cracks and vents in the floors of these deep gorges spew vast ice geysers: the fountains of Enceladus. They were discovered in 2005, when the Cassini spacecraft photographed vast feathery plumes spraying from cracks near the south pole.

The geysers are very powerful. They can shoot as far as 200 miles (300 km) above the surface. The material leaves the vents at jet engine speed. To see one close up would be like looking at a huge rocket engine. Most of the material being ejected is water ice. This leads scientists to believe that Enceladus might have large underground lakes of water. Tidal heating from Saturn keeps this water liquid. Much of the water that's ejected falls back to the surface. But because the material is traveling so fast, some of it leaves Enceladus for good. It then spirals down toward Saturn, where it forms much of Saturn's outermost E ring. The fountains of Enceladus have fascinated me. I think they are one of the more sublimely beautiful things on our solar system and I've done dozens of renderings of them.

The weirdest moons in the solar system, and what it would look like to stand on themS

Callisto is one of the most heavily-cratered places in the solar system, but that's not the feature that most interests me. What draws my attention is a very small area where the surface ice has been worn down in much the same way that rocks have in Chiricahua National Monument. The result is the only place in the solar system that resembles a traditional 50s lunar landscape. The towering ice spires rise 80 to 100 meters (260 to 330 feet) high. Their origin is kind of a mystery. One theory is that the spires were the result of material blasted from Callisto's crust by an impact. As some of the exposed ice turns into vapor in the surrounding vacuum it leaves behind dust that had been bound in the ice. The accumulating dark material absorbs enough heat from the distant Sun to warm the surrounding ice. This keeps the process of erosion going. Whatever the reason for their existence, I like the retro look.