It was only a few decades ago that planetary moons were thought to be boring, crater-riddled worlds, and would be uninteresting to explore. Asteroids fell into the same generalization and comets were all thought to be dirty snowballs. Pluto was even thought to be an icy version of the Moon until probing with earthly telescopes and the Hubble Space Telescope proved otherwise. Once spacecraft began flying past these smaller worlds and the Hubble Space Telescope began to peer deep into the Solar System, it became obvious that no two worlds are exactly alike. There are too many to write about, but the following summarizes outstanding small worlds with at least one distinguishing characteristic, which easily differentiates it from the others.

Moons: There are many amazing moons and some are downright bizarre with one of the most extreme being Jupiter’s moon Io. Massive volcanoes wrack the surface with fountains of fiery hot sulfur flowing across the surface. Plumes soar over 200 miles into the sky and the volcanic activity is so intense that Io may have turned itself inside out several times during its lifetime. Volcanoes are clearly a defining feature that sets it apart from all the other planetary moons. Nearby is Europa with an unusually smooth surface where the highest hill is barely 300 feet tall. The defining feature of Europa is the possibility of a vast global ocean of liquid water beneath the smooth, icy crust. Ganymede is the largest moon in the Solar System and its defining feature is the marbled appearance of its surface - huge slabs of bright, icy crust contrasts with vast dark slabs of rocky crust. Callisto is the outermost of Jupiter’s large moons and its defining feature is the countless craters. The surface is covered with so many craters that new ones can only form by destroying old ones.

Saturn has a family of extraordinary moons led by its largest moon Titan, a world that may resemble a frozen version of primitive Earth where surface temperatures hover at minus 290ºF. There is no single defining feature that sets it apart from the other moons, but instead, there are several, which are dependent on each other. A thick atmosphere and a surface with liquid are probably the two biggest defining features, but other features make Titan truly stand out and rival Io. Titan is covered in a thick atmosphere of organic smog that is 1.6 times thicker than Earth’s atmosphere and a steady east wind blows throughout. The surface is almost like Earth with floodplains, rivers, craters, dunes, mountains, and lakes filled with liquid methane. Methane rain falls from billowy clouds in the smoggy sky creating winding rivers that flow into the floodplains, fill the lakes, and carve the mountains. Cryovolcanoes gushing liquid water and methane may also dot the surface feeding methane into the atmosphere. The liquid methane lakes aid the hydrological cycle, which maintains the atmosphere, influences the climate, and shapes the geology.

Enceladus is closer to Saturn and about ten times smaller than Titan, but it has a dramatic defining feature of thin, but powerful geysers spewing water ice crystals hundreds of miles above the surface. The plumes are so amorphous that they are hard to see, but they are responsible for creating the ghostly E-ring around Saturn between the moons Mimas and Rhea. Mimas is even closer to Saturn and is clearly defined by a huge crater around 80 miles across that is about one-third the size of the moon itself. It nearly destroyed Mimas and gives it the ominous appearance of the Death Star in Star Wars. Hyperion orbits further out than Titan and probably suffered a massive impact that succeeded in shattering it. It is more like a battered hockey puck and its distinguishing feature is its sponge-like appearance. Endless clusters of deeply scalloped craters cover the tortured moon making it appear like a cosmic sponge. Even further out is Iapetus and its distinguishing feature is its contrasting albedo. Around half of the moon is covered with brilliant white ice while the other half is covered with organic material as dark as coal. It continues to be one of the biggest mysteries in the Solar System as to how and why it ended up looking like this.

Uranus has one moon that really stands out. Miranda is hardly 300 miles in diameter and yet it appears as if it were shattered and reassembled with chunks of bright ice and dark rock randomly reassembled. There is one cliff called Verona that is up to 12 miles high and is one of the steepest places in the Solar System. Miranda most likely was shattered and the heavier, rocky blocks were unable to sink down towards the center before the moon cooled leaving them jumbled with the light and brighter icy blocks. Neptune has a moon with real personality, Triton. There are geyser-spewing plumes of nitrogen gas up to five miles high scattered across a vast ice cap of frozen nitrogen and methane where temperatures hover around -390ºF.

Pluto: Even though it may no longer be classified as a planet, Pluto is still a world, and a brutally cold one. It is fittingly classified as a dwarf planet and a Kuiperoid, and often orbits farther than four billion miles from the Sun. Temperatures may fall as low as -420ºF, which is almost absolute zero, -459.67ºF, and almost everything freezes into solids in such intense cold, including gases such as nitrogen, methane, argon, and carbon monoxide. These gases make up Pluto’s thin atmosphere when it is closest to the Sun, less than three billion miles, and a balmy -380ºF, but freeze out onto the surface as frost when Pluto journeys to the distant part of its elliptical orbit. Methane frost on its surface is damaged by the ultraviolet radiation from the Sun, which reddens it giving Pluto a distinct pinkish-tan hue.

Asteroids: These tiny worlds orbit mainly between Mars and Jupiter and are usually classified by composition with metallic, stony, and carbonaceous being the largest classes. Among the asteroids themselves there are a few that have outstanding personalities. Vesta is the most unusual because it has a surface covered with frozen lava. This is unusual for a world so small because it seems impossible for a world so small to generate any internal heat, but the spectral signature indicates rock of volcanic origin. Ceres is also proving that it is not a boring, cratered world. There is evidence that it may contain as much water ice beneath its crust as there is liquid water on Earth. Both of these asteroids are targets for a new spacecraft mission called Dawn, which will orbit Vesta and Ceres at separate times during the next decade. There are hundreds of thousands of other asteroids that are far smaller than Vesta and Ceres and are probably nothing more than boring rocks, but there may still be dozens, if not hundreds, that have characteristics that would set them apart from other asteroids. As exploration of asteroids increase in the decades ahead, there are sure to be major discoveries ahead.

Comets: These are more than just dirty snowballs and as the exploration of comets continues, it is becoming obvious that comets are not all the same. A big clue that they are different from each other is their behavior when seen from Earth as they approach the Sun. The length and size of the tails, composition of the tails, overall color of the comet, and how the nucleus reacts as it is heated are all good indicators that no two comets are alike. Close encounter with spacecraft reveal a nearly black nucleus for Comet Haley and brighter, but cratered nucleus for Comet Temple 1 and Comet Wild 2. Some missions are on their way and others are being planned. It will be interesting to see what kind of geology these dynamic worlds are capable of in spite of their small size.

The worlds within the solar System are as unique as can be with each planet, moon, asteroid and comet displaying a variety of characteristics. These characteristics seem to be governed by the world’s distance from the Sun, size, and chemical composition. The Solar System may be full of mystery and theory, but the hard fact is that no two worlds are exactly alike, therefore giving each one a unique personality.