It is amazing to think about how little we knew about other planetary bodies in the solar system before the dawn of the space age in 1957. Before that time scientists on Earth relied on ground-based radar and passive remote sensing technologies to obtain data. With the acquired ability to send rockets into space, we were able to dispatch satellites to fly-by, orbit, and even land on other planetary bodies. This technology allowed scientists to realize how harsh the environments were on the surfaces of other planets.

Mars is a cold, barren world about one-half the diameter of Earth whose surface conditions were virtually unknown until the 1960s. The Mariner missions beamed back images that depicted a dry, lifeless world. Then in 1976, the Viking Missions corroborated the lifeless aspects of the planet's surface by performing chemistry experiments via two Martian landers. None of these experiments resulted in detection of organics. By that time, it was known that Mars had a mostly volcanic surface, a very tenuous atmosphere (about 1/100th the pressure of Earth's), extremely cold average temperatures (-125 to 23 degrees Fahrenheit), and recurring global dust storms. Curiously, Mars has about the same tilt and rotational period as our home planet does. Recently, scientists have uncovered data that seems to indicate Mars once had liquid water on the surface and possibly a widespread sea.

Venus, on the other hand, is very hot with a thick dense atmosphere. It was once thought to be the sister of planet Earth since it has roughly the same diameter. Science and philosophers conjectured that the surface might be similar to that of a primitive Earth. However, closer inspections revealed an extreme environment of crushing pressures and enormous temperatures. Venus's average global temperatures could melt lead, and the pressure at the surface is about 90 times as great compared to Earth's. The atmosphere is composed mainly of carbon dioxide. Scientists attribute the condition of the atmosphere to a phenomenon known as the runaway greenhouse effect.

The only other terrestrial planet is Mercury. Terrestrial planets are those composed mainly of solid rock and are located closest to the sun. We know much less about Mercury than any of the others mentioned so far. If fact, we have not even imaged the entire surface yet. Mercury also has a hostile environment due to its proximity to the sun. As it slowly rotates (1 Mercury day is about two Earth months) it is thoroughly baked by intense solar radiation and plasma. Mercury's surface looks superficially like our moon. It is heavily cratered and has a volcanic appearance. Mercury is locked in a resonance with the sun so that its revolution around is sun is exactly 1.5 times that of its rotation on its axis. Thus, its year is about one-quarter of an Earth year.

A montage of the Jupiter system;
Europa is the second moon out from Jupiter

As we travel farther out in the solar system, we pass the rocky asteroid belt, gaseous planets, and a lot of space in-between. The "gas giants" are very different from the terrestrial planets. They are much larger than the 4 innermost planets with atmospheres composed mainly of Hydrogen and Helium. Mighty Jupiter has a diameter ten times that of Earth, yet that doesn't lend a proper visualization of the staggering size of this planet. If Earth was shrunk to the size of a peppercorn, Jupiter would be the size of a walnut.

The gaseous planets do have orbiting bodies that are similar to terrestrial planets. These moons are composed primarily of rock and ice. Europa, the second moon out from Jupiter, has an icy surface thought to shroud a subsurface ocean. If this is true, there may be extraterrestrial creatures thriving below the surface. Only further investigation will tell us for sure.