The History Channel gives us a two-for-one special in this episode of The Universe, as we take a look at the first two planets orbiting the Sun — Mercury And Venus: The Inner Planets. The episode is broken down into two parts and takes a look at the more interesting planet Venus, Earth’s “sister” planet, first and is followed by a slightly rushed look at Mercury. As both planets are solid, unlike the gaseous outer planets, the episode and the scientists interviewed are able to talk about the similarities and differences of these inner planets to Earth, and how by studying these two planets has helped to answer questions about our own planet.

Venus, named for the Roman goddess of beauty, is often referred to as Earth’s “sister” planet, due to it similar size and mass, and it was once believed that Venus was covered with oceans before space exploration gave a much more harsh and truthful image of the planet. Venus is a violent world, with temperatures exceeding 900 degrees, billions of lightening bolts arching across the sky, erupting volcanoes and lava flows stretching for miles, and clouds made of sulfuric acid. It is this cloud make up which creates a greenhouse effect on the planet and thus makes the temperature so hot. Interestingly, it was scientists studying Venus and its greenhouse gases that gave way to the discovery of the greenhouse effects currently at work here on Earth.

As the physical make up of Earth and Venus are so similar, scientists can take a look at the active volcanoes here on Earth to examine what the surface of Venus must be like. The islands of Hawaii, which are essentially huge mountains of cooled lava over thousands of years, are studied extensively as a parallel to Venus. Experiments in controlled laboratories studying lightening are also used to recreate how the lightening on Venus works. The air on Venus is so dense, that it is easier for lightening to discharge in the clouds rather than strike the ground.

Mercury, like Earth’s moon, has no atmosphere and currently has no recorded or visible activity happening on the surface. The smallest planet in our solar system (now that Pluto has been demoted), it has been a dead planet for perhaps billions of years. But once it was teaming with volcanic eruptions, which built up the surface of the planet, before being bombarded by asteroids and space debris. One of these asteroids was so huge, that it not only left a crater nine hundred miles across, but cracked open the planet at the exact opposite point of the impact from the seismic waves.

Venus and Mercury are often overlooked in favor of Mars — thanks to the countless associated science fiction stories — and of the giant, more mysterious outer planets, including Jupiter and Saturn. As such, there is a host of great factoids waiting to be discovered in this episode. For instance, did you know that Venus is the only planet that spins on its axis in the opposite direction from the rest of the planets, or that a day on Mercury is equivalent to 180 Earth days? Be sure to tune in to find out more like this, and to see a volcanologist who may be a modern poster child to getting more teens interested in science. Or at least someone to go play hacky sack with at a Phish concert.

The Universe series episodes air on Tuesday at 9pm on the History Channel through September 4, 2007, and will be available on DVD October 30, 2007.