The terrestrial planets formed from rocky planetesimals closer to the Sun. Here it was too hot to allow icy planetesimals (which would otherwise be more numerous because they are made of more common elements). Still there was originally much more hydrogen and helium in the early planets than there is today; it escapes due to insufficient gravity and high temperatures in the upper atmospheres of these planets. The atmosphere that is left is probably due to a combination of venting from inside (volcanoes) and impacts of comets in the early solar system.

Mercury is the closest planet to the Sun. This makes it hard to observe, since it is never very far from sunrise or sunset. Its surface appearance is very much like the Moon - heavily cratered rock, with some lava flows. The Caloris Basin is a hemispheric impact feature; so large that the opposite side of Mercury is jumbled where the shock waves converged at the antipode. Mercury has the highest density of the terrestrial planets. One explanation is that it suffered an even larger impact early on, which removed much of its mantle (leaving the iron core). We used to think that Mercury would be locked by solar tides to keep one face toward the Sun. It turns out to be in a 2:3 resonance: three rotations for every 2 orbits (due to the influence of Venus also). Mercury is too small and hot to have kept any substantial atmosphere.

Venus is in some ways a twin of the Earth (size, density, internal structure). It has an odd rotation (very slow west to east: retrograde) which may have been caused by an early giant impact (but it has no moon). Venus is always shrouded in clouds; we know of its surface features through radar mapping. It has continents with mountain ranges and low basins like the Earth, but less evidence of tectonic plates. There are both impact and volcanic craters, and signs of recent vulcanism. From the surface it looks like barren rocky plains, lava flows, or mountains. Of most interest is Venus' atmosphere. It began like the Earth's, but ended up quite different. Due to its somewhat closer distance to the Sun (0.7 AU) water was kept in vapor form. Near the top of the atmosphere, solar ultraviolet radiation breaks the hydrogen from the oxygen, then the hydrogen escapes. In this way Venus lost its water (on Earth much water is kept safely near the surface in liquid form; the Earth too loses some hydrogen from the upper atmosphere). Because of the lack of liquid water, the carbon dioxide was not dissolved into rocks on Venus as it was on Earth. Thus the carbon dioxide of the Earth resides primarily beneath our feet, while on Venus it would still be over our heads (making the atmospheric pressure 90 times that on Earth). That in turn leads to a runaway greenhouse effect, wherein solar visible light (even though much of it is reflected off the clouds) makes it to the surface, heating it. The heat is radiated as infrared light, but the carbon dioxide traps it. The planet has to get much hotter to radiate away as much energy as it receives through the extra blanket over it, making the surface temperature 750K (hot enough to melt lead)! That, along with sulphuric acid clouds, makes Venus a very unpleasant place.

Mars is the planet with the most Earth-like environment. Although 1.5 AU from the Sun, the temperature can rise to livable ranges, though it is mostly very cold. The interior of Mars likely has a core and mantle, and there used to be substantial volcanic activity. The lack of tectonic plates meant that volcanic hot spots stayed put, giving rise to the Solar Systems largest volcano (Olympus Mons). Early on Mars had a much thicker atmosphere, and there is good evidence that there was running and standing water on its surface for a while (less than a billion years). This includes what look like ancient river beds and outflow channels. There is still some frozen water in the Martian polar caps, and likely below the surface. Because of its smaller mass, Mars could not hang onto the atmosphere very well, and the current surface pressure is only 0.007 that of Earth. Like Venus, what is left of the atmosphere is mostly carbon dioxide. There is still weather on Mars; thin clouds are sometimes seen, and massive dust storms sometimes occur. The surface of Mars is a rocky desert, reddish in color because the iron is oxidized (rusted). Impact craters have not been eroded away as much as on Earth. There is a great rift valley or canyon, Valles Marineris, caused by crustal cracking rather than water erosion. Mars has a day very similar in length to ours, and a year twice as long (with similar seasons). People have thought it might have life since the polar caps and seasonal variations were seen; for a while they imagined that "canals" to bring the polar water equatorward were also visible (spacecraft debunked that). There is evidence that water may have flowed fairly recently on the surface. We still think about Mars as a possible site for at least early life; witness the claims of possible bacterial fossils in a Martian meteorite in 1998. NASA has an ambitious program to further explore Mars. For the most up-to-date images of Mars, check out the Mars Global Surveyor site.