Mercury

• Viewing: Best viewed at dawn or sunset, always close to the Sun; Occasionally transits the Sun [will do so 14 times in the 21st century]; Has a full cycle of phases.
• Motion: Radius 40% of Earth's, 0.4 AU from the Sun, so 1 Mercury year = 88 days, considerably elliptical; Slow rotation (period = 59 day), in resonance with the orbit.
• Exploration: Only flybys by Mariner 10 in 1974; Currently the MESSENGER mission.
• Surface: Dark, similar to the Moon's appearance, with no atmosphere; But the craters are fewer, different, and flatter (stronger gravity? softer material?). Possible ice deep inside craters.
• Features: Very large Caloris basin, probably due to an impact, and "weird terrain"; Very long cliffs or scarps due to tectonic stresses from planet shrinking, after most of the cratering.
• Evolution: Formed about 4.6 Gyr ago; A giant impact may have vaporized most of the original envelope, and left heavier material. Intense early volcanic activity, but the core is now cool.

Venus

• Overall: At 0.7 AU (the bright Morning or Evening Star, associated with the goddess of love and beauty, Venus, Aphrodite, or Ishtar. It can transit the Sun! In closer views, the Blue Planet, smooth and featureless from total cloud cover).
• Planet: Size 95% of Earth's, at 0.7 AU from the Sun [225-day orbit]; Slow, retrograde rotation [with 243-day period], probably due to a past collision; No moons. US flybys in the 1960s; Soviet spacecraft have landed; Several orbiters, including currently Venus Express.
• Surface: Radio maps show mountains, rolling plains, riverbeds, coronas, some large craters and cracks from temperature changes; But the surface is young, less rugged than Earth's; Venus may have been totally resurfaced 500-700 Myr ago, like a pressure cooker.
• Atmosphere: Thick, high pressure (90 atm!), made of CO₂, N, with sulfuric acid clouds; 450°C (850°F) hot everywhere and always, because it traps heat very effectively; Dry, of dusty rocks, with no water (but it may have had water in the past); Only a dim orange light goes through; There is little erosion (not much rain or wind).

  Earth

• Overview: Radius about 6500 km or 4000 miles (to measure it, think about Eratosthenes' way); Bulges at the equator. The interior is made of layers of different density, the thicker ones being in the core; We know mostly from seismic waves.
• Motion: Its distance from the Sun is about 1 AU or 150,000 Mkm, the period of revolution 365 days; Rotation takes 23 h 56 min.
• Unique features: It is like an oasis in the solar system, with its variety of environments and moderate temperature, which allow the existence of a hydrosphere and a biosphere.
• Surface: The crust is divided into about a dozen, roughly 50-km thick plates, that move due to convection in the mantle by a few cm/year, leading to continental drift over billions of years, and cause earthquakes and volcanoes.
• Atmosphere: About 100 thick, Nitrogen (78%), Oxygen (21%), Argon (1%), CO₂ (the right amount); Provides weather; protection from radiation, erosion...

Earth's Moon

• Size: One-fourth the size of Earth, too small to hold onto an atmosphere, 1/6 the gravity.
• Distance: 60 Earth radii [just 380,000 km!] away, in synchronous rotation on 27.3-day orbit.
• Apollo missions: In 1969–1972, 12 astronauts walked on the Moon, conducted experiments.
• Later exploration: Various orbiters from different countries (in addition to the USA and the Soviet Union, Japan, China and India; SMART-1 in 2004-2006, and currently NASA's LRO and GRAIL; Long-term plans include returning people, possibly buildings bases.
• Craters: Of all sizes, produced by impacts that also covered the surface with fine dust.
• Near vs far side: There are almost no maria on far side.
• Interior: Differentiated, with an off-center core.
• Formation: The most likely scenario (from similarities) is impact-ejection from Earth, 4.6 Gyr ago (from analysis of isotopes in Moon rocks).
• Early history: It cooled down 4.4 Gyr ago; Intense cratering until 3.1 Gyr ago; Meanwhile, volcanism filled craters with lava, formed maria.

  Mars

• Visibility: It is brighter than many stars and visible at night also; Reddish (named after the god of war); from Earth one can see white regions near the poles.
• Exploration: Fascinating since a long time ago (see Schiaparelli's "canals" and Lowell); Visited by many orbiters and landers, including 3 orbiters and 2 rovers active right now!
• Planet and orbit: Half as wide as Earth, 1.52 AU from the Sun on an eccentric, almost 2-year orbit; It rotates once every 24.6 hours, with an axis tilted 25°!
• Moons: Two, 28-km Phobos ("fear") and 16-km Deimos ("panic"), probably captured asteroids.
• Surface: Hills and valleys and large-scale features; Volcanoes (Olympus Mons is 21-km high!), canyons (Valles Marineris is 5000-km long!), and many craters; The Northern hemisphere is younger, smoother, probably because of a collision.
• Atmosphere: Less than 1% of Earth's pressure, too thin to protect and for greenhouse effect, but thick enough to erode, with 95% CO₂; Dusty and reddish, probably thicker in the past; Mars is now mostly cold.
• Water: There was flowing water in the past (from channels, gullies, minerals), but it is now frozen or gone.
• Life? The evidence (including Viking landers and meteorite ALH 84001) is inconclusive.