Mars is the most Earth-like other world known, yet the two planets are really a world apart and many mysteries remain. Here's what's known:

  Earth Mars
Diameter About 7,926 miles (12,756 kilometers) but growing, at least at the equator. Roughly 4,222 miles (6,794 kilometers), or 53 percent that of Earth.
Life Yes. Unknown.
Intelligent life Some would say so. No, probably microbes at best.
Distance from Sun 1 astronomical unit (AU), or about 93 million miles (150 million kilometers), on average. Ranges from 1.381 AU to 1.666 AU, due to non-circular orbit.
Temperature Way too hot in summer or way too cold in winter, depending on locale, or both. Planetwide: 47.3 degrees Fahrenheit (8.5 Celsius) over land areas. Warmer if surface air above oceans is figured in. Ridiculously cold most of the time. Freezing in most places. Planetwide: -67 degrees Fahrenheit (-55 Celsius). Can be shirtsleeve during fleeting moments of daytime summer.
Most striking feature

Looks blue from outer space, due to being two-thirds water.

Looks ruddy from outer space, due to being covered in oxidized iron. Earthlings call this "rust."

Cosmic radiation Manageable, protection courtesy a strong magnetic field. Problematic, due to a weak magnetic field.
Weather Hurricanes, typhoons and tornadoes a real problem. Disgustingly interminable fog in coastal California. Utter lack of rain in parts of Africa. Way too wet in many other places. Dust storms abound. Sometimes whole planet is obscured. Dust devils that soar higher into the atmosphere than terrestrial tornadoes and hurricane-like storms as big as Texas.
Heft 1 Earth mass. In kilograms, it's about 6 with twenty-four zeros after it. About 10.7 percent that of Earth.
Biggest mystery How life began. Whether there is life.
Biggest hoax The one suggesting we didn't really go to the Moon. The purported Face.
Day 23 hours, 56 minutes. 24 hours, 37 minutes.
Year 365 days (the time needed to go around the Sun once). 687 Earth-days, or about 670 Mars days.
Gravity Normal. 38 percent of that found on Earth at sea level.
Tilt of rotation axis 23.5 degrees. 25 degrees.
Satellites The Moon, plus several that communicate, and tons of junk as small as paint chips.

Two natural ones, Phobos and Deimos, plus two sent by NASA and more on the way.

Air Quite a bit. About 76 percent of it is nitrogen, with about 21 percent oxygen. Next most common, in order but in very small amounts: Argon, carbon dioxide, neon. Not much. Less than 1 percent the density of Earth's air at the surface, and mostly carbon dioxide (95.3 percent). Trace of oxygen (0.15%).
Volcanic hazards Wrong place, wrong time. Say goodbye. Close? Take a photo. Nothing but remnants of former beasts soaring silently into thin air.
Caves Many, and they're great places for microbial life to hide. Maybe, and they're possible places for humans to hide.
Snow Tons. Like 1,140 inches at Washington's Mt. Baker during the 1998-99 season, a world record. That's 95 feet (29 meters). Yes, surprisingly, but you wouldn't want to ski on it. And it's melting, perhaps due to global warming.
Water ice Yes, often invisible and under moving car tires. Yes, much of it invisible and under the surface or beneath another kind of ice at the south pole.
Dry ice Sure. 4,200 pounds of it is used weekly in just one Vegas show. Tons, covering the water ice at the south pole.
Water Inundates the place, especially in basements and during picnics. Lots in the frozen form. And possibly plenty of the liquid type long ago.
Best photo The Blue Marble. Couldn't decide, so we had experts pick 10.