Mars is a little over half the size of the Earth. Its diameter is 4,222 miles, which is 53 percent of Earth's diameter. The planet's mass is also small, at about 10 percent of Earth's total.
Surface gravity on Mars is only 38 percent of Earth's gravity because of its diminutive diameter and low mass. Atmosphere on Mars is minuscule, reaching only 1 percent of the thickness of the gases that surround the Earth. The orbit of Mars is much longer than Earth's, so a year lasts about twice as long. Days on the two planets, however, are very nearly the same length of time.