As with Mercury and Venus, we can determine the radius of Mars by means of simple geometry. From the data given earlier for the planet's size and distance, we obtain a radius of about 3400 km. More accurate measurements give a result of 3397 km, or 0.53 Earth radii.

Unlike Mercury and Venus, Mars has two small moons in orbit around it; they are visible (through telescopes) from Earth. Named Phobos (Fear) and Deimos (Panic) for the horses that drew the Roman war god's chariot, these moons are little more than large rocks trapped by the planet's gravity. We will return to their individual properties at the end of this chapter. The larger of the two, Phobos, orbits at a distance of just 9380 km from the center of the planet once every 459 minutes. Applying the modified version of Kepler's third law (which states that the square of a moon's orbital period is proportional to the cube of its orbital semi-major axis divided by the mass of the planet it orbits), we find that the mass of Mars is 6.4 × 10²³ kg, or 0.11 that of Earth. (Sec. 2.7) Naturally, the orbit of Deimos yields the same result.

From the mass and radius, we find that the average density of Mars is 3900 kg/m³, only slightly greater than that of the Moon. If we assume that the Martian surface rocks are similar to those on the other terrestrial planets, this average density suggests the existence of a substantial core of higher than average density within the planet. Planetary scientists now believe that this core is composed largely of iron sulfide (a compound about twice as dense as surface rock) and has a diameter of about 2500 km.

Surface markings easily seen on Mars allow astronomers to track the planet's rotation. Mars rotates once on its axis every 24.6 hours. One Martian day is thus very similar in length to one Earth day. The planet's equator is inclined to the orbit plane at an angle of 24.0°, again very similar to Earth's inclination of 23.5°. Thus, as Mars orbits the Sun, we find both daily and seasonal cycles, just as on Earth. In the case of Mars, however, the seasons are complicated somewhat by variations in solar heating due to the planet's eccentric orbit.