showing how three NASA orbiters mapped the gravity field of Mars
North polar early summer water ice cap (1999); a seasonal layer of carbon dioxide ice forms in winter and disappears in summer.
Mars is about 230 million km (143 million mi) from the Sun; its orbital period is 687 (Earth) days, depicted in red. Earth's orbit is in blue.
Animation of the apparent retrograde motion of Mars in 2003 as seen from Earth.