The fall equinox is a significant event in any tipped planet's life. Seasons are caused by a planet's tilted axis. Mars's axis is tipped 25 degrees, almost the same as Earth's, so it also experiences seasons, along with Saturn (27 degrees), Uranus (98 degrees) and Neptune (28 degrees). The length of each planet's seasons varies with its distance from the sun. Something must have happened to tilt its axis to such an extreme — 177 degrees — to account for the contrary spin.