Martian seasons are reflected in the migration of Mars’s polar ice caps. There are permanently frozen water ice caps at the poles that get covered by the Martian snow every winter. But what determines the seasons, when Mars has a very thin atmosphere, very low pressure, and tiny moons?
Martian seasons are detectable through the migration of Mars’s polar ice caps, which are always there. In summer, the northern ice cap is around 1,000 kilometers across, and the southern cap occupies less space but is thicker. In winter, the water ice gets covered by dry ice or carbon dioxide snowflakes.
When it is winter in one hemisphere, the other has summer. Thus, the snowed CO2 sublimates in summer and goes to the other side of the planet, as gas in the atmosphere, to freeze and snow on the other ice cap.
The Mars Global Surveyor mission took detailed images that showed several pits, hills, and cracks in the ice caps. The spiral patterns of dark ice in bright ice is due to the layers created over time. The darker layers were created during dust storms, which are fairly common on Mars.
The layers help scientists study the changes in the tilt of Mars’s rotation pole and its climate. The tilt has ranged from about 15° tilt to 35° in the past 5,000,000 years. Why does the axial tilt change so much?
Learn more about water on Mars and prospects for life.
Axial Tilt Changes
Mars lacks a big-enough moon to keep its angular momentum stable. Also, Jupiter’s strong gravity can affect Mars and change its axial tilt. Mars’s two moons are too small and too close to do anything against Jupiter’s gravity.
When the tilt is high, wider areas of the poles receive sunlight, and much of the ice melts. On the other hand, in lower tilt, more of the polar cap stays colder and in shadow for longer periods, and so the polar cap can grow. Mars has two small moons, but they are not in the best position to affect the tilt. Phobos is about 22 kilometers in diameter, and Deimos is about 12 kilometers in diameter and seven times less massive than Phobos.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.
Phobos, the Bigger Moon
Phobos orbits Mars closer than any other moon orbits a planet in the solar system, at about 6,000 kilometers. Thus, it orbits so quickly that it completes one orbit in less than eight hours—faster than Mars rotates. Phobos orbits in the Mars–Sun plane, and there are two moonrises every day. Thus, every time Phobos passes in front of the Sun, there is a partial solar eclipse on Mars.
Phobos is so close that it experiences significant tidal forces from Mars. The tidal forces will eventually rip the moon into pieces that may form a ring around Mars. The process will take about 50,000,000 years but may have already started. There are grooves over the surface of Phobos that may signal the beginning of structural failure.
Learn more about how the solar system family is organized.
Deimos, the Smaller Moon
Deimos looks much more like a normal moon, orbiting at over 23,000 kilometers from Mars’s surface. It completes one orbit every 30 hours, i.e., longer than Mars’s rotation period. Deimos rises in the east and sets in the west, but as its orbital period is quite close to Mars’s rotation period, it looks as if it does not move much. At the equator of Mars, it would take about 21⁄2 Martian days for Deimos to rise and set.
Deimos also passes the Sun regularly, but it is too far away and too small to cause visible eclipses. However, Phobos can cause lunar eclipses over Deimos. The Curiosity rover observed one from Mars’s surface in 2013. Deimos is not going to shred into a ring around Mars since it is far enough away. Instead, it keeps getting farther from the planet, as Earth’s Moon does.
Learn more about human futures in the solar system.
Origin of Mars’s Moons
The origin of these moons is not known yet. Both have similar compositions to objects in the asteroid belt. Hence, the first theory is that they were captured by Mars, but their orbits are very circular. Captured moons usually have eccentric orbits due to their relative orbital speeds when they get close to the planet.
Even atmospheric drag and tidal forces cannot fully explain the orbits. Maybe they formed from a disk around Mars – as Earth’s Moon did – or maybe they are the survivors of Mars’s once-many asteroid-sized objects. Where the moons come from is still debatable.
Regardless of the origin of the moons and their future, Martian seasons come and go as the axial tilt prefers. The climate on Mars is always compatible with the axial tilt, not the moons.
Common Questions about Martian Seasons
Mars has two moons that have almost no impact on its axial tilt, Martian seasons, and climate. Phobos and Deimos are both too small to have impacts on Mars.
Mars has four Martian seasons, but they vary in length and include a cycle of carbon dioxide moving from the atmosphere down to the ground as snow, and back up to the atmosphere as a gas.
Mars’s orbit is not circular, and its axial tilt is about 25°, making Martian seasons have different lengths. However, the poles always have frozen ice caps.