The Moon’s influence

Astronomy column

It is now well-established that in the past there was a lot of water on the surface of Mars. There are canyons, gullies, river and stream beds, deltas and many other features made by water. Today they are all dry, and the water is now trapped in layers of underground ice. Moreover, many features look as though they were formed more recently than billions of years ago, when we believe there were oceans and lakes on the Red Planet. That’s why a recent discovery by the Mars Reconnaissance Orbiter spacecraft is attracting a lot of attention.

This spacecraft, currently orbiting Mars, is among other things, equipped with two radar systems. Because the surface of Mars is very dry, the radars can penetrate the ground, revealing structures kilometres below the surface. This latest discovery is a huge amount of frozen carbon dioxide. At some point in the past, that carbon dioxide must have been in the atmosphere. This is really interesting because carbon dioxide is a greenhouse gas, inhibiting the ability of the ground to radiate its heat into space. If this frozen carbon dioxide were to evaporate into the atmosphere, it would significantly warm the currently frigid Martian climate. Then of course there could be a lot more frozen carbon dioxide than has been discovered so far, which could make things even warmer. However, what could change the climate enough to evaporate and freeze these deposits of carbon dioxide? To answer this question we need to look first at our own world.

The Earth’s axis is tilted 23 degrees away from being perpendicular to the plane of its orbit around the Sun. Apart from a small wobble that takes about 26,000 years, the Earth’s axis remains pointed in the same direction, currently in the direction of the North Star. That means that as the Earth orbits the Sun, the Earth is sometimes leaning toward the Sun, sometimes leaning away, and at other times neither towards nor away. This is what gives us the rhythm of the seasons: summer, winter and spring/autumn respectively.

The main force dictating how things move in the Solar System and the rest of the universe is gravity. Newton’s equations still do the job beautifully. However, taking into account all the tiny tugs the planets give to each other, as well as the big tug by the Sun requires a lot of computer power, which fortunately we now have. One of the surprises coming out of these calculations is that the little tugs from other worlds can dramatically change the tilt of a planet’s axis, altering the seasons completely. It seems as though this is what happens to Mars, occasionally giving it summer temperatures high enough to get the carbon dioxide evaporated and the greenhouse effect driving changes in the climate. If this happens to Mars, why isn’t it happening to our planet? It turns out that we have the Moon to thank.

Compared with the size of the Earth, the Moon is unusually large, and it is the Moon’s gravitational pull that stabilizes the Earth’s axis, giving us stable seasons. That service is provided in addition to driving the tides, which refresh our coastal ecosystems. Mars has only two tiny moons, Phobos and Deimos, which are far too small to provide these services.

Mercury lies low in the Northwest after sunset. Saturn is in the Southwest, still close to the star Porrima.  Jupiter and Mars rise in the early hours.  The Moon will reach Last Quarter on the 22nd.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton, B.C.