Just how the Moon originally formed is under renewed scrutiny given the finding that it contains larger amounts of water than previously thought. We’ll look at that issue in depth another time, because it’s far from resolved. The generally accepted account of the Moon’s formation involves a giant impact with a planetary embryo that has been called Theia. The name is a nod to the Greek story of the titan that gave birth to Selene, the Moon goddess. After its formation, the Moon would have been closer to a much more quickly rotating Earth, inducing huge tidal forces that may have had repercussions on the evolution of the earliest life on the planet.
All of this has a further bearing on life’s emergence because a large moon can affect the tilt of a planet’s rotation relative to its orbit around the star. The term for this degree of tilt is ‘obliquity,’ and its effects on global climate can be profound. If there is little or no tilt, the poles become colder and heat flows in their direction. Increasing the obliquity means that the poles get more sunlight during half of the year while the equatorial regions cool twice a year. The influence on climate is inescapable, as is the fact that obliquity will be unique for each planetary situation.
A new paper by Sebastian Elser (University of Zurich) looks at this issue in terms of the Earth’s history and the probability of giant impacts among planets in general. What we know now is that the Earth’s tilt varies about 1.3 degrees around the figure of 23.3 degrees, with a period of roughly 41,000 years. Elser and team note that without the Moon, the Earth’s obliquity would experience large variations. Venus, which has no moon, shows a retrograde spin, which the Elser paper finds may have been induced by spin-orbit resonances and tidal effects.
Obliquity can vary enormously with time. The tilt of Mars’ rotation ranges from 0 to 60 degrees in less than 50 million years, and earlier work has indicated that the obliquity of an Earth without its Moon would range from 0 to as much as 85 degrees (complete references on these numbers can be found in the paper, cited below). Large moons, then, may be a major player in keeping climatic conditions stable. The Elser paper explores the impact history of planets to see how many would be likely to have a companion like the Moon, using simulations of planets forming in the habitable zone. The history and evolution of such Moons is then modeled.
The results show that large moons are not unlikely:
Under these restrictive conditions we identify 88 moon forming events in 64 simulations… On average, every simulation gives three terrestrial planets with different masses and orbital characteristics and we have roughly 180 planets in total. Hence, almost one in two planets has an obliquity stabilizing satellite in its orbit. If we focus on Earth-Moon like systems, where we have a massive planet with a final mass larger than half of an Earth mass and a satellite larger than half a Lunar mass, we identify 15 moon forming collisions. Therefore, 1 in 12 terrestrial planets is hosting a massive moon.
Assuming, then, that an Earth-class planet forms in the habitable zone around another star, the chances of its being orbited by a moon large enough to stabilize its orbital tilt is roughly 10 percent. The simulations used here, based on 2010 work by Ryuji Morishima (Swiss Federal Institute of Technology) and colleagues, produce numerous habitable ‘Earths,’ so the question of the importance of the Moon’s stabilizing influence becomes significant. We also have to untangle the issue of the water content of lunar magma, called into play by new work by Erik Hauri (Carnegie Institution of Washington). We’re looking at water levels 100 times higher than first supposed, challenging the giant impact theory of the Moon’s formation, which predicted very low lunar water content. Clearly, untangling all this will involve, among other things, sample returns from planets and other bodies that will teach us more about our system’s history.
Hauri speaks to this question himself:
“Water plays a critical role in determining the tectonic behavior of planetary surfaces, the melting point of planetary interiors and the location and eruptive style of planetary volcanoes. I can conceive of no sample type that would be more important to return to Earth than these volcanic glass samples ejected by explosive volcanism, which have been mapped not only on the Moon but throughout the inner solar system.”
The paper on planet/moon simulations is Elser et al., “How common are Earth-Moon planetary systems?” accepted for publication in Icarus (preprint). On the issue of water on the Moon, see Hauri et al., “High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions,” published online by Science on 26 May 2011 (abstract). On the Moon’s stabilizing effects in general, see Laskar et al., “Stabilization of the Earth’s Obliquity by the Moon,” Nature 361, 615-617 (1993). Abstract available.