Aliens don't need a moon like ours

TALK about being over the moon. It seems planets don't need a big satellite like Earth's in order to support life, increasing the number on which life could exist.

In 1993, Jacques Laskar of the Paris Observatory in France and colleagues showed that the moon helps stabilise the tilt of Earth's rotation axis against perturbations by Jupiter's gravity. The researchers calculated that without the moon, Jupiter's influence would make the current tilt of some 23 degrees wander chaotically between 0 and 85 degrees. That could cause huge climate swings, making it hard for life to survive, especially large, land-based organisms like us.

The result was taken by many to imply that complex life is rare in the universe, since Earth's large moon is thought to have coalesced from the debris of a freak collision between a Mars-sized planet and Earth. Less than 10 per cent of Earth-sized planets are expected to experience such a trauma, making large moons a rarity.

But a study now suggests moonless planets have been dismissed unfairly. "There could be a lot more habitable worlds out there," says Jack Lissauer of NASA's Ames Research Center in Moffett Field, California, who led the research.

The 1993 study showed that the Earth would tilt wildly without the moon because two of its motions would end up in sync, allowing Jupiter to have an outsize influence. The Earth orbits the sun on an elliptical path, and the long axis of this path shifts position over time. The Earth also wobbles like a spinning top as it rotates. Without the moon's gravitational tugs, the rate of this wobbling would be slower, matching up in just the right way with the drifting of its elliptical orbit to magnify Jupiter's effects on Earth's spin axis, leading to big changes in tilt.

However, Laskar's study did not determine how fast these changes in tilt would occur. "The astrobiology community has taken it to mean there will be these really wild variations, and we wanted to test that," says Lissauer. He and his colleagues simulated a moonless Earth over 4 billion years, about the age of the Earth today. They found that our planet's tilt varied between only 10 and 50 degrees, a much smaller range than implied by the earlier study. There were also long stretches of up to 500 million years when the tilt was particularly stable, keeping between 17 and 32 degrees (Icarus, DOI: 10.1016/j.icarus.2011.10.013).

Much larger changes might still occur on timescales longer than 4 billion years, the team admits. But in that case they might be irrelevant for life anyway, they say, because sun-like stars burn out after 10 billion years.

Large moons are not required for a stable tilt and climate, agrees Darren Williams of Pennsylvania State University in Erie. In some circumstances, he adds, large moons can even be detrimental, depending on the arrangement of planets in a given system. "Every system is going to be different."

Jason Barnes of the University of Idaho in Moscow, who co-authored the latest study, is now leading simulations to look at how planet tilts behave in a wider variety of circumstances, including planets arranged in different ways to our solar system.

Why cold, dead moon stayed magnetic

HOW did the moon remain magnetic tens of millions of years after its molten core stopped sloshing?

Early in its life, the moon probably had a core hot enough to churn violently, with the movement of this electrically charged fluid creating a magnetic field. But as the core cooled, the convection should have eased enough to kill the field. So it was a puzzle when Apollo moon rocks suggested the moon still had a magnetic field 4.2 billion years ago, millions of years after the powerful mixing ended. Now two groups have come up with explanations for what could have kept the core stirred up.

The moon is thought to have formed closer to the Earth than it is now and spun faster, slowing down and moving away over time through tidal interactions with Earth. Christina Dwyer at the University of California, Santa Cruz, and colleagues say previous models did not take into account this faster spin, which would have agitated the molten core like water in a washing machine. This could have enabled the magnetic field to last until 2.7 billion years ago (Nature, DOI: 10.1038/nature10564).

Michael Le Bars at the Non-Equilibrium Phenomena Research Institute in Marseille, France, says large meteorite impacts that occurred until about 3.9 billion years ago also could have set the lunar core sloshing for periods of 10,000 years at a time (Nature, DOI: 10.1038/nature10565).

The models might also explain how some asteroids came to be magnetised, says Ben Weiss at the Massachusetts Institute of Technology.

Melissae Fellet

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