Viewing Earth from a different angle

A  cataclysmic collision not only created Earth’s moon, but may have also knocked Earth over on its side, scientists have proposed. In a paper published last week in the journal Nature, the scientists say their numerical simulations indicate that the collision of a Mars-size object with the early Earth left our planet tilted at an angle of 60 to 80 degrees and spinning rapidly, once every 2.5 hours, or almost 10 times as fast as today.

But the simulations also show how the dynamics of the moon and Earth slowed down and straightened up over the next 4 billion years of the solar system, leaving them where they are today. “For the first time, this paper has a model that says we can start in one place and explain all of that without invoking any other follow-on event,” said Sarah T Stewart, a professor of earth and planetary sciences at the University of California, Davis. “And that’s new, and that’s exciting.”

“Where did the moon come from?” has been a persistent question over the eons. Among the rocky planets of the inner solar system, Earth is an anomaly. Mercury and Venus have no moons at all, and Mars has only a couple of potato-shape tiny moons (both less than 15 miles across) that may be captured asteroids. Earth’s moon, by comparison, is a giant, more than 2,000 miles in diameter. In recent years, the preferred explanation for the origin of the moon has been “the big whack”: very soon after the formation of Earth and the rest of the solar system, the Mars-size interloper that astronomers have named Theia bumped into Earth. The resulting slosh of debris coalesced into a slightly larger Earth and the moon in orbit around Earth.

The hypothesis explains a lot, in particular how to create a big moon. But there remained nagging discrepancies between the moon as it exists and the predictions of the big whack model. For one, the composition of the moon is very similar to that of Earth. Planetary scientists would have thought the moon would more closely resemble Theia.

In 2012, Sarah and Matija Cuk, then a postdoctoral researcher, proposed a variation, that Theia slammed into Earth at high speed, scrambling up the materials of the two bodies. The resulting Earth would also have been spinning fast, and they explained how the gravitational interactions with the sun would have then slowed everything. “We changed the impactor,” Sarah said. “We changed the energy. We’re changing momentum. We’re changing the way the moon forms. We’re now changing the whole dynamical sequence. Everything is different except the words, ‘giant impact’.”

Sarah and Matija said the revised calculations explained almost everything about the moon. But there was still a nagging discrepancy — a 5-degree tilt of the moon’s orbit compared with the orbits of the planets and most everything else in the solar system. It’s what astronomers call the plane of the ecliptic. The motion of moons and planets follow an orderly set of rules, Sarah said. “It makes very clean predictions, and when something goes against the orderly set of rules, it requires something special happening,” she said. “The clean prediction is the moon is in the ecliptic. Period. That’s where it should be.”

At its birth, the moon was quite close to Earth, probably within 20,000 miles. Because of the tidal pulls between Earth and moon, the moon’s orbit has been spiraling outward ever since, and as it does, Earth’s pull diminishes and the pull of the sun becomes more dominant. By now, with the moon a quarter of a million miles from Earth, the sun’s gravity should have tipped the moon’s orbit to lie in the same plane as the ecliptic. Last year, two astronomers proposed that planetesimals perhaps as big as the moon itself buzzing through the inner solar system tipped the moon’s orbit through repeated close passes.

Matija, now a scientist at the SETI Institute in Mountain View, California, came up with an alternate idea: Maybe the moon’s orbit is still tilted, because the Earth started off very tilted. The flexing of the Earth and the moon by the gravitational tidal forces dissipates energy, causing the moon to spiral outward. The dynamics can become complicated. “The lunar spin axis does interesting things,” said Matija, the lead author of the new Nature paper. For example, tidal locking — where one side of the moon always faces Earth — is lost for a while before locking in again. It is possible that the far side of the moon was originally the near side. Sarah said some of the transitions in orbits could have heated up the interior of the moon, and signs of that melting might be observable in rocks on the moon.

Alessandro Morbidelli, one of the astronomers who proposed the planetesimals hypothesis, said nothing was proved yet, and both models relied on assumptions. “Certainly it is an interesting model and it will trigger a lot of future work,” he said of the new paper. “I think that our model cannot be ruled out.”