New Method for Estimating Moon's Age Seeks to Put an Old Debate to Rest

A new way of looking at the Earth's formation attempts to answer when the moon was born.

The moon was likely formed when a Mars-size object hit Earth and blew parts of the planet into space.

Earth's moon is some 4.47 billion years old, its birthday having come about 95 million years after the formation of the solar system. This means Earth's closest companion is some 60 million years younger than previously estimated.

The revised figure comes from a new study that sidesteps a long-running debate about the age of our moon and agrees with planetary scientists arguing for a late-forming moon. And the new method researchers used to arrive at their conclusion does away with a lot of the questions about the traditional ways of calculating its age. (See "The Moon's Mystery: Scientists Debate How It Formed.")

Usually, scientists estimate our moon's age by using the radioactive decay of elements like uranium, says John Chambers, a planetary scientist at the Carnegie Institution for Science in Washington, D.C.

By taking an element with a known rate of decay, and knowing its concentration in moon rocks or the Earth's surface, researchers can back-calculate a time for when the material formed. But there are a lot of different radioactive materials that can give you different timelines, says Chambers, who was not involved in the study.

"Geochemists get into a lot of fights with each other trying to determine what these ages mean," says Seth Jacobson, a planetary scientist at the Observatoire de la Côte d'Azur in Nice, France, and co-author of the study.

The new study, published today in the journal Nature, does away with this entirely by coming at the age question from a different angle.

Planetary Weight Gain

It's generally accepted that the moon formed near the end of Earth's construction, says Jacobson, when an object about the same mass as Mars hit our planet and blew parts of both bodies into space. Those bits eventually coalesced into the moon.

After that last giant impact, Earth added weight, but it came from impacts by smaller objects about the size of asteroids like Vesta, he says. Jacobson and colleagues used this later weight gain to calculate when the Mars-size object smacked into the Earth.

Their new method relies on the fact that elements in the Earth's crust that have an affinity for combining with iron, such as platinum or iridium, arrived after the last giant impact.

"When the moon-forming event occurs, this melts the entire surface of the Earth," says Jacobson. All the iron present near the surface sinks into the Earth's core, taking iron-loving elements along with it.

Any such elements now present in the Earth's surface arrived on objects that hit our planet after its core formed.

This later-forming time line for the moon is reasonable, says William Hartmann, a researcher at the Planetary Science Institute in Tucson, Arizona. However, he said that the new study might rely too much on the idea of using the last giant impact as a marker for when such events occurred in Earth's history.

"What people frequently forget in this field is that you never have just one big impact," says Hartmann, who first proposed the giant impact theory of the moon's formation. "We have to worry about how big the next biggest impact was," and whether that impact blurred the effects of the previous giant impact.

A Cascade of Implications

This evidence for a later-forming moon means that Earth's impact history was more violent than previously thought, says Jacobson. (Read "Our Solar System" in National Geographic magazine.)

The change in our planet's bombardment rate, he says, also means a change in the amount of energy that went into Earth's atmosphere and oceans. It also has implications for the history of our planet's surface temperature, which is important because liquid water couldn't form until the surface cooled enough. (Learn about how Earth got its atmosphere and its oceans.)

These results, which suggest the Earth formed nearly 100 million years after our solar system was born, also throw a monkey wrench into how scientists think about the formation of other rocky planets like Mars and Venus, Jacobson says.

Mars formed a few million years after the birth of the solar system, while Earth clearly took longer, he explains. If Venus formed on a timeline similar to that of Mars, then that would be a "huge puzzle" since Venus and Earth are so similar in terms of mass and orbit.

But these are open questions for future research, Jacobson says. For now, he and his colleagues will continue studying how planets in our solar system formed.

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