How a burp made the Man in the Moon

By Roger Highfield, January 12, 2003

This article is reprinted with permission from the UK's The Sunday Telegraph.

To take a peek behind the scenes of this major new study, see Earth & Planetary Science Graduate Student Leads Moon Research. Besides the hard work and dedication, the "story behind the story" describes true scholarly collaboration, a hunch, and a lot of fun.

The moon has been an inspiration throughout history, being the brightest object in the night sky. The human imagination has seen many things lurking in its markings. Some cultures see a woman weaving, an elephant launching off a cliff, or a girl with a basket on her back. Others a rabbit, or even a four-eyed jaguar. We see the Man in the Moon, a face consisting of dark eyes and mouth, with bright forehead, cheeks and chin.

Thanks to research published in the current issue of the journal Nature, scientists now believe that the Man's lopsided and drooping dark features record an ancient catastrophe, a vast burp that shook our natural neighbour billions of years ago.

Through the Apollo moon missions three decades ago, scientists learned that the dark and light regions in the Man's face are two types of terrain. The bright markings are ancient cratered highlands, the terrae, dating back 4.5 billion years, when the moon was born from the debris of a cataclysmic collision between the Earth and a Mars-sized object.

The dark features were formed by younger flows of basaltic lava called maria (singular "mare", from the Latin for ocean). These seas of dark metal-rich volcanic rock cover about 16 per cent of the lunar surface. They must be younger than the basins in which they reside because they are relatively lightly cratered by the impact of meteorites. For example, the impact that formed the Imbrium basin (the Man's right eye) hurled material outwards and sculpted the mountains surrounding the Serenitatis basin (the left eye); thus, Serenitatis must be older.

This image of the Moon shows the dark 'mare basalt' flood lavas. Stegman and his colleagues tie these features in to a model for how the Moon might have briefly generated a magnetic field 4 billion years ago.

But there is one puzzle. Almost all the maria occur on the Earth-facing side of the moon. The reason for this may now have been explained by an effort to answer another puzzle: it is not possible to navigate around the moon with a compass. Although the moon had a magnetic field in the distant past, it has vanished for reasons that were not understood—until now.

Geophysicists at the University of California, Berkeley, have used a computer model of the lunar interior to show that a mighty belch early in the moon's history could explain these puzzling features. The burp could have broken through the surface to paint the Man's dark features. The eructation, like a blob rising to the top of a lava lamp, would also have lifted a blanket that covered the moon's core, triggering the creation of a magnetic field.

The moon has long since cooled off and the global magnetic field disappeared, but the brief burp nearly four billion years ago would explain the old magnetised rocks picked up from the moon's surface during the Apollo missions.

"This 3-D convection model produces an elegant explanation for the magnetic field that astronauts discovered," said Dave Stegman, one of the researchers. "If this model is correct, this would be the first full understanding of the thermal history of any planet, including the Earth, and would be a cornerstone for understanding the histories of all the other planets."

A magnetic field, such as the one that turns compass needles on Earth, requires active convection within a molten iron core, akin to the circulation in a boiling pot of water. The cycling molten metal carries charged particles with it which, as with an electric current, generate a magnetic field.

But in the case of smaller bodies, such as the moon, their cores might not be big enough and hot enough, and the cooling processes efficient enough, to maintain the high heat flow required for convection. The solid crust of these planets seems to act as a blanket to keep heat from escaping, halting convection and quashing any magnetic field.

The idea that convection in the lunar core could have begun 600 million years after the formation of the moon—only then to be switched off suddenly 300 million years later—has been difficult to explain. Stegman hit upon the idea of a blanket of dense material that would briefly insulate and even heat the core before bobbing to the surface to allow a brief period of convection. The resulting burp would break through the surface over one hemisphere, explaining the mare of thorium-rich basalts.

It is this maria that forms the dark features—the eyes and mouth—of our Man in the Moon. As Carl Sagan once put it: "It is a characteristic conceit of our species to put a human face on random cosmic violence."