The moon was watched by millions during Monday's solar eclipse, and scientists have tracked its movements for thousands of years. But new research from a Rutgers University scientist is a reminder that this familiar silvery sphere remains a bit of a mystery.

By heating up samples of lunar rock collected in 1971 during the Apollo 15 mission, Sonia Tikoo and her colleagues revealed new clues to the moon's ancient history — specifically, that of its long-lost magnetic field.

The moon had a magnetic field more than 3.5 billion years ago and now has none, but scientists are not sure when it disappeared. Tikoo, working with partners at the Massachusetts Institute of Technology and University of California, Berkeley, found that the magnetic field lasted at least until 2.5 billion years ago, and possibly as recently as 1 billion years ago.

Scientists are keenly interested in magnetic fields because they enable a planet to maintain an atmosphere, which in turn offers protection from cosmic radiation.

The findings on the moon, published Aug. 9 in the journal Science Advances, were a surprise, said Tikoo, an assistant professor of earth and planetary sciences at Rutgers, in New Brunswick, N.J.

Magnetic fields are generated when a planet or moon cools down, driving "convective motions" — a dynamo — in its core, Tikoo said. Smaller celestial bodies tend to cool faster than larger ones, and therefore are presumed to lose their magnetic fields more quickly.

"The fact that a body as small as the moon can generate such a long-lived magnetic field did come somewhat as a surprise," she said.

First, the researchers calculated the age of the rock samples, using a method that involves measuring the levels of various isotopes of argon gas. That yielded the range of 1 billion to 2.5 billion years ago.

Then the scientists heated up the samples to more than 1,400 degrees Fahrenheit — close to the temperature in which they were formed. The underlying science defies easy explanation, but the heat demagnetized the rock in such a way that the scientists were able to tease out the moon's magnetic field from when the rocks were formed long ago — a value they calculated at 5 microtesla.

This unit of measure for magnetic fields is named for the physicist Nikola Tesla, whose name also inspired the brand of electric cars.

The next step is to test rocks from more recent periods in the moon's history, Tikoo said.

The endurance of the moon's magnetic field suggests that it was fueled by some energy source in addition to the cool-down dynamo process, Tikoo said.

But fear not: the Earth's magnetic field will be protecting us for billions of years to come.

"The Earth is big enough and hot enough that our dynamo is not scheduled to die until probably when the solar system is dying anyway," she said.