NASA, GSFC, MIT, LOLA
The infant moon may have had a thick metallic atmosphere. Intense, super gale-force winds may also have whipped across its surface, raising waves in a fiery ocean of magma.
That’s the conclusion of a new computer model. It ran a series of simulations to test theories of what could have happened early in the moon’s history. It also helped researchers think about the development of some exoplanets — those beyond our solar system — all without leaving Earth’s own neighborhood.
The new simulations pointed to how heat from the young sun, the Earth and the moon’s own hot surface might have vaporized lunar metals to form a thick atmosphere. It could have been as thick as the Martian atmosphere is today.
Researchers reported that assessment online June 22 at arXiv.org.
Most planetary scientists think the moon formed when a Mars-sized protoplanet slammed into Earth. That was some 4.5 billion years ago. The collision flung hot, molten material into Earth’s orbit. That material coalesced, or came together, and eventually cooled into our moon.
At first, the moon would have been covered in a deep, global ocean of magma — hot liquid rock. The post-collision Earth would have been blisteringly hot as well. How hot? Perhaps upwards of 2000° Celsius (3632° Fahrenheit). Earth would have glowed like a small star, specifically a type of star known as a red dwarf.
Prabal Saxena worked on the new study. He is an astrophysicist at NASA’s Goddard Spaceflight Center in Greenbelt, Md. He and colleagues added up the radiation the early moon would have received from its own magma ocean, from the starlike Earth, and from the sun. All of that energy would have vaporized metals in the magma ocean. This would have created an atmosphere about one-tenth the thickness of Earth’s, the model shows.
Previous models had suggested the early moon should have an atmosphere. But the new model is the first to include radiation from the moon, Earth and sun, Saxena’s team says. Pulling all of that information together reveals fresh details. They show how the early moon’s atmosphere and ocean may have interacted.
Saxena and colleagues focused on sodium. It’s an element that vaporizes easily. It also is abundant on the moon. To keep their model simple, they worked with the idea that sodium represented all the components that could contribute to an atmosphere. (In fact, others likely would have played a role too.)
The key to the early moon having an atmosphere was a molten ocean. If it existed, it would have seeded the atmosphere would freshly vaporized atoms from the sodium seas.
Those atoms would have whipped through the metallic air. Why? Because temperature differences on the moon would have had created strong winds. The side facing the sun would have been heated to more than 1700 °C. The side facing away from the sun would have chilled to a frigid ‒150 °C (-238 °F). That temperature difference would have raised winds with speeds over 1 kilometer (0.6 mile) per second. Such winds would probably have whipped up waves in moon’s magma seas.
The moon likely would have experienced snow, too. But not like any snow people have known. When lunar winds hit the zone where hot and cold temperatures meet, the atmosphere would have condensed. A metallic snow would have fallen.
After about 1,000 years, the magma ocean would have cooled enough to solidify into a rocky crust. With no liquid pool to draw from, the entire atmosphere then would have collapsed.
The atmosphere as a ‘rock star’
“The moon’s atmosphere was like a hard-partying rock star,” Saxena quips. “It had a really violent, heavy-metal existence,” he says, after which “it rapidly just fell apart.”
Such a live-fast, die-young picture of the lunar atmosphere sounds like something that could indeed have existed, says Kevin Zahnle. It might even be testable, says this planetary scientist. Zahnle works at NASA’s Ames Research Center in Moffett Field, Calif. The new study’s story is “well within the bounds of the possible,” he says.
Still, he’s not sure if all of its assumptions were good ones. Both Earth and the moon would have had to be “exceedingly dry,” meaning they must not have had much water. If they did, they first should have developed steamy water atmospheres.
One way to test the computer model’s conclusions would be to look for a ring of extra sodium in the rocks around the zone where the sodium snow was supposed to have fallen. That would show that the atmosphere really did have extreme temperature differences and high winds.
Other models of the moon’s formation offer a different scenario. For instance, the moon might have formed from several small impacts instead of a single large one. That would lead to a cooler atmosphere. The winds would not have been as strong, then. There also would have been no sodium snow, Saxena says. So finding that extra sodium could help settle the debate about which kind of impact really happened.
If the scenario with a thick atmosphere, winds and snow holds up, the moon might tell scientists about exoplanets orbiting some far off star. That scenario has Earth glowing like a star. And that would make the early moon a good analog for distant, rocky exoplanets orbiting red dwarf stars, Saxena says. So figuring out what the early moon looked like might offer clues to what is underway on those exoplanets, Saxena says.
analog Something that resembles, stand in for, or is similar to another thing.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
atmosphere The envelope of gases surrounding Earth or another planet.
coalesce The bringing together of many small elements into a combined mass.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
exoplanet Short for extrasolar planet, it’s one that orbits a star outside our solar system.
lunar Of or relating to Earth’s moon.
magma The molten rock that resides under Earth’s crust. When it erupts from a volcano, this material is referred to as lava.
Mars The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
molten A word describing something that is melted, such as the liquid rock that makes up lava.
moon The natural satellite of any planet.
NASA Short for National Aeronautics and Space Administration. (or NASA) Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It has also sent research craft to study planets and other celestial objects in our solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
planetary science The science of planets other than Earth.
protoplanet A consolidating celestial object that might one day turn into a planet — but if any only if that object is large, orbits some star, and during its orbiting will eventually sweep other debris out of its path.
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
red dwarf A type of smallish star that is relatively cool (and hence emits reddish light). Dwarfs are the most common size stars in the Milky Way.
scenario An imagined situation of how events or conditions might play out.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might vary in response to various situations or over time.
sodium A soft, silvery metallic element that will interact explosively when added to water.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
supersonic Moving at somewhere between one and five times the speed of sound in air.
vaporize To convert from a liquid to a gas (or vapor) through the application of heat.