Earth’s Other Moons.
Mini Moons sound crazy...but possible!
Earth usually has more than one moon, according to a team of astronomers from the University of Helsinki, the Paris Observatory and the University of Hawaii at Manoa.
Our 2,000-mile-diameter Moon, so beloved by poets, artists and romantics, has been orbiting Earth for over 4 billion years. Its much smaller cousins, dubbed “minimoons,” are thought to be only a few feet across and to usually orbit our planet for less than a year before resuming their previous lives as asteroids orbiting the Sun.
Mikael Granvik (formerly at UH Manoa and now at Helsinki), Jeremie Vaubaillon (Paris Observatory) and Robert Jedicke (UH Manoa) calculated the probability that at any given time Earth has more than one moon. They used a supercomputer to simulate the passage of 10 million asteroids past Earth. They then tracked the trajectories of the 18,000 objects that were captured by Earth’s gravity.
They concluded that at any given time there should be at least one asteroid with a diameter of at least one meter orbiting Earth. Of course, there may also be many smaller objects orbiting Earth, too.
According to the simulation, most asteroids that are captured by Earth’s gravity would not orbit Earth in neat circles. Instead, they would follow complicated, twisting paths. This is because a minimoon would not be tightly held by Earth’s gravity, so it would be tugged into a crazy path by the combined gravity of Earth, the Moon and the Sun.
Earth captures a minimoon. The path of a simulated minimoon that is temporarily captured by Earth. The object approaches Earth from the right along the yellow line and continues on its trajectory along the orange path and finally escapes capture along the red path to the upper right. The size of Earth and the Moon are not to scale but the size of the minimoon’s path is to scale in the Earth-Moon system.
Inset: Radar image of near-Earth asteroid 1999 JM8 made with NASA’s Goldstone Solar System Radar in California and the Arecibo Observatory in Puerto Rico by a team of astronomers led by Dr. Lance Benner of NASA’s Jet Propulsion Laboratory in Pasadena, California. Minimoons are captured from the much larger population of near-Earth asteroids that pass close to Earth. This two-mile-diameter asteroid is more than a thousand times larger than the biggest minimoons, but it shows the irregular shape and pockmarked surface expected on the much smaller minimoons.
A minimoon would remain captured by Earth until one of those tugs breaks the pull of Earth’s gravity, and the Sun once again takes control of the object’s trajectory. While the typical minimoon would orbit Earth for about nine months, some of them could orbit our planet for decades.
“This was one of the largest and longest computations I’ve ever done,” said Vaubaillon. “If you were to try to do this on your home computer, it would take about six years.”
In 2006, the University of Arizona’s Catalina Sky Survey discovered a minimoon about the size of a car. Known by the unimaginative designation 2006 RH120, it orbited Earth for less than a year after its discovery, then resumed orbiting the Sun.
“Minimoons are scientifically extremely interesting,” said Jedicke. “A minimoon could someday be brought back to Earth, giving us a low-cost way to examine a sample of material that has not changed much since the beginning of our solar system over 4.6 billion years ago.”
The team’s paper, “The population of natural Earth satellites,” appears in the March issue of the journal Icarus. The team used the Jade supercomputer at the National Computer Center for Higher Education (Centre Informatique National de l’Enseignement Supérieur, or CINES) at Montpelier, France.
Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea. Source: NASA/Univ Hawaii
Moon ‘s Creation In Question
The origin of the moon has been challenged, creating a new puzzle as to how the moon formed.
New questions have been raised about the birth of the moon after a new study found samples from Earth and the lunar surface to be virtually identical.
The study reported in the journal Nature Geoscience contradicts the theory that the moon formed after the impact of a Mars-sized object named Theia with the early Earth about 4.5 billion years ago.
At the time both the Earth and Theia were still partially molten, the impact causing Theia’s core to sink into the Earth’s core, while lighter ejecta and debris was thrown into space eventually coalescing to form the moon.
The new study by Junjun Zhang, of the University of Chicago, and colleagues compared isotopic ratios of titanium in lunar and terrestrial samples.
Zhang and colleagues found the ratio of titanium isotopes on both Earth and the moon to be identical to within about four parts per million.
Because the Mars-sized impactor is expected to have been isotopically different, the measurements suggest the moon is either made entirely of material from Earth, or intense mixing occurred on both bodies after impact.
Deutsch: Der Vollmond, fotografiert in Hamois (Belgien) English: Full Moon view from earth In Belgium (Hamois). Français : Pleine Lune vue de la Terre en Belgique à Hamois. (Photo credit: Wikipedia)
Zhang and colleagues say the similarity can’t be explained by both bodies forming in the same part of the solar system because meteorite samples show extensive diversity in titanium isotopic ratios.
They conclude the isotopes are far more likely to have come from Earth rather than another planet.
Planetary scientist Brad Carter from the University of Southern Queensland says while it’s unlikely to have two planets with the same chemical composition, it’s not impossible.
“Despite what the paper says, a planet forming very close to the early Earth could have a similar composition resulting in similar isotopic ratios,” says Carter.
“It’s also possible that Theia was essentially made of ice, something from the Kuiper belt in the outer solar system.”
“This would have provided the energy of impact, but with the ice evaporating away leaving Earth material to eject into space and form the moon.”
Another option suggested by Zhang and colleagues involved the proto-Earth spinning much faster than previously thought, allowing a greater degree of mixing.
“However we don’t really think Earth had spun rapidly enough for that to happen,” says Carter.
“And then you have the problem of slowing down the Earth’s rotation afterwards.”
“Zhang and colleagues suggest a gravitational resonance effect between the Earth, moon and sun may have provided the forces needed to slow down the Earth’s spin rate,” says Carter.
Despite the problems, Carter believes the paper makes a good point about the need for revision of the current theory.
“The simple idea of Theia slamming into Earth remains sound,” says Carter.
“But we need to refine current theories that don’t explain the incredible similarities of isotopic ratios between the Earth and the moon.” Source: Discovery News
Related articles
- Cool animation showing asteroid DA 14′s near miss next year (blogs.discovermagazine.com)
- Findings Cast Doubt on Moon Origins (news.sciencemag.org)
- How the Man in the Moon turned to face us (msnbc.msn.com)
