posted June 21, 2004 07:25 AM              

"Earth Hit by Neighbor in Making of Moon

By Robert Roy Britt
Senior Science Writer, SPACE.com

The leading theory for the Moon's formation has a Mars-sized object slamming into Earth about 4 billion years ago, shortly after our planet formed. The evidence is partly in the Moon's composition, which is similar to the upper portions of Earth.

The theory goes back to the 1970s and is well established as the most likely way to make the Moon.

But where did that other world come from?

Researchers think the impact involved low relative speeds -- like one car merging into another on the freeway rather than a more destructive head-on collision. For that to happen, the impactor must have formed nearby in an orbit similar to that of Earth.

But if it was nearby, why did it hit after Earth formed and not sooner, when things were still under construction and everything else was colliding with something? A new adjunct to the theory offers a simple explanation:

The Mars-sized impactor formed at the same distance from the Sun as Earth in a gravitationally stable spot known as a Lagrangian point, then drifted out of that birthplace -- thanks in part to collisions -- to set up a crash course in satellite development.

The scenario was recently proposed by astrophysicist J. Richard Gott and mathematician Edward Belbruno, both of Princeton University.

Semi-stable

Shortly after the Sun's birth, as Earth was forming, leftover dust and rock congregated in a region of space near Earth's orbit, theorists think. Once there and with Earth in place, the stuff would find some mathematically logical places to gather.

There are five Lagrangian points related to the Earth, Moon and Sun, and each has some gravitational stability. A spacecraft can, for example, orbit a Lagrangian point and with little or no propulsion remain in that configuration for long periods.

The Lagrangian points named L4 and L5 sit along Earth's circular path around the Sun. They move as Earth moves. Each forms an equilateral triangle between itself, the Sun and Earth. Gravity from the Sun and Earth combine to create a state of equilibrium at each of the two points.

Gott and Belbruno poured the raw material for planet formation into a mathematical model, then theoretically estimated what happened at L4 and L5. Material tended to collect there and both locations yielded similar results.

Creeping chaos

Belbruno told SPACE.com how the L4 scenario played out:

"If material flew in there and slowed down a lot by collisions with other objects near these locations, the material would stay, and start to accumulate by gravitational attraction and mutual frictions," he explained.

As initial rocks grow larger, into what scientists call planetesimals, more material is attracted.

"After about 30 million years a Mars-sized object results, Belbruno said. "It is wandering around the L4 region, in a random walk fashion as it is impacted or nearly impacted by planetesimals."

Each impact or close brush gives the object a speed boost.

"Its velocity gradually increases with respect to L4," he continued. "At about 30 million years its velocity is sufficient for it to just barely leak out of the L4 region of stability."

The object moves along a path similar to Earth's orbit but at a different speed. Its movement is chaotic because the Sun and Earth are tugging on it roughly equally. The motion is called "creeping chaotic motion."

"It will approach the Earth either from behind (if it has to catch up), or from the front," Belbruno said. "Once it leaves the L4 vicinity and starts creeping, it can take a few years to reach the Earth. It may fly by the first time and not collide. The flyby will be close. It will return, and almost collide again -- or actually collide."

Splat

The simulations show that a collision is likely.

"Then splat," Belbruno said.

From there, other researchers have detailed how the impacting object turned a chunk of Earth's rock into hot vapor and flung it into space. Some of that orbited the planet and eventually coalesced to form the Moon.

It might be possible to investigate the new piece of the theory by sending a space probe to one of the Lagrangian points.

"Perhaps some type of very small particles may exist at or near L4 or L5 that may be related to the remnants of the Mars sized impactor," Belbruno said. "Although it is not at all clear that even if they were there, they would be directly related, since 4 billion years have gone by. However, it could be intriguing to find out."

The work has been submitted to the Astronomical Journal for review and possible publication.

posted October 01, 2004 12:36 PM              

However isn't that how some say the earth obtained the first signs of life billions of years ago - through these molecules hitting the surface and injecting the first signs of life through carbon-based molecules?

Hmm well now earth has developed it's possibly having an opposite effect - although how can we know fully? Perhaps these deliver new information to earth? - Maybe matter that couldn't attain it's full potential since the earth was underdeveloped in the first stages.. interesting to ponder on.

Hopefully most on the interplantary bombardment will be burnt up in the earth's atmosphere before it reaches us - it's the big suckers that will cause the domino effect on surrounding nature.

Two tons??

Well, have a good weekend. I may not be able to log in for a couple of days, but will check back when I'm on.

K

posted October 01, 2004 02:55 PM              

It's possible we are Martians, yeah. Some unique microbial landing on our planet which with it's life enhancing environment, could have sparked life, or maybe SOME of the life on Earth.

But the 2 tons of Mars debris is only a fragment of what lands on earth.

Here's one explanation -