lpetrich

According to this article and this article, there is an interesting new theory for a curious phenomenon in the early history of the Solar System, the Late Heavy Bombardment.

That is a curious peak in the Moon's bombardment rate that had happened 3.9 billion years ago -- 600 million years after the Solar System's origin.

So what provoked the coming of that swarm of big rocks?

According to John Chambers and Jack Lissauer, there had once been a fifth terrestrial planet, one that had orbited a little beyond Mars's orbit. Being a little beyond make its orbit suffer dynamical chaos over timescales of a few million years ago, and after about 600 million years, it had a near-collision with Mars and got into a much more eccentric orbit whose aphelion extended into the Asteroid Belt.

This caused a lot of asteroids to be deflected into orbits which took them into the inner Solar System, where they proceeded to strike the Earth, the Moon, and Mars, with a few of them striking Mercury.

Thus producing the Moon's Late Heavy Bombardment.

And what happened to this planet afterwards? A likely fate is colliding with some other Solar-System object, most likely Venus or Jupiter.

Finally, I consider the question of what to call this object, if it had existed. I propose Eris, after the Greek goddess of discord. Though Loki, Norse god of mischief, might be a good name. And there is another Eris connection. According to the story, she was not invited to the wedding of Peleus and Thetis, so she took one of the Golden Apples of the Hesperides, wrote "For the fairest" on it, and threw it into the wedding party.

Hera, Athena, and Aphrodite bickered among themselves about who would get this title, and Zeus decided to settle it by having a shepherd named Paris decide. Hera offered him the rule of much of the known world, Athena offered him great military prowess, and Aphrodite the love of the most beautiful woman in the world, a certain Helen. Paris chose that love, but that pissed off Helen's family, causing the Trojan War.

So Eris might be a good name for a planet which once did some "throwing", also with devastating effects.

fando

Eris is excellent!

It would be awesome if we could see the fireworks. Remember Shoemaker-Levy 9? Imagine if a Mars-sized object struck instead. Maybe we'll see some if our telescopes can resolve such activity around other proto solarsystems.

wordsmyth

I've heard that some astronomers believe Mars may have had an atmosphere at one time. Do you suppose its possible that this "near-collision" could have something to do with what happened to that atmosphere.

Also, aren't all the planets named after Roman gods rather than Norse? How about the name "Ares" for this missing planet??

Jayjay

Ares is Greek, and his Roman name is Mars. Anyway, is "Eris" used by Romans? All the other planets have Roman and not Greek names (don't they?) so it stands to reason that the "fifth planet" should also have a Roman name.

Zadok001

"...Mars may have had an atmosphere at one time..."

It doesn't have an atmosphere now? Eh?

wordsmyth

What was I thinking? Yes, you are quite correct, Ares is Greek not Roman. I found this site that lists the names of Greek gods and their corresponding Roman counterparts...

http://www.messagenet.com/myths/names.html

I sort of like the name Discordia which appears to be the Roman equivalent of Eris.

wordsmyth

If Mars has an atmosphere at all, then I have been sorely misinformed, however my point is that some scientists believe it may at one time have had an atmosphere comparable to Earth's and therefore might have had the capacity to support life.

wade-w

You have been sorely misinformed.

Abacus

Mars has an atmosphere. It even occasionally has clouds and global dust storms. IIRC, the atmospheric pressure on its surface is only about 1% the pressure on the Earth's surface, so it is very thin.

Valentine Pontifex

As has already been pointed out, Mars does have an atmosphere though one that is much thinner than the one our planet has.

I might also point out the reason why Mars does not have thickness of atmosphere Earth has is very simple. Mars has a mass only 10.7% as large as the Earth. It lacks the gravity to maintain an atmosphere like Earth has over deep geologic time.