Why do Mercury and Venus don't have moons?

[kucharek]

My little son Lukas just asked me this. As there are plenty of moons in the solar system, and even asteroids have moons, it seems more to be the rule than the exception for celestial bodies to have companions.
Why not Mercury and Venus? To deep inside Sun's gravitational funnel?

Harald

[Vermonter]

'Tis possible. Perhaps just not enough material for a native satellite? It'd be hard for them to snag an incoming satellite, due to Sol's gravity. Also location would have a lot to do with it, I would think.

[Anonymous]

kucharek.


“Why not Mercury and Venus? To deep inside Sun's gravitational funnel?”


I think it has more to do with the greatly increased temperature so close to the Sun. Plus the fact that when our Sun first began to “burn”, it did so much more intensely than today. For a few million years, anyway. This may have been enough by itself to prevent satellite formation.


Another possible reason they have no moons could be that Mercury and Venus do not possess enough mass. Perhaps there is a “minimum mass requirement” in order to build moons. Most of the moons of our solar system are half rock-half ice. But ice could not exist inside the orbit of Mars. That cuts by almost half the potential building material to make a moon.


The reason you suggested (deep inside Sun's gravitational funnel) might also have been important simply because objects close to the Sun are nearly always traveling very fast. Mercury’s battered surface does indicate that the inner solar system was rather violent. Perhaps too violent for a moon to survive. That Earth has a moon may be a rare and freakish exception.

[nebularain]

Keep in mind that the moons of Mars are (most likely?) captured asteroids. Earth's moon, it is believed, resulted from a "Mars' size" body impacting Earth during its protoplanet stage, and the ejected material came together to form the moon.

For all intents and puroses, really, "where moons come from" is still an unsolved mystery; we just have scenarios that are more likely than the others that we can conceive of.

[Kaptain K]

A better question would be "why does the Earth have a moon?" and such a freakishly large one at that, when none of the other inner planets have any (discounting the moons of Mars, which are probably captured asteroids).
The major moons of the gas giants were probably formed the same way planets formed around the Sun. The minor moons of the gas giants are probably captured asteroids and KBOs.
I suspect that when we have more data, we will find that moonlessness is the norm for rocky inner planets.

_________________
"There's a whole lotta things I've never done, but I ain't never had too much fun."
Commander Cody and the Lost Planet Airmen

<font size=-1>[ This Message was edited by: Kaptain K on 2003-03-08 14:13 ]</font>

[ToSeek]

On 2003-03-08 14:12, Kaptain K wrote:
A better question would be "why does the Earth have a moon?" and such a freakishly large one at that, when none of the other inner planets have any (discounting the moons of Mars, which are probably captured asteroids).

I'm inclined to believe that it's an anthropic principle issue (as claimed by the Rare Earth folks): if Earth didn't have such a large moon, we wouldn't be here.

[xriso]

Even more interesting ... are there any second-order moons? i.e. a moon of a moon? This would seem difficult for the gas giant moons - their orbits seem packed too tight for it. Earth's moon is probably too close to have stable satellites.

[The Shade]

On 2003-03-08 10:57, Heathen wrote:

I think it has more to do with the greatly increased temperature so close to the Sun. Plus the fact that when our Sun first began to “burn”, it did so much more intensely than today. For a few million years, anyway. This may have been enough by itself to prevent satellite formation.

Actually, the sun was 30% dimmer in the past, not brighter. It's early heat and solar wind removed volatile chemicals from the inner part of the solar system. Heavier elements that compose the bulk of terrestrial planets were less affected by the sun's solar wind and stayed pretty much where they were.

Kaptain K is right. Earth has a large moon because of a "cosmic" accident. So, it's not the other planets that are feaks, it's us. [img]/phpBB/images/smiles/icon_biggrin.gif[/img]

[Kaptain K]

Actually, the sun was 30% dimmer in the past, not brighter.

True, but! In the very early stages of a main sequence star's life, it is unstable. So, while it is on average dimmer, it is quite variable and between the variations in luminosity and the strong stellar wind, the inner system is not hospitable to light, volatile molecules.

[Anonymous]

The Shade.


“Actually, the sun was 30% dimmer in the past, not brighter.”


Apparently, you’re referring to “the faint young Sun”, a stage reached only after settling onto the main-sequence. This stage follows the T-Tauri stage of stellar evolution. Perhaps I should have wrote “upon initiation of T-Tauri winds” instead of “when our sun first began to “burn”.


During the T-Tauri stage of our Sun, the output in UV and other wavelengths was far more intense than we see today.


From the following link;

“They are Pre-Main Sequence Stars on the HR Diagram. They have temperatures and masses similar to the Sun, but they are much brighter”

“They can reach emission levels 1000 times that of our own sun. The stellar (solar) winds that are produced from these emissions are generally quite strong.”

http://www.lfc.edu/~petermj/astro/ttauri.html


“Heavier elements that compose the bulk of terrestrial planets were less affected by the sun's solar wind and stayed pretty much where they were.”

I agree with this statement with one provision; dust which had already been incorporated into proto-planets was somewhat unaffected but any remaining fine particulates were blown out of the inner system.


“Earth has a large moon because of a "cosmic" accident. So, it's not the other planets that are feaks, it's us.”


I agree. From my above post;

“That Earth has a moon may be a rare and freakish exception.”

[The Shade]

D'oh! Oh well, win some, lose some. [img]/phpBB/images/smiles/icon_wink.gif[/img]