First, attributing actual rarity/probability with how often a model must be worked on is a poor arguement. The system is a bit complicated, and it will take work (and many tries) to get all the details ironed out. One reason it's difficult is we are dealing with one specific situation, possibly one specific collision, and one specific result.<br /><br />The outcomes from the "wrong" models are all possibilities. Earth <i>could</i> have ended up with those moons, if the collision happened that way.<br /><br />A similar situation is ballistics testing, when you want to determine the circumstances for one specific case. You contemplate and model, and try lots of different variations, until you find the one set of circumstances that reproduce the specific case.<br /><br /><br />Now, you could dismiss the impact theory if it's too complicated in general. Say if a straight forward impact couldn't do it, but you needed a high speed retrograde impactor (i.e. head on collision) to do it. The circumstances giving rise to that are hard to come by. But, all we have is an off center, grazing hit from a large planetismal, at a reasonable speed and angle. <br /><br />Now, as far as composition goes: The terrestrial planets do have similar compositions, however they are different enough that you can tell them apart. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>