Earth- Moon System and impacts

[alpha_taur1]

There are numerous theories regarding the Earth-Moon system. I realise that there may not be definitive answers to this question, but here goes: <br /><br />Has the moon on average protected the Earth against impacts, or has it increased the prevalence of impact by effectively attracting more meteoroids and comets?<br /><br />We read of hypotheses that the early earth may have acquired some of its atmosphere through early cometary impacts, and that the increased profile of the Earth-moon system has contributed to a greater atmospheric loading. <br /><br />Conversely other articles speculate that the moon may have (on average) protected the earth by 'catching' some of the meteoroids itself.<br /><br />I can't see how both theories can hold, except perhaps for localised events. Basically it's true that the moon has increased the profile of the Earth, but has also acted as a partial shield. <br /><br />So which effect is the greater? <br /><br />Do the two effects cancel out, so that if you removed the moon from the equation, there would be no overall (average) difference in the number of early impacts on Earth? <br /><br />Where does the incredibly thick atmosphere of Venus fit into all this?<br /><br />Could it be that comets tend to form a tail closer to the sun, and that planets closer to the sun had statistically more of an advantage in capturing some of this tail material?

 

[vogon13]

Probably the definitive answer would be for someone with fast computer to do a Monte Carlo type sim of earth/moon system intercepting a billion impactors from sand grain to Ceres in size, and then do sim again for earth and moon seperately. Compare results and you'll know. I don't know if any one has done this yet.<br /><br />There is a crater chain on the moon (IIRC in Riccioli ?) that if you allow was formed in an event not unlike Shoemaker Levi at Jupiter, then impacting body was probably disrupted by close passage of earth prior to lunar impact. This is just one event, I have no idea on stats for large number of events. <br /><br />Very interesting post, hope someone does sim or provides answer your looking for. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[nexium]

I'm also disillusioned when people with letters after their name make unqualified statements. They should know better. Clearly the moon adds a tiny mass to that of the Earth and increases the effective diameter of Earth by a small amount occasionally. My guess is near misses occur more often because we have a moon, but hits on Earth are scarsely changed at all because the system behaves like a target with a huge hole = the space between the Earth and Moon. <br /> Clearly inbound comets have more material in their tail at Venus distance than at Earth distance from the sun. Outbound comets may have more material in their tail at Earth distance? Mars may sweep up more comet tail material than Venus? The solar wind is blowing the comet mateial outward, and 20%? of the comets that reach Mars orbit turn around before they reach Venus orbit. My guess is less than 1% of the atmosphere of Venus came from comet tails, so we need to look for other reasons why Venus has so much atmosphere compared to Mercury, Earth and Mars. Please comment, refute and/or embelish. Neil

 

[nikshliker]

wouldn't the moon act more attractive than repulsive for a meteor passing by since the meteor will most probably pass by the outside of the earth - moon region. therefore giving more mass to that region? Or are we compareing this to having the moon's mass on the earth ignoring the difference between force due to gravity we would have just one factor... that of the moving system.... plz comment

 

[yevaud]

Lots of reasons, to answer that. For example, during the terminal bombardment phase, there's no reason to assume that Earth received more impacts than Venus did. Earth may be closer to where those bodies were perturbed from, but Venus subtends a small orbit, slightly increasing the possibility of an impact.<br /><br />Second, planets gain a great deal of their initial atmosphere from outgassing during cooling and differentiation, not from during terminal bombardment. What Earth actually gained from this event (approximately 4.5 billion years ago) was free water.<br /><br />Venus, by nature of it's proximity (closer to the Sun), and a weak magnetic field, lost all of it's free H20 due to photodissasociation. This is where heat energy cracked the H20 into oxygen and H. And since it has a sufficiently low magnetic field, the H escaped (as the O2 did later).<br /><br />The smaller the body, the lower the gravity, and the less able the planet is to retain an atmosphere. Mercury is so God-awful close to the sun, and small, that there wasn't a chance of a snowball in hell of it retaining a significant atmosphere.<br /><br />As far as the Earth-Luna system having altered dynamics as far as impacts vs. deflections, that's hard to say. There's plenty of physical evidence on the surface of the moon to show just how many times it has been hit. Earth, with it's constantly evolving surface due to wind, water, plate tectonics, etc., has had 99% + of it's record in that area erased. No way to compare the two.<br /><br />Anyways, isn't it just as likely that given there are two bodies in close proximity, their dual gravitational fields would increase the likelihood, not decrease it? Hard to tell. Vogon's correct...have to a major simulation to figure that out. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>

 

[ehkzu]

So Earth got its free water from its 1AU distance & its mass mainly, huh? And if it hadn't gotten the Moon as well it'd have several miles more depth of water that it now has? Water that I assume would also be a lot less saline.

 

[thalion]

Actually, my understanding is that the hypothesis of the Moon acting as a shield is not probable, for two reasons:<br /><br />1.) The Moon has a much smaller cross-section than Earth.<br />2.) The Earth's increased gravity (and larger cross-section) automatically make it a more likely target for impacts.<br /><br />If the Moon served as any shield, it must have been very early in Earth's history, soon after it was formed. Of course, at that time asteroid bombardment was still intense anyway, so it may have had no real benefit.

 

[yevaud]

Actually, I wasn't looking at it so much as a shield, as a larger dynamical system. When you get down to it, it's not so much as cross-section, as the combined gravity interaction between the Earth and Mood that might increase the probability of "capturing" an object. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>

 

[alpha_taur1]

The other aspect that nobody has mentioned yet (although Thalion hinted at it), is that the moon was once much closer to the Earth than it is now. Here we're talking billions of years ago. <br /><br />If it was closer, it would have also orbited at a faster rate, and acted as a more effective shield. <br /><br />The main emphasis of my original question was from the perspective of the early history of the moon - earth system, because the greatest bombardment period was during that time. <br /><br />I guess that as the moon receded, the overall profile has changed. There is evidence to say that there were many more planetismals at the beginning of its history, but is that true to the same extent for comets?<br /><br />So we're beginning to form a very draft and much more complex picture of an early period where there was perhaps more of a shielding effect against impacts, and a later period where the prevalence of comets was perhaps greater, or at least the ratio of comets to meteorites increased. <br /><br />At the same time, the effect of meteoroids and comets are quite distinct, in that meteoroids have to actually impact to have a consequence, whereas comets have a tail, which can add mass to the atmosphere without impact.