Why Venus, Mars, Mercury no big moons?

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ehkzu

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So Earth's impactor coalesced in a Lagrangian point, wandered over & whacked us, made a moon. Why didn't this happen with any other rocky planet in our solar system?<br /><br />2nd related question: I heard if two moons had formed instead of our one, they'd have lasted no more than 10K years before whacking each other. Izzat so?<br /><br />3rd related question: If a bigger or smaller moon had formed, how would this have affected Earth? Especially, how small a moon would still stabilize Earth's axial tilt? And if the Moon were the size of Titan, would that have locked Earth and it gravitationally by now? (i.e. one side of each always facing one side of the other, with a correspondingly slower Earth rotation--maybe 40 hr. day? Just to pull a number out of the hat. And would that longer day and higher/lower tides affect climate to make it harsher--hotter/colder? Make the equator terrestrially uninhabitable?
 
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tony873004

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Mercury and Venus are less massive than the Earth and they are closer to the Sun. So their Hill Spheres will be much smaller. This may contribute to why they have no moons. Mars is only 10% as massive as the Earth. That probably hurts it quite a bit.<br />
 
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alokmohan

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Thereare many theories on formation of moon .Impact theory is only one of them .Some say moon was hurled out of earth.Again ther is talk of initially we had five or six moonsOrigin of moon is debatable point.But most of our astronomyhas been seeing the moon ,itsphases.If there was no moon things in sky might not have attracted us so much.As we are not so sure of the solar system and formation of planets,theses will require time to explain.Who knows mercury or venus may have moon not yet discovered.We knew KBOs only the other day.
 
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vogon13

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Also had wondered why in some regards, Jupiter, Saturn, Uranus, and Neptune moon systems have similarities, like<br /><br />*rings of varying granduer<br /><br />*inner irregular small satellites (like Amalthea, Pan, etc.)<br /><br />*'regular' satellite zone <br /><br />* outer captured asteroidal (presumably) moons<br /><br />and differences, like<br /><br />*why did Jupiter get 4 big ones, Saturn one, Uranus none and Neptune has a big one in a freak retrograde orbit<br /><br />*Jupiter, Saturn, and Neptune have fairly similar ring systems, and then there's Saturn<br /><br />*Saturn, and Uranus get small size inner satellites like Ariel and Enceladus, Jupiter none and did Triton 'mess up' Neptune"s?<br /><br />*Iapetus has so many weird characteristics as to put it in a class by itself<br /><br />Sorry if this sounds so far off topic, but wouldn't you have suspected maybe some trending characteristics of satellites of outer solar system to be carryied over to inner solar system? Earth's moon always seemed freakishly out of place till Charon turned up. Mars has two close inner satellites I strongly suspect would look like outer asteroidal satellites of Jupiter, Saturn, Uranus, and Neptune, but what are they doing there? <br />Recall Comet Kohoutek was believed to have enormous nucleus (for a comet). Over billions of years would impacts of this class of object have demolished any possible satellite of Mercury? (comets going as fast as they ever go when they do perihelion in Mercury's vicinity )<br /><br />We probably aren't going to learn the 'big picture' about satellites until we receive our copy of Encyclopedia Galactica or write our own version. <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>
 
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newtonian

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ehkzu - Interesting questions.<br /><br />I will research and get back to you on most of that.<br /><br />However, on the effect on climate of a longer day because of slower rotation - I'll start with your 40 hour suggestion:<br /><br />1. Higher day temperatures because of longer day. This would be especially severe in continental climates.<br /><br />In maritime climates it would be more tolerable as powerful, possibly violent, sea breezes moderated the temperatures.<br /><br />In areas where the temperature normally varies by 24 degrees between the morning low and the afternoon high, now the variance would be 40 degrees.<br /><br />So, in the spring when the mean (=average) temperature is 55 degrees and the normal high is 67 and the normal low is 43, now the high would be 75 and the low would be 35.<br /><br />The serious problem with this is that in exposed areas on clear nights 35 degrees causes frost.<br /><br />In other words, this would shorten the growing season for tender crops damaged by frost.<br />In areas with high humidity, like here is south Louisiana, very thick fog would form in mornings- much thicker than normal now.<br /><br />In the summer, the days would become intolerably hot more often.<br /><br />In the winter it would make more of a difference if a storm hit during the day or at night.<br /><br />There would be more solar energy for daily thunderstorms to form, and enough time for daughter storms to for on outflow boundaries.<br /><br />2. The coriolis effect would be weaker. <br /><br />Now, if we had Mercury's rotation speed of 58.7 days we would be in real trouble. Ditto Venus' rotation period of 243 days!<br /><br />Oceans would boil during the day, and the tropics would freeze at night!<br />
 
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alokmohan

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Three related questions has been replied seperately.I hope we pass the quiz .Wanting encyclopedia galactica.
 
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newtonian

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alokmohan - Yes, your post was excellent (good) - notably the different theories as to how our moon was formed.<br /><br />To me it gives evidence of a Creator, i.e. intelligent design.<br /><br />It is just the right mass to stabilize earth's axis tilt and produce beneficial tides and stabilize earth's rotation speed (which recently changed due to the Tsunami).<br /><br />And it is just the right size and distance from earth to produce rare and beautiful solar ecclipses.<br /><br />It is incredibly highly unlikely we would end up with things so perfect for happy human habitation of earth.<br /><br />I doubt it was just a chance impact that gave us our beautiful moon.
 
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vogon13

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Check out the "How rare is a life bearing universe" thread. <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>
 
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ehkzu

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Hmm. Couldn't find the thread. I have read the exceedingly depressing Rare Earth book & find myself agreeing with it pretty much. I also posted a question to the Scientific American website asking what Earth would be like today if we'd never gotten the Moon.
 
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mikeemmert

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Hmm...just found this thread. Really interesting. (Bump)<blockquote><font class="small">In reply to:</font><hr /><p>So Earth's impactor coalesced in a Lagrangian point, wandered over & whacked us, made a moon. Why didn't this happen with any other rocky planet in our solar system?<p><hr /></p></p></blockquote>Who says it didn't? As a matter of fact, where do you think the Caloris Basin impact crater came from? This enormous crater is 1300 km (810 miles) in diameter.<br /><br />Venus has no markings like that. Why not? Maybe because lava is known to have flooded the entire surface of the planet at one point, erasing everything.<br /><br />Mars has the enormous Hellas Basin, 2100 km in diameter. That's a quite a bit larger than most craters.
 
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qso1

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Pure chance whatever the theory or cause...look at it this way. Mars has two moons, why do Earth, Venus, and Mercury not have multiple moons? <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
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qso1

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Sounds good to me and yes, Mars would have a better chance of capturing asteroids as both Phobos and Deimos look similar to asteroid images I have seen (Gaspra and Eros) as opposed to the menacing asteroid of the movie Armageddon. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
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willpittenger

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One thing I should note: I once heard someone state that "Moons are satellites, but the Moon is not a moon." Both Earth and Pluto are considered double planets.<br /><br />Now if there is a professional astronomer that wants to get their two cents in, now is the time to do so. <div class="Discussion_UserSignature"> <hr style="margin-top:0.5em;margin-bottom:0.5em" />Will Pittenger<hr style="margin-top:0.5em;margin-bottom:0.5em" />Add this user box to your Wikipedia User Page to show your support for the SDC forums: <div style="margin-left:1em">{{User:Will Pittenger/User Boxes/Space.com Account}}</div> </div>
 
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qso1

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First off, I'm not a professional astronomer but I thought I'd take a shot at this and if a professional astronomer posts, they can shot me down. A body in orbit around another planetary body is considered a satellite while planetary bodies orbiting stars are planets. Other objects such as asteroids etc. are bodies orbiting the sun but their particular characteristics do not make them planets by current definition.<br /><br />Back to moons. Except when one considers such systems as Earth and Pluto, double planets. A satellite in orbit around a planet could theoretically be larger than the host planet but still be regarded as a satellite. This has not been put to an actual test for obvious reasons. Such an arrangement may not even be possible because the smaller body tends to orbit the larger one so far as we have seen. Smaller stars orbit larger ones in multiple star systems. A theoretical double planet system in which two planets of the same size are present, they would be orbiting a common barycenter which technically, all bodies do if the smaller body is nearly the size of the larger.<br /><br />It all boils down to what the astronomical community decides to call objects in space. A matter of semantics as it were. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
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vogon13

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Good distinction.<br /><br />Had not thought of it that way, but a very good place to draw the line, I think.<br /><br /><br /><br /><br /> <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>
 
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qso1

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Sounds good to me. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>
 
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lsbd

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As to why the Earth has such a big honking moon instead of several little ones, no one knows for sure....that's just the way things turned during the accretion of the solar system. <br /><br />read genesis, something about God creating two lights, one to rule the day (sun) the other to rule the night (moon) ? <img src="/images/icons/wink.gif" />
 
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