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#1 2002-02-05 04:25:38
- Shaun Barrett
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- From: Cairns, Queensland, Australia
- Registered: 2001-12-28
- Posts: 2,843
Re: Planetary Rotation - Mars v. Earth
Just after Proto-Earth was hit by that Mars-size (or bigger) object, its rotation period (day) was approximately 5 hours long. Is there any indication of its rotation period prior to the impact?
Tidal forces and transfer of angular momentum have slowed Earth's rotation and pushed the moon further away; hence our current 24 hour day. But Mars has never had (as far as we know) a moon of sufficient mass to influence its rotation. Does this mean Mars has always had a day of 24 hours, 39 minutes, and 23 seconds?
In addition to these questions, how about a metaphysical one! : Is it not remarkably "spooky" that our next-door planet has practically the same length of day as we do, after 4.6 billion years of totally independent and profoundly different history?!
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The word 'aerobics' came about when the gym instructors got together and said: If we're going to charge $10 an hour, we can't call it Jumping Up and Down. - Rita Rudner
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#2 2002-03-06 20:44:22
- Shaun Barrett
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- From: Cairns, Queensland, Australia
- Registered: 2001-12-28
- Posts: 2,843
Re: Planetary Rotation - Mars v. Earth
Oops!! Looks like I've either asked impossible questions or questions so tedious I've bored everybody into a torpor! ... Sorry.
Incidentally, for those who've fought off terminal boredom and are still awake, if Mars maintains its present rotation rate, an Earth day will have lengthened to equal a Mars day in about 120 million years.
If we postpone colonisation until then, we won't need to bother with changing our watches! And our body-clocks will be in perfect sync.! (Oh ... and our politicians may be approaching a consensus on approving a manned mission by then!! )
The word 'aerobics' came about when the gym instructors got together and said: If we're going to charge $10 an hour, we can't call it Jumping Up and Down. - Rita Rudner
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#3 2002-05-03 23:16:23
- Shaun Barrett
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- From: Cairns, Queensland, Australia
- Registered: 2001-12-28
- Posts: 2,843
Re: Planetary Rotation - Mars v. Earth
Well, since it looks like I've got this topic all to myself anyway, I may as well carry on and enjoy it!
I was gazing idly at my super-duper, highly accurate, MOLA-generated, full-colour, topographically-accurate, "Sky and Telescope" globe of Mars just yesterday, when I happened to find myself contemplating the Hellas impact basin (as one does! ).
It struck me that the basin is not round but elongated somewhat in a roughly east-west direction. Now, I remembered reading somewhere that oblique impacts, even down to 15 degrees from horizontal, still cause circular craters. So it occurred to me that what caused the Hellas basin must have struck Mars at a very low angle, thus converting a lot of its kinetic energy into a change in Mars' angular momentum. Now, I don't know enough about oblique impacts to say which direction, east or west, the impactor arrived from, though my guess from the shape of Hellas is that it came from the East.
If any of this is actually so, then can we deduce that before the Hellas event, Mars was rotating faster and was slowed by the impact? Let's assume the impactor was about 200kms across and travelling at 20kms per second. Let's also assume that its density was similar to that of Mars today. I bet someone out there, like say RobS, might be able to do a rough calculation and work out the length of a Martian day before Hellas formed(? )!!
But then if I'm the only one who ever bothers about this topic, why would anyone care .... sob, sob!!!
The word 'aerobics' came about when the gym instructors got together and said: If we're going to charge $10 an hour, we can't call it Jumping Up and Down. - Rita Rudner
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#4 2002-05-05 20:24:36
- Shaun Barrett
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- From: Cairns, Queensland, Australia
- Registered: 2001-12-28
- Posts: 2,843
Re: Planetary Rotation - Mars v. Earth
Thank you Orodromeus for your interest in this topic!
By way of reply to your comments, I don't think anyone will argue with you over the formation of Argyre; I think most people are agreed that a large impactor caused it.
Again, I'm sure you are correct in saying that the KT impact would have been of insufficient power to have any measurable effect on Earth's rotation.
But the Hellas basin is one of the largest planetary scars we know of and must have delivered a substantial quantity of energy; perhaps enough to influence Mars' rotation rate(? ), especially if it struck at a low angle.
Any Planetary Science graduates out there who can throw in some informed opinions?
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The word 'aerobics' came about when the gym instructors got together and said: If we're going to charge $10 an hour, we can't call it Jumping Up and Down. - Rita Rudner
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#5 2002-05-07 16:34:09
- Canth
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- Registered: 2002-04-21
- Posts: 126
Re: Planetary Rotation - Mars v. Earth
I think maybe the lack of replies is due to people being unable to answer your questions. Not that I am accusing you of anything, I think your questions are great. Who knows maybe there is not any answer to this question to date, I would think that you would have to know a great deal about the impact history of a planet to see how much effect anyone impact had. If you don't know all the impacts a planet has taken and how it started off it would be hard to predict the change in speed. The force of impact shouldn't be nearly so hard, although you must take into account the mass of the objest, its speed, and the mass of the object it is hitting. Good question!
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#6 2002-05-08 05:11:50
- Christina
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- From: UK
- Registered: 2002-05-07
- Posts: 59
Re: Planetary Rotation - Mars v. Earth
Would the angle of impact make a difference too? I mean if the impactor came in with or against the direction of rotation?
[i]the early bird may get the worm, but it's the second mouse that gets the cheese[/i]
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#7 2002-05-08 07:42:03
- Shaun Barrett
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- From: Cairns, Queensland, Australia
- Registered: 2001-12-28
- Posts: 2,843
Re: Planetary Rotation - Mars v. Earth
Yes, Christina, I'm assuming that the direction would make a difference.
I assume, also, that most of the other, smaller impacts all over the planet, coming from every possible direction, would have cancelled one another out when it came to affecting Mars' angular momentum.
But Hellas is just so big, and the impact evidently quite oblique, that I wonder whether it may have had a significant effect on the length of a Martian day.
For all I know, much of the kinetic energy might have been dispersed in the spray of vapourised impactor and Martian crust that must have occurred. Perhaps it is more difficult than I imagine to alter a planet's speed of rotation. I just don't know .... and that's why I'm asking!!
And maybe you're right, Canth. Maybe there are too many variables and nobody knows the answer! But with all those variables and the vast expanse of time over which they've operated, it still seems uncanny to me that Mars and Earth should have such a similar length of day. If you believed in fate, you'd almost have to say it was meant to be .... as if providence is saying: "O.K. You need a challenge? I'll give you a challenge! Your next-door planet is far away, but not too far. It's smaller and colder and drier, but not so small, cold, and dry that you couldn't fix it ... if you tried hard enough. And at least its day is about the right length. So go ahead, make it a new Earth; turn it into a second home!"
I bet this is getting way too schmaltzy for some of you hard-headed realists out there! But still, a man can dream, can't he?
The word 'aerobics' came about when the gym instructors got together and said: If we're going to charge $10 an hour, we can't call it Jumping Up and Down. - Rita Rudner
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