Why are the earth and martian days so similar

[Zo0tie]

This question ties in to the hole question of life in the universe since having too long or too short a day can affect the chances of complex life to evolve and prosper. Why do the two most similar terrestrial planets in the solar system have rotational periods off by only 3%. Yes I know Venus is slower but there are theories to explain that.

Given the range possible, coincidence seems to be a weak argument. Is there some kind of resonance going on. Or perhaps a cataclysmic event in the past that ties them together. Perhaps there is some law of planetary formation that says planets within the habitable zone must have rotation periods of a specific narrow range. Does anyone have an answer?

[NEOWatcher]

Given the range possible, coincidence seems to be a weak argument.

Yet; what would make in not plausible?

Is there some kind of resonance going on.

Actually; the planets started off vastly different. The Earth was much faster, and the moon has slowed it down considerably. The coincidence is that we exist at a time when the rotations are similar, not that the rotations are similar.

[swampyankee]

Coincidence. It works for me.

What I find interesting is that many people have posited that the rate of rotation of the Earth was much, much faster at one time, and only slowed by the Moon. While this may be completely true, it does lead one to ask the question "why does Venus rotate so slowly?"

[KABOOM]

This question ties in to the hole question of life in the universe since having too long or too short a day can affect the chances of complex life to evolve and prosper. Why do the two most similar terrestrial planets in the solar system have rotational periods off by only 3%. Yes I know Venus is slower but there are theories to explain that.

Given the range possible, coincidence seems to be a weak argument. Is there some kind of resonance going on. Or perhaps a cataclysmic event in the past that ties them together. Perhaps there is some law of planetary formation that says planets within the habitable zone must have rotation periods of a specific narrow range. Does anyone have an answer?

Please elaborate as to why complex life will only "evolve and prosper" within a day length at or around 24 hours.

Thank you

[Rhaedas]

We only have a few examples so far of terrestrial sized planets, and only a single example of one harboring life. It's a bit too small of a sample to base anything on other than coincidence.

Our rotation closely matches Mars the same reason the Moon happens to match the Sun's size in the sky for eclipses, or that the North pole points close to the North star. Timing and coincidence. If they weren't closely matched, we wouldn't notice them.

[WayneFrancis]

Coincidence. It works for me.

What I find interesting is that many people have posited that the rate of rotation of the Earth was much, much faster at one time, and only slowed by the Moon. While this may be completely true, it does lead one to ask the question "why does Venus rotate so slowly?"

I suspect that what ever knocked Venus up side down removed most of Venus's rotational velocity.

[WayneFrancis]

We only have a few examples so far of terrestrial sized planets, and only a single example of one harboring life. It's a bit too small of a sample to base anything on other than coincidence.

Our rotation closely matches Mars the same reason the Moon happens to match the Sun's size in the sky for eclipses, or that the North pole points close to the North star. Timing and coincidence. If they weren't closely matched, we wouldn't notice them.

Yup, same reason why many people think street lights flicker because they are walking past them.

[Githyanki]

I suspect that what ever knocked Venus up side down removed most of Venus's rotational velocity.

I too suspect Venus had an impact similar to Earth's Moon impact, but whatever it was, it was larger and hit at a different angle, resulting in the Venus we have today.

[Romanus]

Can't add much, except that--as already said--it's coincidence. Note that a billion years ago their days weren't similar at all, and that in the future (all things being equal) they will go through a temporary time when their days are *truly* equal, due to the steady decrease in the Earth's rotational speed.

[Trakar]

Coincidence. It works for me.

What I find interesting is that many people have posited that the rate of rotation of the Earth was much, much faster at one time, and only slowed by the Moon. While this may be completely true, it does lead one to ask the question "why does Venus rotate so slowly?"

Same reason, massive impact as part of late accretion process. Planetary rotation is the net result of accretion processes.

[grapes]

Same reason, massive impact as part of late accretion process.

Why does it have to be the late accretion process?

[Trakar]

Why does it have to be the late accretion process?

Because, if the massive impact happened early in the accretion process, later additions would have most likely contributed additional net rotational values...or am I missing a smilie angle to this twist?

[grapes]

No smilie, I just never understood why there was a problem. If you have a belt of material in orbit around a central body, and it accretes on one edge of the belt, it has to rotate one way to preserve angular momentum--if it accretes on the other edge, it has to rotate the other way. Unless I'm missing something, and I can already see the possibility of a half dozen.

[Matej Velko]

I don't get why do planets rotate at all... Does anybody know is there a planet in the known universe that does not rotate?

[Matej Velko]

...and I somewhere heard that Venus and Uranus rotate backwards in relation to other planets. Does anybody know why?

[WayneFrancis]

I don't get why do planets rotate at all... Does anybody know is there a planet in the known universe that does not rotate?

They rotate because they preserve the total angular momentum of the material that created them. As smaller matter falls together it tends to spiral inward. This angular momentum has to be conserved. It is just Newtonian mechanics at play.

...and I somewhere heard that Venus and Uranus rotate backwards in relation to other planets. Does anybody know why?

Uranus is not backwards so much as just flipped 97.77 degrees. Venus has been complete flipped both slowing down its rotation and reversing it. Both of these where probably from very large impacts early on in the planets lifetime.

[astromark]

The gravity effect upon the mass of the stellar disk draws the disk together. Rotational energy is always apparent.

The chaotic early solar system contained more than we now see..

collisions and expulsions determined the rate of spin of the Planets. Four and a half billion years of shuffling the mix...

and NO we do not 'know' what happened to Venus and Uranus..

other than the educated guess that they have been in collisions that have determined there now odd rotations..

These are big questions that have many complex issues.. ' Why does Earth and Mars Day seem so much the same.. ?

Because the content mater had rotational velocity that determined that rate of spin., oh and its just the way it is.

[Matej Velko]

Venus has been complete flipped both slowing down its rotation and reversing it.

So you are telling me that neither Venus is rotating backwards in relation to other planets it's just flipped over but it still rotates in the same direction like the other planets.

[Trakar]

No smilie, I just never understood why there was a problem. If you have a belt of material in orbit around a central body, and it accretes on one edge of the belt, it has to rotate one way to preserve angular momentum--if it accretes on the other edge, it has to rotate the other way. Unless I'm missing something, and I can already see the possibility of a half dozen.

No real issue, I was just curious about the phrasing. In general, accretion of a large planetary body generates considerable net angular momentum. It is the planets that lack such momentum or display other than expected rotational range values that require explanation. Looking at first level accretional processes alone we might expect all of the inner planets to have quite fast spins, more in line (proportionally) with the mid-system planets. Mercury's large core and slow spin speak to it's probably having been involved in some collisional process that liberated much of its crust/upper mantle, but it is a smaller planet and close enough to the Sun for it to have other potential explanations for its near orbital lock with the Sun. Venus' slow rate and reverse spin speak to a late formation major impact explanation. In fact with all of the inner planets, it looks like late accretion major impacts are more the rule than the outlier condition, in the case of the Earth our moon is strong supporting evidence and in the case of Mars, the Tharsis bulge provides similar evidentiary support.

[grapes]

Looking at first level accretional processes alone we might expect all of the inner planets to have quite fast spins, more in line (proportionally) with the mid-system planets.

Why? I don't understand that.

[Trakar]

Why? I don't understand that.

I'd be happy to discuss and explain in more detail, but first, perhaps a little background information, which may help to provide a format for that potential discussion.

A nice generalized explanation - "Planet formation" - http://articles.adsabs.harvard.edu//...00129.000.html

Energy Loss and Sticking Mechanisms in Particle Aggregation in Planetesimal Formation - http://physics.ucsc.edu/~bridges/pap...us_123_422.pdf

FORMATION OF TERRESTRIAL PLANETS FROM PROTOPLANETS. II. STATISTICS OF PLANETARY SPIN - http://iopscience.iop.org/0004-637X/...671_2_2082.pdf

Formation of Terrestrial Planets from Protoplanets under a
Realistic Accretion Condition - http://arxiv.org/PS_cache/arxiv/pdf/...003.4384v1.pdf

Planet formation: Statistics of spin rates and obliquities of
extrasolar planets - http://arxiv.org/PS_cache/arxiv/pdf/...004.1406v1.pdf

Not comprehensive by any means but should fill in most of the blanks and lay the ground-work for a solid discussion.

[grapes]

I'd be happy to discuss and explain in more detail, but first, perhaps a little background information, which may help to provide a format for that potential discussion.

A nice generalized explanation - "Planet formation" - http://articles.adsabs.harvard.edu//...00129.000.html

Finally found the pages of discussion on Planetary Rotation on page 158, having browsed through the preceding pages. I don't see anything there that disagrees with me, so I feel better.

I skipped the second one, since I didn't think Sticking Mechanisms were what I was looking for. The third one, in abstract, seemed to agree with me, and the fourth one of the same authors, repeating the results. The last one, in the abstract, says " The distribution of obliquities was found to be isotropic, which means that planets can rotate in direct or indirect sense, regardless of their mass. Our results regarding the primordial rotation periods show that they are dependent on the region where the embryo was formed and evolved." That's pretty much the point I was making.

Not comprehensive by any means but should fill in most of the blanks and lay the ground-work for a solid discussion.

Based upon those references, I conclude that you're just wrong.

OTOH, I probably missed something along the way, but hey, the first article alone was 45 pages! Any help would be appreciated.

[WayneFrancis]

So you are telling me that neither Venus is rotating backwards in relation to other planets it's just flipped over but it still rotates in the same direction like the other planets.

I don't think I said that.

If you look down at the solar system from above, the north pole of the sun, then all the planets rotate in a counter clockwise manner besides Venus and Uranus. Venus rotates in a clock wise manner but given what we know the most like answers are that it had a slow rotation to begin with and tidal effects reversed it or it had a very large impact early on that changed it.

Uranus rotation is much harder to describe via our normal planetary formation models but the impact hypothesis still has big open questions tied to it.

[Jeff Root]

I haven't looked at Trakar's links yet, but a few years ago I figured it
this way:

A portion of a giant molecular cloud collapses from the combination
of gravity, virialization, radiative cooling, collisions between particles,
and some of those colliding particles sticking together. It collapses
into a disk, with the material in very nearly circular orbits.

The orientation of the disk is random, but the distribution of matter
within the disk is determined by the cloud's net angular momentum.
The greater the net angular momentum, and the more closely the
disk happens to align with the direction of net rotation, the more
material tends to end up orbiting far from the central star.

Dust particles tend to clump, forming planetesimals. Each particle
and each clump is in a nearly circular orbit-- but not exactly circular.
When a particle orbiting just outside the orbit of a planetesimal
reaches perihelion, it can collide with the planetesimal. At that
point in its orbit it is moving faster than the planetesimal. It will
most likely hit the night side of the planetesimal, speeding up the
rotation of the planetesimal in the same direction as they orbit.

When a particle orbiting just inside the orbit of the planetesimal
reaches aphelion, it can collide with the planetesimal. At that point
in its orbit it is moving slower than the planetesimal. It will most
likely hit the day side of the planetesimal, again speeding up the
rotation in the same direction that they orbit.

Collisions are forward on the night side; backward on the day side.

Later on, after the dust has cleared, there will still be ocassional
collisions between planets and planetesimals, but in that case
the planetesimals can have had their orbits changed radically from
circular, so there is no greater likelyhood that they will hit one side
of the planet than the other. So a colliision can speed up rotation,
slow it, or change its direction.

-- Jeff, in Minneapolis

[grapes]

Each particle
and each clump is in a nearly circular orbit-- but not exactly circular.
When a particle orbiting just outside the orbit of a planetesimal
reaches perihelion, it can collide with the planetesimal. At that
point in its orbit it is moving faster than the planetesimal. It will
most likely hit the night side of the planetesimal, speeding up the
rotation of the planetesimal in the same direction as they orbit.

When a particle orbiting just inside the orbit of the planetesimal
reaches aphelion, it can collide with the planetesimal. At that point
in its orbit it is moving slower than the planetesimal. It will most
likely hit the day side of the planetesimal, again speeding up the
rotation in the same direction that they orbit.

Collisions are forward on the night side; backward on the day side.

I dunno. From the description, it seems like you are describing a continuous range of particles--those "just inside" to those "just outside". But you say the ones just inside are at aphelion when they reach the circular orbit, so their perihelion is significantly less than the circular orbit, and their semi-major axis is also significantly less than the circular radiius. Conversely, the ones just outside are at perihelion when they reach the circular orbit, so their aphelion is significantly greater, making their semi-major axis significantly greater than the circular radius. There is a wide range of possibilities in between, and I think that is what affects the distributions given in those papers that Trakar refers to.

[Jeff Root]

What do you mean by "possibilities in between" ?

What do you mean by "significantly" ?

There are basically two groups of particles colliding with the
planetesimal: Those in outside orbits and those in inside orbits.
Those in outside orbits can only collide at perihelion, while
those in inside orbits can only collide at aphelion. There will
be some particles in planetesimal-crossing orbits, but they
would be relatively few.

-- Jeff, in Minneapolis

[grapes]

What do you mean by "possibilities in between" ?

Objects with semi-major axes that lie between those two values.

What do you mean by "significantly" ?

Whatever amount would be involved in your distinction of outside or inside. For instance, you have an object orbiting inside the circular orbit colliding at aphelion on the dayside of the planetesimal--increase its semi-major axis just a little bit (without moving its perihelion) and it collides on the nightside, no?

[Jeff Root]

I'm still not sure what you mean. I'll guess that you just mean the
range of possible orbits which intersect the orbit of the planetesimal,
including orbits which cross the planetesimal's orbit.

For instance, you have an object orbiting inside the circular orbit
colliding at aphelion on the dayside of the planetesimal--increase its
semi-major axis just a little bit (without moving its perihelion) and it
collides on the nightside, no?

Yes. I do understand your example!

At any given time, we have a planetesimal in nearly circular orbit
(composed of particles gathered from that orbit and nearby orbits),
particles in nearly circular orbits outside that orbit, and particles in
nearly circular orbits inside that orbit. The orbits are perturbed all
the time. Smaller perturbations happen far more often than larger
perturbations. A perturbation large enough to send a particle from
an inside orbit to the far (night) side of the planetesimal (or farther)
means the particle can hit the planetesimal anywhere. A smaller
perturbation means the particle either fails to hit the planetesimal
or hits it on the day side. So there is a net surplus of particles in
inner orbits hitting the day side, and particles in outer orbits hitting
the night side.

If particles always hit *only* the preferred side, every planetesimal
would form at the maximum rotation speed it could have and still
hold together.

-- Jeff, in Minneapolis

[Trakar]

Finally found the pages of discussion on Planetary Rotation on page 158, having browsed through the preceding pages. I don't see anything there that disagrees with me, so I feel better.

I didn't even realize you had proposed anything to agree or disagree with?!

I skipped the second one, since I didn't think Sticking Mechanisms were what I was looking for...I conclude that you're just wrong.

If, in reading through these four, you think that something I said contradicts or is in disagreement with anything in these papers then I'm obviously wrong! or so poor in my ability to clearly state my understandings that intelligent and knowledgable people cannot properly understand what I am conveying, in which case I'm effectively wrong.

OTOH, I probably missed something along the way, but hey, the first article alone was 45 pages! Any help would be appreciated.

Well, I just grabbed the first four out of my database that seemed to cover the range of planetary formation with a focus on inner system planets. I didn't consider any of them particularly lengthy or complicated, especially given the subject matter.

perhaps the best place to start would be to clarify both of our statements.

What I stated: "...In general, accretion of a large planetary body generates considerable net angular momentum...Looking at first level accretional processes alone we might expect all of the inner planets to have quite fast spins, more in line (proportionally) with the mid-system planets..."

These issues are addressed and confirmed within the references I provided, but I am more than interested, at first, with what you are understanding the above words to mean, and what leads you to disagree with them? I can see that I should have probably said '...Looking at the first several levels of accretional processes alone, we might expect...," as I was trying to emphasize that it is the final, late accretion period where all of the Lunar-size to Mars-size inner system swarm coalesces into the inner planet system. It is this final stage of coalescence and interactions, in a situation where impactor and impacted are relatively comparable in mass, that we get events which can dramatically "spin-down" or "flip" planet-size masses. Spin-ups probably happen, but the process itself and structural integrity issues with large masses, probably set an upper limit to how oblate of a spheroid a roughly earth mass/composition planet can grow to, and so most of the really oblique impacts that would vector into the direction of rotation result in escape velocity debris jets that actually slow rotation and cannabalize mass from both interactors. It is all about the last major impact and the resulting vector of angular momentum, that is where we find the current axis and speed of rotation.

BTW, I may well have been misreading and understanding what you were concerned about. Perhaps if you would care to explain to me in more detail the problems you have with my statements thus far, and why you feel them to be in error and/or out of line with mainstream understanding, then maybe we can both get on the same page.

[grapes]

I'm still not sure what you mean. I'll guess that you just mean the
range of possible orbits which intersect the orbit of the planetesimal,
including orbits which cross the planetesimal's orbit.

I'm not sure what the distinction is there between intersect and cross, but what I mean is the total range of orbits. For instance, you discount the effect of particles that cross the orbit because they are less likely to hit the planetesimal, and say that small perturbations are more apt to produce dayside impacts--but where do you account for those crossing orbits that are just on the other side that only need small perturbations to produce nightside impacts?