Is Moon so crucial for axial tilt stability of the planet?

[Tiger Blood]

I often hear that for planet to have life on it it must have proportionaly large moon to keep it's axial tilt small like Earth is between 21.5° to 24.5° and not like Mars that varies from 15° to 35° which makes destructive effect on possible life that could flourish.

But doesn't Venus have pretty stable axial tilt of 2.7°? I mean I know that it's upside down by 177° but that's only because (apparently) long time ago large body turned it upside down and it usually doesn't wobble?

So can't those experts say that moon is not 'a must' if a planet has retrograde orbit?

 

[PhysicoRaj]

The role axial tilt plays in life-planets is 'seasons.' These seasons influence life forms greatly and hence are crucial for which the axial tilt is necessary. The axial tilt causes seasons. Moon may not be a must, but you just can't take the example of Venus, because let go the seasons on it, the atmosphere is poisonous! And ...are you stressing about retrograde 'orbit' or 'rotation'?

 

[D H]

But doesn't Venus have pretty stable axial tilt of 2.7°? I mean I know that it's upside down by 177° but that's only because (apparently) long time ago large body turned it upside down and it usually doesn't wobble?

Solid body tides, core/mantle interactions, and atmospheric thermal tides explain Venus' current more or less stable configuration. Just because Venus' obliquity is stable now does not mean that it has always been in this orientation. There's no need for a near collision to explain Venus' obliquity. Chaos theory explains how perturbations from the Sun and the planets (even Neptune!) alone would have made practically any initial orientation and rotation rate for Venus would, over time, evolve to a quasi-stable state. There are only four such quasi-stable states; Venus' current orientation is essentially one of those four.References:

Charles F. Yoder, Venus' Free Obliquity, Icarus 117:2 (October 1995), 250-286, doi 10.1006/icar.1995.1156.

Alexandre C.M. Correia, Jacques Laskar, Olivier Néron de Surgy, Long-term evolution of the spin of Venus: I. theory, Icarus 163:1 (May 2003), 1-23, doi 10.1016/S0019-1035(03)00042-3.

Alexandre C.M. Correia, Jacques Laskar, Long-term evolution of the spin of Venus: II. numerical simulations, Icarus 163:1 (May 2003), 24-45, doi 10.1016/S0019-1035(03)00043-5.

 

[mfb]

The role axial tilt plays in life-planets is 'seasons.' These seasons influence life forms greatly and hence are crucial for which the axial tilt is necessary.

Seasons as requirement for life? Can you give any reference for that?
I would be surprised if that is more than pure speculation.

 

[PhysicoRaj]

Seasons as requirement for life?
I would be surprised if that is more than pure speculation.

I said seasons are one of the factors that influence life. They are not the 'only' one. But they do have a great influence.

Can you give any reference for that?

Here's an example:
Can you just imagine Uranus at the place of earth? (for now, let Uranus be a terrestrial planet). The Goldilocks zone would help no more to form life on that obtusely tilted planet.
If still you say life can form, firstly, the temperature on the sun-facing side must by lowered and dark side must by increased(by tidal heating or so..)

 

[Quagz]

What I believe it means is that for life as we have on Earth currently to flourish we require to have a relatively stable axial tilt. As life on this planet has had no 'experience' of and has had no chance to evolve to cope with the more extreme forces that would be exerted on us if Earth had a varying axial tilt.

 

[D H]

Seasons as requirement for life? Can you give any reference for that?
I would be surprised if that is more than pure speculation.

René Heller, Jérémy Leconte, Rory Barnes, Tidal obliquity evolution of potentially habitable planets, Astronomy & Astrophysics 528 (April 2011), A27, doi 10.1051/0004-6361/201015809
arxiv preprint: http://arxiv.org/abs/1101.2156

 

[mfb]

I said seasons are one of the factors that influence life. They are not the 'only' one. But they do have a great influence.

So what?

@D H: If I understand the abstract correctly, the authors say that a planet without moons would align its axis of rotation with the orbital movement, given some conditions (solar mass, distance of the planet, ...). So what? I don't see any problem in life without seasons.

Of course, we have species which need the seasons for their reproduction cycle or other things. But clearly that is an adaption of life to the existing seasons (in regions where they are important). If we would not have seasons, they would not have developed these methods.

 

[PhysicoRaj]

Of course, we have species which need the seasons for their reproduction cycle or other things. But clearly that is an adaption of life to the existing seasons (in regions where they are important). If we would not have seasons, they would not have developed these methods.

Well, okay, Then how would life adapt on a planet's dark side, which is tilted by 90 degrees? A long time in it's orbital period, the same part would observe the night without the sun. Even if there can be life on micro scale, life can't 'flourish' to the macro scale as we have it on earth. (This is what I told in the 'Uranus' example).

 

[mfb]

Wait... Uranus is quite special, and as the paper linked by D H suggests it is quite uncommon for planets in the habitable zone - especially if they don't have moons (!).

And even with this orientation of the rotation, the equator regions have a cycle of "always sun at horizon" -> "day/night cycle" -> "always sun at horizon" -> ... which might be good enough for life.

 

[Drakkith]

Well, okay, Then how would life adapt on a planet's dark side, which is tilted by 90 degrees? A long time in it's orbital period, the same part would observe the night without the sun. Even if there can be life on micro scale, life can't 'flourish' to the macro scale as we have it on earth. (This is what I told in the 'Uranus' example).

I disagree. I don't see any reason to assume that 4-6 months of pure darkness/light keeps life from flourishing. Just look at the polar regions of Earth. I think it's very possible for large photosynthetic organisms to shut down for several months at a time and still survive, after all plenty of plants do so in the wintertime all over the world.

 

[davenn]

I disagree. I don't see any reason to assume that 4-6 months of pure darkness/light keeps life from flourishing. Just look at the polar regions of Earth. I think it's very possible for large photosynthetic organisms to shut down for several months at a time and still survive, after all plenty of plants do so in the wintertime all over the world.

Agreed. and PhysicoRaj, thinks of the deep ocean life that abounds in total darkness not just for a few months but permanently !
and its not just microbes but right through to quite large fish.Dave

 

[PhysicoRaj]

So, in permanent darkness, the ecosystem would lack photosynthetic organisms. So the second-trophic organisms have to find food somewhere else right? On Earth they can eat plankton and others which move about to sunlit (even slightly) areas. But in Those kind of planets, what would become of them????

 

[SHISHKABOB]

So, in permanent darkness, the ecosystem would lack photosynthetic organisms. So the second-trophic organisms have to find food somewhere else right? On Earth they can eat plankton and others which move about to sunlit (even slightly) areas. But in Those kind of planets, what would become of them????

there are extremophiles that derive energy from substances such as hydrogen sulfide, meaning that they are independent from the sun

it's probably not likely for life that evolves under permanent darkness to have much complexity, since the sun is a very large source of energy and there aren't many other sources that can really compete with it

http://en.wikipedia.org/wiki/Hydrothermal_vent#Biological_theories

 

[akhenaten]

According to a recent discovery channel program I watched last night the Moon is essential for life on Earth. It is not just the seasons that are affect but of course the oceans via tides. Plus the moon protects Earth from meteors. As it keep the Earth tilt stable without it the Earth will wobble uncontrollably and you end up with ice on one side and desert on the other. As the moons is moving away from the Earth at 3.5cm a year; this distance increasing year by year, there will come a time when the moon will drift off into deep space. No need for me to worry as it seems this will be in 1.5 billion years. In the past the moon was much closer to Earth than it is now. So it seems "Sans Moon, Sans Life".

 

[mfb]

With increasing distance earth<->moon, the tides become smaller and therefore their effect becomes smaller, too. Where did you read this number of 1.5 billion years? With a constant rate of ~30years/m (upper limit), this is equivalent to 50 million meters or 50,000km. Enough to see it, but not enough to lose the moon.

Plus the moon protects Earth from meteors.

Not much.

As it keep the Earth tilt stable without it the Earth will wobble uncontrollably and you end up with ice on one side and desert on the other.

The other planets without large moons, especially Venus (~13°) and Mars (~25°), don't have this. Why would you expect this with our Earth (~23°)?