Earth's Orbit Unpredictable !

[CraigS]

When minor planets Ceres and Vesta rock the Earth into chaos (http://www.physorg.com/news/2011-07-minor-planets-ceres-vesta-earth.html)



.. also more evidence that points to our inability to rely on pure deterministic physics when speaking on evolutionary macro scale topics like predicting exo-life, the universe and many other things !

Cheers

[CraigS]

Quite extraordinary … especially from the perspective of certain present-day hot topics !

Cheers

[renormalised]

No arguments with that. However, we can roughly determine what the orbit of the planet was at any one particular time by looking at certain sections of the rock record, especially within the sedimentary rocks. Looking at varves or sediments from ancient salt lakes, for example, we can gauge an approximate value for how many days there were in the year at the time of deposition and knowing what the rotational period of the planet was at that particular time (by knowing the co-efficient of tidal drag on the crust/oceans etc etc) we can then give an approximation of the orbit. For example, glacial varves (lake deposits) from the Middle Devonian show that the Earth year was around 411 days in length. Knowing from other research that the day length at the time was around 22 hours, it can be calculated that the orbit of the planet at the time was more elliptical than it is now...most probably the planet swung out to 98-100 million miles and came into about 88-89 million. Meaning the eccentricity of the orbit was around 0.03 instead of being 0.017 as it is at present.

However, despite being able to do that in some instances, as Craig has said, we cannot predict what the orbit will be at any one stage over long periods of time with any certainty, due to those chaotic gravitational influences etc. We can only take approximate snapshots, as above, only if we have enough evidence and such to be able to do so.

[CraigS]

Interestingly, other examples of chaotic motion are:

i) chaotic tumbling of Hyperion complex behaviour in the rotational dynamics.
the obliquity of Mars is currently varying chaotically between 0◦ and ∼ 60◦ on a time-scale of a few million years;
ii) Mercury and Venus' rotational dynamics appears not to be in the chaotic regime, but it is reasonable that these planets may have experienced, in their past, slow chaotic changes in the obliquity of the rotational axis.
iii) The Earth’s case is quite intriguing because this analysis indicates that it should be now in the state of chaotic variation of its obliquity between 0◦ and ∼ 80◦ with a time-scale of a few million years. The moon's torque stabilises the chaotic rotational state, and makes it regular again (ie: the appearance and disappearance of chaos and order due to external influences) .. and that's in addition to the OP in this thread;
iv) the lack of uniformity patterns expected in the spatial distribution of asteroids;
v) Halley's comet orbit is chaotic (the timescale for practical unpredictability could be as short as ∼ 30 years);
vi) most of the orbits of the small planets ... with Mercury found to be able to change drastically, its eccentricity within ∼ 3.5 Gyr;
vii) stellar dynamics in clusters and galaxies (the gravitational n-body problem with large n).

The list goes on and on.
(Source: I can back-track and find it if someone wants it).

We live inside, and around, Chaos !

Cheers

[xelasnave]

I think we make the mistake of measuring time using humans time frame... everything happens so slow by our reference and so appears uneventful..to a degree.. but imagine if one could run a movie condensing say 10 billion years action into a utube movie it would be action packed... things seem so ordered because we observe a small part ...
I do think even within a kcaotic frame work real or geometric parttens appear and repeat now why in such a frame work pattens reoccur and repeat is another matter...the formation of galaxies seems to follow similar pattens and think of the opportunity for options even within a strict framework.
Anyways its great to see numbers on this thanks Craig
alex

[multiweb]

What margin of error do we have from our current excentricity before we run into serious problems with our surface temperature. Is this orbit shape something that can potentially get big enough? Has it in the distant past? I guess how thick is the 'comfort' zone from the sun?

[renormalised]

The habitable zone around the Sun is more than large enough to encompass any conceivable deviation in the Earth's orbit, apart from catastrophic ones. However, a substantial change in the eccentricity of the orbit of the planet would cause problems. At present, the seasonal changes in solar insolation averages out to around 6%. If it were to become substantially larger or smaller, there would be climate consequences for the planet.

Because of chaotic interactions with the other planets over long periods of time, it's impossible to accurately predict what the orbit of the planet will be. So, in a billion years time, the orbit of our planet could be substantially different to what it is now. However, it's believed that the orbits of the major planets will remain stable over very long periods of time...on the order of many billions of years.

[CraigS]

I'd really like to get my hands on this paper.
Unfortunately, its locked up behind A&A's paywall.
:sadeyes:

The statement about Earth's orbit becoming unstable, could also be interpreted to have nothing to do with Ceres' and Vesta's gravitational influences .. the article is not particularly clear on this. (Once again .. I recommend that the journo article be taken with a 'grain of salt').

I notice this 'Laskar' guy has been working on orbit variations throughout the solar system for quite a while. Some of his past papers are on arXiv.

Cheers

[renormalised]

I'll go grab it today:)

[renormalised]

Here it is....

[CraigS]

Thanks Carl.

Will have a good read when I get the chance.

Cheers

[multiweb]

What's your take on paleoclimatology then. Do these new findings throw a spanner in the works or is it still valid?

[renormalised]

No, it's still valid. There are other ways of determining what past climates were like, apart from predicting what the Earth's orbit was at any give moment. It's just that we find that getting an accurate fix on what the Earth's orbit was doing in the dim distant past isn't as easy as one would think.

[multiweb]

Are those past climate changes and massive extinctions event peaks linked to changes in earth orbit in the past you reckon?

[renormalised]

Some of those mass extinction events probably brought about massive climate change and some may have been initially triggered by them. However, the extinction event peaks occur at roughly 30-35 million year intervals, which might be due to the orbit of the solar system about the galaxy. We pass through the area of the spiral arms once every 30-35 million years or so. It appears that the extinction events of medium to large magnitude follow that time span approximately. The really big events, like the K-T or the Permian event seem to occur at around double or a bit more, than that time span. But, it's also probably safe to say that an extinction event can happen at anytime.

Changes in the Earth's orbit (as well as axial tilt, etc) would most certainly cause climate changes and local to regional extinction events. They actually have and the evidence is found in the geological record.

[multiweb]

I don't get that one. Are you saying:

1_ the solar system orbit around the galaxy core is approx 30-35million years. (I didn't realise the galaxy was spinning that fast)

2_ we are passing through spiral arms? I thought all the stars in a spiral arm travel uniformely in the same direction? I mean the arm is a bunch of stars right, so we are a part of the arm?

[CraigS]

As an aside:
This particular paper doesn't say anything about how the resulting chaotic eccentricity manifests itself in temperature models of the past. It leaves this up to the paleoclimatologists.

In a separate paper by the same author (here) (http://arxiv.org/abs/1103.1084) however, they say:


(The last statement being a handy rule of thumb).

They have made all this available on their website here. (http://www.imcce.fr/Equipes/ASD/insola/earth/earth.html)

Need to be cautious about how to interpret this information however .. which is why they refer it all to the paleoclimatologists.

Cheers
PS: The resulting eccentricity seems to manifest itself over ~2.4Myr cycle.

[renormalised]

No...we pass through the spiral arms once every 30-35 million years. As the Sun orbits about the Galaxy (every 250Ma), it bobs up and down with respect to the plane of the Galaxy....roughly +/- 250ly above and below. Right at present, we're about 1-2 million years out and 25ly above the last encounter with a spiral arm...which was in the direction of Centaurus. We're not out of the woods completely, yet. At present we're heading towards what's known as the Aquila-Cygnus Rift, which is a dark, nebula filled region loaded with star formation regions and all sorts of nasty stuff. We won't get there for about 1-1.5 million years. Fortunately, we'll miss the core of the region...just.

The arms themselves travel more slowly than the stars which form in them. If the stars live long enough, they leave the arms to pursue their own orbits about the Galaxy. Most of the stars within a galaxy orbit about it on their own orbits and whilst most orbit roughly in the same direction, their orbits are not all nicely spaced or heading in specifically the same direction. Some orbit the galaxy at high angles, others have almost flat orbits. Some have highly elliptical orbits and others have more or less circular orbits.

[renormalised]

It's higher than that, now. It's precisely the change in solar insolation between the aphelion and perihelion distances which drives the temperature variations in the seasons....it's around 6%. What makes the big difference is the changes in the obliquity of the tilt and which hemisphere is in its summer/winter phase w.r.t. the solstices.

However, an extra 6K difference may cause problems either way, if the other factors come into play as well.

[multiweb]

:lol: And I thought it was a very simple orbit. Sounds more like the merry-go-round from hell. :question: I new that the ecliptic plane was fairly square with the galactic plane but I had no idea how far up and down we went. So we're doing the hoola hoop basically, flying through nasty stuff occasionaly. It's just amazing the earth is still ticking along relatively unaffected by the sound of it. Well except some measly extinction event now and then wiping 70% of biodiversity. But they were all seafood at the time weren't they? Frogs and snails. That kind of stuff :P

[renormalised]

There's one thing I don't like about Laskar's papers...he has a habit of mostly quoting too often from his own previous papers. Whilst that's OK to do, it's not the best thing to do if you're trying to prove a point or show that your work has some veracity. I have not seen any comment on his papers from other researchers, or if they've been cited elsewhere.

[renormalised]

Actually, the ecliptic plane and the plane of the solar system is tilted at about 50 degrees to the plane of the Galaxy. The Earth is hardly unaffected, we just don't see it because of the timescale on which everything occurs and the degree to which things are preserved in the geological record.

[multiweb]

Fascinating stuff. Thanks for answering all my shtoopid questions. ;) :thumbsup:

[renormalised]

No questions is stupid, unless the answer is so obvious and well known that the asking of the original question was done just to annoy others.

[CraigS]

The causes for these extinction events are still very much up for debate, too.

They don't all have to attributed to surface or atmospheric temperature variations.

Cheers

[renormalised]

True....but a large asteroid impact, a global methane belch or increased volcanic activity will cause temp variations.

However, that they happened is fact and we only have a certain number of possible causes. We can take our pick and see which one (or more than one) occurred.

[CraigS]

One dinosaur sneezed on another !
:)
Cheers

[multiweb]

It is well documented that Homer started it all.

[renormalised]

Yeah, but they were all inoculated down at the Bedrock Health Clinic for the flu:):P:P