## April 28, 2009

### The Earth - For Physicists

#### Posted by John Baez

The $n$-Café is buzzing with activity. It’s frustrating! My pal Minhyong is busy explaining how to visualize the algebraic completion of the rational numbers, and my student Alex is busy introducing us to integral transforms in derived algebraic geometry. But I’m too busy to read any of that stuff. I’m supposed to write a history of the Earth for the British magazine PhysicsWorld by May 1st. And it’s supposed to be 3000 words or less! So each word needs to describe 1.5 million years — a heavy responsibility.

I was asked to write this after someone at PhysicsWorld saw week273, where I was describing mineral evolution. I figure this is a good way to get my foot in the door when it comes to popularizing science. They say I can put a copy on my website when it’s done, so I hope it’s okay to put a preliminary version here, too. I wouldn’t have it any other way.

I’d greatly appreciate any comments or questions. I’ll ask some actual earth scientists to correct my mistakes — so what I really could use most of all is feedback about passages that seem obscure or insufficiently entertaining.

Please don’t ask me to cover more material: there just isn’t room! I’ve focused the article on 4 big events in the Earth’s history: the Big Splat, the Late Heavy Bombardment, the Oxygen Catastrophe and Snowball Earth.

Posted at April 28, 2009 12:57 AM UTC

TrackBack URL for this Entry:   http://golem.ph.utexas.edu/cgi-bin/MT-3.0/dxy-tb.fcgi/1956

### Re: The Earth - For Physicists

Hi

I have not read much yet, but here is a typo. I would have emailed you, but I am at a coffee shop with annoyingly frustrating wifi which doesn’t like my email.

In every case, there in interesting physics involved in testing THEESE theories.

Posted by: Alex Hoffnung on April 28, 2009 5:04 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

Some other typos:

The astophysicist Robin Canup has done some remarkable computer simulations of the Big Splat.

The Oxygen Catastophe marks the end of the Archean

characterized by signicant human impact on ecoystems and climate

Posted by: Alex Hoffnung on April 28, 2009 5:10 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

Thanks for catching those typos, Alex. I’ve fixed them.

Posted by: John Baez on April 28, 2009 8:18 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

I’ve fixed them.

But you've still got ‘there in interesting physics’ instead of ‘there is interesting physics’.

Posted by: Toby Bartels on April 28, 2009 8:58 PM | Permalink | Reply to this

First ‘Creationists’ have a problem understanding how the Earth naturally formed. I would suggest including the cloud/rain analogy. In particular changing

The dust circling the early Sun started forming lumps called “planetesimals”.

to

The dust circling the early Sun started condensing into larger particles (much as the droplets or crystals that form clouds in the sky can become raindrops), eventually forming large lumps called “planetesimals”.

The following sentence does not seem to be relevant, and unless you can make it relevant it should probable be removed or given a much smaller in passing mention:

The early Moon is known to have been much closer to Earth than it is now — it’s been receding ever since. For this and many other reasons,

The below seems too specific:

Granite is made in a variety of ways, for example by the remelting of sedimentary rock. Early granite-like rocks were probably simpler.

Maybe change it to:

Granite forms from molten rock and early granite-like rocks were probably simpler than modern forms.

Posted by: RodMcGuire on April 28, 2009 6:14 AM | Permalink | Reply to this

### Re: A few editorial comments.

Rod wrote:

First ‘Creationists’ have a problem understanding how the Earth naturally formed.

The magazine Physics World is published by the Institute of Physics, the main British physics organization, which began life as the Physical Society in 1874. I think the readers of this magazine are mainly physicists. So, I don’t think my job here is to persuade creationists.

Posted by: John Baez on April 28, 2009 6:35 PM | Permalink | Reply to this

### Re: A few editorial comments.

Baez sez

I think the readers of this magazine are mainly physicists.

Don’t overestimate your readers. Write assuming your readers should understand but when possible give some textually-small analogies to help. For me, I didn’t learn anything new (other than you think it is important for some reason that basalt is older than granite).

My impression of scientific articles is that most readers don’t understand them but are just impressed. If your article is popular then it will be passed on. If you can help make it understandable by a bright 13-year old or a corporate middle manager then it is better.

I’m not advocating that you write in Scientific American style - I find that magazine unreadable because most articles are reduced to supposedly understandable idiot mush. But there should be some happy medium.

Anyway, enough ranting.

The only slightly major thing I find lacking from your ‘history of the earth’ is some mention of magnetic reversals - not because they are cool but instead because they are used in dating the history of the earth.

Posted by: RodMcGuire on May 1, 2009 10:03 AM | Permalink | Reply to this

### Re: A few editorial comments.

By the way, I wanted to distinguish between granite and basalt, the two most common types of rock. Both are made from molten rock, but basalt is “primitive” while granite is “sophisticated” — it’s made from the remelting of sedimentary rock, so it could only appear after there were oceans, roughly half a billion years after the Earth formed. In fact, it probably took 1.5 billion years for granite formation to really get going, since that’s when plate tectonics got rolling, which pushed ocean plates down in the mantle where they remelt.

Posted by: John Baez on April 28, 2009 8:27 PM | Permalink | Reply to this

### Re: A few editorial comments.

John actually answered this nicely, but as a “Creationist” I want to add that while religion is about truth, physics is about models which explain experiment. IMHO John is right not to go there.
And since the article is for people interested into physics, I think people reading it are more interested in explaining observations than in unobservable theological truths, and one needn’t turn science into a political tool.
Indeed, I thought maybe the last paragraph was overstated, because global warming is a politically volatile issue- and not directly relevant to the content of the article. I would cut that part out. It has nothing to do with physics, for one thing…

Posted by: Daniel Moskovich on April 29, 2009 7:47 AM | Permalink | Reply to this

### Re: A few editorial comments.

Global warming has a lot to do with physics — we need physics to understand whether global warming is happening, how it works, and how various actions we might take would affect it. My whole piece is gently attempting to get physicists interested in the Earth. I start by saying that the Earth is just as full of interesting physics as the Big Bang and black holes, and I want to conclude by saying that we need physicists to help us understand both the past and the future of the Earth. I need to polish the ending a bit to make that point clearer — but I certainly won’t cut it.

The reason global warming is “politically volatile” is because it’s an important issue with potentially enormous consequences. That’s not a reason to avoid talking about it. That’s a reason to talk about it!

I would hope that everyone of good will, whether they believe humans are causing global warming or not, would agree that this issue is worth understanding and discussing — and that physicists can help.

Posted by: John Baez on April 29, 2009 3:52 PM | Permalink | Reply to this

### Re: A few editorial comments.

So chondrites can be formed either by accretion of chondrules with unspecified other stuff? or by collision of planetesimals?

How was dust circling the sun melted by those energetic jets from the POLES?

Posted by: jim stasheff on April 30, 2009 5:46 PM | Permalink | Reply to this

### Re: A few editorial comments.

Jim wrote:

So chondrites can be formed either by accretion of chondrules with unspecified other stuff? or by collision of planetesimals?

I guess both processes are possible: chondrules stuck together, first in small lumps, then in bigger ones — but then some of these bigger lumps collided and shattered. But I don’t know if most of the chondrites we see are the original small lumps, or the result of bigger lumps shattering. I don’t even know if anyone knows!

I don’t think “unspecified other stuff” is required: I think the chondrules just stuck together, especially when molten.

This abstract suggests that turbulence and shock waves in the early solar nebula are responsible for chondrite formation:

This paper suggests that material originally from the planet Mercury may be present in chondrites:

This paper admits “The conditions and mechanism of chondrule formation remain poorly understood”:

So, I guess that one of my main jobs should be to avoid suggesting this story is better understood than it is!

How was dust circling the sun melted by those energetic jets from the POLES?

I wasn’t trying to say the jets are what melted the dust. Indeed, I think accretion disks tend to have jets shooting out the poles because that’s where there’s the least junk in the way! But it’s not only hot near the poles: the accretion disks get hot all over, near their center.

I was mainly just trying to let people know that protostars involve highly energetic processes that could easily melt stuff. But, I’ll rewrite this passage a bit to make it clearer. Thanks!

Posted by: John Baez on May 4, 2009 3:53 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

One theory is that Jupiter and Saturn moved into their current 2:1 resonance around this time

I thought it was 5:2. Let’s turn to Wikipedia:

The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun, or 5.2 AU) and it completes an orbit every 11.86 years. This is two-fifths the orbital period of Saturn, forming a 5:2 orbital resonance between the two largest planets in the Solar System.

On the other hand, apparently there might have been a 2:1 resonance once:

A past resonance between Jupiter and Saturn may have played a dramatic role in early Solar System history. A 2004 computer model by Alessandro Morbidelli of the Observatoire de la Côte d’Azur in Nice suggested that the formation of a 1:2 resonance between Jupiter and Saturn created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune’s distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt as Neptune approached it could explain the Late Heavy Bombardment 600 million years after the Solar System’s formation and the origin of Jupiter’s Trojan asteroids.[10] An outward migration of Neptune could also explain the current occupancy of some of its resonances (particularly the 2:5 resonance) within the Kuiper belt.

Earlier in the article such resonances are played down:

A number of near-integer-ratio relationships between the orbital frequencies of the planets or major moons are sometimes pointed out (see list below). However, these have no dynamical significance because there is no appropriate precession of perihelion or other libration to make the resonance perfect

Posted by: David Corfield on April 28, 2009 9:02 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

David wrote:

I thought it was 5:2.

Yikes — you’re right! How embarrassing. There’s a nice description of the Nice model on Wikipedia, but it didn’t say anything about Jupiter and Saturn going from a 2:1 resonance to a 5:2 resonance, so I got lulled into thinking they stayed in the 2:1… even though I’ve known they’re now in a 5:2.

From week222:

The orbits of Jupiter and Saturn are almost in 5:2 resonance, but this is a “shallow” resonance, and Saturn wiggles back and forth around this resonance with a period of about 880 years - an effect called the “Great Inequality”. The first person to study this was Laplace. I read elsewhere that:

The dynamics of the Sun-Jupiter-Saturn system was recognized as problematic from the beginnings of perturbation theory. The problems are due to the so-called Great Inequality (GI), which is the Jupiter-Saturn 5:2 mean-motion near-commensurability.

This is from:

30) F. Varadi, M. Ghil, and W. M. Kaula, The great inequality in a planetary Hamiltonian theory, available as chao-dyn/9311011.

This shallow 5:2 resonance is related to the continued fraction

1/(2 + 1/(2 + 1/(14 + 1/(2 + …. ))))

which is close to 2/5.

The Pluto-Neptune 3:2 resonance, on the other hand, is a “deep resonance” and related to the continued fraction

1/(2 - 1/(2 + 1/(10 + …. )))

which starts out close to 2/3.

I never did manage to fully understand the difference between a shallow resonance and a deep one. But anyway, thanks a lot!

Posted by: John Baez on April 28, 2009 6:14 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

Luckily evolution found a way out of this impasse: now many of us need oxygen.

Is that not just a little too casual? Who is the ‘us’ there?

The opposite sort of feedback is happening now, as melting ice makes the Earth darker and thus even warmer.

Presumably area rather than volume is important, so melting per se is a bit vague. And what do you mean by ‘now’? Not the past three months it seems.

Posted by: David Corfield on April 28, 2009 9:18 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

David wrote:

Is that not just a little too casual?

It was an attempt at humor.

Who is the ‘us’ there?

I need oxygen, and so do you.

And what do you mean by ‘now’?

Okay, I should be clearer.

Posted by: John Baez on April 28, 2009 6:24 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

I would like to see more elaborations on the origin of life. You could point out some theories such RNA World, or the Citrus Cycle.

Posted by: Thomas Wang on April 28, 2009 7:17 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

Alas, I said “Please don’t ask me to cover more material: there just isn’t room!” I wanted to talk about theories of the origin of life, but I decided that it wasn’t good to quickly summarize them. I can imagine including them and leaving out the Oxygen Catastrophe, especially since I know of more work by physicists on the origin of life than on the Oxygen Catastrophe. But I think I’ll see what the editor says.

Posted by: John Baez on April 28, 2009 7:57 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

“Abiogenesis for Physicists” would make fascinating reading, but I guess we’ll just have to be patient! :-)

Posted by: Blake Stacey on April 29, 2009 4:42 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

No need to wait, Blake — try this.

When we get time we should try some of these more technical papers:

• Morowitz, H. J. 1999. A theory of biochemical organization, metabolic pathways, and evolution. Complexity 4:39–53
• Smith, E., and H. J. Morowitz. 2004. Universality in intermediary metabolism. Proceedings of the National Academy of Sciences of the U.S.A. 101:13168–13173.
• Morowitz, H. J., and E. Smith. 2007. Energy flow and the organization of life. Complexity 13:51–59.
• Srinivasan, V., and H. J. Morowitz. 2009. The canonical network of autotrophic intermediary metabolism. Biological Bulletin. In Press.
Posted by: John Baez on May 4, 2009 3:26 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

Although you may want to link to Wikipedia articles too, the images really should link to the Wikimedia Commons files that they're ultimately taken from:

One reason is that a Wikipedia page might change its chosen images, in which the case the links there will no longer apply to your images; another is that these image description pages have the copyright information that PhysicsWorld editors will want if you expect the images to be in their version of the article.

Note that the last is in Wikipedia under fair-use guidelines that may or may not be good enough for PhysicsWorld, while the others are free (public domain or Creative Commons by-sa); in any case, the editors can track them all down through the links above.

Posted by: Toby Bartels on April 28, 2009 9:15 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

Thanks for the links. For my own webpages I always try to eventually put copies of the necessary images on my own website, since I don’t trust anyone else to maintain them forever.

(I’m not eternal either: I have no idea what’ll happen to my website when I die. I should start working on this issue, but this sort of thing is very easy to put off, since I won’t be around when it matters. Someday there will be big businesses that promise to maintain people’s websites after they die, but so far it’s hard to know who to trust.)

The folks at PhysicsWorld are aware of copyright issues, and will obtain the legal right to use any images they decide to use. But you’re right, their job will be made easier if I let them know the Wikimedia Commons URL’s. So thanks again!

I hope the folks at PhysicsWorld will also draw a nice timeline for me. They said I can put a PDF of the final version of my paper on my website. Someday I’ll give links to that here.

Posted by: John Baez on April 29, 2009 6:27 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

Having your own copies of the pictures seems like a good idea (and I see that you've already done so for one of them), but I also think that you ought to link directly to the Wikimedia versions too. For one thing, this complies (for the two for which this is relevant) with the Creative Commons licence; for all of them, it provides more complete information about where the image came from. (Of course, one might link from the image itself, from some text like ‘(source)’ in the caption, or wherever.)

Posted by: Toby Bartels on May 1, 2009 11:13 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

So I take it you are not going to discuss the black hole that seeded the formation of the Earth in Riofrio’s quantum cosmology.

Posted by: Kea on May 1, 2009 11:21 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

My information may be out of date, but I was under the impression that there were zircons dating to 4.4 Gya in the Jack Hills formation. While it isn’t customary to call them rocks, they do present positive evidence of early crust solidification, possibly followed by a thorough pulverizing. Also, I think there is some (not entirely conclusive) evidence based on oxygen and carbon isotope distributions in the zircons that there was liquid water and possibly life on Earth when the crystals were formed. The low temperature is consistent with Stefan-Boltzmann combined with reasonable estimates, assuming the Earth wasn’t undergoing extremely rapid convection.

Posted by: Scott Carnahan on May 2, 2009 7:29 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

Just to follow up, there was a Nature article about this last July. One of the authors uploaded it here (pdf). I should note that the zircon crystals containing the diamonds were formed at least 4250 Mya, but I guess they couldn’t put dates on the diamond inclusions they sampled (which, they argue, must have been formed earlier).

Posted by: Scott Carnahan on May 2, 2009 7:58 AM | Permalink | Reply to this

### Re: The Earth - For Physicists

Thanks for the info. I’m not succeeding in accessing that PDF file right now, but I’ll check out the Nature article. Thanks for the information! I’ll have to figure out how this modifies the story I’m telling here, if at all.

Posted by: John Baez on May 3, 2009 11:18 PM | Permalink | Reply to this

### Re: The Earth - For Physicists

I’ve updated this article based on some suggestions by Joao Medeiros, the features editor of PhysicsWorld, in which it will appear.

The changes are small — just a little extra information here and there.

Interestingly, he wanted more stuff on current-day climate change and how it relates to earlier episodes in the Earth’s history. I told him climate change deserves an article of its own, or even a whole issue. (And they should get an article by a respected expert.)

Posted by: John Baez on June 18, 2009 9:27 PM | Permalink | Reply to this

### IOP emailed it!; Re: The Earth - For Physicists

Let me be the first to congratulate you: IOP just emailed me (and many others) your revised article.

Posted by: Jonathan Vos Post on August 1, 2009 2:30 AM | Permalink | Reply to this

### Re: IOP emailed it!; Re: The Earth - For Physicists

Cool! Thanks for pointing that out!

Posted by: John Baez on August 1, 2009 3:18 PM | Permalink | Reply to this

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