Yey, Moon?

July 27, 2009 8:51 AM

Planetary magnetic fields and life: Is there any reason to believe that strong magnetic fields on rocky inner planets are relatively uncommon?

If so, is there any reason to believe that the hypothetical giant impact 4.5 billion years ago that created the moon could have spun the earth up sufficiently to create that magnetism?

Finally, assuming the first two questions aren't ridiculous, couldn't the combination help explain the apparent dearth of other complex life in the universe?

Not an expert in the field, I'm just curious if I'm completely off base in seeing a connection here.

posted by leotrotsky to science & nature (13 comments total)

Sorry that this isn't a direct answer to your question, but given our meagre abilities with regard to observing the universe I wouldn't say that we have much grounds for believing there is any kind of dearth of complex life in the universe. About all we're able to say for now is that our instruments haven't picked up any communicating life within the timeframe we are capable of observing.
posted by fearnothing at 9:12 AM on July 27

Is there any reason to believe that strong magnetic fields on rocky inner planets are relatively uncommon?

The Earth's magnetic field is caused by charged particles in the rotating liquid metal outer core (the inner core is solid). If planets are truly rocky, made entirely of solid rock with no charged particles, then they will not have a magnetic field.

If so, is there any reason to believe that the hypothetical giant impact 4.5 billion years ago that created the moon could have spun the earth up sufficiently to create that magnetism?

Planets are created spinning. The hypothetical impact that created the Moon may have increased or decreased or decreased this speed, but only by a very, very small amount.

Finally, assuming the first two questions aren't ridiculous, couldn't the combination help explain the apparent dearth of other complex life in the universe?

It is a combination of Earth's magnetic field and the atmosphere that protects us from the solar wind. The Core would have you believe that life on Earth would be impossible without a magnetic field but this isn't true. Periods on Earth when the magnetic field strength was very low (during a pole reversal) do not seem to correlate with any mass extinction.

On a side note: there is very little agreement about the dearth of life in the Universe. The apparent dearth is more likely due to our limited observational capacity and the distance/time scales involved.
posted by alby at 9:19 AM on July 27 [1 favorite]

I think the key is not so much the spin, which all planets do, as whether the planet has a liquid core, which not all planets have. Mars and Venus don't have a magnetic field, Mercury does.
posted by IanMorr at 9:20 AM on July 27

My (limited amateur) understanding is that our sister planet Venus, also has a molten center but not as much iron in it's composition to create a magnetic field. Mars has nearly the same rotation rate as Earth but I believe it as a solid core (maybe because it only has about 1/3 the mass of the Earth?)

Of course, I am not your astronomer.
posted by bonobothegreat at 9:29 AM on July 27

...the key is not so much the spin, which all planets do...

Unless they are tidally locked. We know of some satellites that are. We thought Mercury was tidally locked, but it isn't. Hard to know what, if anything, this says about magnetic fields.
posted by Guy_Inamonkeysuit at 9:31 AM on July 27

Mars and Venus and Mercury don't have magnetic fields. (I'm not absolutely certain about Venus.)

The Earth is an unusual planet. When it first formed, it was a near twin of Venus, with no moon. But huge amounts of stray material collected in one of the Earth's trojan points, and eventually formed a body about the size of Mars. The gravitational trick that makes trojan points work doesn't apply to something which begins to rival the mass of the primary, and so it eventually fell out of the trojan point and was attracted to the Earth. Best evidence from computer simulations is that it grazed the Earth, went on past, then fell back in and the two bodies merged.

The initial graze knocked a lot of rocky surface material off both bodies, which went into orbit and eventually formed the Moon. Earth (our Earth) ended up with most of the metal.

So Earth is no longer a near twin of Venus; we've got a lot more metal than Venus does, including a lot more Uranium and Thorium in our planetary core. That, plus tidal flexing from the Moon, means our liquid core is bigger and moves more, and that appears to be where the magnetic field comes from.

Early on Venus probably had a magnetic field too; but as the crust solidified and the liquid part of the core shrank, it eventually collapsed.

Or so I understand it from the reading I've done. I am not a planetary geologist.

The problem with your question is that we can't judge probabilities on this. We don't have a statistically significant sample of planetary systems yet from which to judge. Planet/moon combinations like Earth/Moon could be very common or extremely rare.

We have found dozens of planets around other stars, but the way we currently look for them biases the sample in favor of massive planets in tight orbits. The sample is not a fair one.
posted by Chocolate Pickle at 9:33 AM on July 27

I think the key is not so much the spin

Not true. The magnetic field is due to the dynamo effect, a dynamo doesn't work without movement.
posted by alby at 9:34 AM on July 27

Planetary magnetic fields are the result of a rotating liquid core of a magnetic material -- iron in the case of Earth and Mercury, metallic hydrogen in the case of Jupiter -- creating a dynamo. That magnetic field creates the magnetosphere, which shields the planet from solar wind and cosmic rays. Venus, Mars and the Moon don't have magnetospheres, and are not presently tectonically active, which is to say that their cores are probably not molten iron.

Liquid cores not only allow for magnetospheres, they allow for plate tectonics. I believe it's been theorized that plate tectonics are required for life to exist (in terms of cycling elements to and from the surface). I've read that it's possible that the Earth represents the bare minimum in terms of size, in that if it were much smaller, its core would have cooled and solidified -- no significant magnetosphere to protect water vapour in the atmosphere from being blown away by the solar wind, no plate tectonics. (Note that Venus isn't that much smaller than Earth.)

Larger terrestrial planets -- such as the super-Earths of two or three or more Earth masses that have been discovered orbiting other planets -- might be better candidates for habitability from that perspective.

(Note: I am not an astronomer. I am a historian who dabbles enthusiastically.)
posted by mcwetboy at 9:35 AM on July 27

I'm not absolutely certain about Venus

Venus does have a magnetic field, but its induced by the interaction of the solar wind rather than created by the planet itself.
posted by alby at 9:36 AM on July 27

Your question appears to be a mashup of two components of what is commonly called the Rare Earth hypotheis. This asserts that simple life is very common but that complex life was only able to appear on the Earth because it is an exceptionally stable place. The two points of interest to you would be:

1. The Earth's magnetic field protects it from dangerous radiation. Other planets do have magnetic fields, and the Moon is not part of that; a liquid core and relatively quick rotation are necessary. However, the Earth's magnetic field does sometimes drop off to nearly nothing during reversal events, and those don't seem to correspond to mass extinction events, so it may not be all that important.

2. The Moon stabilizes the Earth's axis of rotation. Since the Earth does have a liquid core (see #1) without the Moon, its axis of rotation could drift over time to just about any position, and those inevitable times when it might heel over like Uranus would create very difficult climatic extremes during the part of the year when a pole is facing the Sun.

It's worth mentioning that the Moon was created in very low orbit and has been spiralling outward over the eons, taking some of the Earth's rotational energy via tidal friction to loft itself ever higher. Had the impact set the Moon orbiting in the opposite direction, those same tides would have eventually crashed it back into the Earth.
posted by localroger at 9:36 AM on July 27

I don't think we can yet say that "strong magnetic fields on rocky inner planets are relatively uncommon." It's true that in our own solar system, Venus, Mars, and Mercury all show little or no global field (though Mars probably had a stronger field in the past). But we can't generally probe the fields directly on other planets -- even if we had detected lots of rocky inner planets around other stars, which we haven't. (It's actually quite tough to measure magnetic fields even on stars other than our Sun -- usually you can only infer their presence indirectly, for instance by looking at the way they heat stellar atmospheres. And those fields are generally much much stronger, at least in places, then anything we see on planets.)

Physically speaking, you expect a field to be generated (by what's called a "magnetic dynamo") whenever you have convective motions -- basically, stuff moving around in order to transport heat. To generate a large-scale field, you probably need rotation too. Neither of those things is probably especially rare (though, again, who knows). For instance, very close-in planets may be tidally locked to the star they're orbiting -- the same process that leads to the Moon's rotation rate being locked to Earth's -- which would typically imply not-so-small rotation rates.

You might be interested to hear that there are some scientists who think that magnetic fields might conceivably play a role in making life rare -- but not in the sense you're talking about here. The specific line of thinking is that most stars (by number) are lower-mass than our Sun, so their "habitable zones" are closer in; if those stars are magnetically active, as some of them certainly are, that activity might be commensurately a lot more harmful than the Sun's activity is to us. This is all fairly speculative, though.

Anyway, neat question.
posted by chalkbored at 9:45 AM on July 27

The following is just my own speculation: the Moon serves to mediate the Earth's atmosphere, and maintain it at about it's current level. The Moon is the reason that the Earth doesn't have an atmosphere like Venus (where surface atmospheric pressure is 90 times Earth) or like Mars (2%).

How? Two effects. First, tidal flexing prevents the Earth's tectonic plates from freezing together. Tectonic movement means volcanoes, and that's where new atmosphere comes from.

But the other effect is even more important. There's a point between Earth and Moon which is gravitationally in balance between the two. A gas particle that falls off that ridge on the Moon's side then can go into orbit around the Moon and get a free ride all the way to the far side of the Moon, outside the Earth's magnetic field where it can be blown away by the solar wind.

So the Moon is constantly causing new atmosphere to be created here, and at the same time it is constantly vacuuming up existing atmosphere and funneling it into the solar wind to be taken away. When those two are in equilibrium, you get about as much atmosphere as we currently have, which turns out to be conducive to life.
posted by Chocolate Pickle at 10:51 AM on July 27

By the way, the primary important function of Earth's magnetic field is that most of the time it protects the atmosphere from the solar wind. Periodic brief collapses of the field during reversals don't represent a substantial problem, as long as the field is strong most of the time.

But if there's no field for very long periods of time (a billion years or so), then you get an atmosphere like that of Mars.
posted by Chocolate Pickle at 10:53 AM on July 27