Coincidence? I think not...
February 16, 2010 6:39 PM Subscribe
Please help me fact-check the following claims about the solar system.
I'm preparing a discussion section for an astronomy class tomorrow morning. One of the activities I'd like to have is to give students a list of facts about the solar system. For each fact, they have to decide whether that fact has a simple explanation, or if it's a coincidence. Since I'm not an expert in the history of the solar system, I would like to use the hive-mind to check my assumptions. Moreover, if you could supply additional coincidences (or facts that might be thought to be coincidences) about the solar system I would be thrilled.
Here goes:
1. All the planets orbit in the same plane (the 'ecliptic')
My answer: That's not a coincidence. All the planets formed from the solar nebula's accretionary disk, which is flat because motion cancels through collisions in all directions, except that of the original angular momentum of the gas cloud.
2. Even though Kepler's First Law predicts that a planet could orbit the Sun in any elliptical path, the planets' orbits are actually nearly circular.
That's not a coincidence. Highly eccentric orbits were disfavored by the collisions of planetesimals.
3. The Moon is the same apparent size as the Sun, so that the Moon exactly covers the Sun during Solar eclipses.
That's a coincidence. Since the Moon is moving away from the Earth, that won't be the case a couple million years from now.
4. Our Moon is larger than Mars' moons.
That's not a coincidence. It is due to the different history of our Moon (giant impact hypothesis). Neither the Earth nor Mars would be capable of gravitationally capturing an object as heavy as our Moon.
5. The Earth only has one moon.
I think that's a coincidence... Is it?
6. Our Moon is denser than Mars' moons. [3,346 kg/m³ vs. 1,887 kg/m³ for Phobos; Deimos is even less dense]
That's also a consequence of the giant impact hypothesis. During the impact, material from the Earth's low-density surface was scraped off and thrown into space to eventually form the Moon.
7. There is quite a bit of water on Earth.
That's a coincidence. We think icy comets that collided with the Earth brought a lot of the water with them.
8. There is a star, Polaris, that the Earth's rotation axis points towards, making it the North Star.
That's a coincidence. Due to precession, there isn't always be a star aligned with the Earth's rotation.
9. The Earth has an atmosphere, but the Moon doesn't.
Not a coincidence. The Earth is a lot more massive than the Moon, and so can retain an atmosphere. This is also why Mars' atmosphere is thinner than the Earth's.
10. The frost line for the early Solar System was estimated to be 2.7 AU from the Sun. That's right in the middle of the Asteroid belt.
I think that's a coincidence. The Asteroid belt's location is determined by the presence of Jupiter, and has nothing to do with the location of the frost line. Is that correct?
11. The terrestrial planets are close to the Sun, while the jovian planets are further away.
Not a coincidence. Terrestrial planets were within the frost line.
12. Jovian planets are larger and heavier than terrestrial planets.
Not a coincidence. The seeds of jovian planets were larger than those of terrestrial planets because they included frozen hydrogen compounds. Heavier seeds resulted in heavier planetesimals and eventually heavier planets.
13. The Moon's orbit is nearly circular (it has low eccentricity), just like the planets' orbits around the Sun.
Same answer as in 2.
14. Mercury, the closest planet to the Sun, is also the smallest planet in the Solar System.
Not a coincidence. Similar answer to 12. Fewer rocks were able to condense so close to the Sun, so the seeds of planets near Mercury's current location were smaller.
15. The planets orbit and rotate counter-clockwise as viewed from the North. Even the Sun rotates in the same direction.
Not a coincidence. I think this is a consequence of conservation of angular momentum. Is there a simple explanation for this?
I'm preparing a discussion section for an astronomy class tomorrow morning. One of the activities I'd like to have is to give students a list of facts about the solar system. For each fact, they have to decide whether that fact has a simple explanation, or if it's a coincidence. Since I'm not an expert in the history of the solar system, I would like to use the hive-mind to check my assumptions. Moreover, if you could supply additional coincidences (or facts that might be thought to be coincidences) about the solar system I would be thrilled.
Here goes:
1. All the planets orbit in the same plane (the 'ecliptic')
My answer: That's not a coincidence. All the planets formed from the solar nebula's accretionary disk, which is flat because motion cancels through collisions in all directions, except that of the original angular momentum of the gas cloud.
2. Even though Kepler's First Law predicts that a planet could orbit the Sun in any elliptical path, the planets' orbits are actually nearly circular.
That's not a coincidence. Highly eccentric orbits were disfavored by the collisions of planetesimals.
3. The Moon is the same apparent size as the Sun, so that the Moon exactly covers the Sun during Solar eclipses.
That's a coincidence. Since the Moon is moving away from the Earth, that won't be the case a couple million years from now.
4. Our Moon is larger than Mars' moons.
That's not a coincidence. It is due to the different history of our Moon (giant impact hypothesis). Neither the Earth nor Mars would be capable of gravitationally capturing an object as heavy as our Moon.
5. The Earth only has one moon.
I think that's a coincidence... Is it?
6. Our Moon is denser than Mars' moons. [3,346 kg/m³ vs. 1,887 kg/m³ for Phobos; Deimos is even less dense]
That's also a consequence of the giant impact hypothesis. During the impact, material from the Earth's low-density surface was scraped off and thrown into space to eventually form the Moon.
7. There is quite a bit of water on Earth.
That's a coincidence. We think icy comets that collided with the Earth brought a lot of the water with them.
8. There is a star, Polaris, that the Earth's rotation axis points towards, making it the North Star.
That's a coincidence. Due to precession, there isn't always be a star aligned with the Earth's rotation.
9. The Earth has an atmosphere, but the Moon doesn't.
Not a coincidence. The Earth is a lot more massive than the Moon, and so can retain an atmosphere. This is also why Mars' atmosphere is thinner than the Earth's.
10. The frost line for the early Solar System was estimated to be 2.7 AU from the Sun. That's right in the middle of the Asteroid belt.
I think that's a coincidence. The Asteroid belt's location is determined by the presence of Jupiter, and has nothing to do with the location of the frost line. Is that correct?
11. The terrestrial planets are close to the Sun, while the jovian planets are further away.
Not a coincidence. Terrestrial planets were within the frost line.
12. Jovian planets are larger and heavier than terrestrial planets.
Not a coincidence. The seeds of jovian planets were larger than those of terrestrial planets because they included frozen hydrogen compounds. Heavier seeds resulted in heavier planetesimals and eventually heavier planets.
13. The Moon's orbit is nearly circular (it has low eccentricity), just like the planets' orbits around the Sun.
Same answer as in 2.
14. Mercury, the closest planet to the Sun, is also the smallest planet in the Solar System.
Not a coincidence. Similar answer to 12. Fewer rocks were able to condense so close to the Sun, so the seeds of planets near Mercury's current location were smaller.
15. The planets orbit and rotate counter-clockwise as viewed from the North. Even the Sun rotates in the same direction.
Not a coincidence. I think this is a consequence of conservation of angular momentum. Is there a simple explanation for this?
# 4 - Earth's moon is larger than Mars' moons.
I would label that a coincidence, precisely because of the giant impact hypothesis. Earth's size and mass is roughly similar to that of Mars, so unless there are factors specific to Earth that make it more likely to have been hit by a really-big-thing, there's no reason that Mars couldn't have had a massive impact and formed big moon.
posted by Wulfhere at 7:06 PM on February 16, 2010
I would label that a coincidence, precisely because of the giant impact hypothesis. Earth's size and mass is roughly similar to that of Mars, so unless there are factors specific to Earth that make it more likely to have been hit by a really-big-thing, there's no reason that Mars couldn't have had a massive impact and formed big moon.
posted by Wulfhere at 7:06 PM on February 16, 2010
1) Pluto is only loosely in the ecliptic. It's off by about 17 degrees.
posted by Tell Me No Lies at 7:10 PM on February 16, 2010
posted by Tell Me No Lies at 7:10 PM on February 16, 2010
On (5), Neil Comins has a book called What if the Moon Didn't Exist?, which explores the condition of the title and other variations on solar systems. Following the giant impact hypothesis, he draws quite a number of consequences, at least one of which might have prevented our own evolution-- namely, Earth might have retained a suffocating Venus-like atmosphere.
If that's true, then it's not exactly coincidence that Earth has a large moon: if it didn't we might not be here.
On (3) you might point out annular eclipses as showing that the apparent size of the sun and moon aren't identical.
On (8) you might want to say "...a bright star". Some bright kid might point out that there are very likely plenty of stars at the celestial pole that are too dim to see.
On (14), what about Ceres and other asteroids? The solar system doesn't care about our definition of "planets". :)
posted by zompist at 7:10 PM on February 16, 2010
If that's true, then it's not exactly coincidence that Earth has a large moon: if it didn't we might not be here.
On (3) you might point out annular eclipses as showing that the apparent size of the sun and moon aren't identical.
On (8) you might want to say "...a bright star". Some bright kid might point out that there are very likely plenty of stars at the celestial pole that are too dim to see.
On (14), what about Ceres and other asteroids? The solar system doesn't care about our definition of "planets". :)
posted by zompist at 7:10 PM on February 16, 2010
Actually Mars is much smaller than the Earth -- Mars' diameter is 53% of Earth, and its mass is 11% of Earth. (source)
Mars' moons are likely captured asteroids from the Asteroid Belt -- it seems like only Mars or Jupiter would be in a position to do that. (I know Jupiter has 60+ moons ... don't know if any of the tiniest are thought to be captured asteroids.)
posted by lisa g at 7:15 PM on February 16, 2010
Mars' moons are likely captured asteroids from the Asteroid Belt -- it seems like only Mars or Jupiter would be in a position to do that. (I know Jupiter has 60+ moons ... don't know if any of the tiniest are thought to be captured asteroids.)
posted by lisa g at 7:15 PM on February 16, 2010
15. Uranus rotates on an axis that's closer to parallel with the ecliptic than not.
posted by Zed at 7:20 PM on February 16, 2010
posted by Zed at 7:20 PM on February 16, 2010
Best answer: Here are my comments on your assumptions. This is not my primary field of research, but I TA'd for a solar system class a couple of times way back when in grad school. Hope this helps:
1. I don't think you need to invoke collisions to get a flat disk. Just gravity and conservation of angular momentum get you a flat disk. Because of gravity, a particle at any radius wants to move directly to the center, in this case the sun. However, moving to the plane is the closest it can get and still conserve angular momentum.
5. I don't think this is entirely a coincidence. By that I mean that if you made multiple simulations of our solar system, the Earth would statistically have fewer moons than, say, Jupiter. There just weren't that many really big things to ram into the Earth and create lots of moons like the one we have. Jupiter's moons were probably created out of circumplanetary material, which Jupiter was able to accrete because it has a huge mass. It has a huge mass because it is outside the snowline and had access to abundant ice. Earth is within the snow line, so it makes sense that it has a smaller mass, and therefore a smaller circumplanetary disk, and therefore fewer moons.
7. I wouldn't necessarily classify this as a coincidence, either. I'm not sure that there is a consensus on the origin of Earth's water, but according to this paper, "water is accreted in comparable amounts from a few planetary embryos in a ``hit or miss'' way and from millions of planetesimals in a statistically robust process. Variations in water content are likely to be caused by fluctuations in the number of water-rich embryos accreted, as well as from systematic effects, such as planetary mass and location, and giant planet properties."
10. I don't think this is a coincidence. Jupiter was able to grow to such a large mass because it was outside the snow line. The large mass of Jupiter in turn explains why a planet was never able to "coagulate" at the position of the asteroid belt. That's why it's a bunch of asteroids instead of a planet.
15. I think the simple explanation is that the primordial cloud out of which the solar nebula formed had some net rotation, and angular momentum was conserved as it collapsed under the influence of gravity. Why did it have some net rotation? Wouldn't it be weirder if it didn't, and all the bulk motions perfectly canceled out?
posted by pizzazz at 7:38 PM on February 16, 2010
1. I don't think you need to invoke collisions to get a flat disk. Just gravity and conservation of angular momentum get you a flat disk. Because of gravity, a particle at any radius wants to move directly to the center, in this case the sun. However, moving to the plane is the closest it can get and still conserve angular momentum.
5. I don't think this is entirely a coincidence. By that I mean that if you made multiple simulations of our solar system, the Earth would statistically have fewer moons than, say, Jupiter. There just weren't that many really big things to ram into the Earth and create lots of moons like the one we have. Jupiter's moons were probably created out of circumplanetary material, which Jupiter was able to accrete because it has a huge mass. It has a huge mass because it is outside the snowline and had access to abundant ice. Earth is within the snow line, so it makes sense that it has a smaller mass, and therefore a smaller circumplanetary disk, and therefore fewer moons.
7. I wouldn't necessarily classify this as a coincidence, either. I'm not sure that there is a consensus on the origin of Earth's water, but according to this paper, "water is accreted in comparable amounts from a few planetary embryos in a ``hit or miss'' way and from millions of planetesimals in a statistically robust process. Variations in water content are likely to be caused by fluctuations in the number of water-rich embryos accreted, as well as from systematic effects, such as planetary mass and location, and giant planet properties."
10. I don't think this is a coincidence. Jupiter was able to grow to such a large mass because it was outside the snow line. The large mass of Jupiter in turn explains why a planet was never able to "coagulate" at the position of the asteroid belt. That's why it's a bunch of asteroids instead of a planet.
15. I think the simple explanation is that the primordial cloud out of which the solar nebula formed had some net rotation, and angular momentum was conserved as it collapsed under the influence of gravity. Why did it have some net rotation? Wouldn't it be weirder if it didn't, and all the bulk motions perfectly canceled out?
posted by pizzazz at 7:38 PM on February 16, 2010
I think you need to consider a little more what you mean by coincidence.
Do you mean: imagine a billion solar systems with 10 planets [and life on the 3rd from the sun]. Which of the following are likely to hold, and which are probably coincidental for our system?
Bigger objections:
4&6) Echoing Wulfhere, I don't think you've justified these being not coincidence
15) The story is that the same angular momentum which created the L of their orbit is the source of their Spin.
posted by tintexas at 7:44 PM on February 16, 2010
Do you mean: imagine a billion solar systems with 10 planets [and life on the 3rd from the sun]. Which of the following are likely to hold, and which are probably coincidental for our system?
Bigger objections:
4&6) Echoing Wulfhere, I don't think you've justified these being not coincidence
15) The story is that the same angular momentum which created the L of their orbit is the source of their Spin.
posted by tintexas at 7:44 PM on February 16, 2010
1. All the planets orbit in the same plane (the 'ecliptic')
As you can see in this chart, the planets' orbits are inclined as much as 7 degrees from the ecliptic. But yes, pretty close.
6. Our Moon is denser than Mars' moons. [3,346 kg/m³ vs. 1,887 kg/m³ for Phobos; Deimos is even less dense]
That's also a consequence of the giant impact hypothesis. During the impact, material from the Earth's low-density surface was scraped off and thrown into space to eventually form the Moon.
This explanation doesn't make sense to me - if our moon is MORE dense, why is it made of the LOW-density material from the Earth's crust?
8. There is a star, Polaris, that the Earth's rotation axis points towards, making it the North Star.
That's a coincidence. Due to precession, there isn't always be a star aligned with the Earth's rotation.
I think it makes more sense to say "there are a LOT of visible stars, so the Earth's axis has got to be pointed pretty close to at least one of them."
12. Jovian planets are larger and heavier than terrestrial planets.
Not a coincidence. The seeds of jovian planets were larger than those of terrestrial planets because they included frozen hydrogen compounds. Heavier seeds resulted in heavier planetesimals and eventually heavier planets.
I don't think this is correct. The Earth is much denser than Jupiter on average, and it was formed from approximately the same materials. I think the reason the outer planets are so much bigger is because their orbital paths swept up massively more material; there was just more matter out there in those large orbits to get balled up into a planet.
9. The Earth has an atmosphere, but the Moon doesn't.
Not a coincidence. The Earth is a lot more massive than the Moon, and so can retain an atmosphere. This is also why Mars' atmosphere is thinner than the Earth's.
This isn't quite correct. It's true that the Martian atmosphere is much thinner than the Earth's, but it's TOO thin for this explanation to make sense. The reason the Martian atmosphere is so much thinner is because its molten core solidified long ago, which halted the magnetic dynamo that generates planetary magnetic fields. Once Mars's magnetic field disappeared, the Martian atmosphere was no longer shielded from the solar wind, and was slowly stripped away into space.
Here's a non-coincidence:
The moon always shows the same face to the Earth.
This is not a coincidence - the Moon has long been subject to tidal forces from the Earth, which basically means it gets stretched/squashed as it rotates in the Earth's gravitational field because the Earth's gravitational field gets weaker across the diameter of the Moon. This effect slowly drags on the Moon's rotation until the Moon is "tidally locked", which means it rotates around its axis at the same frequency as it orbits the Earth; thus it shows the same face to the Earth at all times. There are many satellites in the solar system that are tidally locked to their parent, for example: Pluto/Charon, Mars/Phobos, Mars/Deimos, plus many of the gas giants' moons.
posted by Salvor Hardin at 7:48 PM on February 16, 2010
As you can see in this chart, the planets' orbits are inclined as much as 7 degrees from the ecliptic. But yes, pretty close.
6. Our Moon is denser than Mars' moons. [3,346 kg/m³ vs. 1,887 kg/m³ for Phobos; Deimos is even less dense]
That's also a consequence of the giant impact hypothesis. During the impact, material from the Earth's low-density surface was scraped off and thrown into space to eventually form the Moon.
This explanation doesn't make sense to me - if our moon is MORE dense, why is it made of the LOW-density material from the Earth's crust?
8. There is a star, Polaris, that the Earth's rotation axis points towards, making it the North Star.
That's a coincidence. Due to precession, there isn't always be a star aligned with the Earth's rotation.
I think it makes more sense to say "there are a LOT of visible stars, so the Earth's axis has got to be pointed pretty close to at least one of them."
12. Jovian planets are larger and heavier than terrestrial planets.
Not a coincidence. The seeds of jovian planets were larger than those of terrestrial planets because they included frozen hydrogen compounds. Heavier seeds resulted in heavier planetesimals and eventually heavier planets.
I don't think this is correct. The Earth is much denser than Jupiter on average, and it was formed from approximately the same materials. I think the reason the outer planets are so much bigger is because their orbital paths swept up massively more material; there was just more matter out there in those large orbits to get balled up into a planet.
9. The Earth has an atmosphere, but the Moon doesn't.
Not a coincidence. The Earth is a lot more massive than the Moon, and so can retain an atmosphere. This is also why Mars' atmosphere is thinner than the Earth's.
This isn't quite correct. It's true that the Martian atmosphere is much thinner than the Earth's, but it's TOO thin for this explanation to make sense. The reason the Martian atmosphere is so much thinner is because its molten core solidified long ago, which halted the magnetic dynamo that generates planetary magnetic fields. Once Mars's magnetic field disappeared, the Martian atmosphere was no longer shielded from the solar wind, and was slowly stripped away into space.
Here's a non-coincidence:
The moon always shows the same face to the Earth.
This is not a coincidence - the Moon has long been subject to tidal forces from the Earth, which basically means it gets stretched/squashed as it rotates in the Earth's gravitational field because the Earth's gravitational field gets weaker across the diameter of the Moon. This effect slowly drags on the Moon's rotation until the Moon is "tidally locked", which means it rotates around its axis at the same frequency as it orbits the Earth; thus it shows the same face to the Earth at all times. There are many satellites in the solar system that are tidally locked to their parent, for example: Pluto/Charon, Mars/Phobos, Mars/Deimos, plus many of the gas giants' moons.
posted by Salvor Hardin at 7:48 PM on February 16, 2010
Best answer: 2. Most of the extraterrestrial planets located so far are in highly elliptical orbits. To a great extent that's selection bias; the way we detect planets outside our solar system tends to prefer finding big planets in close orbits. Still, it does mean that it's not impossible for elliptical orbits to exist. And I remember reading that it's been worked out that the gravitational influence of Jupiter is so strong that it controls the orbits of all the other planets. (Jupiter is more massive than all the rest of the planets combined.) It happens to be in an orbit which isn't very eccentric, and it forces all the others to do the same.
5. Not coincidence, just timing. Just after the impact, the debris in orbit would have been like the asteroid belt except in miniature. Over time pieces of it accreted, and the logical conclusion of the accretion process was a single body.
6. your argument contradicts itself; you're making an argument for why Luna should be less dense. The reason Luna is more dense than Phobos and Deimos is that the original impacting body contained a lot of metal. Most of that metal ended up on Earth in the impact but a lot of it ended up mixed in the orbital debris field which eventually formed the moon. As to Phobos and Deimos, they are probably captured asteroids. So the moon has a lower percentage of metal than Earth, but a lot higher than asteroids. (And Phobos is spiraling in; in 11 million years it will be gone.)
7. Not coincidence. That's a consequence of process by which the moon was formed. First, Earth ended up with a lot more uranium and thorium in its core, which have been doing their atomic power thing ever since, making heat. Second, the moon causes tidal flexing. The combination of those two means that plate tectonics continues to operate here, which means we have active volcanoes. Nearly all of our atmosphere, including the water, comes from volcanoes.
11. Not coincidence, but also not related to the frost line. The original solar nebula contained a mixture of elements, but in the inner system the Sun scooped up most of the hydrogen. In the outer system that didn't happen as much.
12. The reason they're larger and heavier is because they have more gas, which in the inner system was far more likely to fall into the sun.
14. Not yet clear either way. The extraterrestrial planet search has found planets with mass much greater than Jupiter in orbits closer than that of Mercury.
15. Mercury is tide-locked. (It's stuck in a 3:2 resonance, but it's because of solar tides.) Venus counter-rotates. Uranus is on its side; at one point in its orbit its north pole almost points at the sun.
I'd add one more to your list: The day length on Earth and Mars are nearly the same. Seems to be coincidence.
posted by Chocolate Pickle at 7:49 PM on February 16, 2010 [2 favorites]
5. Not coincidence, just timing. Just after the impact, the debris in orbit would have been like the asteroid belt except in miniature. Over time pieces of it accreted, and the logical conclusion of the accretion process was a single body.
6. your argument contradicts itself; you're making an argument for why Luna should be less dense. The reason Luna is more dense than Phobos and Deimos is that the original impacting body contained a lot of metal. Most of that metal ended up on Earth in the impact but a lot of it ended up mixed in the orbital debris field which eventually formed the moon. As to Phobos and Deimos, they are probably captured asteroids. So the moon has a lower percentage of metal than Earth, but a lot higher than asteroids. (And Phobos is spiraling in; in 11 million years it will be gone.)
7. Not coincidence. That's a consequence of process by which the moon was formed. First, Earth ended up with a lot more uranium and thorium in its core, which have been doing their atomic power thing ever since, making heat. Second, the moon causes tidal flexing. The combination of those two means that plate tectonics continues to operate here, which means we have active volcanoes. Nearly all of our atmosphere, including the water, comes from volcanoes.
11. Not coincidence, but also not related to the frost line. The original solar nebula contained a mixture of elements, but in the inner system the Sun scooped up most of the hydrogen. In the outer system that didn't happen as much.
12. The reason they're larger and heavier is because they have more gas, which in the inner system was far more likely to fall into the sun.
14. Not yet clear either way. The extraterrestrial planet search has found planets with mass much greater than Jupiter in orbits closer than that of Mercury.
15. Mercury is tide-locked. (It's stuck in a 3:2 resonance, but it's because of solar tides.) Venus counter-rotates. Uranus is on its side; at one point in its orbit its north pole almost points at the sun.
I'd add one more to your list: The day length on Earth and Mars are nearly the same. Seems to be coincidence.
posted by Chocolate Pickle at 7:49 PM on February 16, 2010 [2 favorites]
I'm wrong about #11.
posted by Chocolate Pickle at 7:56 PM on February 16, 2010
posted by Chocolate Pickle at 7:56 PM on February 16, 2010
Oh, here's another one:
Around 75% of (the baryonic matter in) our universe is made of hydrogen, and another 23% is made of helium. Somehow, on the Earth, we got all these awesome heavier elements, like silicon, and iron, etc. How convenient!
There's a reason for this - our solar system was formed from the ashes of dead stars! Inside a star, stellar fusion converts light elements like hydrogen and helium into heavier elements, which explains some of the heavier elements. But there's another puzzle - fusion is only energetically favorable for producing things up to iron - after that, you LOSE energy when you fuse atoms, which means no star can produce any noticeable amount of any element heavier than iron. So what explains the other 80-something awesome elements that we have? Turns out, when fusion shuts off in a star because it's used up all its light elements as fuel, it implodes, and in its last gasp, when it has reached a critical density while imploding, in one mad burst, it fuses massive quantities of heavy elements together to make everything heavier than iron, then explodes. Couple billion years later, a solar system condenses out of the mixed up remnants of supernovae, leading to the development of an ecosystem which leads to a species the cultural pinnacle of which is MetaFilter.
posted by Salvor Hardin at 8:03 PM on February 16, 2010 [3 favorites]
Around 75% of (the baryonic matter in) our universe is made of hydrogen, and another 23% is made of helium. Somehow, on the Earth, we got all these awesome heavier elements, like silicon, and iron, etc. How convenient!
There's a reason for this - our solar system was formed from the ashes of dead stars! Inside a star, stellar fusion converts light elements like hydrogen and helium into heavier elements, which explains some of the heavier elements. But there's another puzzle - fusion is only energetically favorable for producing things up to iron - after that, you LOSE energy when you fuse atoms, which means no star can produce any noticeable amount of any element heavier than iron. So what explains the other 80-something awesome elements that we have? Turns out, when fusion shuts off in a star because it's used up all its light elements as fuel, it implodes, and in its last gasp, when it has reached a critical density while imploding, in one mad burst, it fuses massive quantities of heavy elements together to make everything heavier than iron, then explodes. Couple billion years later, a solar system condenses out of the mixed up remnants of supernovae, leading to the development of an ecosystem which leads to a species the cultural pinnacle of which is MetaFilter.
posted by Salvor Hardin at 8:03 PM on February 16, 2010 [3 favorites]
8. There is a star, Polaris, that the Earth's rotation axis points towards, making it the North Star.
Moreover, it's a coincidence that Polaris happens to be a relatively bright star, without a lot of other nearby bright stars to confuse you, which makes it a particularly good useful navigational aide. As a double (triple?) coincidence, there is a bright, clearly defined constellation, the Big Dipper, which makes an excellent "pointer" to Polaris.
posted by Cool Papa Bell at 8:12 PM on February 16, 2010
Moreover, it's a coincidence that Polaris happens to be a relatively bright star, without a lot of other nearby bright stars to confuse you, which makes it a particularly good useful navigational aide. As a double (triple?) coincidence, there is a bright, clearly defined constellation, the Big Dipper, which makes an excellent "pointer" to Polaris.
posted by Cool Papa Bell at 8:12 PM on February 16, 2010
7.Like chocolate pickle says, most of the water has outgassed from the molten earth itself. Earth has a strong magnetic field and this keeps the water from being stripped away by the solar radiation. Venus, on the other hand has no magnetic field and a very hot, deep atmosphere where the water molecules were driven high up and stripped away. This is a good page of info on Venus.
posted by bonobothegreat at 8:14 PM on February 16, 2010
posted by bonobothegreat at 8:14 PM on February 16, 2010
You could change the wording of #14 to clarify that it's not a coincidence that the smallest planet *formed* closest to the central star. This would accommodate the observations of extrasolar hot Jupiters, since most folks believe that hot Jupiters migrated to their observed position.
posted by pizzazz at 8:37 PM on February 16, 2010
posted by pizzazz at 8:37 PM on February 16, 2010
8. There is a star, Polaris, that the Earth's rotation axis points towards, making it the North Star.
I don't think this is coincidence or explanation, at least not based on the answer you're looking for/what I know of precession. You're basically saying this statement is false, which does not make it a coincidence. Maybe it should say "There is *currently* a star" or "that the Earth's rotation axis *currently* points towards" instead?
posted by kro at 8:38 PM on February 16, 2010
The magnetic field of the Earth is strong because Earth's core is molten, which is due to the large amount of uranium and thorium down there and because of tidal flexing by the moon.
The more interesting question is why Earth's atmosphere isn't as thick as Venus, since we've had even more vulcanism. I have my own idea about that, but I don't know whether it's scientific orthodoxy.
Earth's atmosphere extends out a long way, at decreasing pressures. There's a point between the Earth and Moon of gravitational equipotential, and Earth's atmosphere isn't yet negligible at that point. Gas particles which reach there can go into orbit around the Moon instead, which means they can get a free ride all the way around the moon to the other side -- which is outside the Earth's magnetic field, exposed to the solar wind. So the moon acts like a big vacuum cleaner, sucking up atmosphere from Earth and feeding it into the solar wind.
Meanwhile, the moon's tides help keep vulcanism going here. The two work against each other. New atmosphere is produced at a steady rate, but the Moon's efficiency at getting rid of old atmosphere is proportional to how much air the Earth has. When the Earth's atmosphere is thicker, the Moon is more efficient at getting rid of it. The two effects are at equilibrium now, and the equilibrium point is a lot lower than what Venus has.
If Earth didn't have a moon, our atmosphere would be like Venus.
Interestingly, it may stay close to this. As Earth ages, the fissionables in the core are getting used up, and vulcanism is slowing, as is production of new atmosphere. But the moon is getting further from Earth, which makes it less effective at getting rid of old atmosphere.
posted by Chocolate Pickle at 8:52 PM on February 16, 2010
The more interesting question is why Earth's atmosphere isn't as thick as Venus, since we've had even more vulcanism. I have my own idea about that, but I don't know whether it's scientific orthodoxy.
Earth's atmosphere extends out a long way, at decreasing pressures. There's a point between the Earth and Moon of gravitational equipotential, and Earth's atmosphere isn't yet negligible at that point. Gas particles which reach there can go into orbit around the Moon instead, which means they can get a free ride all the way around the moon to the other side -- which is outside the Earth's magnetic field, exposed to the solar wind. So the moon acts like a big vacuum cleaner, sucking up atmosphere from Earth and feeding it into the solar wind.
Meanwhile, the moon's tides help keep vulcanism going here. The two work against each other. New atmosphere is produced at a steady rate, but the Moon's efficiency at getting rid of old atmosphere is proportional to how much air the Earth has. When the Earth's atmosphere is thicker, the Moon is more efficient at getting rid of it. The two effects are at equilibrium now, and the equilibrium point is a lot lower than what Venus has.
If Earth didn't have a moon, our atmosphere would be like Venus.
Interestingly, it may stay close to this. As Earth ages, the fissionables in the core are getting used up, and vulcanism is slowing, as is production of new atmosphere. But the moon is getting further from Earth, which makes it less effective at getting rid of old atmosphere.
posted by Chocolate Pickle at 8:52 PM on February 16, 2010
9. The Earth has an atmosphere, but the Moon doesn't.
Pragmatically true, but not strictly true. The moon has a constantly replenished atmosphere made up of some 80,000 atoms per cubic centimeter. This is a one hundred trillionth the atmosphere of Earth, but significantly more dense than what, say, 100 miles away from the lunar surface has to offer.
posted by wackybrit at 9:01 PM on February 16, 2010
Pragmatically true, but not strictly true. The moon has a constantly replenished atmosphere made up of some 80,000 atoms per cubic centimeter. This is a one hundred trillionth the atmosphere of Earth, but significantly more dense than what, say, 100 miles away from the lunar surface has to offer.
posted by wackybrit at 9:01 PM on February 16, 2010
#3 -- The moon is in an elliptical orbit around the earth so its apparent size changes. If there's an eclipse at its highest point, its apparent size is smaller than the sun, and you get what's known as an "annular eclipse".
posted by Chocolate Pickle at 9:07 PM on February 16, 2010
posted by Chocolate Pickle at 9:07 PM on February 16, 2010
If Earth didn't have a moon, our atmosphere would be like Venus.
This is not the consensus view, which holds that the atmosphere of Venus is so thick because it of the runaway greenhouse effect.
posted by pizzazz at 9:30 PM on February 16, 2010 [1 favorite]
This is not the consensus view, which holds that the atmosphere of Venus is so thick because it of the runaway greenhouse effect.
posted by pizzazz at 9:30 PM on February 16, 2010 [1 favorite]
Pizzazz, I think you have cause and effect backwards. They think Venus has a runaway greenhouse effect because its atmosphere is so thick.
posted by Chocolate Pickle at 9:50 PM on February 16, 2010
posted by Chocolate Pickle at 9:50 PM on February 16, 2010
Chocolate Pickle, it's a positive feedback loop, so cause and effect are intertwined. Venus is slightly closer to the sun than the Earth, so it had a slightly higher surface temperature, and therefore a higher initial fraction of water in vapor form. Water is a greenhouse gas, so this caused an increase in temperature, which vaporized more of the water, which made the atmosphere thicker, which caused an increase in temperature. And so on.
posted by pizzazz at 9:58 PM on February 16, 2010
posted by pizzazz at 9:58 PM on February 16, 2010
Meanwhile, the moon's tides help keep vulcanism going here.
Tidal friction is not a very significant contribution to the heat flow of the earth, unlike the moons of Jupiter. Tidal friction in the earth amounts to at most 5% of the heat. The major contributors to the earth's heat flow are the original heat of formation, gravitational energy of density differentiation, and radioactive decay.
posted by JackFlash at 10:21 PM on February 16, 2010 [1 favorite]
Tidal friction is not a very significant contribution to the heat flow of the earth, unlike the moons of Jupiter. Tidal friction in the earth amounts to at most 5% of the heat. The major contributors to the earth's heat flow are the original heat of formation, gravitational energy of density differentiation, and radioactive decay.
posted by JackFlash at 10:21 PM on February 16, 2010 [1 favorite]
Jack Flash, I think the most important aspect of tides WRT plate tectonics is that it flexes the joints between the plates and helps keep them loose. But yeah, those other factors are more important.
Pizzazz, good point. I don't really insist on my idea about the moon, in any case.
posted by Chocolate Pickle at 10:33 PM on February 16, 2010
Pizzazz, good point. I don't really insist on my idea about the moon, in any case.
posted by Chocolate Pickle at 10:33 PM on February 16, 2010
By the way, regards #15, Triton is in a retrograde orbit. For that reason among others, Triton is now thought to be a capture, rather than a body formed out of Neptune's planetary nebula.
posted by Chocolate Pickle at 10:45 PM on February 16, 2010
posted by Chocolate Pickle at 10:45 PM on February 16, 2010
Chocolate Pickle, the "without our heavy moon to thin the atmosphere, we'd be Venusian" theory has been mainstream at some point (I remember it from Larry Niven novels) but AIUI it's no longer current. I don't know why it's fallen out of favor though.
pizzazz: You do need collisions or some sort of lossy process. If you imagine the pre-disk cloud as containing particles with basically random velocities (but with a slightly nonzero average angular momentum) then they'll all be orbiting in different planes. Each particle will be moving in a plane, yes, but in order to get everything in the same plane you need interactions among them. Collisions are the most obvious interaction. Gravitational ejection seems unlikely to be a big factor for a dust cloud.
Re (2): I think elliptical orbits are also less stable in the long term against perturbations by other planets. See also Bode's law: coincidence or explainable? This is also related to (5), I think: our moon is big enough that there aren't (many?) stable orbits in which we could have another moon.
posted by hattifattener at 1:40 AM on February 17, 2010
pizzazz: You do need collisions or some sort of lossy process. If you imagine the pre-disk cloud as containing particles with basically random velocities (but with a slightly nonzero average angular momentum) then they'll all be orbiting in different planes. Each particle will be moving in a plane, yes, but in order to get everything in the same plane you need interactions among them. Collisions are the most obvious interaction. Gravitational ejection seems unlikely to be a big factor for a dust cloud.
Re (2): I think elliptical orbits are also less stable in the long term against perturbations by other planets. See also Bode's law: coincidence or explainable? This is also related to (5), I think: our moon is big enough that there aren't (many?) stable orbits in which we could have another moon.
posted by hattifattener at 1:40 AM on February 17, 2010
Something that I've always found cool:
There's a high concentration of asteroids 60 degrees ahead of and 60 degrees behind Jupiter (http://en.wikipedia.org/wiki/Jupiter_Trojan)
Not a coincidence - these are the Lagrange points where the gravitational pull of Jupiter and the sun are in balance: http://en.wikipedia.org/wiki/Lagrangian_point
posted by primer_dimer at 1:55 AM on February 17, 2010
There's a high concentration of asteroids 60 degrees ahead of and 60 degrees behind Jupiter (http://en.wikipedia.org/wiki/Jupiter_Trojan)
Not a coincidence - these are the Lagrange points where the gravitational pull of Jupiter and the sun are in balance: http://en.wikipedia.org/wiki/Lagrangian_point
posted by primer_dimer at 1:55 AM on February 17, 2010
I can't speak to the science like the others previous, but for numbers 5 & 7 (and possibly a few others) the word "coincidence" isn't as apt as "happenstance"; a coincidence implies two or more things aligning in some way, whereas those assertions concern the existence of a single item/material.
posted by kittyprecious at 5:58 AM on February 17, 2010
posted by kittyprecious at 5:58 AM on February 17, 2010
What a lot of good discussion! I don't have anything solar-systemy to add to it, but I want to play grammar nazi and student advocate for a minute.
8. .. there isn't always be a star
yes, I know your text is just a draft. I just want to make sure you catch this before cut/pasting it to the final version.
Also, from the student point of view, the fact that these statements generated so much discussion and disagreement from the participants here might lead to the conclusion that a graded test with these same questions on it might lead to a lot of argument in the classroom. Even the definition of "coincidence" itself has been debated, and I can see that some students might mean it one way and others a different way.
IMHO, I think you should be prepared to grade this not as strict right/wrong type of answers, but use 'how well can you defend your answer?' as the grading criteria.
posted by CathyG at 7:51 AM on February 17, 2010
8. .. there isn't always be a star
yes, I know your text is just a draft. I just want to make sure you catch this before cut/pasting it to the final version.
Also, from the student point of view, the fact that these statements generated so much discussion and disagreement from the participants here might lead to the conclusion that a graded test with these same questions on it might lead to a lot of argument in the classroom. Even the definition of "coincidence" itself has been debated, and I can see that some students might mean it one way and others a different way.
IMHO, I think you should be prepared to grade this not as strict right/wrong type of answers, but use 'how well can you defend your answer?' as the grading criteria.
posted by CathyG at 7:51 AM on February 17, 2010
hattifattener, I was indeed wrong, and you are correct that you need some kind of dissipation to form a disk. A complete answer would include three main ingredients:
1. gravity
2. angular momentum
3. viscosity (aka collisions between gas particles) in the gas (plus a little dust) cloud from which the solar nebular formed. When the original poster used the words "motion cancels through collisions", I wrongly thought she was referring to collisions between solid bodies. Once most of the (gas) mass is in a basically disky configuration, and solid bodies have had time to form, collisions among them may play some additional role in setting the thickness of the disk, but I'd have to think a bit more about that to comment.
posted by pizzazz at 8:31 AM on February 17, 2010
1. gravity
2. angular momentum
3. viscosity (aka collisions between gas particles) in the gas (plus a little dust) cloud from which the solar nebular formed. When the original poster used the words "motion cancels through collisions", I wrongly thought she was referring to collisions between solid bodies. Once most of the (gas) mass is in a basically disky configuration, and solid bodies have had time to form, collisions among them may play some additional role in setting the thickness of the disk, but I'd have to think a bit more about that to comment.
posted by pizzazz at 8:31 AM on February 17, 2010
I was going to say that a little more lead time would have been helpful for the hive-mind here, but everything I noticed was brought up by 1:40am at the latest. Go Mefi! How did your class go?
Just one clarification, to hattifattner's comment:
#13 The Moon's orbit does indeed get circularized by the same tidal effects that are causing it to recede. I'm not sure how strong a factor that is in #2; most tidal forces weaken as r^4 and I think the circularization effect is even worse at r^6. But it would explain why the really eccentric orbits seem to be out among Pluto and kin.
posted by roystgnr at 9:58 AM on February 17, 2010
Just one clarification, to hattifattner's comment:
#13 The Moon's orbit does indeed get circularized by the same tidal effects that are causing it to recede. I'm not sure how strong a factor that is in #2; most tidal forces weaken as r^4 and I think the circularization effect is even worse at r^6. But it would explain why the really eccentric orbits seem to be out among Pluto and kin.
posted by roystgnr at 9:58 AM on February 17, 2010
What a spectacularly good idea for a class!
4. Our Moon is larger than Mars' moons.
That's not a coincidence. It is due to the different history of our Moon (giant impact hypothesis). Neither the Earth nor Mars would be capable of gravitationally capturing an object as heavy as our Moon.
I'm sure you know this and are merely stating things a little infelicitously, but 2-body capture is impossible in principle without a collision in non-relativistic mechanics, regardless of the mass of the bodies involved.
posted by jamjam at 10:35 AM on February 17, 2010
4. Our Moon is larger than Mars' moons.
That's not a coincidence. It is due to the different history of our Moon (giant impact hypothesis). Neither the Earth nor Mars would be capable of gravitationally capturing an object as heavy as our Moon.
I'm sure you know this and are merely stating things a little infelicitously, but 2-body capture is impossible in principle without a collision in non-relativistic mechanics, regardless of the mass of the bodies involved.
posted by jamjam at 10:35 AM on February 17, 2010
Most important lesson: All this is subject to change because that's how science works. We make hypotheses and test them and, if necessary, go back and revise. I would probably talk about earlier theories about the origin of the moon, water on Mars, how many planets we thought we had over time, tectonic drift, and anything else that got revised.
posted by chairface at 3:22 PM on February 17, 2010
posted by chairface at 3:22 PM on February 17, 2010
Response by poster: Thank you for all your feedback and suggested coincidences. I removed some of the more contentious claims (or just plain wrong, like what I said about the Moon's density, which I now believe I just made up), and added Chocolate Pickle's length of a Martian day coincidence. I also made sure to bring up hot jupiters and other exceptions to the 'rules' (e.g. Venus and Uranus). The class went well and sparked some interesting discussions.
jamjam: My understanding is that Mars' moons were believed to have been captured through friction with Mars' previously thicker atmosphere. Friction causes the total energy of the asteroids to drop below zero, and hence to enter a bound orbit.
I hope you all had fun pondering these ideas. Thanks again for your help.
Extra credit for metafilter: Is there a reason why our Moon's orbit would be more or less in the ecliptic (5° inclination)? It seems to me that the giant impact hypothesis should allow the inclination to be random. Any ideas?
posted by spaghettification at 3:22 PM on February 17, 2010
jamjam: My understanding is that Mars' moons were believed to have been captured through friction with Mars' previously thicker atmosphere. Friction causes the total energy of the asteroids to drop below zero, and hence to enter a bound orbit.
I hope you all had fun pondering these ideas. Thanks again for your help.
Extra credit for metafilter: Is there a reason why our Moon's orbit would be more or less in the ecliptic (5° inclination)? It seems to me that the giant impact hypothesis should allow the inclination to be random. Any ideas?
posted by spaghettification at 3:22 PM on February 17, 2010
Is there a reason why our Moon's orbit would be more or less in the ecliptic (5° inclination)? It seems to me that the giant impact hypothesis should allow the inclination to be random. Any ideas?
My guess is that the impact was not from a random direction. It was a collision with another object in the plane of the accretionary disk. The impact object's orbit was in the plane of the ecliptic just like all the other planets.
posted by JackFlash at 6:04 PM on February 17, 2010
My guess is that the impact was not from a random direction. It was a collision with another object in the plane of the accretionary disk. The impact object's orbit was in the plane of the ecliptic just like all the other planets.
posted by JackFlash at 6:04 PM on February 17, 2010
The way I heard it, computer analysis yielded this scenario: Mass accumulated in one of Earth's trojan points. When it reached approximately the size of Mars, the gravitational trick that holds things into trojan points stopped working for it and it started drifting around. Eventually it moved close enough to the Earth for a collision.
Someone spent a bunch of time trying different kinds of collision scenarios with computer simulations to see what would happen. They only found one whose outcome matched what we observe: the other body made a glancing blow on Earth's equator, went past a bit, then fell back and hit the Earth square and was absorbed.
The first glancing blow knocked a lot of mass off of both bodies -- in the ecliptic, and that mass is what eventually became the Moon. And because of the nature of the glancing blow, what mainly came off was surface rock, which is why the Moon has a lower average density than the Earth. As a result of the collision, Earth ended up with a lot more metal and a lot less rock than it had previously had.
posted by Chocolate Pickle at 12:12 AM on February 18, 2010
Someone spent a bunch of time trying different kinds of collision scenarios with computer simulations to see what would happen. They only found one whose outcome matched what we observe: the other body made a glancing blow on Earth's equator, went past a bit, then fell back and hit the Earth square and was absorbed.
The first glancing blow knocked a lot of mass off of both bodies -- in the ecliptic, and that mass is what eventually became the Moon. And because of the nature of the glancing blow, what mainly came off was surface rock, which is why the Moon has a lower average density than the Earth. As a result of the collision, Earth ended up with a lot more metal and a lot less rock than it had previously had.
posted by Chocolate Pickle at 12:12 AM on February 18, 2010
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2. Pluto (dwarf planet, I know) has an elliptical orbit that intersects with Neptune's.
15. Venus rotates in the opposite direction from the other planets.
posted by lisa g at 6:56 PM on February 16, 2010