Subject: E.01 How did the solar system form? Author: Joseph Lazio <jlazio@patriot.net>
Any theory of the formation of the solar system must explain at least the following two observations: First, the planets, with the exception of Pluto, orbit in almost the same plane (the "ecliptic"). Second, the inner four planets are small and rocky, while the outer four planets are large and gaseous. One theory that does a reasonably good job of explaining these observations is the disk model.
The Sun is thought to have formed by the collapse of a large interstellar gas cloud. The original cloud was probably thousands of times larger than the present solar system. Initially the cloud had a very slow rotation rate (it's essentially impossible for one of these clouds to have a rotation rate of exactly zero). As it collapsed, it began rotating faster (much like a skater will spin faster if she pulls her arms to her sides---this principle is known as the "conservation of angular momentum"). The collapse process is not 100% efficient, though, so some of the material did not fall into the proto-Sun. This rotating gas that was left behind settled into a disk.
In addition to gas, interstellar clouds can also contain dust. Therefore, the rotating disk consisted of dust grains and gas. In the process of settling into a disk---and even after the disk had formed---the dust grains began to collide and stick together. Initially quite small, this process of colliding dust grains sticking together (known as "accretion") began to build up larger dust grains. The accretion process continued with large dust grains accreting to form small pebbles, small pebbles accreting to form large pebbles, pebbles forming rocks, rocks forming boulders, etc. Initially this process is quite random: Two dust grains collide only if their paths happen to cross. However, as particles became larger, they exert a larger gravitational force and attract smaller particles to them. Hence, once started, the accretion process can actually speed up.
The collapse process itself can generate considerable heat. Furthermore, as the Sun's mass grew, it eventually reached the point at which fusion reactions in its core could be sustained. The result was that there was a heat source in the middle of the disk: the inner parts of the disk were warmer than the outer parts.
In the inner part of the disk, only those materials which can remain solid at high temperatures could form the planets. That is, the dust grains were composed of materials such as silicon, iron, nickel, and the like; as these materials accrete they form rocks. Farther from the early Sun, where the disk was cooler, there were not only dust grains but also snowflakes---primarily ice flakes of water, methane, and ammonia. In the outer parts of the disk, not only could dust grains accrete to form rocks, but these snowflakes could accrete to form snowballs.
Water, methane, and ammonia are relatively abundant substances, particularly compared to substances formed from silicon, iron, etc. In the inner part of the solar system, where only rocks could remain solid, we therefore expect small planets, whereas in the outer solar system, where both rocks and ices could remain solid, we therefore expect large planets. (Not only did the gaseous planets form from more abundant substances, they also had more raw material from which to form. Just compare the size of Earth's orbit to that of Jupiter's orbit.)
The formation of the giant planets, particularly Jupiter and Saturn, deserves an additional comment. It is currently thought that they formed from a run-away accretion process. They started accreting slowly and probably initially were quite rocky. However, once their mass reached about 10--15 times that of Earth, their gravitational force was so strong that they could attract not only other rocks and snowballs around them, but also some of the gas in the disk that had not frozen into an ice. As they attracted more material, their gravitational force increased, thereby attracting even more material and increasing their gravitational force even more. The result was run-away accretion and large planets.
One of the problems with this scenario for the formation of Jupiter, though, is that it seems to take longer than the disk may have existed. The conventional scenario predicts that Jupiter might have taken several million years to form. Alan Boss (2000, Astrophysical Journal, vol. 536, p. L101) has suggested that the conventional model for the formation of Jupiter is wrong. His work indicates that a giant planet might also form from small, unstable clumps in the disk. Rather than being "bottom-up," like the conventional model, his "top-down" idea is that an entire region of the disk might become unstable and collapse quite quickly, perhaps in only a few hundred years.
One of the results of finding planets around other stars is the realization that this model does not require the planets to always have been in the same orbits as they have today. Interactions between the planets, particularly the giant planets, and the disk of material could have resulted *migration*. The giant planets may moved inward or outward from their current locations during their formation. If planets can migrate during or shortly after their formation, it makes it easier to explain the presence of Uranus and Neptune. A straightforward application of the above model encounters a slightly embarrassing problem: The time to form Uranus and Neptune is longer than the age of the solar system. If, however, these planets formed at a closer distance, then migrated outward, it may be easier to understand why Uranus and Neptune are at their current distances from the Sun. (See Science magazine, vol. 286, 1999 December 10 for more details.)
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Subject: E.04 When will the Sun die? How? Author: Erik Max Francis <max@alcyone.com>
The Sun is a yellow, G2 V main sequence dwarf. Yellow dwarfs live about 10 billion years (from zero-age main sequence to white dwarf formation), and our Sun is already about 5 billion years old.
Main sequence stars (like our Sun) are those that fuse hydrogen into helium, though the exact reactions vary depending on the mass of the star. The main sequence phase is by far the most stable and long-lived portion of a star's lifetime; the remainder of a star's evolution is almost an afterthought, even though the results of that evolution are what are most visible in the night sky. As the Sun ages, it will increase steadily in luminosity. In approximately 5 billion years, when the hydrogen in the Sun's core is mostly exhausted, the core will collapse---and, consequently, its temperature will rise---until the Sun begins fusion helium into carbon. Because the helium fuel source will release more energy than hydrogen, the Sun's outer layers will swell, as well as leaking away some of its outer atmosphere to space. When the conversion to the new fuel source is complete, the Sun will be slightly decreased in mass, as well as extending out to the current orbit of Earth or Mars (both of which will then be somewhat further out due to the Sun's slightly decreased mass). Since the Sun's fuel source will not have increased in proportion to its size, the blackbody power law indicates that the surface of the Sun will be cooler than it is now, and will become a cool, deep red. The Sun will have become a red giant.
A few tens or hundreds of millions of years after the Sun enters its red giant phase (or "helium main sequence"; the traditional main sequence is occasionally referred to as the hydrogen main sequence to contrast the other main sequences that a massive star enters), the Sun will begin to exhaust its fuel supply of helium. As before, when the Sun left the (hydrogen) main sequence, the core will contract, which will correspondingly lead to an increase in temperature in the core.
For very massive stars, this second core collapse would lead to a carbon main sequence, where carbon would fuse into even heavier elements, such as oxygen and nitrogen. However, the Sun is not massive enough to support the fusion of carbon; instead of finding newer fuel sources, the Sun's core will collapse until degenerate electrons---electrons which are in such a compressed state that their freedom of movement is quantum mechanically restricted---smashed together in the incredible pressures of the gravitational collapse, will halt the core's collapse. Due to the energy radiated away during the process process of the formation of this electron-degenerate core, the atmosphere of the Sun will be blown away into space, forming what astronomers call a planetary nebula (named such because it resembles a planetary disk in the telescope, not because it necessarily has anything to do with planets). The resulting dense, degenerate core is called a white dwarf, with a mass of something like the Sun compressed into a volume about that of the Earth's.
White dwarfs are initially extremely hot. But since the white dwarf is supported by degenerate electrons, and has no nuclear fuel to speak of to create more heat, they have no alternative but to cool. Once the white dwarf has cooled sufficiently---a process which will take many billions of years---it is called an exhausted white dwarf, or a black dwarf.
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When it first formed, the Moon probably did not always show the same face to the Earth. However, the Earth's gravity distorts the Moon, producing tides in it just as the Moon produces tides in the Earth. As the Moon rotated, the slight elongation of its tidal bulge was dragged a bit in the direction of its rotation, providing the Earth with a "handle" to slow down the Moon's rotation. More specifically, the tidal bulge near the Earth is attracted to the Earth more strongly than the bulge away from the Earth. Unless the bulge points toward the Earth, a torque is produced on the Moon.
If we imagine looking down on the Earth-Moon system from the north pole, here's what we'd see with the Moon rotating at the same rate as it goes around the Earth:
Earth Moon
__
/ \ ____ ^
| | / \ |
\__/ \____/ Orbiting
this way
Tidal bulge *greatly*
exaggerated.
What if the Moon were rotating faster? Then the picture would look like:
Earth Moon
__
/ \ ___ ^
| | / ) |
\__/ (___/ Orbiting
this way
Rotating
counterclockwise;
Tidal bulge *greatly*
exaggerated.
If it isn't clear why the tidal bulge should move the way the picture shows, think about it this way: Take the Moon in the top picture, with its tidal bulges lined up with the Earth. Now, grab it and rotate it counterclockwise 90 degrees. Its tidal bulge is now lined up the "wrong" way. The Moon will eventually return to a shape with tidal bulges lined up with the Earth, but it won't happen instantly; it will take some time. If, instead of rotating the Moon 90 degrees, you did something less drastic, like rotating it one degree, the tidal bulge would still be slightly misaligned, and it would still take some time to return to its proper place. If the Moon is rotating faster than once per orbit, it's like a constant series of such little adjustments. The tidal bulge is perpetually trying to regain its correct position, but the Moon keeps rotating and pushing it a bit out of the way.
Returning to the second picture above, the Earth's gravitational forces on the Moon look like this:
___ F1 <-----/ ) F2 <-------(___/
F2 is larger than F1, because that part of the Moon (the "bottom" half in the drawing, or the half that's "rearward" in the orbit) is a bit closer to the Earth. As a result, the two forces together tend to twist the Moon clockwise, slowing its spin. Over time, the result is that the Moon ends up with one face always facing, or "locked," to the Earth. If you drew this picture for the first case, (where the Moon rotates at the same rate that it orbits, and the tidal bulges are in line with the Earth), the forces would be acting along the same line, and wouldn't produce any twist.
Another way to explain this is to say that the Moon's energy of rotation is dissipated by internal friction as the Moon spins and its tidal bulge doesn't, but I think the detailed force analysis above makes things a little clearer.
This same effect occurs elsewhere in the solar system as well. The vast majority of satellites whose rotation rates have been measured are tidally locked (the jargon for having the same rotation and orbital periods). The few exceptions are satellites whose orbits are very distant from their primaries, so that the tidal forces on them are very small. (There could be, in principle, other exceptions among some of the close-in satellites whose rotation rates haven't been measured, but this is unlikely as tidal forces grow stronger the closer to the planet the satellite is.)
Pluto's satellite Charon is so massive (compared to Pluto) that it has locked Pluto, as well as Pluto locking Charon. This will happen to the Earth eventually too, assuming we survive the late stages of the Sun's evolution intact. :')
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Yes, at a rate of about 3--4 cm/yr.
The tidal bulges on the Earth (largely in the oceans), raised by the Moon, are rotated forward (ahead of) the Earth-Moon line by the Earth's rotation since it is faster than the Moon's orbital motion.
Using a similar picture as from the previous question, we'd see (looking down from the north pole):
Earth Moon
____
/ ) ___ ^
/ / / \ |
(____/ \___/ Moon's orbit &
Earth's rotation
(Ocean) Tidal bulge this way
*greatly* exaggerated.
The gravity from these leading and trailing bulges impels the Moon mostly forward along the direction of its motion in orbit (the Moon's orbit is not exactly in the plane of the Earth's equator). This force transfers momentum from the rotating Earth to the revolving Moon, simultaneously dragging the Earth and accelerating the Moon.
In addition to causing the Moon to recede from the Earth, this process also causes the Earth's rotation to slow and days to become longer (at a rate of about 0.002 seconds every century). Eventually the result will be that the Earth will show only one face to the Moon (much like the Moon only shows one face to the Earth). A lower limit to how long it will take for the Earth and Moon to become tidally locked is 50 billion years, at which point the month and the Earth's "day" will both be approximately 50 (of our current) days long. However, this estimate is based on the assumption that liquid water seas would be present on Earth's surface to provide the tidal interactions necessary. But as the Sun evolves, the seas will evaporate and tidal interactions will be much slower (solid planet distortions only). The oceans will evaporate about 1--2 billion years from now, so the actual time will probably be much longer.
Considerably more detail on the evolution of the Earth-Moon system can be found in an article by J. Burns in the book _Planetary Satellites_ (ed. J. Burns [Tucson: University of Arizona]) and in Sir Harold Jeffries' _The Earth_, 3rd ed (Cambridge Univ Press, 1952).
It is also interesting to consider what would happen if a satellite orbits its planet *faster* than the planet rotates. This is not the case for the Earth and Moon, but it is true for Mars and Phobos. In this case, Phobos also raises (crustal) tides on Mars. But now, Phobos is in front of the tidal bulge, so the gravitational action of the tidal bulge slows Phobos and Phobos moves *inward*. Thus, at some point in the future, Phobos will hit Mars. The most recent estimate is that the impact will occur in 40 million years, by A. T. Sinclair (1989, Astronomy & Astrophysics, vol. 220, p. 321).
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The Earth is constantly pelted by bits of cosmic debris. Most of this simply burns up in the atmosphere (as one can attest by simply watching meteors on a dark night). However, if an object is big enough it can survive passage through the atmosphere. The damage done by a meteorite (an object that strikes the Earth) depends upon its initial size.
10--100 m: Objects in this size range can produce devastation similar to that of an atomic blast (leading to them occasionally being called "city-busters"). Effects include severe damage to or collapse of standing buildings and the ignition of flammable materials leading to widespread fires. The radius over which such effects occur would vary depending upon the size and composition of the object, but could easily exceed 10 km. The Tunguska event, in Siberia, of 1908 is thought to have been from an object about 60 m in size; it led to trees being flattened out to 20 km and trees 40 km away being damaged.
At the small end of this size range, objects about 10 m strike the Earth about once a decade. Fortunately, only the densest objects, those containing iron, survive to the surface; most of the objects of this size explode sufficiently high in the atmosphere that there are no effects (other than maybe a loud noise) on the ground. At the larger end of this size range, it is estimated that the Earth is struck several times a millennium or about 1 impact every 100--200 yr.
100 m--1 km: Objects in this size range are likely to cause severe damage over a regional area, possibly as large as a continent (hence the name "continent-busters"). If they strike land, they will almost certainly produce a crater, while an ocean impact will generate large tidal waves. A 150 m object might produce a crater 3 km in diameter, an ejecta blanket 10 km in diameter, and a zone of destruction extending much farther out. For a 1 km impactor the zone of destruction might reasonably extend to cover countries. The death toll could be in the tens to hundreds of millions. A 1 km impactor could begin to have minor global consequences, including global cooling caused by vast amounts of dust in the atmosphere.
Estimates from the geologic record suggest that craters are formed on the Earth roughly once every 5000 yr.
1--10 km: Objects in this size range are likely to cause severe global effects ("species-busters"). An impact 65 million years ago by an object of 5--10 km in diameter is thought to have been partially or fully responsible for the extinction of half the living species of animals and plants at the time, including the dinosaurs. The crater alone from such an impact will be 10--15 times larger than the object itself. World-wide crop failures from dust injected into the atmosphere could imperil civilization, and the largest-sized objects could make the human species extinct.
The frequency with which the Earth is struck by such objects has to be estimated from the geological and paleontological record. At the low end of this size range, estimates are that such impacts occur roughly every 300 000 yr; at the upper end of the size range, impacts occur about every 10 million years.
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