Astro 28L: Field Astronomy in Southern Big Sur
The inner planets are small and rocky. Basically, this is because they are close to the sun and warm. Letís see how this works. †Warm temperatures mean the gas molecules in the atmosphere are moving around rapidly. A general principle is that,† molecules all have the same average energy over time. That means that heavy molecules† move slow, and light molecules move fast. Now, 90% of all atoms in the universe are hydrogen atoms Ė the lights of all! And most of the rest are helium atoms; the second lightest.
Now, for every planet you can define an escape velocity at the surface. This escape velocity is directly related to the mass of the planet and to the radius of the planet; these are the two terms that show up in the law of gravity. The escape velocity is the velocity that a thrown ball would require in order to be† moving so fast that gravity could never slow it to a halt and it would escape completely into space. For the earth, this is 7 miles per second. Any gas atoms in the upper most levels which are moving faster than this will escape and be a net loss to the planet. Hydrogen and helium are so lithat that† at typical inner planet temperatures of several hundred degrees Kelvin, that they will escape the atmosphere fairly quickly. This is true even if the planet is† massive and so the escape velocity is higher. Therefore, since well over 90% of all atoms are these two escapeeís, the inner planets are essentially boiled down remnants of what could have been giant gas planets if it had been colder. For Mercury, not even the heaviest of atmospheric molecules Ė carbon dioxide Ė can be gound. Venus was more massive and able to hang on to a thick CO2 atmosphere. Earth lost most of itís CO2 atmosphere due to the emergence of life and the oceans, which turned the CO2 into limestone and buried it. So, the earth has a relatively thin atmosphere of mostly nitrogen. Mars has had a rough history, battered by asteroids, and lost most of itís atmosphere. Itís got a bit of CO2 left over and thatís it.
The giant planets are more like the primordial composition youíd expect. Small rocky cores surrounded by thick mantles of hydrogen and some helium. Jupiter is over twice as far away from the sun as Mars, and itís much colder there; hence itís been able to hang on to the light gases.
Inner planets formed by the collisions of smaller planetesimals and rocks. Violent collisions with speeds typical of interplanetary orbits; 30 km/sec or so. Collisions of these speeds between rocks will melt or vaporize them. So the earth was a molten ball of lava to start with. It cooled to form a crust which is now about 50 miles thick.
Hereís a general principle: Inner planets have a heat content which is proportional to their mass, which is proportional to their volume, which is proportional to their radius cubed† (given thereís all made of some sort of rock). But they have an ability to cool which is proportional to their surface area, which is proportional to their radius squared. Thus, bigger planets cool slower and have thinner crusts. Thin crusts (New York-style, if you will!) are easier to crack due to internal convection in the mantle. Thick crusts lose that ability. The earth then, itís not suprising, is the most geologically active of the inner planets, and the most geologically active body in the solar system except for Jupiterís Io (which is bizarre). California is one of the most interesting and geologically active areas in the world.
††† Weíll be camping on the edge of the Salinian Block. This is a wedge of land between the San Andreas Fault on the east, and the Hosgri and Naciemento Faults on the west, at the coast. There is currently a controversy about the origin of the rock here. Some evidence suggests it was formed and solidified far to the south, in tropical conditions. Whatís for sure is that the western margin of California, west of the San Andreas Fault, is part of the Pacific Plate and is moving northward relative to the rest of America, by about 2cm per year. This average motion actually happens all at once, with long intervals of quiet. Weíre currently in a period when the central California area is undergoing movement. We had a 6.0 and 5.0 earthquakes in Parkfield, a few miles east of our camp, just a couple of weeks ago. Less than a year ago, we had another powerful earthquake at San Simeon, on the Hosgri/Naciemento Fault which is the opposite margin of the Salinian Block.
Ponderosa Campground is in an area of Mesozoic sandstone and shale about 100 million years old or so. Itís near a wedge of metamorphic granitic rock, which is igneous rock which cools slowly underground. This wedge expands northward and comes to dominate most of the high peaks of the Santa Lucia (Big Sur) mountains farther north. This is typical of the fault-dominated margin of California Ė wedges of very different rock which have been moved to unusual locations by the lateral action of the Pacific and North America plates grinding against each other. Iíll give a lecture on the geology of California as a high light for our Saturday adventure as we overlook the dramatic coastline.