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. Let’s 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 – 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 escapee’s, 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 – carbon
dioxide – can be gound. Venus was more massive and able to hang on to a thick
CO2 atmosphere. Earth lost most of it’s 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 it’s atmosphere. It’s got a
bit of CO2 left over and that’s it.
The giant planets are more like the primordial
composition you’d 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 it’s much colder there; hence it’s been able to hang on to the light
gases.
Crustal
Geology.
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.
Here’s 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 there’s 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, it’s not suprising, is the most
geologically active of the inner planets, and the most geologically active body
in the solar system except for Jupiter’s Io (which is bizarre). California is
one of the most interesting and geologically active areas in the world.
We’ll
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. What’s 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.
We’re 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. It’s 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 – 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. I’ll give a
lecture on the geology of California as a high light for our Saturday adventure
as we overlook the dramatic coastline.