Solar System Matter
It is useful to simplify the more-than-100 elements of the periodic
table into the following four catgories.
- Gas. The sun and the gas giant planets are, by mass, 70%
hydrogen and 28% helium. Both of these are very light gases that have
difficulty "sticking" to a small planet.
- Ice. Ices are also referred to as "volatiles". You should
know the names of the common ices: Water, H2O; Carbon
Dioxide, CO2; Carbon Monoxide, CO; Ammonia, NH3;
and Methane, CH4. Water ice is the most common. "Volatile"
is a good work because none of these ices can be heated very much or
they turn to gas. The solar system started out very cold (tens of Kelvin to
about 100 Kelvin in the early solar nebula).
- Rock. Rocks solidify/sublimate at pretty high temperatures, 500 -
1000 Kelvin. When there is ambient pressure, such as in volcanos on
earth, rocks also have a liquid phase (lava).
- Metal. Pure iron, pure magnesium, pure nickel,
etc. Metals solidify/sublimate at the highest temperatures of all:
1000-2000 K.
Rocks are mixtures of little grains of relatively pure chemical
substances called minerals. You can write a chemical formula
down for a mineral, but you almost always have to write many chemical
formulae to describe the content of a typical rock. The most common
rocks are silicates such as SiO4 and lots of closely
related oxides of silicon, aluminum, and magnesium. Rocks on earth
come in three basic families. And we add "primitive rock" to the list
for rocks that have never melted or been significantly altered (and
therefore cannot have been part of earth, originally, since earth
started molten).
- Igneous rocks solidified from a melt. Volcanic rocks are
igneous, but rocks can cool more slowly underground as well. The two
most common igneous rocks on earth are basalt and granite.
- Sedimentary rocks form in several steps. First, wind and
water pulverise existing rocks (igneous, sedimentary, or
metamorphic). Then the bits of broken rocks are washed to a sea or
lake, where they are deposited in horizontal layers of muck. Over
time, these muck layers will compress and cement together to become
rocks. A variant on this theme is when minerals that are completely
dissolved in water precipitate out to the lake bed when, for instance,
the lake dries up.
- Metamorphic rocks are rocks of either of the above two
types that are brought below the surface far enough to undergo such
heat and pressure as to change (i.e. metamorphose) the
crystal structure and appearance of the rock.
- Primitive rocks may be found in meteorites, asteroids, and
comets. Primitive rocks have never been melted or chemically altered
very much from the time they condensed from the interstellar medium.
Differentiation is an important word in planetary science. It
refers to a simple idea: what if you had a molten planet. The densest
material would naturally fall toward the center of the planet, while
the fluffiest material would naturally buoy to the surface. So one
would get a planet that is chemically different from core to
surface, i.e. differentiated. Our earth is layered, starting
from the core, metal - heavy rock - light rock - hydrosphere -
atmosphere. Is it just a "planet thing?" No, big asteroids can also
differentiate.
Basic composition of the outer solar nebula in either gaseous form
(H and He), or
little solid grains:
| Material |
Percent by mass |
| Hydrogen |
77.0 |
| Helium |
22.0 |
| Water |
0.6 |
| Methane |
0.4 |
| Ammonia |
0.1 |
| Rock and Metal |
0.3 |