Geoscience Reference
In-Depth Information
Barflies
The giant impact theory remains alive and well, if not quite stable. If as seems
likely, scientists are able eventually to explain the isotopic similarities, will we be
able to declare giant impact one of the great theories of science, like evolution and
plate tectonics? To meet that standard, a theory not only has to explain the obser-
vations that gave rise to it but also other observations that it was never intended
to explain, especially those that arose after scientists originally proposed the the-
ory. Plate tectonics, for example, explains not only the origin and motion of the
seafloor—not only continental drift per se—but most of the major features of con-
tinental geology.
Our solar system has 180 observed moons, most of them discovered in the last
few decades. Moons must result not from repeated improbable events but be an in-
evitable, “class-predictable” outgrowth of solar system evolution. If giant impact
is to be more than ad hoc, with a claim to greatness, it must be a general theory
that can explain the origin of all the moons of the solar system and much more as
well.
And there is a great deal more that needs explaining. The planets in our solar
system are like the regulars at the
Star Wars
cantina, where Luke and Kenobi meet
Han Solo and Chewbacca and where no two barflies are alike. One of the most
fundamental facts about a planet is the tilt of its rotational axis. Come to think of it,
why are the axes of the planets tilted in the first place? Why is each not perpendic-
ular to the plane of the ecliptic? And why does each planet have a different axial
tilt? Jupiter's axis, for example, is barely inclined, while Uranus lies on its side.
Earth's axis leans at the well-known 23.5 degrees on average, to which we owe the
seasons. Without the axial tilt, temperature would vary only with latitude, and life
on Earth would be very different, if indeed we were here to experience it.
But cast your eye around those barflies. Consider the four inner, terrestrial plan-
ets and the asteroid belt between Mars and Jupiter, starting with the midget on the
last barstool: Mercury. It is the smallest planet in the solar system, having only
about 5.5 percent of the mass of the Earth. Mercury has no moon, no atmosphere,
and the highest density of any body in the solar system. Moving up one stool,
Venus is about the same size as the Earth, also has no moon, has a dense atmo-
sphere, and rotates so slowly that a day on the Morning Star is longer than a year.
All the other planets orbit the Sun counterclockwise as viewed from the Sun's
north pole, but contrary Venus rotates clockwise. Earth has an atmosphere and a
uniquely large moon depleted in iron. Mars is just over half as large as the Earth
and has two small, insignificant moons and hardly any atmosphere. Beyond Mars,
