Summary

 Despite having armies of geologists crawling
over Earth for about a century now, it was only 25 years ago that
plate tectonics-perhaps its most prominent and important process-was
beginning to be understood. Recognition of impacts as a major
planetary process has developed only in the past 25 years, and
only in the past few years did we first see our sister planet's
surface in detail. So it would be very presumptuous of us to think
we know very much of these other bodies that, in most cases, we
have barely seen. Many exciting discoveries remain-signs that
water ice may exist at our Moon's south pole, the possibility
that oceans may lie under Europa's surface, and perhaps proof
that primitive life existed on Mars. New discoveries continue
to be made even on our home planet: it was recently shown that
Earth's core is rotating faster than the rest of the planet, and
only in the last few years have we begun to recognize the effect
of geologic processes on the development of life; it is therefore
not likely that we will have a complete understanding of the Solar
System geology after our first brief glance at it.