----- The following copyright 1991 by Dirk Terrell
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 Lecture #12  "Mercury - The Neglected Little Planet"

   I suspect that many of you have never seen the planet Mercury with the 
naked eye, even though it can become quite bright. Most of you have probably 
seen the other bright planets (even if you weren't able to identify them). 
The are two main reasons why Mercury is so hard to see. One is its proximity 
to the sun. Mercury is in a reasonably eccentric orbit around the sun 
(e=0.206) with a semi-major axis of only 58 million kilometers. Thus the 
planet always appears close to the sun in the sky, never appearing outside 
of either the bright glare of the sun or twilight. The second reason why it 
is hard to see is because it is a small planet. Mercury has a diameter of 
4878 km which makes it slightly larger than 1/3 the size of the earth 
(0.38). In spite of this, Mercury can look very bright once you find it.

   Observations of the planet's surface from the earth are difficult to 
make. A testament to this is the fact that the rotation period was not known 
until 1965! For about 200 years, the rotation period was thought to be the 
same as the orbital period of 87.97 days (by the way, unless stated 
otherwise 'day' means earth day). In 1965 the rotation period was measured 
by bouncing a radar beam off the planet. Due to the rotation, one side of 
the planet is moving toward the earth, while the other moves away from us. 
This causes a change in the radar beam (via the Doppler effect) that can be 
analyzed to deduce the rotation period. It was found that Mercury rotated 
with a period of 58.65 days. If you look carefully at the two numbers, you 
will see that they are in a 3/2 ratio. That is, Mercury rotates 3 times for 
every 2 times it goes around the sun. This whole number ratio indicates what 
we refer to as a resonance. The earth's moon rotates once for every orbit 
and we say that it is in a 1/1 resonance (usually referred to as 
'synchronous rotation'). We will be seeing all sorts of resonances in the 
solar system. One interesting thing that results from the eccentric orbit is 
that near perihelion (closest approach to the sun) the sun appears to move 
eastward in the sky, in spite of the fact that the planet rotates in a 
prograde sense. This is because the planet moves so fast near perihelion. If 
you could stand on Mercury, you might see the sun rise, stop, and go back 
down! (Can you imagine the myths that might have developed if that happened 
on Earth?)

   Most of what we know about Mercury came from flybys of the planet by the 
Mariner 10 spacecraft, which first arrived there in March of 1974. During 
its three encounters with the planet, Mariner 10 photographed a little more 
than half the surface, at a resolution comparable to ground-based 
photographs of the moon. Infrared instruments measured the surface 
temperatures, and found that it reached 700 K (about 800 deg. F) at the 
subsolar point. You could probably get a nice suntan there, but take lots of 
Gatorade.

   Perhaps the most unexpected finding by Mariner 10 was the presence of a 
magnetic field. Although not large (only about 1% that of earth), it is 
strong enough to form a magnetosphere and deflect the solar wind. The reason 
for Mercury's magnetic field is not well understand. It might be a remnant 
field (like a bar magnet), but that would require anomalously high amounts 
of ferromagnetic elements. It is possible that the planet has a molten 
interior that is heated by the decay of radioactive materials. This would 
enable the planet to generate its magnetic field via the dynamo effect. 
Although the latter idea is more popular among planetary scientists, we do 
not know for sure.

   In appearance, the planet is very similar to the moon, being covered with 
impact craters. In fact, most people cannot distinguish a picture of Mercury 
from a picture of the moon. Mercury's craters are different from those on 
the moon, in certain respects. The ejecta blankets of Mercurian craters are 
much smaller than those on the moon, because the surface gravity is larger 
on Mercury. Because gravity is stronger, the material thrown out by an 
impact cannot travel as far on Mercury as it can on the moon.

   One surprising feature seen in the Mariner 10 photographs was a type of 
rounded cliff called scarps. Scarps are thrust faults where one mass of 
material rides over another, giving the surface a wrinkled look. We think 
that the scarps were formed when the planet cooled and shrank, kind of like 
the wrinkles that form on an apple when it dries and shrinks. This shrinkage 
probably occured over a long period because some scarps have displaced 
craters while others have been covered by craters.

   So that's a quick look at a planet that has some interesting puzzles but 
has not received as much attention as its larger siblings.

 Dirk