Planet Mercury
Fun facts:
closest planet to sun --- .387 AU's
only Pluto is smaller-- Mercury is .38 the diameter of Earth (Luna is .27 the diameter of Earth)
Mercury's mass is only 5.5 % that of Earth. But its gravity is 37 % that of Earth! That's about the same as Mars's gravity. How come???
Well, its density is 5.44 g/cc. That's nearly as high as Earth's!
What
's going on???
Answer: Mercury must contain a LOT of iron. Does this make sense, considering its position in the solar system? (Fe is particularly "refractory")
Mercury probably has an iron core as large as 74 % of its radius. Some think that this is probably due to a collision with another large planetesimal, knocking much of Mercury's possible earlier mantle away. (Like the collision of a Mars-size body with Earth that is thought to be the cause of Luna.)
Mercury does have a weak, but significant magnetic field. Perhaps a portion of its core is still molten, or perhaps the field is just a remnant field, still present because of the large amount of iron.
Mercury of course has no atmosphere; its gravity is not high enough to hang on to released gases. Like Luna, there is some leakage of vapor from its sunlight-bombarded rocks. This vapor appears to be mostly Na, O, and K. But the vapor doesn't stick around to form an atmosphere.
Temperatures:
798 oF on the day side; -279 oF on the night side
Length of day vs night? Well, one rotation (day) takes 59 Earth days. (Mercury spins quite slowly.)
Another interesting thing is that a year for Mercury (one orbit) takes 88 Earth days. That's a 2:3 spin-orbit coupling. That is, there are 2 mercurian years for every 3 days. Another way to say this is-- Mercury spins once for every 2/3 orbit. This is most probably the result of tidal tugging from the sun.
Terrains of Mercury:
1) cratered terrain & intercrater plains, 2) smooth plains
The cratered terrain & intercrater plains are not saturated with craters (unlike Luna). There are plains materials interspersed among the craters. One idea is that volcanism continued during the Late Bombardment Period, depositing these intercrater materials.
15 % of Mercury's surface consists of the second terrain type: "smooth plains". These plains resemble the lunar maria, but appear to be considerably thinner. These plains are concentrated in the northern hemisphere; perhaps the crust is thinner there(?). Some of the plains materials on Mercury may not be lava, but instead, debris surge deposits from impacts. We've only had one spacecraft visit Mercury (Mariner 2), and we don't have very high resolution imagery.
There are a few other specific features on Mercury warranting our attention:
One is the Caloris Basin. This is a large multi-ring basin, about 1300 km in diameter. Directly on the opposite side of the planet from this impact site is antipodal terrain. This is a peculiar landscape made up of knobby looking hills and valleys in a specific region measuring 500 km across. Again, this pecular terrain is 180 o around the planet from the center of the Caloris Basin. The idea is that a shock wave from the asteroid impact traveled through Mercury and out its opposite side, jumbling the terrain as it passed through. (There is similar "antipodal terrain" opposite the Imbrium and Orientale Basins on Luna.) See page 124 in the text on reserve for visuals.
There are also some interesting tectonic features on Mercury. Extending very widely across Mercury's surface are fault scarps. These generally trend N-S, and are seen from pole to pole. They cut across craters in many cases. See page 132 in the text on reserve. Recall our discussion of faulting; normal vs reverse faults and their respective relations to tensional vs compressional regional crustal stresses.
The faults on Mercury are thrust faults, so clearly compressional forces were widespread at some point in that planet's history. See page 131 of your on-reserve textbook for a discussion of the possible origin of these faults. Which hypothesis do you favor? Why?