In Class:
Question to Ponder
Why are our solar system's inner planets (e.g., Mercury, Venus, Mars,
and Earth) deficient in hydrogen and helium?
- a) They formed form a part of the solar nebula that had little
hydrogen and helium.
- b) It was too warm for the hydrogen and helium to freeze out into
ices that could become parts of these planets.
- c) They don't have enough mass to hold atmospheres of hydrogen and
helium.
- d) Trick question: the composition of these planets is that same as
that of the Sun and the outer planets.
What's Going on on Mercury?
- Not much.
- Mercury is about five times bigger than the Moon, but by
planetary standards, it's still pretty dinky.
- Size matters for internal heating, which drives lots of stuff on
Earth.
What We can Learn from Mercury's Density
- Average density is quite high for such a small planet: 5400
kg/m3.
- Surface seems like normal rock (i.e., with density more like
2500-3000 kg/m3).
- Therefore, it's likely that Mercury's core is dense -- probably
iron.
- Thus, Mercury is differentiated, and it probably has an iron core.
Mercury's Surface
- Pock-marked with craters just like the Moon's.
- No real atmosphere to speak of, so very little surface erosion.
- One big impact basin -- Caloris Basin
- Possibly caused by a really big impactor that cracked the crust
and allowed molten material to seep out (much like the maria on the
Moon).
- Such an impactor might have also blown off some of
Mercury's rocky outer crust, giving a reasonable explanation for why
the ratio of dense iron core mass to outer rocky material is so high.
- Scarps are long linear cliff features.
- Scarps are likely caused by the cooling of the planet's interior, which
makes it shrink.
- Something like "wrinkles" seen on a really old orange.
|
