1. Craters in the Solar System.
Go to the Solar System Collisions
program at http://janus.astro.umd.edu/astro/impact/.
a) Bits of rocky and icy debris continually strike the inner planets
at high speeds. Launch your own rocky impactors at Mercury, Venus,
Earth and Mars. Chooses sizes of 10cm, 1m, 10m, 100m and 1km and
record what happens (shooting star, atmospheric explosion also called
an airburst, or surface crater) for each planet. For craters, record
the crater diameter as well. Make a table that summarizes all of your
results.
b) For Venus, Earth, and Mars, work out the smallest rocky body that
can penetrate the planet's atmospheric shield. Get the diameter to two
significant figures (e.g. 9.1 not 9.08). Record the diameter of the
impactor, the diameter of the crater that it creates, and how often
this happens. (Note, we actually do see smaller craters on Venus, but
these are from debris thrown up by the largest impacts.)
c) If Venus is 4.5 billion years old and all quantities remain
unchanged over time (i. impact rate, ii. atmosphere of Venus,
iii. surface of Venus), then roughly how many of these smallest
craters should we see on its surface? We actually see far fewer than
expected. How might you imagine changing each of the three factors to
explain the observed number of craters on Venus?
d) Finally, evidence from the Moon shows that the flux of debris to
the planets was larger in the past and a study of crater sizes on
Venus shows that its atmosphere was probably never dramatically denser
than it is today. This suggests that the surface of Venus has been
altered somehow. Take the largest crater on Venus (Mead Crater,
diameter 280km) and use the Collision Calculator to work out the size
of the rocky impactor and how often such events occur. What age do
you infer for the surface of Venus?
2. Planetary Atmospheres
A planetary atmosphere has a
characteristic height at which the atmosphere pressure (and density)
roughly halves. On Earth, this height is around 5 km, so 5km up there
is only 1/2 as much atmosphere, 10km up there is only 25% as much,
etc.
a) To experience the pressure in Mars' atmosphere (assume 128
times less than Earth), how high should we go in our atmosphere?
Compare to the height of Mt. Everest.
b) Inside deep mineshafts, the atmospheric pressure is greater
than at sea-level because of this effect. If we wished to experience
the atmospheric pressure of Venus (assume 64 times that of Earth), how
deep a hole would we need to dig into the Earth? Compare to the
deepest borehole ever drilled (12.2 km by the Russians) and the
thickness of continental crust (about 35 km)
c) Roughly how far up is the edge of space, if we define this as
the point where the atmosphere is only one billionth (10-9)
that of Earth's surface? Make the approximation
210 = 1024 ~ 1000 = 103.
3. Planetary Formation. (3-4 sentences each)
a) From the most likely scenario of Solar System formation
discussed in class, do you expect rock and iron cores for Jupiter and
Saturn? Why or why not?
b) Explain the leading theories for why Mercury has a large iron
core and the Moon has almost no iron.
c) Explain the theory for why Jupiter is
the most massive of all of the planets.