Download Group 1 Notes for Week 8 - UGA Physics and Astronomy

Luke Boggs, Will Abney, Neal Amos, Matt Bestic, Paristu Alizadeh, Guthrie Akins
Notes for 2/27/12
Inner Solar System
= Terrestrial Planets: small; rocky/metallic; 0-2 moons; high density
vs. Outer Solar System
= Jovian Planets (“Gas Giants”): lots of moons; ring systems; low density
So, there is an enormous difference between the inner and outer planets: quantitatively and
qualitatively. So, an explanation for the origin of the solar system must take into account these
differences. There is also a gap in between these planets, physically – the asteroid belt, between
2-4 AU from the Sun. The planets beyond that are fairly well spaced out.
So, the raw materials are different for each type: rocky vs. icy particles. There is a temperature
gradient: the further away from the Sun, the colder it gets. The objects further away can accrete
ice particles: the rocky metallic cores further than 5 AU can get an ice layer, and when they get 5x
the size of the Earth, they can start pulling in the Hydrogen and Helium gas by their larger
gravity. You get bigger worlds, but lower densities than the inner planets. The pressure from the
ice will change the central rocky/icy cores, but that will be dealt with later.
But this isn’t the whole picture. Young stars have powerful STELLAR WINDS: streams of charged
particles continually leaving the surface of the star at very high speeds. It’s not powerful enough
to blow a planet off of its course/orbit, but it can move dust particles around very easily. So, for
about 50 million years after the birth of a star, the dust particles haven’t all be blown away by the
Stellar Winds. But we need more than 50 million years to form the outer planets.
GRAVITATIONAL INSTABILITIES could account for the difference: disturbances in mass
layers that cause the layer to collapse gravitationally. So, it could be that gravitational
disturbances in the Accretion Disk itself could produce Jovian planets as they are, instead of
requiring 50+ million years to acquire the dust and gas necessary (which would be impossible due
to Stellar Winds). These are 2 competing models.
Extrasolar planets and systems help us to figure out more possible models for solar system
formation. At this point, there are at least 2000 systems. Before taking actual images of extrasolar
planets, you had to look at the gravitational effects of the planets surrounding a star. This affects
the movement of stars. Planets move stars, but only a small amount. It causes an orbit of the
center of mass between the star and the planet(s). For our solar system, the center of mass is
actually inside the Sun. For other systems, it provides a little wobble in the other stars’ orbits. You
can detect it in a couple of ways: check it over a period of several years; or check the spectrum –
sometimes it’s going away from you (a red shift) and other times it’s going towards you (a blue
shift). It is the same thing that happens in a binary star system. If 2 stars are orbiting each other,
Luke Boggs, Will Abney, Neal Amos, Matt Bestic, Paristu Alizadeh, Guthrie Akins
the shift is easy to measure; if a planet is orbiting a star, though, the shift is tiny. It took more
sensitive instruments to be able to detect these small shifts, which didn’t happen until the late
1980s. You can also see the luminosity of a star decrease if a planet passes in front of it, but only if
it is in the same plane as your line of sight.
Most of the solar systems that were being found in this fashion were not like our solar system at
all. The problem was that the planets were very close to the sun and Jovian. The bulk of these
systems were composed entirely of Jovian planets. This goes against our ideas – they should only
be able to form further away from the planets because it’s too hot to form these icy planets so
close to the star. The theory had to be reexamined. This is the beginning of the idea of
PLANETARY MIGRATION: planets don’t necessarily stay in the spots where they were formed.
They can migrate inwards, towards the Sun, or outwards, away from it.
So, is our solar system normal or abnormal? There doesn’t appear that there is much migration in
our system. If you’re looking for star movement, then you’re going to want big objects in those
star systems so that you can see the movement. The method of looking for planets, then, was
skewed towards finding stars with Jovian planets. So, it’s to be expected that all of these early
discoveries found systems that were composed primarily of Jovian planets. Even more sensitive
instruments have found terrestrial planets, but we still can’t answer the question about whether
our system is normal.
Notes for 2/29/12
For our purposes, we are adding the Moon into our discussion of the terrestrial planets. It is quite
similar to Mercury. Some of the moons of Jupiter are also the same size – they could be called
planets, but that definition is reserved for things only orbiting the Sun. Since the moon and these
Jovian moons are not orbiting just the sun, then they can’t be called be planets.
Comparative Planetology: Why do some of the terrestrial planets look alike? Why are there
differences? What dictates the way they look today is their size – this is the most important thing.
Why would this change their surface bodies?
The Era of Bombardment (heavy cratering) took place in the first 500-600 million years.
Mercury and the Moon: craters, still easy to find; haven’t changed much in the way they looked
around 4 billion years ago
Earth and Venus: craters are harder to find because of atmosphere, erosion, etc.; the surface has
changed a lot since it formed
Mars: somewhere in the middle
Luke Boggs, Will Abney, Neal Amos, Matt Bestic, Paristu Alizadeh, Guthrie Akins
Conclusions about these craters: planets that have tectonic activity are able to resurface
themselves; this relates back to the size of planets. How? The volume holds the heat, but the
surface area dictates the cooling. Look at the following proportions:
Cooling / Heating · Surface Area / Volume · R2/R3 · 1/R
This means that large things cool more slowly. Mercury and the Moon are smaller than Venus
and Earth. All of them were hot early on, but the heat was lost earlier on Mercury and the Moon
because they had less surface area. Venus and Earth kept heat longer because they had more
surface area.
Venus presents a problem. It has a massive cloud cover, so much that we can’t see the surface of
the planet. Soviet spacecraft landed there, but they didn’t last long, so they didn’t send back many
pictures. Radar Imaging through the Magellan spacecraft lets us see what Venus’ surface looks
like. Timing of radar echo allows you to see elevation.
Notes for 3/3/12
Surfaces of the terrestrial planets tell us how much they have been renewed: heavy cratering tells
us that the surface is old (Moon and Mercury); lack of cratering indicates a relatively new surface
(Venus and Earth); Mars has some craters and extinct volcanoes.
How do you renew a surface? Volcanic activity, Tectonic activity, Erosion, Impact Cratering
Why does this happen? Things cool by themselves, unless you pump more energy into them. Hot
interiors and cold exteriors cause the heat flow to move outward, radiating from the planet’s
surface. This movement drives volcanism and tectonic activity. This Temperature Gradient varies
by size: Mercury and the Moon have a small gradient (change), which thickens the crust. It’s hard
to get through the crust. Venus and the Earth have large gradients, which cause thin crusts.
Since Mars is in between, it deserves some special comment. There are huge canyons, which
indicate some geologic activity at some point in the past. Mercury and the Moon could have
canyons of this size. Conversely, there are huge craters on Mars that would have erased if they
had occurred on Venus or Earth. So, on Mars, geologic activity probably ended about 2 billion
years ago, which means that it lasted for about 2 billion years from the creation of the solar
system. Martian volcanoes are the largest in the solar system. This is no coincidence: Earth’s
gravity would break down a formation of this size because of the difference in gravitational force
from the top to the bottom of the formation. Because gravity is weaker on Mars, it won’t be
broken down. Summing up: Mars was once geologically active (which you can see from the
Luke Boggs, Will Abney, Neal Amos, Matt Bestic, Paristu Alizadeh, Guthrie Akins
volcanoes), but it has ceased to be so (which you can see from the many craters). It is possible that
Mars once had a larger atmosphere, but lost it, because it died geologically.
Summary:
Monday February 27
Inner Planets: Mercury, Venus, Earth, Mars
-Terrestrial
-Small, rocky, metallic
-1 or 2 moons
-Earth has biggest moon.
Outer Planets: Jupiter, Saturn, Uranus, Neptune
-Jovian
-Giant gas planets with ring systems
-Many moons
-Low density
Solar/stellar winds: A stream of charged particles continually leaving the surface of a star at very high
speeds.
-Two equal masses have central mass right between them.
-Stars drag each other around with very small shift in velocity; only a few meters per second.
Wednesday February 29
-Mars is considered a terrestrial planet because of abnormally large body.
-Gynemede is size of Mercury but is considered a “moon” because it orbits a planet instead of Sun.
-Mercury and Moon have remarkably similar looks.
-Size determines surface properties.
-Earth’s surface is geologically active, so we do not see the many craters that we see on the Moon or
Mercury, which looks the same way they did 4 billion years ago.
-Surface area dictates cooling of planet.
-Large things cool more slowly.
Luke Boggs, Will Abney, Neal Amos, Matt Bestic, Paristu Alizadeh, Guthrie Akins
-Earth and Venus have resurfaced themselves to retain cleaner look in present.
Friday March 2
Surfaces of terrestrial planets:
-Moon and Mercury’s current surfaces formed 4 billion years ago.
-Earth and Venus have relatively new surfaces.
-Mars is in between.
Four ways to change surface:
1. Volcanic activity
2. Tectonic activity
3. Erosion
4. Impact cratering
-Planets have hot interior and cold exterior.
-Mercury and the moon had their surfaces geographically die four billion years ago. Mars had its surface
die most likely around two billion years ago.
-The largest volcano in the solar system is located on Mars and is 25 km high.