This lecture will introduce all the different objects in the solar system
There are nine planets in the solar system (although the
classification of Pluto as a planet is maybe wrong)
These are artist impressions of them
This shows their relative size, Earth is the largest inner planet,
Jupiter the largest of them all
Planets can be split up as terrestrial (like earth) - small, rocky,
close to the sun, have a surface, quite dense (like rock), few moons
Or Jovian (like Jupiter) - gaseous, large, far from the sun, low
density, lots of moons
Mercury is the innermost plant. Mariner 10 managed to visit it but not
that much data on it. It is slightly elongated cos of the Sun's
gravitational pull. It precesses as it orbits - test of GR (see later).
Venus. It has retrograde rotation (rotates backwards). Probably due to
it's very last interaction with a large planetesimal. Has a thick
atmosphere of CO2 and sulphur - corroded the first satellites to visit it.
It also has the hottest surface temperature, hotter than mercury, due
to the greenhouse effect.
Venera 13 made it down there though. Found it has a rocky grey
surface. It's atmosphere is the most interesting - see later.
Mars is our neighbour the other way. It has Ice polar caps. Huge
volcano called Olympus Mons which is 3x as high as Everest and a
4000km long rift valley called Vallas Marineris
Ancient flood patterns on mars. Suggestion that water once flowed on
the surface.
The ice cap in the north (south not visible).
The view from the Pathfinder mission
Rocky terrain. Frost layer of freezing CO2
Has there been life on mars? This 'thing' is very small.
Next planet out is Jupiter - the largest. Has it's giant red spot -
a storm that has been going on as long as we've been looking at mars
(> 300 years). Very strong winds beneath it.
Jupiter probably does have a rocky core
Saturn - has lovely rings.
Saturn casts a shadow on the rings.
Next comes Uranus and Jupiter.
Uranus is weird - it is flipped on it's back i.e. it's axis of
rotations is 82 degrees (cf earth = 23.5 degrees). Weird seasons (the
diagram is wrong, at one it should say winter in north, summer in
south and vice versa at the other). I
refer to it as the most laid back planet in the Universe!
Uranus also has rings but not as prominent as Saturn
Neptune has a funny banded structure. Neptune was actually discovered
via prediction from Uranus' orbit.
The final planet is Pluto but with it's weird eccentric orbit, it's
elongation out of the plane, it's size and composition and it's sister
moon that is almost the same size (Charon) it is a wield planet
There are also moons in the solar system. This is Titan, it has an
atmosphere. Mercury and Venus are the only planets without a moon.
Let's look at our nearby planets in more detail
Venus has a very hot surface temperature of 800K. Clouds form in the
48-68 km layer, we thought these would reflect the sun and the
temperature was surprising.
The atmosphere of earth has more complicated structure. Sunlight warms
the earth. Air is a bad conductor so temperature decreases through the
troposphere. Ozone is present in the stratosphere. It is good at
absorbing the suns UV (good for us!). There is no convection here so
smooth air (this is where planes fly). No ozone in the mesosphere so T
drops again. In the thermosphere there are atoms that can absorb short
UV radiation so T increases
Note mars has a very thin atmosphere but what there is is made up of CO2.
Venus is so hot cos of the greenhouse effect. Light enters the planets
atmosphere, is absorbed by the planet, reradiated at longer wavelengths
and then most of it is traped by the CO2 in the atmosphere.
This heats up the planet.
Warning to earth!? Our levels of CO2 are increasing.
and the temperature of earth is increasing.
Earth, Venus and Mars started with similar atmospheres. Mars used to
be warmer and more earth like in the past. The CO2 and the GH effect
on Earth increased the initial temperature by 35 degrees. On Mars only
by 5 degrees.
On Venus, the greenhouse gases never escaped from the atmosphere. It
was never cool enough for water to condense out of the atmosphere so
it never rained and the CO2 remained in the atmosphere. There are no
plate techtonics on Venus but not needed. There are volcanoes that are
probably still active.
On Earth we have water in the oceans and CO2 in the
atmosphere. Initially the level of CO2 in the Earth's atmosphere would
have been as high as on Venus.
When it rains, CO2 is dissolved out of the air, reacts with
rocks and is stored there so only a small amount of CO2 in the
air. The amount is sustained by plate tectonics - volcanos liberate
CO2 and rejoins the atmosphere. Process takes millions of years. With
out the plate techtonics, rain would quickly wash the CO2 out of the air in a
few 1000 years then we'd loose the greenhouse effect.
On Mars it rained 4 billion years ago and washed most of the CO2 from
the atmosphere. Stored in rocks. There are no plate tectonics on mars
(there were volcanos but they are now thought to be extinct). Water
and CO2 mostly
escaped - reversed greenhouse effect. Once atmosphere is this, UV can
penetrate and give energy to the nitrogen which the escaped. O2
molecules also escaped but on their way out, they gave rise to the rusty
appearance of mars.
There are asteroids in the solar system. Ceres is the largest. It is
900km in diameter. They rotate the sun in roughly circular orbits.
The asteroids are located between 2-3.5 AU in the Asteroid belt
between the orbits of Mars and Jupiter. Bodes law - ratio of two
neighbouring planets semi-major axis = 1.5. Exception is
Jupiter/Mars. Wondered if a planet at the asteroid belt location -
no. Sum of the mass of the asteroids not enough to make a
planet. Caused due to the effect of Jupiter - no large planet and a
5th terrestrial planet forms. Jupiter prevents it.
See gaps in the asteroid belt - Kirkwood gaps. Gaps are where the
orbital periods are in phase. Another special group are the trojans.
Asteroids have funny appearances - their gravity is not high enough to
make them spherical. Craters from interactions. They are made of
of carbonous or rocky material, some have traces of metals. The
largest is Ceres ar 940 km in diameter.
Some even have their own moon.
End of the dinosaurs?
Iridium which is rare on earth but found in meteorites is found in a
layer on Earth in many places that dates back to 65 million years. An
asteroid that was 10km across could have given rise to this layer
Meteorites are 100m across. See a brief flash of light as they burn up
in the earths atmosphere - shooting star. 300 tons of extra
terrestrial rock falls on the earth each day. A meteor is the
meteorite as it burns up in the earth's atmosphere. Meteors can be
asteroids but there are many more small things than big things so most
are from meteorites. Often these are the debris of comets too.
Cause damage!
3 types: Stony (95%)
- when polished up see some iron.
Stony Iron 1%
Iron
Have weird markings that are hard to fake.
Other heavenly body is the comet
Chunks of dusty ice, partly vapourise as they pass the sun. Nucleus is
hard to see - darker than coal (4% albido). When far from the sun,
this is all that exisits and we cannot see them. Probably a few km in
diameter. Close to the sun it's
icy surface gets heated and expands. Have a H envelope that is
not visible. We actually see the coma - evaporated gas, extends
several tens of thousand km in size.
The Ion tail sweeps directly out from the sun - blue in
colour. Fells the effect of the solar wind.
The dust tail lags behind as gravity gives the shape.
The dark nucleus of Halley's comet.
Motion of the comets tail
Comets are found in two places - the Kuiper belt and the Oort
cloud. The Kuiper belt is just past the orbit of Pluto and extends to
500 AU. Orbits are eccentric and inclined out of the plane. 35,000
comets in the KB with diameter > 100km. Short period comets found here
and this is where they were formed. Other place is the Oort cloud - a
spherical cloud that extends 10^4-10^5 AU from the sun (1/5 way to
nearest star). Created 4.5 billion years ago when planetesimals were
ejected out of the Solar System by interactions with larger
lumps. Very elliptical, eccentric, inclined, long period orbits.
A comet detection.
A comet sometimes fragments and you see a meteor shower (the Leonids etc).
Comet Shoemaker Levey 9
Smashes into Jupiter (the great vacuum cleaner in the sky).
