ISP205 Lecture #11, Feburary 13, 2001
- Review:
- Inverse Square Law:
brightness decreases with the square of the distance
- Solar System: most things orbit/spin counterclockwise
Exceptions: spins of Venus and Uranus
- Density = Mass / Volume
- Typical densities:
Water: 1 g/cm3
Rock: 2.5-3.5 g/cm3
Iron: 8 g/cm3
- There are 2 (3) types of planets
- Terrestrial Planets
(Mercury, Venus, Earth, Mars) +Moon
density: 3.9 - 5.5 g/cm3
made of rock and iron
- Giant Planets
(Jupiter, Saturn, Uranus, Neptun)
density: 0.7 - 1.6 g/cm3
made of gas, ice, some rock
- Pluto
density: 2.0 g
- Review Radioactivity/ Dating
- Radioactivity is the decay of atomic nuclei into
another
element via emission of a particle (electron or alpha particle)
- Radioactivity is a random process
- The half-life is the time it takes for half the nuclei
in a sample
to decay (start counting at any time)
(plot of data from radioactive dating exercise)
- Each material has a characteristic probability for the
decay
of a nucleus and therefore a characteristic half-life that
is often well known from laboratory measurements
- The ratio of undecayed nuclei to decayed nuclei tells
you
how many half-lives the sample is old.
- Examples for radioactive nuclei used for dating:
(the number indicates the total number of protons and neutrons
in the nucleus)
235U decays into 207Pb half-life: 700 Million Years
40K decays into 40Ar half-life:
1.3 billion years
14C decays into 14N
half-life: 5730 years
- Radioactive dating works best for ages that are of the
same
order of magnitude than the half-life.
(otherwise either the number of decayed nuclei or the number
of undecayed nuclei is too small to be measured).
- Radioactive dating of Meteorites yields an age of the
solar
system of (at least) 4.5 billion years.
- Other objects in the solar system than the Sun and Planets:
- Moons satellites orbiting a
planet
earths moon sometimes listed among terrestrial planets
(density 3.3 g/cm3)
- Rings: dust and
rocks orbit in disk around equator of a planet
all giant planets have rings - saturns
rings are most easily visible
see this picture for other planets rings
- Asteroids
rocky objects that orbit the sun, mainly
in asteroid belt
most of them are in the Asteroid belt
(between
Mars and Jupiter)
more than 10000 known (known orbits)
size up to 1000km (Ceres)
NEAR mission finished data taking of Eros -
controlled crash next week
Should hit ground Feb 12, 3:04 pm Eastern Time
See Image of Eros;
(size: 33km X 13km X 13km)
Binary
Asteroids !
Movie of crashing probe
Last images
- Comets
ice (water, carbon dioxide,
carbon monoxide, methyl alcohol)
orbit the sun on highly elliptical orbits
come from the Oort cloud, ~50000AU away
from sun (pluto: 40 AU !)
like Hale
Bopp - will be back in 2380 years !
and from Kuiper Belt just
beyond Plutos orbit
(see picture)
more than 1000 known (5-10/yr discovered)
- Dust
tiny grains, for example rock
we have special names for things that hit a planet:
- Meteors dust grains, burning up in
earths atmosphere
- Meteorites any bigger piece hitting a planet or
moon
- Geological Activity
- Another method for age determination: Crater Counts
- Example: Moon or Mercury (picture)
Example: a 10km crater on the moon
(depends on size of object) should
occur every 1 million years.
if we see 3000 that would mean the moon's surface
would be 3 billion
years old.
-
- Craters on Earth ? Redshift DEMO
Why are there less
craters on earth ?
(there are a few: example)
- Crater counts measure time since last rebuilt of surface
This time can be short for large geological activity
(then few craters)
- Examples for geological activity:
- Volcanoes
- Erosion (Wind, Water)
- Sedimentation
- Plate Tectonics
(picture of
continental plates)
Continental plates float on "liquid" mantle
Drift can be several cm per year
Drift is not contineous - sudden shifts cause earthquakes
If plates drift apart, gap gets
refilled with fresh rock
it takes ~100 Mio years to replace ocean
floors
completely
- Planet Earth
- Radius: 6378 km (largest terrestrial planet)
- How can we learn about earths interior ?
- Density measurement
- Seismology
- Detect waves triggered by earthquakes at
different
locations on earth
- S-waves: shear waves do not penetrate liquid
- P-waves: presure waves do penetrate liquid
demo s,p waves
(picture seismology)
- Volcanoes
- Magnetic field
DEMO: fieldlines
picture earth magnetic field
- Earth has a magnetic field with the north and
south
poles roughly aligned with the rotational axis
- Magnetic field traps charged particles from the
sun
because they spiral round the field lines
this causes Northern Lights (aurorae)
(picture, polar lights !!! experiment !!)
- Origin: Liquid iron and earths rotation lead to
currents that
generate a magnetic field (Dynamo effect).
DEMO: current generates magnetic field
- Basic properties and composition:
- Crust, continental: 20-70 km, solid rock
ocean
floor: 6 km, solid rock
- Mantle: 2900 km, somewhat liquid rock (flows
slowly)
Temperature up to 3000 oK
- Core: remaining 3500 km, iron
with some nickel and sulfur
Temperature: 5000-7000 oK
- Outer core: Liquid
- Inner Core: (inner 1200 km radius) probably
solid
because of high pressure
- Origin of heat in earths interior:
Earth is cooling but slowly because of size
Initial heating through:
- Gravitational energy release during formation
and fractionation
- Large impact rate of "planetesimals"
- Radioactivity
- Atmosphere:
- Thin layer of gas around earths surface
- Properties:
- Pressure at bottom: 1 bar (1 kg/cm2)
- Composition today: 78% Nitrogen, 21% Oxygen,
1% Argon plus traces of other gases (CO2, Water)
- Earths gravity just strong enough to keep heavier
molecules on earth without too much leakage at
current temperatures.
Hydrogen and Helium escaped !
- Layers:
- 0 - 10 km: Troposphere (wheather, clouds, ...)
Temperature at 10km: 50 oC below freezing
- 10 km - 50 km: Stratosphere
- Ozone Layer at top of Stratosphere: shields
against
UV radiation
- above 50 km : Mesosphere
- above 80 km : Ionosphere (gas ionized from
UV
radiation)
- Origin of Atmosphere
- Origin of first atmosphere (most likely)
- , water etc. incorporated in earth by
impact of comets or planetesimals
(small pieces of matter that stuck together to
form the planets in the early solar system)
- Gases are then released by volcanism and
heating from high impact rate of planetesimals
- The first atmosphere was very hot (steamy)
- The first atmosphere was most likely very
rich in
CO2 with no oxygen
- Since about 2 billion years oxygen has been
added
as waste product of plant life
- The Greenhouse effect
- Visible light can penetrate the atmosphere. On dark
ground
it heats and is converted to infrared radiation that is blocked
by the atmosphere. This is called the greenhouse effect.
picture
- Water vapor and CO2 gas block infrared, but not
visible light
and are the main greenhouse gases on earth.
- Without the greenhouse effect the earths
temperature would be
below freezing
- Runaway greenhouse effect:
High temperatures release water vapor and CO2 from rocks.
That leads to a stronger greenhouse effect and higher temperatures.
An additional factor can be the melting of polar ice that reflects
sunlight as visible light.
- Earths distance from the sun is just right for a
moderate greenhouse
effect.
- For the first time human activities contribute to
the greenhouse
effect (picture)