ISP205 Lecture #10, Feburary 8, 2001
- Inverse Square Law
- intensity = number of photons / area
- the intensity decreases with the square of the distance
- Applies to radiation (including heat), sound, gravity ....
(if not collimated)
- Bad news: it is harder to see distant objects (need larger telescopes)
- Good news: if the intrinsic brightness of a star is known one can
determine
its distance from the apparent brightness.
- Properties of Solar System
- All planets rotate in plane counterclockwise around the sun
- Their orbits are only slightly elliptical
(Except: Mercury, Pluto)
- Most moons orbit in plane counterclockwise around planets
- The sun and most planets and moons spin counterclockwise
(Except: Venus, Uranus, Pluto)
(picture from
other book)
- The composition of the planets varies with distance to sun:
- Density tells you something about the composition
density = mass / volume
- density of a mixture is the average of the density of the
individual components (weighted according to their proportions)
- typical densities: iron : 7.9
g/cm3
rock : 2.5-3.5 g/cm3
water : 1 g/cm3
water ice: 0.9 g/cm3
air : depends
on temperature and pressure
typical on earth: 0.0013 g/cm3
- Density of Planets: Mercury 5.4 g/cm3
Venus 5.2 g/cm3
Earth 5.5 g/cm3
Mars 3.9 g/cm3
Jupiter 1.3 g/cm3 (1400
earths !!!)
Saturn 0.7 g/cm3 (!) Saturn
would float !!!
Uranus 1.3 g/cm3
Neptune 1.6 g/cm3
- Conclusion ?
Planets fall in 3 groups:
1) Terrestrial Planets: (Mercury, Venus, Earth, Mars) + Moon
2) Giant Planets: (Jupiter, Saturn, Uranus, Neptune)
3) Weird Planets: Pluto
see images
- Other objects in the solar system than the Sun and Planets:
- Moons satellites orbiting a
planet
- Rings dust and
rocks orbit in disk around equator of a planet
all giant planets have rings - saturns
rings are most easily visible
- 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 !
- Comets ice (water, carbon dioxide,
carbon monoxide, methyl alcohol)
orbit the sun on highly elliptical orbits
come from the Ort cloud, ~50000AU away from sun (pluto: 40 AU !)
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
- The solar system formed 4.5 billion years ago out of same material
Where do we know that from ? Radioactive dating
- Unstable atomic nuclei decay into a "daughter" nucleus,
which is
a different element. It emits an electron or a helium nucleus (alpha
particle)
- Example: alpha decay of 235U (pic)
- Decay happens randomly - there is no way to predict which nucleus
decays next and when it decays
- BUT: The average time it takes for the next decay is a fixed number
characteristic
of the material.
- Usually given: Half-life - the time it takes for half the unstable
nuclei in a given sample
to decay. Exampe: 235U has a half-life of 700 million years
- Age determination: Compare number of unstable nuclei that did not yet
decay to
the number of daughter nuclei that decayed already
- Note: after 1 half life: 1/2survive
2
1/4 survive
3
1/8 survive
.... never zero ! (except if only a few left ...)
- This gives the age since the rock was last molten
(only then the number of unstable nuclei + their daughter nuclei remains
fixed)
Therefore: rocks on earth can be much younger than the solar system or
earth itself
Age of solar system from radioactive dating of meteorites that
solidified soon after the formation
of the solar system..
- Other dating methods: crater counts
- Assuming a known, constant rate of impacts, one can determine
the age of a surface from the
number of impact craters.
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.
- BUT: only age of surface since last major geological activity
Example: Mercury, Mars, and Jupiter
have the same age, but have very different crater counts
- Formation of the solar system - brief overview
- Swirling gas clound collapsed into a rotating disk
- Sun and planetesimals formed - planetesimals stuck together forming
planets
- Further away from sun its colder - more ice, gas - close to the sun
only rock and iron
can survive
- Overall composition of solar system (=
composition of original cloud)