Reading - Chapter 5 - and the Hubbard chapter (see HW5)
First a recap of the first stages of solar system formation......
-> "flakes", grains, dust
- composition varying with temperature (i.e. distance from Sun)
- evidence of earliest phases of condensation of the solar nebula disk from Calcium-Aluminum inclusions in meteorites which suggest 4.7 billion years ago millimeter grains were formed very close to the young star but got blasted out to the asteroid belt by strong gets of wind coming from the star (these pieces of asteroid later broke up and came to Earth as meteorites)
- amount condensing depends on cosmic abundance of materials - importance of "snow line"
- What causes initial sticking?
- electrostatics?
- "organic goo"?
- for small scales the cross-section for collisions is just the geometric cross-section of the body
- Keplerian orbits - low eccentricities -> low speed impacts -> accretion
- high eccentricities -> high speed impacts -> break up to smaller pieces
- TIMESCALE? See HW4 calculations - time to grow by simple accretion to 1000 km ~ 2.5 million years
Now, the new material........
- Enhanced cross-section as mass of planetesimal grows
- HW4 problem 1 - and see box on page 114
- Effective cross-section due to gravitational focussing
- dM/dt is proportional to cross-section which is proportional to R2 [ 1 + (Vesc2/Vapp2)]
- THE BIG GETS BIGGER!!
- Why can't we form Jupiter and the Giant Planets the same way as binary stars?
- Primary issue is TIDAL FORCES - when there is a large mass ratio between the 2 binaries, the gravitation of the larger one causes tidal forces on the secondary
- Mass of Sun / Mass of Jupiter ~ 1000
- Secondary issue - Jupiter is enhanced by ~X3 in heavier elements over the solar nebula - hard to supply with planetesimals after collapse - easiest as nucleus
- Jupiter "core" ~ 20 Earth-masses of planetesimals - ice, rock, iron
From Hubbard reading:
Timescales -
[Remember - the timeframe for building Uranus and Neptune are guesses - current models cannot make Uranus and Neptune on this timescale in their current locations - see below]
TWO ISSUES
(1) How to make Uranus and Neptune?
(2) How to put ESP giant planets in close to the star?
In the outermost solar system:
- too little material in nebula
- Keplarian motions too slow
-> hard to make Uranus and Neptune in their current locations within the age of the solar system.
Hal Levison's simulation of making Jupiter and Saturn from 5X minimum mass solar nebula - but not Uranus and Neptune
However, the standard model works for the terrestrial planets! So, what is
different?
* Gravitational scattering cross-section varies with heliocentric distance
- the size of the giant planet remains the same
* "Sphere of Influence" - see HW2 answers - increases with distance from the Sun
* If objects are close to the Sun then ratio is small.
* If objects are far from the Sun then ratio is large.
* At 20AU scatterings are much more common than collisions, so the system gets
excited - rather than accreting into a single planet.
How to make Uranus and Neptune -
- Hal claims "In roughly 50% of the cases we have run, we get a reasonable solar system" - as in these cases.
- Net result is that tossing planetesimals INWARDS (to bombard inner planets - see cratering on the Moon "Late Heavy Bombardment which ended ~3.5 Billion years ago) causes Uranus and Neptune to move OUTWARDS
- But, you say, the simulation shows Uranus and Neptune scattering planetesimals out to the Kuiper Belt - true - what is hard to see in the simulation is that more are scattered inwards to the inner solar system.
How do we get giant planets so close to the star?
- need to make them out beyond the frost line >3 or so AU
- how do they loose (gravitational potential) energy and move inwards? Gas drag on remaining gas in nebula - at least, the current theory. Similar to atmospheric drag on satellites -> re-entry to Earth.
New moons of Jupiter discovered by Dave Jewitt in 2000 and in 2001 - mostly (especially the retrograde- irregular - ones) captured.
See The Jovian Satellites Page for further information.
Impact formation - our Moon, for example - as shown by Robin Canup's movie.
- "T-Tauri phase" of star
- not clear when this happens - probably AFTER making the regular satellites the capture of irregular moons.
- BUT - for ESP's we need to keep the gas around long enough for gas drag to bring the giant planets in close to the star.