Section 7 - Formation of the Solar system Flashcards
1) Describe the nebula hypothesis.
2) What evidence supports this?
1) - The Sun and planets believed to have formed from the gravitational collapse of a local over-density in the interstellar medium.
- Collapsing nebula formed a disc around the rapidly rotating proto-Sun due to conservation of angular momentum.
- Planets and smaller bodies condensed out of this protoplanetary disc.
2) Supported by all planets orbiting in approximately circular orbits in the same plane, aligned with the spin of the Sun.
- What is the believed surface density profile of the protoplanetary disc?
- What is the proposed lower limit of the protoplanetary disc?
- Mass is dominated by _____, but proportion of solids jumps at the _______ where the disc is _____ for ____ to form.
- What is the expected lifetime of a protoplanetary disc? Why?
- What are debris discs?
- Capital sigma(r) directly proportional to r ^(-3/2)
- M_disc >= 0.01 M_Sun
- Gas, snowline, cool, ices
- A few million years, the proportion of young stars with an excess of infra-red emission from a protoplanetary disc drops rapidly with age.
- Less massive discs of dust around older stars, dust believed to be replenished by collisions between asteroids.
Describe the 7 steps of the core-accretion scenario.
1) Gravitational collapse of over-dense region in interstellar mediu to form proto-star with protoplanetary disc.
2) Condensation of sub-micron sized dust particles within disc.
3) Growth of solid particles by pairwise collisions from dust to 10km sized plaetesimals, initially assisted by electrostatic forces.
4) Further growth by pairwise collsions from planetesimals to protoplanets assisted by mutual gravity. Growth to isolation mass, where Hill sphere of a protoplanet has swept out an annulus in disc.
5) Larger cores form beyond the snowline in disc, where temperature low enough for ices to form.
6) Gas from disc accreted by these large cores to form the giant planets. Limited by photo-evaporation of the disc by the young Sun.
7) Terrestrial planets assemble on a longer timescale by orbital permutations leading to giant collisions.
2) Condensation of sub-micron sized dust particles within disc
- What elements form at the highest temperatures? What do these form?
- What forms at slightly lower temperatures? What do these form?
- What forms at much lower temperatures?
- What is the relationship between the temperature of the disc and the radius?
- Ca and Al, resulting in the calcium-aluminium-rich inclusions (CAIs) that are the oldest minerals found in meteorites.
- Silicates, and then metal alloys/oxides that form chonrules in meteorites and the terrestrial planets.
- Snowlines of different ices, especially water, ammonia, and then methane.
- T_disc is directly proportional to r^(-3/4)
3) Growth of solid particles by pairwise collisions, from dust to 10km sized planetesimals, initially assisted by electrostatic forces.
- What can collisional growth build? What are two properties of these?
- When does erosion become important?
- Why do ice particles and silicate particles have much more efficient growth of particles?
- How do particles grow past the 1m barrier in the protoplanetary disc?
- Macroscopic dust cakes. Low density, open fractal structure.
- When impact velocity and dust cake size increases.
- Because ice particles are sticky, and silicate particles have ice coatings.
- Growth has to be rapid, aided by instabilities in disc dynamics. Overall pressure gradient in dic is negative, leading to inward drift of meter-scale objects, density waves can lead to pressure maxima and local outward drift. Traps particles in local disc structures, increasing the rate of particle growth through the meter barrier.
6) Gas from the disc is accreted by these larger cores to form the giant planets. Limited by the photo evaporation of the disc by the young Sun.
- When can gas accretion begin?
- Why is it initially slow?
- When does gas accretion become a runaway process? What does this form?
- What causes an ice giant to form rather than a gas giant?
- Once core sufficiently massive that the escape velocity of the core is greater than the mean thermal velocity of the gas.
- Because gas fills the Hill sphere of the core and only contracts through cooling, which is inefficient for diffuse gas.
- Once mass of envelope exceeds mass of core, so gas becomes compressible. Gas giants.
- Timescale to reach runaway gas accretion is similar to the lifetime of the disc of some planets will not reach this phase before the gas disc is dispersed. This leaves an ice giant (with a less massive core and less massive gaseous envelope.
- What causes the gas disc to disperse? Planet migration: - Before the disc is dispersed, what can lead to planet migration? - What is type one migration? - What is type 2 migration?
- X-ray driven photo-evaporation once the Sun turns on as an active star.
- Gravitational interactions.
- Gravity of protoplanet excites spiral density waves in the disc, and gravitational interaction with excess mass in these density waves can transfer angular momentum, allowing the planet to migrate inwards.
- Migration continues but slows after the planet has cleared a gap in the disc.
7) The terrestrial planets assemble on a longer timescale by orbital perturbations leading to giant collisions.
- How were the terrestrial planets formed?
- How did Mercury lose most of its mantle?
- How was the Moon formed?
- What has radio-isotope dating of lunar rock samples shown?
- When gas disc dispersed, inner Solar System thought to have contained many rocky protoplanets that had each grown to isolation mass. Protoplanets likely to have had very eccentric orbits as there was no damping from the gas disc. Secular resonances grew eccentricities until orbits crossed and protoplanets collided to form the 4 terrestrial planets.
- In the final collision.
- From Earth’s mantle following collision with a Mars-sized proto-planet.
- These collisions continued for around 10^8 yr, 10x longer than lifetime of protoplanetary disc.
