Pebble accretion and the early Earth

[windy miller]

TL;DR Summary

Pebble accretion and the early Earth

It is being reported in the press that there is new evidence for pebble accretion models https://www.bbc.co.uk/news/science-environment-51295365
Normally when we think of the very early Earth we think of being formed violently . This model seems more gentle. I am wondering what that means for the appearance of the night sky on the very early Earth. As I understood it, if one stood on the early Earth it wouldn't be possible to see the night sky because of 1) the debris of the early impacts and 2) the vaporisation of water from violent impacts . However if this new pebble accretion model is correct would that still be so. Would the view fo the night sky be blocked out ?

 

[mfb]

It sounds like they talk about the meter size barrier. After an object reaches a size of several kilometers gravity leads to further accretion (that's largely beyond the growth of Arrokoth). By the time you could think of standing on a planet and having a night sky gravity is so strong that everything impacts at high speed.

 

[davenn]

and for the complete opposite and more common theory

https://advances.sciencemag.org/content/advances/6/7/eaay7604.full.pdfwhich is one I prefer

 

[Ian J Miller]

The oligarchic accretion model predicts that cores either cannot accrete beyond about 10 A.U., or it is very slow. In the grand Tack model, or variations thereof, Uranus and Neptune started within 10 A.U. and moved out by gravitationally sending planetesimals in. However, LkCa 15b suggests this model is wrong. This is about 5 times Jupiter's mass and is situated about 15 A.U. from a star that is slightly smaller than the sun, and the star is about 2 My old.

The oligarchic model needed about 10 My to form Jupiter's core. To me, this suggests that the actual mechanism is most probably a monarchic growth model, where one body sweeps up a continuous supply of feed from material falling into the star. As I see it, the core grows in the region where the ice is near its triple point, i.e. the core grows like a snowball until it is big enough for gravity to take over. If so, the other giants will grow where similar ices are near their triple point (albeit occluded in water ice - such occluded ices have been formed in the laboratory under very low pressures.) If so, the cores, and the planetary moons, will have the composition required for that temperature.

Our four planets are roughly where they are supposed to be, assuming our accretion disk followed relationships similar to disks we have observed. Unfortunately, our data on compositions of moons of Uranus and Neptune are inadequate, but the model predicts no significant atmospheres for the Jovian Moons, but atmospheres of nitrogen and methane for Saturnian moons that are big enough that arise from chemical processing of ammonia and methanol.

Interestingly, the only example of which I am aware of streaming instability is a feed into LkCa 15b, and this suggests that it is the giant and its gravity that creates the instability.

 

[Dragrath]

and for the complete opposite and more common theory

https://advances.sciencemag.org/content/advances/6/7/eaay7604.full.pdfwhich is one I prefer

Yeah there is growing evidence that planets likely form very quickly also comparing the linked BBC source to the primary source I noticed neither mentions pebble accretion at all rather they describe that determines that Arrokoth (2014 MU69) must have formed gently at low velocities with models based on observational constraints suggesting it formed from direct collapse and ruling out collision based hierarchical accretion as playing a role in 2014MU69's formation . They even make the point that this only refers to this one object. Personally I suspect there are likely multiple competing processes since the resulting bodies can have a wide array of different properties and each model thus far has its own problems.

https://www.nasa.gov/feature/new-ho...tical-piece-of-the-planetary-formation-puzzle