,
16 tweets,
6 min read
Read on Twitter
Fun conversation at dinner tonight, since my oldest daughter Jess participated in pebble accretion research in the lab of @joshuacolwell and @astroaddie at @UCF two years ago. Josh, Addie and Julie Brisset are doing reduced gravity experiments to understand planet formation.../1
2/ The earliest part of planet-formation is well-understood, because molecules fuse to form dust (like snowflakes growing in a cloud), then the dust particles easily stick to each other by cohesion. But after they grow to the size of pebbles they will bounce instead of stick...
3/ This is the "bouncing barrier" of the planet formation process. So Josh, Addie, Julie, and others in their team have been devising experiments to study how gravel-sized particles can form ever-larger clumps until they are large enough for gravity to take over...
4/ Once they are large enough then gravity helps them smash together when they collide, so they can keep getting bigger all the way up to become planets. So the Microgravity Lab at @ucf has been focusing on the growth of pebble-sizes. E.g., read more here: arxiv.org/pdf/1706.08625…
5/ They have been doing experiments with a drop-tower at UCF, in rockets with @blueorigin, in space planes with @virgingalactic, in CubeSats to be deployed by NASA, and in @ISS_Research. My daughter participated by analyzing data downlinked from experiments in Space Station.
6/ Jess presented a conference paper about it: ascelibrary.org/doi/10.1061/97… … It also became her high school science fair project. So when we saw the picture from @NewHorizons2015's flyby, we got excited because it hints at how pebbles formed into planets.
7/ When gravel particles collide the bounce is not perfectly elastic so they lose kinetic energy. They move TOWARD each other faster than they move AWAY. So a cluster of gravel particles orbiting the sun will "on average" have a net statistical motion TOWARD each other.
8/ In granular physics, this is called "granular collapse". This clustering behavior is seen in many types of experiments. When sand is blowing across the desert, it will cluster into sheets. I'm not sure, but I wonder if rain forms into sheets for similar reasons...?
9/ I used a similar argument to prove that the particles blown by a lunar lander are colliding with each other during flight, causing granular collapse, so the collision of sand particles must be included in computer models or else the predictions will be inaccurate.
10/ In this image you can see the sandblasting of lunar soil on the Surveyor 3 spacecraft. It produced a "mottled" appearance because the impinging dust specks were clumped where they struck the Surveyor. It is not obvious at first how this proves granular collapse, so here it is 
11/ You can measure the width of the sandblasting splotches. Project lines from the left and right sides of a splotch back to the lunar surface under the centerline of the Apollo Lunar Module (LM) engine. That forms a pie-shaped wedge that is the angular width of the splotch...
12/ The soil was eroding within a few meters of the Apollo LM, and in that region, the width of this wedge would be narrower than a single dust speck. So all the particles that must have hit the Surveyor within a single splotch couldn't lift off the soil at the same time & place.
13/ This tells us they must have formed into a cluster of dust specks during flight, after leaving the lunar surface but before hitting the Surveyor. This tells us the blowing dust cloud is highly collisional and that granular collapse was taking place.
14/ So to tie this back to planet formation, granular collapse is a common phenomenon whenever lossy collisions are taking place in a particle cloud, but is this what happened during planet formation? As the picture from @jtuttlekeane at the top of the thread shows, maybe YES!
15/ For two Kuiper Belt objects to join together soooo slowly that they just kiss without disrupting each other on contact, they had likely formed out of the same local cluster of gravel particles. This suggests that there WAS indeed a cluster of gravel particles!
16/16 You gotta love it when a spacecraft flies 13 years to the far distant range of the solar system, sends back a single picture, and so much can be gleaned from the overall shape in one picture! This shows the value of spacecraft missions. And there's so much more to come!
