What Would a Collision Between Two Icy Worlds Look Like?

[Aanker]

I've been casually thinking about this for a while. Imagine that we have two Europas, or perhaps something even more extremely water/-ice rich, colliding as part of the formation process of some theoretical solar system.

What would, visually, such a collision look like? Would there be a gigantic flash - and if so, from what - or would we just see massive slabs of ice be ripped apart or vaporized and ejected at insane speeds in various directions? In most animations of the early Earth being impacted by a Mars-sized body (the collision that led to the formation of the Moon), large swaths of the planet are portrayed as turning into lava and molten material. I would presume that an icy world could not have such an appearance. Would it turn into a steamy fuzzball instead?

[HoloYolo]

Maybe a lot of steam? Space Sauna!

[MartGonzo]

Like a giant's snowball fight.

[lajoswinkler]

There would be one heck of a flash. Any matter glows if heated above a certain point, and there's enough energy in collisions of planetary objects to cause blinding light. A piece of ice at a reentry will flash. Now imagine objects with orders of magnitude more surface colliding directly, lithosphere to lithosphere. Regardless of the composition, it would be an abominable blinding light.

Images like these are hilariously incorrect.

Absolutely downgraded in every way, with fancy stuff flying around. In reality, if two Europas would collide at several tens of kilometres per second at the distance of our Moon, it could very well look like this.

It would be some time before the material cools down by radiation, which you'd feel on your skin.

Also, remember that planetary objects would behave like liquids, rocky or icy, doesn't matter. They would both turn into blinding ejecta, swirling very slowly in the sky.

Edited July 16, 2015 by lajoswinkler

[Nuke]

i think you would see a flash but this would quickly be obscured by massive clouds of steam.

[peadar1987]

The heat generated would probably be enough to not only boil the water, but crack it into hydrogen and oxygen, which would then recombine explosively. It would be very hot and very bright!

[Aanker]

Great replies so far!

The heat generated would probably be enough to not only boil the water, but crack it into hydrogen and oxygen, which would then recombine explosively. It would be very hot and very bright!

Enough to fuse the hydrogen thus produced?

[Godot]

...

Enough to fuse the hydrogen thus produced?

Surely not enough to create a self sustaining fusion reaction ...

icy/rocky planets are tiny compared to gas planets ... which are small compared to stars

[Aanker]

Surely not enough to create a self sustaining fusion reaction ...

icy/rocky planets are tiny compared to gas planets ... which are small compared to stars

Not sustained, but sporadic. Basically, as the planets/moons collide and the temperature reaches its maximum, could some of the freed hydrogen theoretically fuse?

Merely curious.

[lajoswinkler]

The heat generated would probably be enough to not only boil the water, but crack it into hydrogen and oxygen, which would then recombine explosively. It would be very hot and very bright!

Breakup would occur and would be homogeneous, not leading to parts with one gas dominance, but explosive recombining would be highly unlikely as we're talking about an open thermodynamic system. The event is happening in vacuum where gases behave as gases actually behave - like a bunch of tiny balls flying around. Gases don't creep, cavort or linger unless under pressure.

You'd simply have massive amount of gaseous mixture of water, hydrogen and oxygen (dominantly water) being in various stages of chemical equilibrium in the depths of the ejecta chaos. As the heat is transfered into space, equilibrium is shifted towards water. In outer parts of the catastrophe, equilibrium could not occur because pressures are too low and particle collisions are scarce.

We actually witnessed something close to this.

Icy dirtball impacting a mixture of hydrogen and helium. Close enough. Fireball larger than Earth.

[Rune]

Breakup would occur and would be homogeneous, not leading to parts with one gas dominance, but explosive recombining would be highly unlikely as we're talking about an open thermodynamic system. The event is happening in vacuum where gases behave as gases actually behave - like a bunch of tiny balls flying around. Gases don't creep, cavort or linger unless under pressure.

You'd simply have massive amount of gaseous mixture of water, hydrogen and oxygen (dominantly water) being in various stages of chemical equilibrium in the depths of the ejecta chaos. As the heat is transfered into space, equilibrium is shifted towards water. In outer parts of the catastrophe, equilibrium could not occur because pressures are too low and particle collisions are scarce.

We actually witnessed something close to this.

https://upload.wikimedia.org/wikipedia/commons/1/1b/SL9ImpactGalileo.jpg

Icy dirtball impacting a mixture of hydrogen and helium. Close enough. Fireball larger than Earth.

Quoted for thruthiness. Basically, big flash, and when the overexposure fades, you are left with a cloud of really hot gas and ejecta around some chunks of solid/fluid material glowing softly in the infrared as they collapse gravitationally into a single body, hating further in the process (because if two planet bring a lot of energy into the collision, they are also huge heatsinks to absorb the heat generated, not everything would turn to steam, and forget about fusion). The relative speed of the impact is also important. Two icy bodies can merge slowly at a few hundred m/s in the Oort cloud, or they can slam at cometary speeds (50+km/s), and the energy release is orders of magnitude different.

Rune. Shoemaker was of the energetic kind.

[kerbiloid]

Any explosive turns into a water steam, carbon oxides and nitrogen oxides when bursts.

TNT give 4.2 MJ/kg which is equal to kinetic energy for sqrt(4.2e6*2) ~= 3 km/s speed.

So, the ice worlds collision would look just like a burst, nothing special.

[peadar1987]

Enough to fuse the hydrogen thus produced?

Nope, not even close.

[lajoswinkler]

Any explosive turns into a water steam, carbon oxides and nitrogen oxides when bursts.

TNT give 4.2 MJ/kg which is equal to kinetic energy for sqrt(4.2e6*2) ~= 3 km/s speed.

So, the ice worlds collision would look just like a burst, nothing special.

There's much more energy in two Europas colliding at some 40 km/s than there is in chemical explosives ever fired and currently stockpiled. Hell, even everything nuclear fired and stockpiled is puny compared to it.

As you can see, there's a decent flash with TNT. The camera has a filter, otherwise the flash would look like a white, washed out ball.