[YeGoblynQueenne]

Also, if you play the animation slowly, you can see a brighter circle forming at the point of impact, before the impact. Is that part of the simulation or an artifact of the animation, I wonder?

[magicalhippo]

My interpretation is that the surface magma, cooled down hence the dull orange, is blown away by the bow shock in the atmosphere from the other body entering the atmosphere. This exposes the hotter, brighter, underlying magma.

Could of course be entirely wrong.

[mcherm]

Wouldn't that require an unreasonably thick atmosphere? Our current atmosphere is only around 10 miles high. (Obviously, it all depends on what you consider as your cutoff.)

[magicalhippo]

Possibly, I'm no expert.

I didn't think about timescale though, the bow shock would require very short timescale (if at all plausible like you said).

If it happens on longer timescales, then maybe it's just surface heating from the emission of the other body.

[Rexxar]

Gravity that attract mater imho (and make hot matter go on surface). There is another effect : the incoming moon is not a sphere any more before impact.

[grenoire]

I'm guessing it's the low energy light from the rendered sphere.