The impactor device was very similar to a HEAT warhead; a high explosive shaped charge lined with copper. Detonation of the shaped charge turns the copper lining into what's called an explosively-formed penetrator of EFP [1]. Note that the device was separated from the main probe before use and the probe itself was below the horizon when the impactor was fired in order to avoid damage from debris.

[1] https://en.wikipedia.org/wiki/Explosively_formed_penetrator

Explosively >:)

https://www.lpi.usra.edu/meetings/lpsc2013/pdf/1904.pdf