I believe the moon in question is Miranda, as James K suggested. It is hypothesized that it suffered a collision with another large body sometimes in the past. There are several large features that suggest this:
- Three large grooves, each > 200 km long, cover parts of the surface (the "scar" you talked about). These are the coronae.
- Observations indicate that some parts of the surface appear less than 100 million years old.
- Verona Rupes, a 10 km-high cliff, is the largest in the Solar System. Other, smaller, rupes exist elsewhere on Miranda and across the Solar System, but not quite as high as this one.
Alternate explanations include heavy crustal activity in the past. Tidal effects from Uranus may also have played a role (either at different times than the putative impact, or instead of it entirely). Gas giants strongly interact with their moons through tidal forces; cases include Io's volcanism, driven by Jupiter, and the covered oceans of Europa and Enceladus.
Here is a partial map of Miranda, displaying the coronae and several other features: