My guess is methane as well, Mars still "breathes" out methane occasionally[1]. Given that chunks of Mars have been kicked off the planet and landed on Earth, I wonder if you modeled a chunk of Titan's methane ice being kicked off Titan and hitting Mars would have the effect of filling out enough of Mars' atmosphere to warm it.

[1] http://science.sciencemag.org/content/306/5702/1758