1 Martian 2: [dingbats]
1 [caption]: *Translated from Martian: How goes the plans for the invasion of Earth?
2 Martian 3: [dingbats]
2 [caption]: Pretty good, except we transmitted some taunts by radio...
3 Martian 1: [dingbats]
3 [caption]: ... and we think the Earthlings have deciphered our language.
4 Martian 2: [dingbats]
4 [caption]: You dingbats!

2012-02-06 Rerun commentary: Well the joke is pretty obvious here if you look at the dialogue transcript.

The Martian control table is still in one of its many preliminary forms. I like that some of the controls are, if you look closely, some sort of giant analogue gauges like you'd find on the side of a steam engine.

So that's why Mars has so many canals! The Martians need access to lots of water for their steam-powered civilisation!