I didn't have a clue with this puzzle when it was published. Then a couple of weeks later I did the Martian Heritage trail and afterwards had another look.
An idea came to me almost immediately this time. It looked promising so I continued on along that path and within half an hour I'd got the answer. I was a bit annoyed with myself in fact for not doing that the first time. Another good puzzle though (have a fave).
This was the third one of the day. An easy find - and I could have a rest while I signed the log.