On Mars, there are spider-looking features near the South pole that might be caused by CO2 sublimation. When were these features first observed on the surface of Mars?
I have the impression these were first observed in images taken by the Mars Orbiter Camera (MOC) of the Mars Global Surveyor, probably sometime between 1997 (arrival at Mars) and 1999 (when the main mapping program began) -- or possibly as late as 2000 (when they are first mentioned).
In Piqueux et al. (2003), a paper specifically focused on the spiders, they say
The term ‘‘spiders’’ was used by Kieffer  (although the term may have originally been coined by Andy Ingersoll [H. Kieffer, personal communication, 2003]) and we use this term in this paper.
In Kieffer (2000) we find this statement
Dark radially converging dendritic patterns are visible in MOC images of some portions of the spring polar cap ; these have been termed "black spiders" by the MOC team .
Unfortunately, the published version of that short conference proceeding article is missing all of the references after number , so I don't know if there is more specific useful information available. The conference at which Kieffer (2000) was presented was in August, 2000, so I'd put mid-2000 as the latest possible date.