Science Fiction - Alien race discovered on Mercury - Language Machine

[Josiah]

Can't remember what the cover looked like, read in the 1990's (not sure when). I think that book was older than that though.
IIRC, the story started on Mars, male protagonist, possibly had a brother. eventually ended up on Mercury and discovered an alien race living in the habitable belt that made up the boundary between the light and dark side of the planet. The aliens had no skeletons and eyes with no pupils. Human scientist used a language learning machine to communicate with them. Hooked up an alien and a human and had them point and name objects.
Any assistance is appreciated, thanks all!

[Kris]

Josiah, can you tell us more about the hero - his age, job/skills/interests (e.g., linguist/programmer/professor/explorer), parents/family, birth planet, etc.? What work did he do on Mars?

[Lobstergirl, au gratin]

Josiah, are you still looking for this or did you find it?