Can decomposition occur without other lifeforms? If an astronaut was to die on a planet complete with moderate temperatures and devoid of life would decomposition be possible? Could the bacteria living on or within the astronaut's body do the job?

Yes. If there are bacteria on or in the astronaut, they'll do it. If, somehow, he's totally sterilized (gamma rays or something), he'll just stick around a LONG time, while his body breaks down through abiotic means like erosion, radiation destroying chemicals in his cells, cracking from heat and cold, etc. He'd be sorta like a small, squishy rock.

 

Mokele

It's the bacteria that exist in our bodies that are mainly responsible for decomposition anyway. Decomposition always begins in the gut and the abdomen becomes discoloured blue/green after about 12 hours.