Given that there are no organisms on other planets/moons, if we bury a dead human (or other organisms), will it decay? I think the body will be intact as on earth there are several organisms/micro organisms that help in decaying. Is my understanding correct?
This could be off-topic here, but I have an answer, so I might as well post it.
Yes and No
According to Wikipedia, decomposition occurs due to two factors: the body's own chemicals and the effects of bacteria.
Autolysis is the process of a bit of organic matter breaking itself down. The lysosomes in a cell use digestive enzymes that normally would break down food to inadvertently eat away at the cell. These proteins slowly start to destroy the organelles inside each cell. Eventually, this effects the entire body.
You might be more familiar with putrefaction. This is the process bacteria use to break down proteins in the body's tissue and get energy from it. Fungi also contribute. together, these organisms decompose organic matter, absorbing the nutrients for themselves.
So, would this happen on another planet? Well, autolysis might. After all, the body's chemicals are still there. True, the vacuum of space (if the planet lacks an atmosphere) could cause some damaging effects, but perhaps the body would be okay and decompose. Putrefaction, on the other hand, might not happen. Unless extraterrestrial bacteria or other organisms exist to bite away at the tissue, there will be nothing there to aid in putrefaction.
There's always the chance that the environment will act like a peat bog and preserve the body, but to date, no subjects have volunteered to test this hypothesis.