This question assumes that in the future a sizeable colony of humans might be established on Mars, or elsewhere.
On Earth, one common way to deal with the dead is burial. This system relies on microbes consuming the soft tissues and thereby transferring nutrients back to Earth.
Life on Mars has not been discovered. So for now, it can be assumed there would be no natural way to decompose the bodies of deceased colonists. Sending the bodies to Earth would be expensive and would require procedures to ensure unpleasant conditions did not eventuate aboard the vessel taking the bodies to Earth. Cremation would use vital oxygen supplies. Burial would most likely result in corpses remaining as they are, either frozen or desiccated, similar to the Inca mummies found in Argentina in 1999. One idea I came across was to have the bodies composted and the nutrients used a fertilizer for plants.
Have any protocols been developed by anyone that addresses what would be done with deceased colonists?