On the Earth, a dead body is quickly decomposed by bacteria, its remains reduced to bones and the material essentially recycled by nature. On other planets, however, there are no such bacteria – and so bodies do not decompose. Following our experiences leaving objects on the Moon in the late 1960s and early 1970s, it is tempting to say that because no decomposition would occur, a dead body on Mars would simply lie on the surface forever. Moreover, the Martian atmosphere is mostly carbon dioxide and is dry thanks to the near-total absence of water from the surface – which would seem conducive to preserving organic material on the surface.
However, this is not the case. On Mars, a dead body would probably gradually be stripped bare and disappear not through the actions of bacteria, but due to the extremely harsh conditions on the Martian surface.
Mars has a day that lasts a similar period to Earth’s own (at least compared to the ten-hour day on Jupiter or the 243-Earth-day “day” on Venus). However, within that day, in part because of its very thin atmosphere, temperatures vary much more wildly than they do here, where at a single location the daily change in temperature will often be no more than a few degrees. For brief periods during the day, the temperature at a given point on Mars will actually resemble a warm day on Earth. However, at night the temperature plunges over a hundred degrees. A body exposed to these conditions would freeze at night, and then thaw during the day.
The result would be a process of desiccation, followed by regular thawing and breakup of organic material. What was left probably would not resemble a human body after very long. Concerned about the possibility that bacteria on space probes could “contaminate” the Martian surface and interfere with the search for indigenous Martian microorganisms, scientists have tested bacteria’s capacity to survive in the dryness and temperature changes they would encounter on the Martian surface. Even the samples of the hardiest bacteria, like E. coli, were entirely dead after just a few days.
Moreover, any body left on the surface of Mars would also be regularly exposed to the devastating dust storms which sometimes circle the planet. Martian dust storms are equivalent to hurricanes or tornadoes on Earth, but are particularly frequent during the planet’s storm season. The Spirit and Opportunity probes rely on these storms to scour clean their solar panels, but a body left exposed in a particularly strong storm might well be stripped of clothing and tissue.
The issue in the Martian dust storms is not so much the force of the wind as the dust which is blown by it. The Martian atmosphere is so thin that even a very strong wind would have much less force than a wind of equivalent velocity here on Earth. However, as the storms appear over the sand and rock surface of the planet, they blow large clouds of dust at extremely high velocities. This scouring effect might well pick clean a body as efficiently as any bacteria. Even if a body on Mars were not damaged by the sandstorm, it might still be effectively buried, disappearing from view (at which point it could probably survive under the surface effectively forever).
In short, because Earth-like bacteria do not exist and could not decompose a body on Mars, that body could theoretically remain in place forever, much as it would on the Moon. However, because of the harsh climate and sandstorms, it is possible the body still would not survive. If it were buried under sand, it might still survive forever, hidden from view but also protected from harmful forces above.