why do scientist belive that life may have existed in the past on mars?

Answer 1

Because of the condition it is in. The poles are actually frozen water .
And They have found riverbed formations and signs of water were there. Also The worm-like structures present in that (supposed) Martian meteorite ALH 84001 superficially resemble miniature bacteria. On this basis NASA loudly proclaimed that they might be the fossils of primitive Martian life forms. (SF#108, #110)

Some scientists were suspicious of this claim. J.P. Bradley et al, in a recent letter to Nature, declared the putative fossils to be only the fractured surfaces of lifeless crystals. Responding, K. Thomas-Keprta insisted that her group was not so stupid that it would mistake crystals for fossils! Her group, too, had noticed the crystals. The claimed fossils are much larger and more numerous than the crystals. To settle the matter, Thomas-Keprta and associates plan to dissect the suspect structures.

Answer 2

They have found evidence of water, at least in the Martian past. Due to a scarcity of atmosphere, the water very likely has boiled off, and the only water remaining is frozen beneath the surface. But, if water did exist as a liquid in the past, and if it still exists as liquid or moisture beneath the surface, then where there is water, life may be found also.

Answer 3

There is significant evidence that water has been on mars. Water means that all the essentials for life were available at one point. They don't have any evidence that there was life other than the conditions were there to support it.

Answer 4

The planet shows river beds and canyons that appear cut by water. Where their is water there is a possibility of life.

Answer 5

By what has been discovered and observed by various probes over the years. Try space.com.

Answer 6

Cos they're dumb