Why is mars considered the planet most likely to have living organisms?

Answer 1

Because it is roughly in the habitable zone. Plus we have found a meteor containing signs of microorganism life (not conclusive proof), along with minerals in Mars' crust that on Earth are formed by oceanic microorganisms. Venus while technically in the habitable zone as well has a runaway greenhouse effect that bolsters the planet's surface to over 800 degrees F or 400 C on both the day and night sides and a crushing atmospheric pressure that for the most part nothing could survive. Usually this is hotter than the day side of Mercury but that depends on where Mercury is in its orbit as Mercury's orbit is elliptical. Some scientists contend though that Europa and Ganymede (moons of Jupiter) or even Enceladus (moon of Saturn) may harbor life because these moons are either suspected or proven to have pools of liquid water under their surfaces (depending on which of the moons we are talking about) but for Jupiter's moons this is a high radiation environment (Ganymede might combat this with its weak magnetic field) and in Enceladus' case the water is shallow in comparison to the other two. If we do find life it probably will be microorganisms on Mars. Probably nothing fancy, simple bacteria.

Answer 2

Mars is considered the planet most likely to have living organisms because astronomers and scientists think that there is liquid water beneath Mars's crust.