Other than the incredibly hostile environment, and the fact that Mars is pretty darned big.
Ok, the
incredibly hostile environment thingy:
Extremophile - Wikipedia
In the 1980s and 1990s, biologists found that
microbial life has an amazing flexibility for surviving in extreme environments—niches that are extraordinarily hot, or acidic, for example—that would be completely inhospitable to complex organisms. Some scientists even concluded that life may have begun on Earth in
hydrothermal vents far under the ocean's surface.
[3] According to astrophysicist Dr. Steinn Sigurdsson, "There are viable bacterial spores that have been found that are 40 million years old on Earth—and we know they're very hardened to radiation."
[4] On 6 February 2013, scientists reported that
bacteria were found living in the cold and dark in a lake buried a half-mile deep under the ice in
Antarctica.
[5] On 17 March 2013, researchers reported data that suggested
microbial life forms thrive in the
Mariana Trench, the deepest spot on the Earth.
[6][7] Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet below the sea floor under 8500 feet of ocean off the coast of the northwestern United States.
[6][8]According to one of the researchers, "You can find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are."
[6]
Astrobiologists are particularly interested in studying extremophiles,
[16] as many organisms of this type are capable of surviving in environments similar to those known to exist on other planets. For example, Mars may have regions in its deep subsurface permafrost that could harbor
endolith communities.
[16] The subsurface water ocean of
Jupiter's moon
Europa may harbor life, especially at hypothesized hydrothermal vents at the ocean floor.
Recent research carried out on extremophiles in
Japan involved a variety of
bacteria including
Escherichia coli and
Paracoccus denitrificans being subject to conditions of extreme gravity. The bacteria were cultivated while being rotated in an
ultracentrifuge at high speeds corresponding to 403,627
g (i.e. 403,627 times the gravity experienced on Earth).
Paracoccus denitrificans was one of the bacteria which displayed not only survival but also robust cellular growth under these conditions of hyperacceleration which are usually found only in cosmic environments, such as on very massive stars or in the shock waves of
supernovas. Analysis showed that the small size of prokaryotic cells is essential for successful growth under
hypergravity. The research has implications on the feasibility of
panspermia.
[17][18]
On 26 April 2012, scientists reported that
lichen survived and showed remarkable results on the
adaptation capacity of
photosynthetic activity within the
simulation time of 34 days under
Martian conditions in the Mars Simulation Laboratory (MSL) maintained by the
German Aerospace Center (DLR).
[19][20]
On 29 April 2013, scientists at
Rensselaer Polytechnic Institute, funded by
NASA, reported that, during
spaceflight on the
International Space Station,
microbes seem to adapt to the
space environment in ways "not observed on Earth" and in ways that "can lead to increases in growth and
virulence".
[21]
On 19 May 2014, scientists announced that numerous
microbes, like
Tersicoccus phoenicis, may be resistant to methods usually used in
spacecraft assembly clean rooms. It's not currently known if such resistant microbes could have withstood
space travel and are present on the
Curiosity rover now on the planet Mars.
[22]
On 20 August 2014, scientists confirmed the existence of
microorganisms living half a mile below the ice of
Antarctica.
[23][24]
And Mars is a pretty big place thingy:
If a colony of bacteria grow at a rate of doubling every day, and we spread them over a one meter surface, at the end of:
7 days the bacteria spread over 128 square meters
14 days the bacteria spread over 16,384 square meters
21 days the bacteria spread over 2,097,152 square meters
30 days the bacteria spread over 1,073,741,824 square meters
38 days the bacteria spread over 274,877,906,944 square meters
45 days the bacteria spread over 35,184,372,088,852 square meters
52 days the bacteria spread over 4,503,599,627,370,496 square meters, or
31 times the total surface area of Mars (144,798,500,000,000 square meters)
The surface of Mars would be covering the entire surface of Mars half way through the
47th day with the bacteria spread over 199,032,864,766,430 square meters.
Never underestimate the power of exponential growth friend.
And doubling daily is a rather slow growth rate for bacteria from what I gather.
Considering that these hypothetical bactgeria would have no predators, it is a very conservative estimate, I suspect.