Watch live: NASA's Perseverance rover set to land on Mars

Why do I think there was once life on Mars?  Well, the rust on the surface seems to me to be one hell of a big clue.  You see, when the earth cooled from a molten ball of rock and seas formed, life began very quickly and long before there was oxygen in the atmosphere in any appreciable amount.  Oxygen is so active it easily compounds with very many other elements such that there would be little if any of it free in the air.  

My guess is that this would also be the case on Mars, water, an atmosphere but very little free oxygen.  So then, where would all that rust come from?  On earth we had a similar circumstance.  In our case cyanobacteria released ozygen.  Then the oxygen combined with iron in the water and it coated the earth in rust deposits.  I would imagine that pretty much the same would have happened on Mars.  

After the earth cooled enough to have liquid water it didn't take very long for life to begin, so I'd suspect that early life isn't all that difficult to get started. Cyanobacteria would probably develop just as quickly and so there's only a need for Mars to hold its water and atmosphere for only a few hundred million to a billion years to reach the point where it can rust the iron out of the water. Otherwise one needs to find another source for all that free oxygen to rust all that iron. It didn't happen on earth without the assistance of life, I'm pretty sure it wouldn't happen on Mars either.

As the planet cooled its water would, following what we see here, sink into the soil and become a permafrost layer.  The planets volcanic/tectonic activity would probably create aquafers, lava tubes and erosion would create caves and caverns.  Some of these voids could fill with water and be held there for long periods of time.  Assuming that the interior of the planet didn't cool all that quickly, this buried water could have remained liquid for a very long time, perhaps as much as a couple of billion years.  So it will be very interesting to see what these explorations can find.