NASA's Curiosity rover is helping scientists close in on a Martian mystery: Why does a mountain jut out of a barren crater?

Scientists said Monday that rock images indicate that 3-mile-high Mount Sharp may have formed in a big lake bed over a million or even tens of millions of years. Deposits of sediment seem to have shaped the mountain.That begs the question as to whether microbial life may have existed there in those wet ancient times.

"This lake was large enough, it could have lasted millions of years — sufficient time for life to get started and thrive, sufficient time for lake sediment to build up to form Mount Sharp," said Michael Meyer, lead scientist of NASA's Mars exploration program.

Curiosity's latest observations could mean Mars' warm, wet period occurred about 3.5 billion years ago, more recent than had been thought.

In addition, Martian lakes could have lasted longer than previously suspected. Scientists are uncertain whether this wet period was continuous or interrupted by dry spells.

Meyer acknowledged that even here on Earth, "we don't have a very good handle" on how long it takes life to originate and how long a conducive environment needs to be in place beforehand. So addressing the possibility of whether life once existed on other planets is made all the more complicated.

Curiosity has been exploring Gale Crater since its arrival on Mars two years ago; the rover reached the base of Mount Sharp a few months ago. The crater, 96 miles across, was caused by an asteroid impact.