Mars may now be considered a barren, icy desert but did Earth's nearest neighbour once harbour life?
It is a question that has preoccupied scientists for centuries and fired up sci-fi imaginings.
Now three space exploration projects are gearing up to launch some of the most ambitious bids yet to find an answer.
Scientists believe that four billion years ago the two planets both had the potential to nurture life—but much of Mars' intervening history is an enigma.
The new Mars probes from the United States, United Arab Emirates and China will launch this summer.
Their goal is not to find Martian life—scientists believe nothing would survive there now—but to search for possible traces of past lifeforms.
These vast and costly programmes could prove futile. But astrobiologists say the red planet is still our best hope for finding a record of life on other planets.
To read more, click here