Mars today is a dry, inhospitable place. There is hardly any liquid water on the surface, though there is water ice particularly around the poles. But scientists know that liquid water is a key ingredient for life as we know it to develop.
Scientists are pretty certain that there is nothing alive on Mars today. But they have long speculated about whether there could have been life, most likely microbial, on Mars millions of years ago. To find out if that were possible, they needed more information about whether Mars ever had surface water, and if so, for how long.
Spirit and Opportunity revolutionized Mars science by finding small spheres nicknamed “blueberries” on the planet’s surface. These were clumps of a mineral called hematite, which forms in acidic water, showing that there must once have been liquid water on the surface. The pair also discovered soil rich in silica which must have formed in water, further providing evidence of Mars’s watery past.
Even now, scientists are still debating exactly how long that water stuck around for, but it is thought that Mars had plentiful liquid water up until around 4 billion years ago. That finding was the basis for sending the Perseverance rover to the Jezero crater on Mars, in order to look for evidence that there could once have been microscopic life on the planet. The crater is the site of an ancient lake, making it potentially ideal conditions for life to have thrived billions of years ago.
