How will Nasa scientists know if life existed on Mars?

Perseverance rover carrying cargo of advanced instruments to detect evidence of ancient microbes

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Nasa scientists are closer than ever to being able to answer one of the most enduring Martian mysteries: has the red planet ever given rise to life?
One of the primary goals of Nasa's Perseverance rover is to scour the surface of Mars for signs of ancient life, evidence which would show for the first time that life had existed beyond Earth.
Previous missions to Mars like the Curiosity rover, which is still active after arriving in 2012, have shown that the planet was once significantly wetter and warmer, and could have provided a habitable environment for microbial life forms.
Despite the great advances in our understanding of Mars made in recent years, scientists have never found proof that life has ever existed there.
Perseverance, Nasa's largest-ever rover and the first designed specifically to find evidence of ancient life, could change that.


The hunt for ‘extraordinary’ evidence

But how will scientists know if they have found evidence that life once existed on Mars?
"To quote Carl Sagan 'If we see a hedgehog staring in the camera', we would know there's current and certainly ancient life on Mars, but based on our past experiences, such an event is extremely unlikely," Gentry Lee, chief engineer for the Planetary Science Directorate at Nasa's Jet Propulsion Laboratory (JPL), said.
"Extraordinary claims require extraordinary evidence, and the discovery that life existed elsewhere in the universe would certainly be extraordinary," he added.
Often described by Nasa scientists as a "robotic astrobiologist", Perseverance is carrying a payload of instruments and sensors which will allow scientists to study the planet's surface and hunt for evidence of ancient microbial life.

A pair of cameras known as Mastcam-Z work like human eyes to produce 3D images and can be used to zoom in on areas of interest to scientists. Engineers had tried to equip the Curiosity rover with a similar instrument, but found themselves unable to meet the tight space requirements with the technology available in 2011.
Sitting just above the rover's eyes on the mast, the SuperCam instrument can fire a laser beam at rocks, generating small clouds of plasma that scientists can analyse to find out what they are made of.
Two more devices on the rover's robotic arm – a tiny but powerful X-ray beam and another laser – will be used to look for concentrations of organic molecules.
Using these instruments, scientists will be able to hone in on areas of the Martian surface most likely to contain biosignatures – evidence of past life.
Scientists will also search for surface features that could have been made by ancient lifeforms, like stromatolites – special layered mineral formations sometimes caused by bacteria. However, even this evidence would not be a smoking-gun.
"Yes, there are certain shapes that form in rocks where it's extremely difficult to imagine an environment devoid of life that could cause that shape to form. But that said, there are chemical or geologic mechanisms that can cause domed layered rocks like we typically think of as a stromatolite," Ken Williford, deputy project scientist for the Perseverance mission, said.
Scientists are also hoping to find fossilised microbes, and Nasa is targeting an ambitious landing in Mars's Jezero Crater, which 3.5 billion years ago was the site of a large lake and a river delta.
"We expect the best places to look for biosignatures would be in Jezero's lake bed or in shoreline sediments that could be encrusted with carbonate minerals, which are especially good at preserving certain kinds of fossilised life on Earth," said Mr Williford.
Samples collected by the Perseverance mission will be essential to finding any evidence of life, but even Nasa's most advanced rover will be unable to analyse them fully on Mars.
The agency is instead planning to return the samples to Earth with a later mission, meaning the rover alone may not be able to prove that Mars once supported life.
Bobby Braun, who manages the JPL's Mars Sample Return programme, said: "The instrumentation required to definitively prove microbial life once existed on Mars is too large and complex to bring."

Nasa scientists believe the rover will make a valuable contribution to solving the enduring mystery of life on Mars, even if its samples come back negative.
Mr Williford said: "We have strong evidence that Jezero Crater once had the ingredients for life. Even if we conclude after returned sample analysis that the lake was uninhabited, we will have learnt something important about the reach of life in the cosmos.
"Whether or not Mars was ever a living planet, it's essential to understand how rocky planets like ours form and evolve," he added. "Why did our own planet remain hospitable as Mars became a desolate wasteland?"
Scientists and space enthusiasts will have to endure the nail-biting "seven minutes of terror" on Thursday as the mission touches down.
If the landing is successful, scientists will be one step closer to answering a question that has fascinated astronomers since Galileo's discovery of Mars in 1609.