NASA's latest Mars rover, Perseverance, is scheduled to lift off tomorrow on its seven-month journey to the Red Planet.
If it survives its nail-biting landing in February next year, it will become NASA's 10th spacecraft in 45 years to touch down successfully on Mars.
Perseverance is one of three Mars missions setting off this month and the first rover ever tasked with finding evidence of past life on Mars, said Abigail Allwood, an Australian geologist at NASA's Jet Propulsion Laboratory who is in charge of PIXL, one of the seven instruments onboard the rover.
"No mission has ever been given the mandate to look for evidence of past life," Dr Allwood said.
Finding that evidence will be challenging, but Perseverance is the most sophisticated rover yet, according to Aileen Yingst of the Planetary Science Institute.
Dr Yingst has worked on several Mars rover missions, including Curiosity, and is the co-principal of another of the instruments onboard Perseverance called SHERLOC/WATSON.
"This is … probably the most complex operational mission since Apollo," she said.
What we know about Mars up to now
We've been fascinated about the potential of life on Mars ever since the 1800s when Giovanni Schiaparelli discovered "canali" or channels criss-crossing the surface of the Red Planet.
It wasn't for another 100 years, in 1965, that we got our first close-up of the planet when the Mariner 4 spacecraft revealed a cratered world with a thin atmosphere.
Since then, a more detailed picture has emerged of a warm, wet world that was once similar to Earth.
"We can see that Mars is the Earth that almost was," Dr Yingst said.
"At one point, Mars had on its surface every habitable environment that Earth had.
"That has profound implications for the search for where life might have arisen."
Extensive mapping of the Red Planet by orbiting spacecraft has revealed deposits of ice at the poles, in craters and underground, marks of ancient lakes and rivers cutting across the surface, and the possibility of liquid water under the south pole.
A series of rovers and landers on the ground have surveyed the landscape and found ripple marks where water once passed over the surface, and detected hints of organic compounds in the rocks.
"We have gotten to the point where we look at Mars as a world of sediment — that is, a place that was very geologically active," Dr Yingst said.
In fact, signs of tectonic activity and fleeting bursts of methane in the atmosphere indicate the planet may still be active today.
A rover with a difference
While these discoveries indicate that Mars had — and may even still have — conditions that could support life, it is not evidence that life ever existed.
None of the rovers previously sent to Mars have been equipped to specifically investigate features in enough detail to show they were created by biological processes, Dr Allwood said.
"The problem with these rovers is that they have these great high-resolution cameras and everyone kept seeing these beautiful little structures, rocks, crystals with zones around them and veins," she said.
"All these things that tell a rich geologic history, but it couldn't be interpreted."
That's where Perseverance comes in
While the new rover looks almost identical to its predecessor Curiosity, five of the seven tools it is carrying are specifically designed to explore the geology and chemistry of Mars, hunting for tell-tale signs of past microbial life.
Dr Allwood's tool PIXL is designed to scan the chemical composition of rocks to identify microscopic structures as small as a single grain of sand.
"It doesn't detect organics, but it's very much part of the bread and butter measurements of what we have to do to understand the background from which you can even … begin to attempt to address evidence of biology," she said.
Meanwhile, SHERLOC is designed to detect organics that contain building blocks of life such as carbon, while its camera WATSON, which is what Dr Yingst is working on, is designed to image the minute detail of grains of sand in an area the size of a postage stamp.
"WATSON allows us to see the surface clearly and SHERLOC allows us to see the surface in … wavelengths the human eye can't see," Dr Yingst said.
Two other cameras called Supercam and Mastcam-Z on the rover's mast will look for evidence at the macro scale, such as outcrops.
The hunt for ancient life
What exactly does the team expect to find?
"We actually don't know," Dr Allwood said.
But Earth may hold some clues.
Stromatolites, such as those found in the shallow waters off the coast of Western Australia, are created by layers and layers of microbes.
Fossilised stromatolites discovered in 3.34 billion-year-old rocks in the Pilbara by Dr Allwood in 2006 are the earliest undisputed evidence of life on Earth.
PIXL was instrumental in the confirmation of that discovery three years later.
The chosen landing site for Perseverence is Jezero Crater, which is thought to offer the best chance of finding macrostructures like stromatolites or chemical signatures associated with ancient life.
We've studied the planet from orbit and landed enough times now that we have a more sophisticated understanding of the geology of Mars than any other alien planet, Dr Allwood said.
"We know … enough to say at the most basic level, this could be a habitat for life."
Of all the sites the team considered, Jezero Crater is the only one where there was a standing body of water on the surface of Mars for a significant period of time, Dr Allwood said.
From orbit, the fan-like features of an ancient delta system running into a lakebed are clearly visible.
Environments where sediments are laid down over time offer the best chance of preserving both fossil and chemical evidence of past life.
"It's like the large sheets of a book that you might press a flower in," Dr Allwood said.
But Dr Yingst said finding hints of ancient life, even in areas that are potentially habitable, is hard.
"When you are trying to command a robot millions of miles away you have to look at points.
"If the ... evidence that life leaves behind is outside that little postage-stamp area, it will be very difficult to see it."
Finding hints is just the first step. Clinching the case for the existence of ancient life is complex — and can be controversial even on Earth.
Many structures or organic compounds that might hint at biology can also be created by non-biological processes.
For example, ripples in rocks that look like stromatolites can also be created by processes such heating or folding, or layers of non-biological compounds laid down over time in a lake.
The discovery of organic molecules is meaningless without geological context, Dr Allwood said.
All the pieces of evidence, from all the different tools, need to come together.
"If you have one line of evidence here to say 'Hey we've got some organics', that's interesting," said Dr Yingst.
"But does WATSON tell you that particular texture is what you'd expect? Is Supercam looking at this and seeing the same thing? Is Mastcam-Z seeing this at the centimetre or the metre scale?
"To be frank, it's not going to be easy."
Dr Allwood believes the knowledge of Mars built up by previous missions gives the team a good shot, however.
"We now know where the good sites are and we also know from terrestrial studies on Earth what to look for — and how to study it, when you find it, to unambiguously figure out what it is."
"For the first time in history we … can credibly address the question of whether we are, or always have been, alone in the universe."
First, though, Perseverance must reach its destination.
Seven minutes of terror
Like Curiosity before it, Perseverance will go through its own "seven minutes of terror" when it reaches Mars in February next year.
While NASA has a good track record of landing spacecraft on Mars, there are no guarantees.
"There are a lot of dead spacecraft on Mars," Dr Allwood said.
Dr Yingst has worked on missions at both extremes.
She was a graduate student working on the Mars Polar Lander mission, the last US spacecraft to crash on Mars in 1999.
But she was also in the control room when the Curiosity mission successfully landed in 2012.
"The success of a landing is way better," she said.
"There's this moment and you've landed, and then you get the first picture."
"It's hard to express the joy that comes from taking everything you thought you knew and throwing it right over your shoulder because it was absolutely not what you were expecting.
"It's an amazing thing to be able to see that surface for the first time and know that I can roll up my sleeves and get to work."
