The successful launch of Nasa’s Perseverance rover mission to Mars means an unprecedented flotilla of spacecraft is now headed for the Red Planet.
Led by the UAE’s Hope mission - which took off earlier this month, followed by China’s Tianwen-1 - the three probes will arrive next February and gather clues to the mystery of whether Mars has ever harboured life.
It is a question that has tantalised scientists for over 150 years, with the Red Planet playing a game of now you see it, now you don’t.
It began with supposed sightings of “canals” on the surface, conjuring up images of a race of alien engineers. In what has become something of a theme, these proved to be illusions which said more about the preconceptions of scientists than reality.
In the mid-1970s, Nasa landed two small laboratories designed to detect signs of life in the Martian soil. Some of the tests gave positive results, but sceptics claimed they came from lifeless chemical reactions.
But now there is growing excitement that scientists are finally closing in on the answer. And it comes from changing the question. Instead of asking if there's life on Mars, attention is focusing on whether there's life in it.
It has long been clear that the surface of Mars is a very hostile place. Even in summer, the typical daytime temperature is -43°C , plunging to -90°C at night. Its thin atmosphere is choked with carbon dioxide and its surface is blasted by ultraviolet light.
Could life be lurking under the surface of Mars?
It is hard to imagine any life-forms thriving in such an environment. But recent missions to Mars have revealed more benign conditions below-ground, including the existence of liquid water – widely regarded as vital for the existence of life.
Along with shelter from temperature extremes and radiation, life-forms have a better chance of surviving inside Mars than on its surface.
But they also need some source of energy capable of keeping them alive. For humans, it’s called food – whose energy ultimately comes from sunlight. Deprived of that, organisms sheltering inside Mars have to find some other source of energy – but what ?
As reported earlier this week, an intriguing answer has now been put forward by a researcher at New York University Abu Dhabi.
According to Dr Dimitra Atri, radiation from deep space known as galactic cosmic rays (GCRs) may smash through the top few metres of the Martian surface. Made up of fast-moving sub-atomic particles, GCRs could trigger chemical reactions from which life-forms can extract the energy they need.
The bizarre idea of organisms using radiation to stay alive is backed by the discovery of life-forms deep inside our own planet. In 2008, researchers reported finding bacteria in the pitch blackness 2.8 km underground in a South African goldmine.
Deprived of any source of sunlight-based energy for millions of years, they had found a way of extracting energy from chemical reactions triggered by the natural radioactivity of the surrounding rock.
New missions could unravel mysteries
Are life-forms lurking inside Mars doing something similar using cosmic radiation ? Intriguing hints have been detected by several Mars missions, in the form of bursts of methane gas emerging from the surface. Methane is a highly reactive gas whose main source on Earth is bacteria.
In 2018, Nasa announced that its Curiosity Mars rover had found methane concentrations peaked in the summer and then declined in winter. The cause is still being investigated, but one possibility is the seasonal ebb and flow of Martian bacteria living off cosmic radiation.
The probes now en route to the Red Planet may help resolve such mysteries. While not designed to detect life directly, the UAE’s Hope orbiter will provide detailed insights into the planet’s atmosphere – including seasonal changes.
Nasa’s mission aims to land the car-sized Perseverance rover into a crater and use radar, camera and other instruments to look for signs of life. It will also collect samples for return to Earth by a future mission, and fly a mini helicopter over the area.
Meanwhile, China’s Tianwen-1 mission to Mars - the nation’s first - has an orbiter, lander and a rover and will carry out a broad survey of the planet, including analysis of soil samples.
The discoveries made by all three probes will then feed into the joint European-Russian ExoMars mission, set for launch in 2022.
This will attempt to land a rover on the planet capable of drilling up to two metres into the surface, and test the soil samples for signs of organic molecules linked to the existence of life.
This will put the idea of life in Mars to its first proper test. And it is just possible the results will put an end to the guessing game the Red Planet has teased us with for so long.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK