Advanced climate modelling has been applied to the planet's past, suggesting Venus once harboured liquid water. Picture / Getty
On the surface, the second planet from our sun — Venus — should be uninhabitable.
The few probes we have sent there have had to contend with metal-melting 450C surface temperatures and the immense pressure of its thick atmosphere — some 92 times more than Earth's.
But it wasn't always so.
Advanced climate modelling has been applied to the planet's past, news.com.au reports.
They suggest Venus once harboured liquid water.
Its surface conditions could have been habitable for a period of up to 2 billion years.
That's more than enough time for life to establish a foothold.
But something changed.
Planetary scientists don't know exactly what.
A runaway greenhouse effect was initiated.
All of the water on Venus evaporated.
This made things even worse — further adding to the thick blanket around the planet that snared all the Sun's heating rays.
It's long been thought this catastrophic change in climate would have killed off any living thing — be it microbe or Venusian monster.
But a new research paper published in the science journal Astrobiology argues there may be indications this was not entirely the case.
Some microorganisms may have clung on to life — in the clouds.
Cloudy lifeline
Coasting in the sky above Venus' hellish surface, such microbes may be enjoying a surprisingly balmy lifestyle.
Air pressure reduces with altitude, as does temperature.
And there appears to be a distinct band some 48km to 52km high in Venus' atmosphere where conditions are ideal for life as we know it.
Temperatures vary between 0 and 60C. Atmospheric pressure ranges from half to twice that of Earth's. And there's a scattering of vital nutrients such as sulphur and carbon dioxide.
"Together, our lines of reasoning suggest that particles in Venus' lower clouds contain sufficient mass balance to harbour microorganisms, water, and solutes, and potentially sufficient biomass to be detected by optical methods," the study reads.
Now the astrobiologists of the University of Wisconsin's Madison Space Science and Engineering Center say evidence of life could have been staring us in the face all along.
Strange, dark splotches in Venus' skies
Discovered about a century ago, it has long been wondered what it is about these lingering clouds that cause them to absorb ultraviolet light. We now know part of the reason is centrated sulfuric acid and iron chloride.
Lead author Sanjay Limaye says we should be staring at these Venusian clouds to see what's going on there.
The University of Wisconsin's researchers point out we already know of microbe blooms here, on Earth, that eat carbon dioxide and excrete sulfuric acid. Some Earth-based bacteria also absorb light. And living bacteria have been found as high as 41km above our surface.
Could feeding frenzies of living microorganisms such as these be the cause of Venus' strange, dark clouds?
"Venus has had plenty of time to evolve life on its own," Limaye says. It's two-billion-year window of opportunity was "much longer than is believed to have occurred on Mars."
Head in the clouds
The idea of life in the clouds of Venus is not new. Astronomer and famous science advocate Carl Sagan made just that suggestion in 1967.
However, no evidence supporting the idea has since been found.
But the prospect has been intriguing Nasa for some time.
And Limaye says he hopes his research is the spur it needs to get its project off the ground.
Nasa has designed and developed a system called the Venus Atmospheric Manoeuvrable Platform (VAMP). It's a drone that both flies like a plane and floats like a balloon.
It's believed capable of coasting through the acidic skies of Venus for up to a year, testing samples and studying its makeup.
It's hoped the glider can 'piggyback' a trip to Venus by Russia's Roscosmos Venera-D mission, which is due to launch in the 2020s.
At the moment, our understanding of Venus is surprisingly limited.
It's long been the Cinderella of space-exploration missions.
Our understanding and knowledge of its surface is little more than what we had of Mars in the 1970s.
Planetary scientists are keen to push beyond the clouds to examine samples of the planet's surface — particularly the oldest stuff.
This could reveal whether Venus once had the climate necessary for microbial life to get established, before its supposed retreat to the clouds.
"On Earth, we know that life can thrive in very acidic conditions, can feed on carbon dioxide, and produce sulfuric acid," says study co-author and professor of biological chemistry at California State Polytechnic University Rakesh Mogul.
"To really know, we need to go there and sample the clouds. Venus could be an exciting new chapter in astrobiology exploration."
New schedule details will follow after assessing the situation.