Extract from ABC News
Venus is the third-brightest object in the sky, yet we know very little about our mysterious neighbour.
But this hellish world shrouded by clouds of sulphuric acid holds answers to questions about what makes a planet habitable, and the fate of our blue planet.
"All of us love a good mystery," said Stephen Kane, an Australian planetary scientist at the University of California, Riverside.
"And that's primarily because it has this thick atmosphere that prevents us from being able to easily see the surface."
But we're about to find out more.
Earlier this week, NASA gave the green light to two new missions — DAVINCI+ and VERITAS — to explore our closest neighbour (bumping out missions to Jupiter's moon Io and the ice giant Neptune and its moon Triton.)
While VERITAS will send an orbiter to map the planet in more detail than ever before, DAVINCI+ will drop a probe through the planet's thick clouds to explore its atmosphere before landing on the surface.
Due to arrive sometime near the end of the decade, they will be the first missions to Venus since the 1990s.
Dr Kane will be working on both missions.
"We didn't anticipate that both of them would be selected, but Venus has been neglected for quite some time," he said.
Here are some of the big questions scientists are trying to find out.
Just when did Venus turn into the planet from hell?
One of the primary things that Dr Kane is interested in is "what makes a planet habitable?"
Today, life is not possible on the furnace-like surface of Venus, where temperatures exceed 470 degrees Celsius — that's hot enough to melt lead — created by a runaway greenhouse effect.
But scientists such as Dr Kane are trying to work out whether or not Earth and Venus were once twins.
"I'm obsessed with answering that particular question; not just for Venus, but for planets around other stars," he said.
"What are the conditions that can cause planets to go down either a habitable pathway or uninhabitable pathway?
Did Venus always have a runaway greenhouse effect? Or did something trigger the effect later in its evolution? And if so, most importantly, when did it stop?
"That would be an incredible revolutionary thing for Venus because it would confirm that indeed, Earth and Venus did have a shared history up until a certain point," Dr Kane said.
"Both the atmospheric chemistry and the geology of the surface are two very important pieces in the puzzle of 'why Venus turned out the way that it did'."
Was there ever water on Venus?
The DAVINCI+ probe will collect information about temperature, atmospheric pressure and chemistry.
"One of the fundamental challenges we have at the moment is that we don't understand the chemistry," Dr Kane said.
Venus's atmosphere is primarily made up of carbon dioxide with droplets of sulphuric acid, and is around 90 times thicker than Earth's atmosphere.
DAVINCI+ will measure the atmosphere all the way down to the surface.
One of the things it will look for is the ratio of hydrogen to deuterium, which can tell scientists about the amount of water lost from the planet.
Today there is next to no water vapour in the atmosphere, but it could have contributed to the greenhouse effect in the past.
"There are ideas [water vapour] remained in clouds in the atmosphere for long periods of time, but it never actually condensed down to the surface and formed oceans," Dr Kane said.
Instead it was eventually lost to space, leaving behind the thick blanket of carbon dioxide.
Another idea is that Venus may have been covered in oceans 2 billion years ago, but as the planet warmed the oceans evaporated, creating a thick blanket that got hotter and hotter.
When the oceans and water vapour disappeared, carbon dioxide continued to build up in the atmosphere.
"So what we're really trying to do with these new missions is trying to figure out at what point in Venus's history [the greenhouse effect was triggered]," Dr Kane said.
Could life exist in the clouds?
Studying the atmosphere could also help scientists work out if anything could still – if it ever did – exist in the clouds.
Last year, the idea that life could exist in the clouds swirling above Venus was reignited by research that indicated the presence of phosphine in the atmosphere.
On Earth, this compound of phosphorus and hydrogen is generally only produced by biological processes indicating microbial action.
Since then, the biological origin of phosphine on Venus has been widely debated, Dr Kane said.
And, although the team has repeated their observations — albeit with weaker results — other teams believe the chemical detected in the clouds was simply sulphur, a component of sulphuric acid which makes up the atmosphere.
As a result, Dr Kane says whether or not the DAVINCI+ probe will explore the atmosphere for phosphine or phosphorus is still up in the air.
What's under the clouds?
What we know about the surface of Venus is patchy.
The best data we have was captured by the Magellan mission, which mapped the planet's surface in the 1990s.
"But that data is very poor resolution," Dr Kane said.
"VERITAS will be able to fix a lot of these problems."
Getting better data about the surface of Venus will help scientists understand whether or not the planet ever underwent any tectonic activity.
"We don't see a lot of evidence for plate tectonics on Venus, but that may just be because we don't have good enough data," Dr Kane said.
Geological processes such as plate tectonics on Earth are critical for the storage of carbon.
"So it could be that Venus lost the ability to have plate tectonics or a process of subduction, so it couldn't recycle its carbon," Dr Kane said.
"Therefore, it all ended up in the atmosphere and that might have been the linchpin which caused it to enter its present state."
What's under the ground?
We also know very little about what the interior of Venus looks like.
Knowing that is important, according to Dr Kane, because we want to know whether Earth and Venus formed the same way.
Venus rotates very slowly — in fact only once every 243 Earth days — so we don't know if it has a liquid core like Earth.
It doesn't have a moon, so it's very hard to work out its gravitational pull.
"Some of the spacecraft that have gone to Venus in the past have tried their hardest to obtain gravity measurements as they orbit the planet, but those are very poor," Dr Kane said.
And it has almost no magnetic field.
On Earth, the magnetic field protects us from being bombarded by solar radiation and helps maintain our protective atmosphere.
"The lack of magnetic field is almost certainly related to perhaps the lack of a liquid core," Dr Kane said.
"So that would be really great to get more detailed measurements for those, so that we can model that better."
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