Artist impression of ESA’s EnVision mission at Venus
Venus is a tough planetary nut to crack. Decades after ground-based radar surveys first pierced our sister planet’s dense, carbon dioxide-dominated atmosphere, we still know don’t know whether Venus ever had lakes or oceans. Whether it was habitable and for how long. And whether it could possibly have harbored life. Nor do we know the timing nor the exact trigger for the onset of the planet’s runaway greenhouse of a climate. But we do know that temperatures at its surface reach some 450 degrees Celsius and that Venus has surface pressures some 92 times that of Earth.
In contrast, we now know a lot more about Mars. The consensus among the planetary science community is that Mars at least had large lakes and rivers if not an ocean of water. Mars was likely habitable and probably even developed some sort of microbial life, even if we may never detect it.
Venus will remain more of a puzzle for decades to come. But the European Space Agency’s estimated $500 million Euro EnVision orbital mission to Venus, due for launch early in the next decade, should totally change the current paradigm of what is currently known about Venus’ surface history.
A few of the questions, EnVision will address, notes ESA, include:
How have the surface and interior of Venus evolved?
How geologically and tectonically active is Venus today?
Could evidence of past water remain in the oldest rocks found on Venus’ surface?
And when and why did the runaway greenhouse begin at the planet?
Expected to launch on an Ariane 62 rocket as early as 2032, EnVision’s four years in science orbit will mark the first mission to Venus to have a subsurface radar sounding instrument (SRS). During EnVision four-year science mission, SRS will directly measure the planet’s subsurface features to depths of a kilometer.
A NASA-provided radar will also image and globally map the surface, says ESA, while a radio science experiment will probe the planet’s internal structure and gravity field as well as probe the atmosphere.
Located beyond the inner edge of our solar system’s habitable zone, researchers still debate whether Venus may have had abundant liquid water or even life, says ESA; that is, before developing the runaway greenhouse warming which rendered it uninhabitable.
The EnVision mission to Venus will explore why Earth’s closest neighbour is so different.
There’s also still much debate as to whether Venus ever had any sort of atmospheric processing that enabled the planet to sequester its carbon dioxide in carbonate rocks, as does Earth. But understanding Venus is crucial to understanding terrestrial planets everywhere, from our own Earth to those in orbit around distant sunlike stars.
It also could help us better understand climate change here on Earth.
It’s important to understand when and how Venus reached its climatic tipping point into a runaway greenhouse, as Anne Grete Straume, EnVision’s project scientist, told me at last month’s Europlanet Science Congress in Granada, Spain.
No one is suggesting that Earth would ever turn into as inhospitable planet as Venus, but a better understanding of the mechanisms that led to Venus’ runaway greenhouse might help us understand to how to best mitigate climate change on Earth.
But that’s no easy task.
“Looking at Venus in the past and figuring out how it got to where it is now is difficult,” Colin Wilson, a planetary scientist at ESA ESTEC in The Netherlands, told me at last month’s Europlanet Science Congress in Granada, Spain. “Everything is extrapolated from what we see currently.”
On geological timescales, most of Venus’ surface is less than a billion years old which is thought to be due to some sort of catastrophic volcanic resurfacing mechanism.
Even though its closer orbit to the Sun gives it twice as much sunlight, Venus today absorbs less sunlight than Earth, says Wilson. That’s because it’s completely enveloped in very reflective clouds, which reflect away 70 percent of the light which falls on it, he says. But as you descend through the clouds, then it just gets hotter and hotter, says Wilson. That’s because of the thick atmosphere and the greenhouse effects that it creates, he says.
EnVision will use three different spectrometers to analyze minerals on the surface and trace gases from both the surface and from Venus’ atmosphere. That, in turn, could indicate whether Venus ever had large amounts of liquid water such as an ocean.
Rewind four and a half billion years to when Venus and Earth were formed and both would have had a magma ocean, and a steam atmosphere, maybe 100 Earth atmospheres of steam, Wilson says.
But the data we have today does not help us categorically decide whether Venus had a warm, wet ocean phase, or it never had a liquid water ocean phase, says Wilson. But if you’re running planet simulations, it’s a lot easier to end up with an inhospitable planet, than with a habitable planet, he says.
Understanding exactly how far from a star you can expect habitable conditions is a complicated calculation, says Wilson. It depends on the cloud cover; on the surface composition; and, on the recycling of atmospheric material into the planet, he says.
What puzzles Straume and Wilson most about Venus?
Why you have this this type of very high-pressure atmosphere, Straume wonders, and is the atmosphere really fed by volcanic activity?
From NASA’s Magellan radar mission, we were able to detect volcanoes, lava flows, rifts and all kinds of fascinating geological features, says Wilson.
“But we effectively have got a static snapshot of Venus,” said Wilson. “I’d be fascinated to know if that geological activity is going now.”
Going from a static picture of Venus to one of a dynamic planet is going to be the most exciting thing for the next missions there, says Wilson.
