The conditions on Venus are hard to describe. Many planetary scientists say "Start by imagining Hell and work up from there." It's an environment where words like "over 500 degrees Celsius" get thrown around, and it's flat-out crushed every probe we've sent into it. As the sun increases in brightness over time, Earth will move into a runaway greenhouse stage — but not for a billion and a half years or so. Still, it inspired the a team of astronomers to conclude, “As the solar constant increases with time, Earth’s future is analogous to Venus’s past.”
In the runaway greenhouse stage, a planet absorbs more solar energy than it can give off to retain equilibrium. As a result, the world overheats, boiling its oceans and filling its atmosphere with steam, which leaves the planet glowing-hot and forever uninhabitable, as Venus is now.
One estimate of the inner edge of a star’s “habitable zone” is where the runaway greenhouse process begins. The habitable zone is that ring of space around a star that’s just right for water to remain in liquid form on an orbiting rocky planet’s surface, thus giving life a chance.
Revisiting this classic planetary science scenario with new computer modeling, the astronomers found a lower thermal radiation threshold for the runaway greenhouse process, meaning that stage may be easier to initiate than had been previously thought.
“The habitable zone becomes much narrower, in the sense that you can no longer get as close to the star as we thought before going into a runaway greenhouse,” said Tyler Robinson, a UW astronomy postdoctoral researcher and second author on the paper. The lead author is Colin Goldblatt of the University of Victoria.
Though further research is called for, the findings could lead to a recalibration of where the habitable zone begins and ends, with some planets having their candidacy as possible habitable worlds revoked.
“These worlds on the very edge got ‘pushed in,’ from our perspective — they are now beyond the runaway greenhouse threshold,” Robinson said.
Subsequent research, the astronomers say, is needed in part because their computer modeling was done in a “single-column, clear-sky model,” or a one-dimensional measure averaged around a planetary sphere that does not account for the atmospheric effect of clouds.
Venus was created at about the same time as Earth, in about the same place, and it's roughly the same size - it would therefore have started with the same materials as us, drawn together from the same region of the planet forming dust left over from the sun. But Venus now has only 0.001% of our water content, and a couple of flybys by the dynamically named Venus Express may have revealed the reason.
In 2008, the European Space Agency’s Venus Express discovered hydrogen and oxygen streaming off the night side of the planet in a 2:1 ratio, which you might recognize as the ratio in H20. If not, we're sure you can now deduce it. It seems that what little water Venus has left is being blasted apart in the atmosphere by the solar wind, a vast stream of charged particles blown out by the sun. Now, the Express has passed by the dayside and measured almost three hundred kilograms of hydrogen a day being lost into space. It hasn't found any oxygen yet, but the search continues.
Data gathered from Venus Express is invaluable to climate scientists modeling Earth’s climate to predict its future. The climate of our two neighbors is in stark contrast to Earth with Venus is a cloudy inferno and Mars is a frigid desert.
“It seems that Venus started out much more like Earth," says David Grinspoon, Denver Museum of Nature and Science, and one of Venus Express’s interdisciplinary scientists. "To members of the public it must seem like climate models are crystal balls, but they are actually just complex equations."
The more scientists look at those equations, the more they realize just how complicated Earth’s climate system is. Grinspoon puts the predicament like this: “In fifty or a hundred years, we will know whether today’s climate models were right but if they are wrong, by then it will be too late.”
To help increase confidence in the computer models, Grinspoon believes that scientists should look at our neighbouring planets. “It seems that both and Venus started out much more like Earth and then changed. They both hold priceless climate information for Earth,” says Grinspoon.
The atmosphere of Venus is much thicker than Earth’s. Nevertheless, current climate models can reproduce its present temperature structure well. Now planetary scientists want to turn the clock back to understand why and how Venus changed from its former Earth-like conditions into the inferno of today.
Climate scientists believe that the planet experienced a runaway greenhouse effect as the Sun gradually heated up. Astronomers believe that the young Sun was dimmer than the present-day Sun by 30 percent. Over the last 4 thousand million years, it has gradually brightened. During this increase, Venus’s surface water evaporated and entered the atmosphere.
“Water vapor is a powerful greenhouse gas and it caused the planet to heat-up even more. This is turn caused more water to evaporate and led to a powerful positive feedback response known as the runaway greenhouse effect,” says Grinspoon.
As Earth warms in response to manmade pollution, it risks the same fate. Reconstructing the climate of the past on Venus can give scientists a better understanding of how close our planet is to such a catastrophe. However, determining when Venus passed the point of no return is not easy. That’s where ESA’s Venus Express comes in.
The spacecraft is in orbit around Venus collecting data that will help unlock the planet’s past. Venus is losing gas from its atmosphere, so Venus Express is measuring the rate of this loss and the composition of the gas being lost. It also watches the movement of clouds in the planet’s atmosphere. This reveals the way Venus responds to the absorption of sunlight, because the energy from the Sun provides the power that allows the atmosphere to move.
Understanding Mars’ past is equally important. ESA’s Express is currently investigating the fate of the Red Planet. Smaller than the Earth, is thought to have lost its atmosphere to space. When Martian volcanoes became extinct, so did the planet’s means of replenishing its atmosphere turning it into an almost-airless desert.
“What happened on these two worlds is very different but either would be equally disastrous for Earth. We are banking on our ability to accurately predict Earth’s future climate,” says Grinspoon.
Earth is defended from the atmosphere-flensing radiation by its magnetic field, and the data set indicating how vital that is may be short, but it's quite compelling. Earth, magnetic field, alive. Venus, none, dead. Mars, none, dead. We may want to hold on to this thing. Not that we have the tech to do anything about it should it stop or decide to flip or something. Luckily, it would never do such a thing.
Wait, it does do that? Oh. Well, good luck,to us.
The Daily Galaxy via University of Washington, Venus Express and ESA
Image credit: NASA