Venus has had a somewhat complicated — and unfortunate — history. Planetary scientists suspect that billions of years ago, Venus was rather more like Earth: warm, pleasant, and flush with liquid water. But sometime in the past it lost its water and turned into a scorching-hot wasteland.
According to a recent paper, Venus was doomed from the start, but may have been accelerated in its path to dry-town through the gravitational influence of none other than the great bully of the solar system: Jupiter.
A wandering giant
The early solar system was a total Wild West: lawless and chaotic. For starters, astronomers now strongly suspect that the giant worlds didn’t form in their present orbits. We can identify this through the shaping and sculpting of orbits in the asteroid belt and in the distribution of the icy remnants past the orbit of Neptune. By piecing together the gravitational clues from the holdovers of the formation of the solar system, it’s clear that the giant planets first formed much farther out, then migrated inward closer to the sun.
However, we don’t have a clear picture of exactly how that migration happened. In some models, Jupiter slowly inches closer over the course of hundreds of millions of years, followed by Saturn and the rest. But in other models, Jupiter jumps almost to the orbit of Mars before slinking back out to its present position.
Either way, a dancing Jupiter caused havoc for the inner planets. The mass of that planet is so great — it’s 2.5 times more massive than all the other planets combined — that any little shift in its orbit pulls and plucks on anything else in the solar system.
Take, for instance, Venus. Currently, Venus has one of the most perfectly circular orbits in the entire solar system. Its eccentricity (the measure of how elliptical an orbit can get) is only 0.007, meaning that at closest approach Venus is 66.5 million miles (107 million kilometers) from the sun, and at its farthest it’s … 67.7 million miles (109 million km) from the sun.
But according to a recent paper appearing in the preprint journal arXiv, if Jupiter happened to migrate inward closer to the sun, it could have tugged Venus into an extremely elliptical orbit, creating an eccentricity of up to 0.3.
Since Venus no longer has that great of an eccentricity, something must have happened to circularize its orbit, and the authors of the paper suggest that it was ocean tides. If Venus had great liquid water oceans (which we suspect it did, since Venus and Earth are about the same size and had similar formation histories), then the tides on the oceans could have provided enough friction to stabilize the orbit of that planet into a nice, steady circle.
But that elongation of the orbit due to Jupiter may have had another catastrophic consequence: It could have hastened the transformation of Venus from tropical wetland to hellish nightmare.
Our solar system: A photo tour of the planets
A watery Venus
If you’re a planet trying to hold on to your liquid water oceans, an eccentric orbit is a real pain in the neck. We already know from studies of Earth’s own history that variations in our planet’s eccentricity (due to, you guessed it, gravitational tweaks and tugs from the other planets) triggered ice ages and glaciation events. Indeed, some deep-time climate variations are directly connected to changes in our eccentricity.
But still, despite the occasional frozen moment, Earth has been able to hang on to its water. Poor Venus suffered a worse fate, however. If Venus got sent into a highly elliptical orbit due to the presence of Jupiter in the early days of the solar system, it spent some of its year far away from the sun (nice and cool) and some of its year way too close for comfort.
All told, the researchers calculate, Venus may have suffered from more heat exposure than is healthy. The problem is that radiation intensity increases rapidly for even small inward shifts in orbital position. The more time that Venus spent closer to the sun, the worse it suffered.
Even worse, the closer Venus was to the sun, the more it was susceptible to ultraviolet radiation blasts from solar flares, which were especially prominent when our sun was a young upstart.
The combination of increased heat and increased exposure to high-energy radiation set in motion Venus’ downward, hellish spiral.
As Venus lost its oceans, the water vapor in the atmosphere trapped heat. The trapped heat caused more water to evaporate, which put more water in the atmosphere, which trapped more heat, and round and round it went in a vicious greenhouse cycle. With no liquids to lubricate Venus’ joints, plate tectonics stopped, allowing carbon dioxide to leak into the atmosphere to dramatic excess, locking in its fate.
Eventually, Venus turned itself inside out and cooked itself to death, leaving our neighbor a nightmare world. And it may have been accelerated on that path by a wandering, plundering Jupiter.
Venus is more than a cautionary tale for our own greenhouse gas emissions. You may have noticed that astronomers are very interested in exoplanets — worlds outside the solar system — and whether they might be homes for life. Venus sits just on the inside edge of what’s called the habitable zone of our sun, the range where the intensity of light is just right to allow for liquid water on a world’s surface.
But Venus’ surface is very much not habitable at all (though its clouds may be another matter), and this may be the fault of Jupiter. When we go to examine the possibility of life on other worlds, the authors of the paper conclude, we have to pay attention to any giant planets in those systems. They may have performed similar acrobatics as Jupiter did, ruining any chances of life persisting on the inner worlds.