Hi! I’m Giada Arney, a new blogger at Pale Blue Blog from the University of Washington. Even though I’m an astrobiologist, I’ve spent a lot of time thinking about the least habitable planet in our solar system: Venus! Venus has a similar size and bulk composition to Earth, but there the similarities end. Picture this: Venus has a surface hot enough to melt lead, clouds made of corrosive sulfuric acid, and a surface pressure equivalent to being almost 1 km underwater! Why would an astrobiologist be interested in a planet that has would happily melt, corrode with acid, and crush anything living? Let me tell you a story…
Once upon a time there was a balmy world with sparkling oceans, sunny skies, and swimsuit-worthy temperatures. You might imagine yourself enjoying a beach vacation here, but this isn’t Hawaii or Cancun. This is what Venus may have been like in the distant, distant past.
Scientists are working hard to unravel clues and piece together our sister planet’s lost history. How to we know Venus might once have been a hot vacation destination? To get the answer, let’s do what all good astrobiologists do: let’s follow the water!
When you imagine earthlings piecing together our planet’s early history, you probably picture geologists scurrying from field site to field site, sifting through a treasure trove of ancient rocks. Unfortunately for any would-be Venusian geologists, it’s hard to learn much about the very distant past of our hot neighboring planet by studying its terrain because Venus’ surface is relatively young. We know this because it has no craters older than about half-a-billion years (which sounds old, but in terms of planetary science, that’s pretty young!). This may be because Venus’ thick, rigid crust does not allow the gradual release of internal heat that Earth has via its plate boundaries and volcanoes. The heat therefore builds up over time until the planet can’t take the stress anymore and then BAM — you get what is known in planetary parlance as a planet-wide “catastrophic resurfacing event” when the heat is released, well, catastrophically.
This catastrophic resurfacing event would have obliterated all surface evidence of ancient Venus: that’s why Venus has no craters older than about 500 million years. This is bad news if you’re a geologist! Venus has no ancient riverbeds for geologists to scratch their heads over, and there are certainly no fossilized critters waiting to be dug up (IF Venus ever had them). Fortunately, there’s more to a planet than just its surface, and Venus reigns as queen of the terrestrial atmosphere game. Its 90% carbon dioxide atmosphere is about 100 times thicker than Earth’s and boasts an astonishingly complex array of chemical cycling processes. Could we learn about ancient Venus by studying its atmosphere?
Let’s use our imaginations. Picture yourself in a tropical cabana on an ancient Venus beach. Unfortunately for you, a freak storm rolls in, and the weather takes a turn for the worst. Strong winds start whipping the sand around. Now, (and this is important) imagine the beach is just a little bit rocky with larger pebbles mixed in with the sand grains: the storm winds are going to have an easier time blowing around the little grains of sand than the big pebbles, yeah? Stay with me, this is going somewhere…
If the wind blew for a long time, you might imagine that the beach would become noticeably depleted in small sand grains relative to bigger pebbles because small grains are blown away more easily than big grains. In other words, the stormy beach would have a higher ratio of big pebbles relative to small sand grains compared to similar beaches with nicer weather. If an observant Venusian was strolling along this beach and noticed its high ratio of big pebbles compared to small grains and knew of this beach’s frequent wind storms, she might conclude that the beach had lost a lot of sand over time.
Ok, now hold that thought in the back of your mind as I tell you the next part of the story. (Again, I promise this is going somewhere!)
A little known fact outside of astronomy and astrobiology is that stars get brighter as they age. This means our sun in the distant past was dimmer than it is now, and the planets of our solar system all received less sunlight billions of years ago. It’s possible that this reduction of sunlight in ancient times allowed Venus to have pleasant surface temperatures early in its history. And, being different distances to our star, as the sun evolved and got brighter, Venus and Earth would have been affected differently. Early Earth may have started out a little bit chilly and then warmed up as the sun’s thermostat rose, but early Venus might have started out warm and ended up way too hot. (Don’t confuse this process with the climate change occurring on our planet now — the timescale and cause are completely different!)
Now, it’s time to follow the water! Hotter temperatures will to cause more evaporation of a planet’s water supply. Think back to those oceans ancient Venus may have had. As the sun heated up, the oceans started to evaporate. Water vapor is a potent greenhouse gas, meaning it traps heat efficiently, so lots of water vapor in the atmosphere would have heated up the surface more. As the surface heated up, more water would have evaporated, causing more heating, causing more evaporation, and so on until the oceans were completely dry and nobody would ever want to vacation on Venus again.
The uppermost layers of a planet’s atmosphere are susceptible to the whims of outer space, and in Venus’ now very hot atmosphere, the upper layers would have been rich in evaporated ocean water. High energy ultraviolet sunlight striking a water vapor molecule can break it apart in a process known as ‘photodissociation’, separating H2O’s hydrogen from its friend oxygen. Hydrogen, the most lightweight element, is hard for small planets (like Venus and Earth, compared to big guys like Jupiter and Saturn) to hold on to it. In the upper layers of a small planet’s atmosphere, free hydrogen atoms flying around unbound to anything to weigh them down can easily escape to space just like little grains of sand can easily escape a windy beach. And so, through the processes of photodissociation and hydrogen escape, Venus lost its water to space.
But not all hydrogen is created equal. There is a rare type of heavy hydrogen called deuterium that has a proton and a neutron in its nucleus (regular hydrogen just has a proton). The extra neutron in deuterium gives it some heft: it’s about twice as heavy as regular hydrogen, so just like the big pebbles on a windy beach, it’s harder for deuterium to get swept away.
When we measure the amount of deuterium relative to hydrogen in Venus’ atmosphere, we find that Venus has a lot more of the heavy stuff compared to the lightweight stuff: the ratio of deuterium to hydrogen in Venus’s atmosphere is about 150 times greater than the ratio in Earth’s atmosphere. This suggests that Venus lost a lot of hydrogen in the past, and if the source of that hydrogen was water photodissociation, this also means Venus lost a lot of water in the past. Estimates of the quantity of water lost by Venus range from anywhere between a few meters of surface water to an entire ocean’s worth.
Of course, in all areas of science, there are subtleties and complications. We don’t know if the deuterium-to-hydrogen (D/H) ratio Venus formed with to begin with was different from Earth’s (analogous to the rocky beach starting out rocky rather than losing its sand over time). We also don’t know if some other source besides water photodissociation enriched Venus in deuterium: comets, for instance, can have higher D/H ratios than Earth, and if a big one hit Venus in the past, that may have caused the enrichment we see. All we have are tantalizing clues hiding in the atmosphere telling us that Venus could have once been a gorgeous vacation spot.
There are billions of other planetary systems out there! Those other worlds are all orbiting stars that are slowly getting brighter with time, just like our sun. This slow process of heating gradually desiccates Earth-like worlds, leaving billions of Venus-like husks in their places across the galaxy. This may be a depressing idea, but it suggests that Venuses are common types of planets. Studying the world next door and learning how she got to her present hellish state will help us in the future when we run up against her many cousins in our search for another Earth.