About 4 billion years ago, Venus (far left), Earth (middle), and Mars (far right), all had water. The relative planet sizes above are roughly to scale, though how they looked that long ago is speculation. But just knowing a time existed when we had three planets in our own solar system with liquid water is pretty cool. So I wanted to dive into that topic, and see what it might mean for life beyond our own planet.
Let’s start with the timeline chart below. In the top row in brown is the age since present day in billions of years. So on the far left you’ll see the solar system formed -4.5 billion years ago. Then each column is half a billion years, until on the far right you reach +2 billion years in the future. Next we have rows for Venus, Earth, and Mars respectively, where blue color shows when each planet had liquid water. Around 4 billion years ago you can see the triple play for wet planets. Additionally for Earth I labeled a few key events in the evolution of life. Finally in the bottom row is Sun luminosity as a percent of present day. If you weren’t aware, 4.5 billion years ago the Sun only had 75% of today’s luminosity. Then following normal star evolution the Sun has become bigger and brighter over time. On the far right 2 billion years from now, it’ll hit 120%, or 20% brighter than today.
Venus had water for at least 600 million years early in its history, though more recent work has suggested that water might have lasted another 1 or 2 billion years. Hence the light blue section in the Venus row, where we don’t know for sure. Then as the Sun got brighter, the runaway greenhouse effect kicked in, and Venus’ water boiled off. Now the surface of Venus is hot enough to melt lead at nearly 900°F.
The first surprise for Earth is it had liquid water 4 billion years ago despite the Sun being so much less luminous. The exact reasons are still debated as the “Faint Young Sun paradox”, but one likely factor was more greenhouse gasses than we have today. A second surprise is that life shows up immediately with water. The next major event occurs a billion years later, when cyanobacteria evolve and suddenly start kicking out oxygen (a poison) as a waste product. This transforms Earth to an oxygenated atmosphere, and life had to radically evolve just to survive the change. Finally, 3.5 billion years after life first gets started (!), we see multicellular life appear in the Cambrian. That’s roughly half a billion years ago. And human intelligence evolved rather quickly after that, at least on this kind of timescale. Projecting another billion years forward, the Sun will get bright enough to burn off all our water. At that point Earth will be as toasty as Venus is now.
For Mars, the Noachian period from 4.1 to 3.7 billion years ago was the wet era. This means the Mars curiosity rover looking around right now at riverbeds (very cool pics) is looking at geology laid down 3.7 billion years ago. Unlike Venus, or eventually Earth, Mars didn’t lose its water to the greenhouse effect. Mars is just too small. Not enough gravity to retain its own atmosphere. And too small for tectonic activity to release additional gas from the crust. So while early on it had lots of greenhouse gas making liquid water possible far from the Sun, that gas was lost to space after a few billion years. Then Mars became what it still is today: dry and lifeless (at least on the surface).
What does this mean for life in our solar system? If you go back to the Earth timeline, singled celled life evolved as soon as we had liquid water. And in recent decades scientists have found every niche on Earth that has water and an energy source has single celled life. No matter how extreme. This implies early in our solar system’s history we might have had microbial life on Venus, Earth and Mars all at the same time. And it also means the liquid water oceans beneath Jupiter’s moon Europa and Saturn’s moon Enceladus would be great places to send a robotic probe, since unlike Venus or Mars they are still wet.
But looking at the Earth timeline we also see that multicellular life evolved very late in the game. This means all complex life we care about – trilobites, dinosaurs, arthropods, trees, flowers, fish, and even humans – are squeezed into a tight half billion year wide box. In fact if multicellular life had evolved another billion years later it might have missed the wet Earth window entirely.
Looking more broadly, what does this mean for life in our galaxy? If multicellular life is the most unlikely step, taking 3.5 billion years instead of only .5 for intelligence, then we might be able to explain the Fermi paradox. The Fermi paradox is the contradiction between so many potentially wet and life supporting planets in the galaxy, but no aliens. A quote from biologist J.B.S. Haldane might help here. A clergyman asked Haldane what could be concluded about the Creator from the study of creation. Haldane replied “God has an inordinate fondness for beetles”, alluding to the fact that there are 30 times more beetle species than mammal. An updated reply might be “God has an inordinate fondness for microbes.” We may inhabit a galaxy swarming with such life. Untold millions of planets spinning through the dark, eerily full of single celled silence.