When it comes to planets in our Solar System’s habitable zone, one world is too close to the Sun, another is too far and Earth is just right. But maybe things would be different if Venus and Mars simply swapped orbits. What makes these planets impossible to live on right now? What changes would they undergo when they trade places? And why would you need Jupiter to give these planets a chance at supporting life?
Our Solar System’s habitable zone, otherwise known as the Goldilocks zone, is the distance from a star where liquid water could exist on the surface of a planet. As long as conditions aren’t too hot or too cold, life could be possible. Just like on Earth. But simply occupying prime real estate in the habitable zone isn’t a guarantee that a rocky planet is actually habitable. Both Venus and Mars orbit within this region too. And yet, no life.
Despite being of a similar size and with roughly the same surface gravity as Earth, Venus is too hot to hold liquid water. And dense. Its atmosphere is 90 times denser than our own and the surface temperature is a scorching 462 °C (863.6 °F). On the other end of the Goldilocks zone, Mars is also quite Earth-like. Except way colder, drier and dustier.
Its average surface temperature is – 46 °C (- 51 °F) and it has an atmosphere about 1% as thick as Earth’s. So if we swapped things around, would Venus and Mars transform themselves into perfectly livable planets? Bringing the red planet into Venus’ orbit would put it at a distance of approximately 108 million km (67 million mi) from the Sun.
This would bring it halfway closer to the Sun than it was before. Being so much closer, Mars would get a significant boost in the amount of sunlight it receives. And that alone could make the surface temperature climb to a balmy 32°C (90°F). Just like Earth’s tropics. But without a thick atmosphere or oceans, all this new heat wouldn’t stay on the planet.
Just like in Earth’s deserts, Mars would see extreme temperature differences between day and night. The thin Martian atmosphere would make it difficult for water to stay liquid. With such a low surface pressure, water would boil at a lower temperature. Evaporation would occur much quicker. Looks like if you want to see life flourish on Mars, you’d have to take a slightly more hands-on approach. But first, let’s check out how Venus is doing in its new neck of the woods.
You’d now find Venus located 228 million km (142 million mi) away from the Sun. That’s twice as far as it used to be. And naturally, you’d think surely this scorching planet would be starting to cool down. As it turns out, the planet’s high surface temperatures have a lot more to do with its thick atmosphere than its overall closeness to the Sun.
Venus has a very high albedo, which means that it reflects about 75% of the incoming sunlight. So it could be that the planet’s temperature remains more or less just as hot as before. Even as the upper parts of the atmosphere slowly cool enough for the albedo effect to decrease, that would mean that Venus could now absorb a larger amount of incoming sunlight.
So that temperature wouldn’t start to drop all that much. Another setback on the road to becoming a life-supporting planet would be that Venus’ atmosphere is mostly composed of carbon dioxide with small amounts of nitrogen and clouds of sulphuric acid. What you still wouldn’t find nearly enough of is water. The concentration of water in Venus’ atmosphere is 100 times too low for even the most resilient organisms back on Earth to survive.
But since you went through all the effort of bringing both planets from one end of the habitable zone to the other, why not take things a little further? You could try terraforming the planets to make them a little more livable. One way you could increase the surface pressure of Mars would be to try releasing all the carbon dioxide found in its soil and minerals.
The only problem here is that the red planet would need to reach temperatures above 300 °C (572 °F). Despite this new location so close to the Sun, Mars wouldn’t ever get this hot. Besides, even if you could release all of that precious greenhouse gas, it would only amount to an atmospheric pressure of about 10 to 14% of Earth’s. There simply isn’t enough carbon dioxide on the red planet.
Even if you’d been successful with that effort, you’d still be unable to breathe on Mars. An option for dealing with this would involve bringing special microorganisms to the planet that could convert the hostile Martian atmosphere into breathable air. This would be similar to what Earth was like 2.5 billion years ago. Back then, cyanobacteria converted our methane and ammonia-rich atmosphere into the oxygen-rich one you enjoy today.
Ultimately, this would be a big waste of time and resources. You’d find that with Mars’ weak magnetic field, it wouldn’t be able to hold onto any atmosphere you could create. That would be thanks to the power of solar winds that would constantly strip away whatever atmosphere you’d manage to generate. And in Venus’ old orbit, solar winds would be almost five times as dense as Mars is used to.
Way out on Venus, the question of creating conditions for life could come down to whether you could manage to form an ocean on the planet. And that would entail bombarding the atmosphere with hydrogen. As hydrogen is introduced to the atmosphere, it would react with the carbon dioxide, leading to the formation of water. Introduce enough hydrogen and you could eventually see Venus with 80% of its surface covered by water.
For this to work, you’d need a lot of hydrogen. So much that you’d probably need to find a way to harvest it from a gas giant like Jupiter or Saturn. Or maybe one of their moons. I’m not sure all this planet moving and hydrogen bombarding was really worth it. You’d still be left with only one Goldilocks planet capable of supporting life.
- “Mars Compared To Earth”. Matt Williams. 2015. phys.org.
- “Venus Compared To Earth”. Matt Williams. 2016. universetoday.com.
- “Venus’ Atmosphere: Composition, Climate And Weather”. Nola Taylor Tillman. 2018. space.com.
- “Planet Distance Chart”. 2022. jpl.nasa.gov.
- “The Habitable Zone | The Search For Life – Exoplanet Exploration: Planets Beyond Our Solar System”. 2022. exoplanets.nasa.gov.