Written by Alex Torres
Something that has captured my curiosity, and indeed the curiosity of many astronomers, is why the Earth is the only terrestrial planet we know to currently harbor life. Looking at the other potential candidates in the habitable zone around our Sun, Venus and Mars, the answer seems rather obvious. Venus is often described as a hellscape with surface temperatures hot enough to melt lead, a surface pressure equivalent to being one mile underwater on Earth, as well as a host of other characteristics that make it about as inhospitable to life as we can think of. On the other hand, Mars has an average surface temperature of -85°F (-65°C) with a surface atmospheric pressure equivalent to less than one hundredth that of the Earth’s at sea level. Weirdly enough, despite these two planets being almost complete opposites in just about every way, they are the same with respect to one crucial characteristic: neither are capable of supporting liquid water at their surfaces. Both Mars and Venus are bone dry, the water having evaporated due to lack of pressure on the former and extraordinarily high temperatures on the latter. Mars, however, is capable of supporting long term frozen water on its surface, giving it a slight advantage over Venus on its water content, but regardless, neither has surface conditions capable of supporting life as we know it on Earth whose chemistry relies on liquid water. Though, interestingly enough, it is possible and even likely that both planets had liquid water on their surfaces at some point in the past. While this may be surprising given their current state of affairs, it is important to consider that both planets were not always as they currently are, and they both owe their transformations over time to the Sun.
Let’s begin with Venus shortly after its formation. Soon after Venus’s surface solidified, it would have had surface temperatures very similar to modern-day Earth. Without its incredibly thick carbon dioxide atmosphere, Venus with its closer proximity to the Sun coupled with the Sun’s lower luminosity in the past would have put the planet smack dab in the middle of the habitable zone, allowing liquid water to sustainably exist on the surface. Estimates for how long this liquid water remained on Venus range from about one billion years after its formation to just a billion years ago that Venus lost its water. Regardless of how long it could have sustained liquid water on its surface, given that Venus formed in a similar manner as the Earth in the same environment, we are relatively confident that Venus had surface oceans at one point. What makes this a truly interesting fact to consider is that we know life on Earth developed pretty much right after our planet’s surface solidified and the oceans formed. This means that before Venus became the hot, greenhouse world it is today with sulfuric acid rain and surface pressures capable of smashing a person like a pancake, it could have supported life. Of course, as the Sun became more luminous over time, any oceans on Venus would have evaporated, allowing for all the carbon locked up in that water and the underlying surface rock to escape into the atmosphere and form the extremely thick CO2 blanket that envelopes Venus today. What’s more, as the planet underwent this runaway greenhouse transformation, its atmospheric temperature increased which, in conjunction with Venus’s lack of intrinsic magnetic field to deflect solar wind, caused the gaseous H2O on Venus to reach escape velocity in the upper atmosphere and leak from the planet until pretty much no water remained. Therefore, even though it was the evaporation of its oceans that kick started its runaway greenhouse transformation, very little water remains in the Venusian atmosphere at all.
On the other hand, even though Mars would have received even less energy from the Sun in the past than it does presently, it too would have had more favorable conditions for liquid water right after formation due to more significant internal heat and a more substantial atmosphere. When Mars was still molten inside, it likely had a magnetic field much like the Earth does currently which would have protected it from high energy solar radiation. Therefore, it could have hosted a much thicker atmosphere than it does now without the solar wind stripping it away, and with the presence of that atmosphere and its associated greenhouse effect, Mars would have had a higher surface temperature than it would otherwise. This higher surface temperature above the freezing point of water in addition to an atmospheric pressure at the surface capable of keeping water from boiling could have allowed Mars to host liquid water on its surface in the past. There is even well-studied evidence for surface water having existed Mars in the form of visible river deltas, valleys potentially carved out by liquid water, and basins where water could have resided in the form of lakes and oceans. However, due to Mars having a significantly smaller volume than Earth, and therefore a larger surface area to volume ratio, Mars radiated away its internal heat much faster than the Earth. As the mantle and core of Mars cooled off, the dynamo powering its magnetic field shut down exposing the Martian atmosphere to high energy solar wind capable of stripping away gas from the planet with its relatively low gravity. Over time, this means that the surface of Mars would have cooled off without its insulating atmosphere, forcing much of the surface water to freeze with any remaining liquid water doomed to boil away due to the low atmospheric pressure.
In the end, this means that our two next door planetary neighbors, Venus and Mars, both likely had everything they needed to host life just like the Earth. However, by the time the Earth would go on to develop multicellular life with the Cambrian Explosion a few hundred million years ago, Venus and Mars would already have been dry, desolate wastelands with little hope of harboring any form of life at all on their surfaces. It is a little disappointing to think of all we could have missed out on with two planets right next door hosting life just as cool, crazy, and complex as that on Earth, but at the same time, perhaps Venus and Mars never had life at all. Maybe Earth really is unique in its development of life, and even in the billions of years that Venus and Mars had oceans, all of their water never hosted anything more interesting than lifeless molecules. The point is that we can speculate all we want about the enigmatic histories of our planetary neighbors, but what is truly interesting is our ability to actually visit these alien worlds for ourselves and figure out if they truly ever did host aliens of their own.
References
Atmosphere of Mars. (n.d.). Wikipedia. Retrieved November 6, 2023, from https://en.wikipedia.org/wiki/Atmosphere_of_Mars
Brooks, F. (n.d.). Mars. Wikipedia. Retrieved November 6, 2023, from https://en.wikipedia.org/wiki/Mars
Choi, C. Q. (2016, October 14). Water on Mars: The Story So Far | News | Astrobiology. NASA Astrobiology. Retrieved November 6, 2023, from https://astrobiology.nasa.gov/news/water-on-mars-the-story-so-far/
Climate of Mars. (n.d.). Wikipedia. Retrieved November 6, 2023, from https://en.wikipedia.org/wiki/Climate_of_Mars
Dynamo theory. (n.d.). Wikipedia. Retrieved November 6, 2023, from https://en.wikipedia.org/wiki/Dynamo_theory
Magnetic field of Mars. (n.d.). Wikipedia. Retrieved November 6, 2023, from https://en.wikipedia.org/wiki/Magnetic_field_of_Mars
McCall, R., Hammer, J., & Robison, M. (2019, September 23). Venus May Have Been Habitable for 3 Billion Years. Newsweek. Retrieved November 6, 2023, from https://www.newsweek.com/venus-habitable-three-billion-years-before-runaway-greenhouse-effect-1460686
New Models Show That Venus Was Likely Habitable Four Billion Years Ago. (2019, March 9). Forbes. Retrieved November 6, 2023, from https://www.forbes.com/sites/brucedorminey/2021/09/24/new-models-show-that-venus-was-likely-habitable-four-billion-years-ago/?sh=523478f41caf
Scudder, J. (2015, February 12). The sun won’t die for 5 billion years, so why do humans have only 1 billion years left on Earth? The Conversation. Retrieved November 6, 2023, from https://theconversation.com/the-sun-wont-die-for-5-billion-years-so-why-do-humans-have-only-1-billion-years-left-on-earth-37379
Solar System Temperatures. (n.d.). NASA Science. Retrieved November 6, 2023, from https://science.nasa.gov/resource/solar-system-temperatures/
Sutter, P. (2019, August 7). How Venus Turned Into Hell, and How the Earth Is Next. Space.com. Retrieved November 6, 2023, from https://www.space.com/venus-runaway-greenhouse-effect-earth-next.html
Tomaswick, A. (2020, August 23). Did Jupiter Push Venus Into a Runaway Greenhouse? Universe Today. Retrieved November 6, 2023, from https://www.universetoday.com/147535/did-jupiter-push-venus-into-a-runaway-greenhouse/