At this very moment, there are a total of eleven robotic missions exploring Mars, a combination of orbiters, landers, and rovers. As the second most habitable planet beyond Earth in the Solar System, these missions are busy answering vital questions about Mars' past — foremost of which is whether or not it once supported life (or if it still does!).
These efforts are expected to lead to significant advancements in just a few years and culminate with crewed missions in the next decade. On top of that, some people are entirely sincere about creating a new home on Mars, one which will enable long-term human habitation and maybe even the birth of a Martian civilization.
This raises all kinds of questions about the conditions on Mars and whether humans could truly live there long-term. While there are strategies to ensure food, water, shelter, and radiation protection in the short term, there are those who argue that long-term habitability would require some serious ecological engineering — aka. Terraforming.
Perhaps with some serious work, Mars could truly live up to its nickname: "Earth's Twin." But with all the focus dedicated to humans living on Mars, we tend to overlook Earth's other neighbor. Also known as Earth's "Sister Planet," Venus has a lot going for it that might make it a better candidate for long-term human habitation.
But what exactly can Earth's "Sister Planet" offer that "Earth's Twin" cannot? As it turns out, there are a few benefits, assuming we're willing to put in the work. With all apologies in advance to Robert Zubrin, James Lovelock, and Elon Musk, let's review what they are:
Life on Mars
Beyond Earth, Mars is the most habitable planet in the Solar System. Hence why our efforts to find evidence of extraterrestrial life are overwhelmingly focused there right now. However, being the next-most habitable planet doesn't mean that Mars is a playground for life as we know it.
For starters, the average temperature on Mars is -81 °F (-62.7 °C), which is significantly lower than what we're used to here on Earth, 57.2 °F (14 °C). The temperature on the surface of Mars is also subject to a greater level of variation, ranging from 68 °F (20 °C) during the summer in the equatorial region (at midday) to −243 °F (−153 °C) during winter at the poles.
The air is a toxic fume composed overwhelmingly of carbon dioxide (96%) with trace amounts of argon, nitrogen, and water vapor, and the atmospheric pressure is less than 1% that of Earth's. So, in addition to being toxic, Mars' air is also way too thin to breathe in.
Then there's the matter of radiation. Here on Earth, people living in developed nations are exposed to an average of 0.17 mSv a day, or 62 mSv in a year. Meanwhile, the Martian surface is exposed to an estimated 0.73 millisieverts (mSv) a day, or 266 mSv a year. That's over four times as much, and it's even worse during periods of heightened solar activity.
Lastly, there's the issue of gravity. Here on Earth, all terrestrial organisms evolved in an environment where objects fall towards the surface at a rate of about 32 ft/s² (9.8 m/s²). On Mars, the gravity is 37.5% of what life on Earth experiences 12.208 ft/s² (3.721 m/s²).
As ongoing research has shown, exposure to microgravity can take a serious toll on the human physique and psyche. While there is very little research concerning the effects of low-g — such as Lunar (16.54%) and Martian gravity — it is a foregone conclusion that long-term exposure will have similar consequences.
Terraforming to the Rescue?
Naturally, there are strategies for dealing with just about all of these issues, with the possible exception of gravity. For example, the extremes in temperature and high radiation levels can be mitigated by creating structures on the surface that can maintain an atmosphere and provide sufficient radiation shielding.
Space agencies and commercial space entities worldwide are currently exploring ways to use Martian regolith, ice, and other materials — a process known as In-Situ Resource Utilization (ISRU) — to create habitats that have outer walls capable of limiting radiation exposure. These would be paired with a pressurized interior structure that would keep the atmosphere in.
Food, water, and energy could also be harvested locally using various ISRU methods. This includes using local soil to grow food, local ice to provide drinking and irrigation water, and solar panels and wind farms to generate electricity. It's also suggested that Mars could be ecologically transformed to accommodate Earth life forms (a process called terraforming).
As we explored in a previous article, there are many ways that Mars can be terraformed. In all cases, three general steps are involved: thickening the atmosphere, warming it up, and melting the polar ice. Luckily, these three things are all complimentary to one another and can be accomplished by doing one thing: triggering a greenhouse effect.
One way to do this would be to import volatile compounds like ammonia and methane, both of which are in abundance in the outer Solar System. Both are powerful greenhouse gases, and ammonia (mostly nitrogen by weight) could be broken down to provide a buffer gas to thicken the atmosphere.
Other powerful greenhouse gases, like fluorine and chlorofluorocarbons, have also been recommended as a means of heating the surface. Another idea is to cover the Martian surface with low albedo (dark) material or plants, which would cause more heat to be absorbed on the surface. Carl Sagan himself recommended this be done in the polar regions to melt the ice caps.
Some more radical ideas include using an orbital mirror to direct sunlight onto the ice caps. This would be especially useful when directed at Mars' southern polar ice cap, which is largely composed of dry ice (frozen carbon dioxide). The release of sublimated CO2 and water vapor would have a profound greenhouse effect.
Other ideas include hurling meteors, asteroids, or iceteroids (especially those lots of volatile elements) at the surface. This would kick up dust that would allow the atmosphere to absorb more solar radiation. Some have even suggested using nuclear devices (looking at you, Musk) to melt the polar ice caps and kick debris up into the air.
To ensure that this replenished atmosphere isn't stripped away over time, NASA scientists have proposed positioning an artificial magnetic shield at the Sun-Mars L1 Lagrange Point. Along with a denser atmosphere, this shield would also drastically reduce the amount of radiation the Martian surface is exposed to.
Once all that is done, the process of converting the atmosphere to something breathable will commence. A popular idea here is to introduce photosynthetic organisms, like cyanobacteria and lichens, to convert CO² into oxygen gas. Artificial atmospheric processors could also be built to help the process along.
Beyond that, the only remaining unknown is gravity, for which there is no long-term solution. On the one hand, humans may have access to medical treatments in the future that address the physical and mental effects of living in Martian gravity. Alternately, rotating habitats could be built in orbit Mars that would simulate the feeling of 1 g.
Ah, but there's another drawback to Mars' lower gravity. Assuming we were to create an atmosphere equal to Earth in terms of air pressure (101.325 kPa), it would only hold on to 38% of this over time (38.44 kPa). This means that the air on Mars would always be too thin for comfort, and people would still need to carry oxygen packs with them.
The Case for Venus
Like Mars, Venus was once a vastly different place. According to data gathered by various missions, it is believed that until recently (in geological terms), Venus was a warm and wet planet where oceans covered 80% of the surface. This is actually close to what scientists thought Venus was like until the Soviet Venera and NASA Mariner probes revealed what a hellish place it is today.
What's more, it is theorized that just 700 million years ago, Venus was a temperate planet covered in oceans. This came to an end, apparently due to a near-global resurfacing event that occurred 500 million years ago, where large amounts of magma bubbled up from the mantle and released massive amounts of CO² into the atmosphere.
This magma would have solidified before reaching the surface and created a barrier preventing the atmospheric CO² from being reabsorbed by the crust. What followed was a runaway Greenhouse Effect that caused severe climate change, leading to the hostile environment that we see there today.
However, if the planet could be restored to its former self — by reversing the Greenhouse Effect (which is possible) — then humanity would have a planet closer to Earth that is roughly equal in size, mass, and gravity. Let's compare:
Venus is the closest planet to Earth, ranging from a minimum distance of about 23.7 million miles (38.2 million km) to a maximum of around 62 million miles (261 million km). Because of the nature of our orbits, Earth and Venus make their closest approach every 584 days (1 year and 7 months), which is known as an "inferior conjunction."
In contrast, the average distance between Earth and Mars is about 140 million miles (225 million km), ranging from 34.6 million miles (55.7 million km) to around 249 million miles (401.3 million km). Our two planets make their closest approach every 26 months (2 years and 2 months), which is known as an "opposition" since the Sun and Mars are on opposite sides of the sky (when viewed from Earth).
So not only does Venus get closer to Earth than Mars, but it also makes its closest approach to us more often. This means that missions to Venus could launch more often and would take less time to get there.
Then there's the matter of Venus' gravity, which is the equivalent of 90% to what we experience here on Earth - 8.87 m/s² (0.904 g). Compare this to Mars, where the gravity is roughly 38% of Earth's (0.3794 g). This means that for potential settlers, the health-related risks associated with lower gravity would be much lower.
Of course, Venus (as it is today) has its share of challenges that make the prospect of living there very difficult! These make terraforming not only a good idea but a potential necessity, assuming people want to live there in large numbers. Otherwise, they will need to be happy living in floating cities among the clouds (an actual possibility!)
For starters, Venus is the hottest planet in the Solar System, with an average surface temperature of 867 °F (464 °C) — which is hot enough to melt metals like lead and zinc. The atmosphere is also a toxic fume, composed overwhelmingly of carbon dioxide with trace amounts of nitrogen, sulfur dioxide, and water vapor.
However, unlike Mars, atmospheric pressure on Venus is a whopping 9100 kPa - that's 90 times the pressure of Earth's atmosphere. To experience that kind of pressure here on Earth, a person would have to venture over 3,000 feet (910 meters) under the sea. So unless you have a vehicle that can withstand extreme heat and pressure, you're not getting anywhere near the surface.
As if that weren't enough, Venus' atmosphere is also permeated by clouds of sulfuric acid rain. These have been observed in Venus' upper atmosphere and may not condense closer to the surface. But spacecraft attempting to land on the surface must first penetrate this acidic shroud.
Venus also has the slowest rotation period of any major planet, taking roughly 243 Earth days to rotate once on its axis. On top of that, Venus rotates in the opposite direction as the Sun (retrograde rotation), which is something astronomers have only ever observed with one other planet (Uranus).
Between its slow retrograde rotation and the fact that Venus takes close to 225 days to orbit the Sun, a single "solar day" on Venus lasts 116.75 days. This means that for an observer on the surface of Venus, it takes close to four months for the Sun to set and rise again (compared to 24 hours here on Earth).
Venus is also isothermal, which means that it experiences virtually no variation in temperature. This is due to its dense atmosphere, but also its slow rotation and its low axial tilt (3° vs. Earth's 23.5°), which essentially means that Venus doesn't experience seasons or anything we might consider a day-night cycle.
If you're thinking that this is starting to sound like something out of Dante's Inferno, then you're on the right track! But with the right kind of work, it could be made into something more akin to a tropical island paradise.
The Long Rain
Luckily, with the right kind of ecological techniques and some serious elbow grease, Venus could be terraformed into an ocean planet with mild temperatures and endless beachfront property. As with Mars, it comes down to three major goals. They include:
- Reducing the atmospheric pressure
- Lowering the temperature
- Converting the atmosphere to something breathable
Much like terraforming Mars, these three goals are complementary, even if they are the complete opposite. Luckily for us, Venus has a lot to work with, and the outcome would be easier for humans to adapt to. The first proposed method was made by none other than Carl Sagan in 1961 in a paper titled "The Planet Venus."
It was in this paper that Sagan argued that seeding the atmosphere of Venus with genetically engineered cyanobacteria could gradually convert the atmospheric carbon dioxide to organic molecules. Unfortunately, the subsequent discovery of sulfuric acid clouds and the effects of solar wind made this proposal impractical.
It would be another thirty years before another proposal for terraforming Venus was made, which was done by British Paul Birch in his 1991 paper "Terraforming Venus Quickly." According to Birch, flooding Venus' atmosphere with hydrogen would trigger a chemical reaction, creating graphite and water. The graphite would be sequestered while the water would fall as rain and cover 80% of the surface in oceans.
This proposal reminded many scientists of what Venus was thought to be like before the Space Age — that its dense canopy was once thought to be made up of rainclouds. As such, "The Long Rain" (a science fiction short story about Venus by Ray Bradbury) has become synonymous with terraforming Venus.
Another idea was to flood Venus' atmosphere with refined magnesium and calcium, put forward by planetary physicist Mark Bullock and astrobiologist David H. Grinspoon. In a paper titled "The stability of climate on Venus," they ventured that this would sequester carbon dioxide in the form of calcium and magnesium carbonates.
Another proposal is to use solar shades, something that was recommended by Birch and famed aerospace engineer and space exploration advocate Robert Zubrin. This concept would involve using a series of small reflective spacecraft in Venus' atmosphere to divert sunlight, thereby reducing global temperatures.
Alternately, a single large shade could be positioned at the Sun-Venus L1 Lagrangian point to limit the amount of sunlight reaching Venus. This shade would also block solar wind, preventing Venus' atmosphere from being stripped and also shielding the planet from solar radiation.
This would trigger global cooling, resulting in the liquefaction or freezing of atmospheric CO², which would then be deposited on the surface as dry ice (which could be shipped off-world or sequestered underground).
In 2003, NASA scientist Geoffrey A. Landis released a study titled "Colonization of Venus," where he indicated how cities could be built above Venus’ upper cloud layer. At this altitude, temperatures would be tolerable for human beings, and the atmospheric density would allow the cities to remain afloat.
These cities could act as solar shields and processing stations while providing initial living space for colonists. Over time, as Venus' atmosphere grew less dense, the cities would migrate to the surface and become part of the landscape.
Another suggestion is to speed up Venus' rotation, which could have the added benefit of generating a planetary magnetic field. There are a number of ways to do this, like striking Venus' surface with large asteroids or using mass drivers or dynamic compression members to impart transfer energy and momentum to the surface.
This would allow for the creation of an Earth-like diurnal cycle and could also help remove some of Venus' dense atmosphere. Similarly, mass drivers or space elevators could scoop clouds from Venus' atmosphere and eject them into space, gradually thinning it out over time.
An Ocean Planet
The end result of this would be a Venus very much like its former self. This would mean a planet covered predominantly by oceans. Due to the nature of Venus' geological features and small variations in elevation, the surface would essentially be a giant archipelago with a few larger continents.
One of these would be Ishtar Terra, a continent in the Northern Hemisphere roughly between Australi and the continental United States in size. It is here that the highest point on Venus, Maxwell Montes (indicated in white), is located. This behemoth mountain stands 36,000 ft (11,000 m) tall — taller than Mt. Everest (29,000 ft; 8,850 m) — and measures almost 500 mi (800 km) in diameter.
Another major landmass is Aphrodite Terra, which is located along the equator and is about four times the size of Ishtar Terra. A third landmass, Lady Terra, is slightly smaller than Aphrodite Terra. The rest of the planet's surface, according to research performed by the NASA Goddard Institute for Space Science (GISS), would be covered in oceans ranging in depth from 30 ft (10 m) to about 1000 ft (300 m).
Think of the Caribbean, Polynesia, and the Greek Isles, but on a planet-wide level! To get a better idea of what this would look like, check out this cool map! And — as mentioned earlier — the planet would have a surface gravity pretty close to that of Earth. This would make adapting to the new world all the more pleasant.
Over time, humans could introduce terrestrial organisms like plants, trees, bacteria, and aquatic species to Venus. With some modifications, this could lead to an explosion of life and the development of a tropical planet, with biodiverse jungles on the larger landmasses and more coastline than you can shake a stick at!
You may have already figured out what the downside of all of this is. If you guessed that it would take a massive effort to transform Venus, and it would be very challenging to create a settlement there in the meantime, you'd be absolutely right! While Venus could be terraformed to become what it once was, the commitment in time, energy, and resources would be nothing short of herculean.
Elon Musk once summarized the challenge of creating a self-sustaining city on Mars using a real-estate metaphor. In a 2012 interview with CBS This Morning, he said that: "You need to live in a dome initially, but over time you could terraform Mars to look like Earth and eventually walk around outside without anything on... So it's a fixer-upper of a planet."
If we were to view Mars as a future prospect, "fixer-upper" would certainly be an apt description. If we want to live there someday in large numbers, we are likely to have to alter the environment. Even then, there are long-term challenges that we may never be able to solve (i.e., it's lower gravity).
However, Mars is also "move-in ready," which is to say that cities with a few hundred inhabitants could be established there in the near future. Over time, these settlers could gradually transform the Martian environment, making it suitable for more and more inhabitants. By the time it became fully settled, there could be solutions to all of its hazards.
Comparatively speaking, Venus is a "tear-down and rebuild" with some serious asbestos issues! We know the plot could be turned into a wonderful home, and the property value would skyrocket. But we're talking about a huge commitment in terms of money and time before that can happen. In the meantime, the best we can do is move a few people into the existing structure's anteroom.
Of course, if we're willing to think in terms of the long-run and assume that humanity does have an "interplanetary" future, there's no reason to assume we can't eventually make Mars AND Venus second homes for humanity. To use the same old metaphor, we can purchase the fixer-upper next door and gradually get it to the point that it's fully furnished.
Then we can devote our attention and our efforts to the tear-down lot on the other side of us and refurbish it into a third lovely home for ourselves and our children. Once people move in, we can rest assured that if any one of the houses burns down, it won't take us all with it!
Disclaimer: A previous version of this article stated that the sci-fi short "The Long Rain" was authored by Olaf Stapledon when it should have been Ray Bradbury.