Posted on November 5, 2018
"When I suggested this topic, I wondered if there were two inhabited planets (Earth and Venus) if Mars and Venus formed in opposite locations," said Chris Colose, a climate scientist at NASA's Goddard Institute for Space Studies. "Being in the orbit of Mars would avoid the uncontrolled greenhouse and a planet the size of Venus would not have its atmosphere removed as easily as Mars."
What would happen if you switched the orbits of Mars and Venus? Would our solar system have more habitable worlds?
It was a question raised in "Comparative Climatology of Terrestrial Planets III"; a meeting held in Houston at the end of August. It brought together disciplinary scientists that included astronomers, climate science, geophysics, and biology to build a picture of what affects the environment on rocky worlds in our solar system and beyond.
The question about Venus and Mars was proposed as a gedanken experiment or "mental experiment"; a favorite of Albert Einstein to conceptually understand a topic. Leaving such a problem before the interdisciplinary group in Houston was the flesh before the lions: the elements of this question were about to be destroyed.
The Earth's orbit is between Venus and Mars, with Venus orbiting closer to the Sun and Mars orbiting farther. While both our neighbors are rocky worlds, they are also not top picks for vacation destinations.
Mars has a mass of only a tenth of Earth, with a fine atmosphere being removed by the solar wind; a stream of high energy particles flowing from the sun. Without significant gas coverage to retain heat, temperatures on the Martian surface are averaged at -80 ° F (-60 ° C). Notably, Mars orbits within the limits of the classical habitable zone (where an Earth-like planet could maintain surface water), but the small planet is not able to regulate its temperature, just as the Earth could be in the same place.
Unlike Mars, Venus has almost the same mass as Earth. However, the planet is suffocated by a thick atmosphere composed mainly of carbon dioxide. The heat retention capabilities of these gases rise to surface temperatures above 460 ° C (860 ° F).
But what if we could change the orbits of these planets to put Mars on a warmer path and Venus in a colder? Would we discover that we were no longer the only habitable world in the solar system?
"Modern Mars in the orbit of Venus would be reasonably warm by Earth standards," Colose said. Dragging the current Mars into the orbit of Venus would increase the amount of sunlight reaching the red planet. As the thin atmosphere does little to affect surface temperature, average conditions should rise to about 90 ° F (32 ° C), similar to the Earth's tropics. However, the thin atmosphere of Mars continues to present a problem.
Colose noted that without a thicker or oceanic atmosphere, the heat would not be transported efficiently around Mars. This would lead to extreme seasons and temperature gradients between day and night. The thin atmosphere of Mars produces a surface pressure of only 6 millibars, compared with 1 bar on Earth. At such low pressures, the boiling point of the water falls to leave all the pure water of the surface frozen or vaporized.
Mars has ice caps that consist of frozen carbon dioxide, with more greenhouse gas sunk in soils. A brief glimpse of hope for the small world came into the discussion with the suggestion that they would be released at the highest temperatures in the orbit of Venus, giving Mars a thicker atmosphere.
However, recent research suggests that there is not enough carbon dioxide trapped to provide a substantial atmosphere on Mars. In an article published in Nature Astronomy, Bruce Jakosky of the University of Colorado and Christopher Edwards of the University of Northern Arizona estimate that the melting of the polar ice caps would provide a maximum atmosphere of 15 millibar.
The carbon dioxide trapped in the Martian rocks would require temperatures above 300 ° C to be released, a very high value for Mars, even in the orbit of Venus. 15 millibars doubles the pressure of the current atmosphere on Mars and exceeds the so-called "triple point" of water that should allow liquid water to exist. However, Jakosky and Edwards note that evaporation would be rapid in the dry Martian air. Then we come across another problem: Mars is not good at holding the atmosphere.
Orbiting Mars is NASA's Mars Atmosphere and Volatile Evolution Mission (MAVEN). Data from MAVEN revealed that the atmosphere of Mars was removed by the solar wind. It is a problem that would be exacerbated in the orbit of Venus.
"Atmospheric loss would be faster at Venus' current position, as the dynamic pressure of the solar wind would increase," said Chuanfei Dong of Princeton University who modeled the atmospheric loss on Mars and extrasolar planets.
This "dynamic pressure" is the combination of the particle density of the solar wind and its velocity. Speed does not change much between Mars and Venus, "Dong explained," but the proximity of Venus to the Sun increases the density by almost a factor of 4.5. That would mean that the atmosphere on Mars would be lost even faster than in its current position.
"I suspect it would be just a warmer rock," Colose concluded.
While Mars does not seem to do much better in locating Venus, what if Venus were towed into Mars' current orbit? Set in the habitable zone, would this earth-sized planet become a second habitable world?
Surprisingly, cooling Venus may not be as simple as reducing sunlight. Venus has a very high albedo, which means that the planet reflects about 75% of the radiation it receives. The suffocating temperatures on the surface of the planet are not due to a high level of sunlight but to the thickness of the atmosphere. Conditions on the planet may therefore not be immediately affected if Venus orbits in the coldest place on Mars.
"The atmosphere of Venus is in balance," said Kevin McGouldrick of the University of Colorado and collaborator of the Akatsuki mission in Japan to explore the atmosphere of Venus. "It means that its current structure depends on the radiation from the sun. If you change that radiation, the atmosphere will eventually adjust, but it probably will not be fast. "
Exactly what would happen to the 90-bar atmosphere of Venus in the long run is not obvious. It may be that the planet will cool slowly to more temperate conditions. Alternatively, the planet's bright albedo may decrease as the upper atmosphere cools. This would allow Venus to absorb a larger fraction of the radiation that would reach its new orbit and help maintain the stifling surface conditions. To really cool the planet, Venus may have to be dragged beyond the habitable zone.
"After about 1.3 au, carbon dioxide will begin to condense in clouds and also on the surface as ice," said Ramses Ramirez of the Earth-Life Sciences Institute (ELSI) in Tokyo, who specializes in modeling the edges of habitable area. (An "au" is an astronomical unit, which is the distance from our sun to Earth.)
When carbon dioxide condenses, it can no longer act as a greenhouse gas and retain heat. Instead, ice and clouds usually reflect surface heat. This defines the outer edge of the classical habitable zone when carbon dioxide should have condensed mainly outside the atmosphere to about 1.7 au. The result should be a quick cooling to Venus. However, this outer boundary for the habitable zone was calculated for an Earth-like atmosphere.
"Venus has other things going on in its atmosphere compared to Earth like clouds of sulfuric acid," Ramirez noted. "And it's much drier, so this point (where carbon dioxide condenses) may be different for Venus."
If Venus were continually dragged out, even the planet's considerable heat supply would wear out.
"If you shoot Venus out of the solar system as a rogue planet, it will eventually cool down!" Pointed out Max Parks, a research assistant at NASA Goddard.
It seems that simply changing the orbits of the current Venus and Mars would not produce a second habitable world. But what if the two planets formed in opposite places? Mars is unlikely to have fared better, but will Venus have avoided forming its lead-melting atmosphere and becoming a second Earth?
At first glance, this seems very likely. If the Earth were pushed into the orbit of Venus, the water would begin to evaporate rapidly. Like carbon dioxide, water vapor is a greenhouse gas and helps to retain heat. The temperature of the planet, therefore, would continue to increase in an escape cycle until all the water had evaporated. This "uncontrolled greenhouse effect" is a possible story for Venus, explaining its terrible surface conditions. If the planet had formed within the habitable zone, this escape process should be avoided as it had been for Earth.
But the discussion within the group has revealed that it is very difficult to offer any assurance that a planet will become habitable. An example of the resulting roulette game is the crust of the planet. The Venus crust is a continuous cap and not a series of plates as fragmented as on Earth. Our plates allow a process known as tectonic plates, whereby the nutrients are cycled through the surface of the Earth and hold the mantle to help sustain life. However, it is not clear why the Earth formed this way, but Venus did not.
One theory is that the warmer Venusian crust ruptures rapidly, preventing the formation of separate plaques. However, a survey by Matt Weller of the University of Texas suggests that the formation of tectonic plates can be overwhelmingly bad. Small, random fluctuations can send two identical planets to different evolutionary paths, one developing tectonic plates and the other a stagnant cap. If true, even forming the Earth in exactly the same position could result in a less tectonic planet.
A rotating globe with tectonic plate boundaries indicated as cyan lines. The warmer orbit of Venus may have shortened the length of time that plate tectonics could develop, but moving the planet into Mars' orbit offers no guarantee of a nutrient-moving crust.
A rotating globe with tectonic plate boundaries indicated as cyan lines. (NASA / Goddard Space Flight Center)
However, if plate tectonics is definitely required for habitability, it is also not known. It was pointed out during the discussion that both Mars and Venus show signs of past volcanic activity, which may be sufficient action to produce a habitable surface under the right conditions.
Of course, moving a planet's orbit is beyond our technological skills. There are other techniques that can be tried, such as an idea by Jim Green, chief scientist at NASA and Dong involving the artificial protection of the atmosphere of Mars against the solar wind.
"We have reached the conclusion opposite Bruce's role," Dong observed cheerfully. "It may be possible to use technology to give Mars an atmosphere. But it's fun to hear different voices and this is the reason why science is so interesting! "
The Daily Galaxy via NASA's Elizabeth Tasker / Astrobiology