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Where Could Humans Survive in our Solar System?

Habitability in our solar system. Credit: UPR Arecibo, NASA PhotoJournal

Habitability in our solar system. Credit: UPR Arecibo, NASA PhotoJournal

If humans were forced to vacate Earth, where is the next best place in our solar system for us to live? A study by the University of Puerto Rico at Arecibo has provided a quantitative evaluation of habitability to identify the potential habitats in our solar system. Professor Abel Mendez, who produced the study also looked at how the habitability of Earth has changed in the past, finding that some periods were even better than today.

Mendez developed a Quantitative Habitability Theory to assess the current state of terrestrial habitability and to establish a baseline for relevant comparisons with past or future climate scenarios and other planetary bodies including extrasolar planets.

“It is surprising that there is no agreement on a quantitative definition of habitability,” said Mendez, a biophysicist. “There are well-established measures of habitability in ecology since the 1970s, but only a few recent studies have proposed better alternatives for the astrobiology field, which is more oriented to microbial life. However, none of the existing alternatives from the fields of ecology to astrobiology has demonstrated a practical approach at planetary scales.”

His theory is based on two biophysical parameters: the habitability (H), as a relative measure of the potential for life of an environment, or habitat quality, and the habitation (M), as a relative measure of biodensity, or occupancy. Within the parameters are physiological and environmental variables which can be used to make predictions about the distribution, and abundance of potential food (both plant and microbial life), environment and weather.

The image above shows a comparison of the potential habitable space available on Earth, Mars, Europa, Titan, and Enceladus. The green spheres represent the global volume with the right physical environment for most terrestrial microorganisms. On Earth, the biosphere includes parts of the atmosphere, oceans, and subsurface (here’s a biosphere definition). The potential global habitats of the other planetary bodies are deep below their surface.

Enceladus has the smallest volume but the highest habitat-planet size ratio followed by Europa. Surprisingly, Enceladus also has the highest mean habitability in the Solar System, even though it is farther from the sun, and Earth, making it harder to get to. Mendez said Mars and Europa would be the best compromise between potential for life and accessibility.

n Oct. 5, 2008.  Image credit: NASA/JPL/Space Science Institute  Cassini came within 25 kilometers (15.6 miles) of the surface of Enceladus o

n Oct. 5, 2008. Image credit: NASA/JPL/Space Science Institute Cassini came within 25 kilometers (15.6 miles) of the surface of Enceladus o

“Various planetary models were used to calculate and compare the habitability of Mars, Venus, Europa, Titan, and Enceladus,” Mendez said. “Interestingly, Enceladus resulted as the object with the highest subsurface habitability in the solar system, but too deep for direct exploration. Mars and Europa resulted as the best compromise between habitability and accessibility. In addition, it is also possible to evaluate the global habitability of any detected terrestrial-sized extrasolar planet in the future. Further studies will expand the habitability definition to include other environmental variables such as light, carbon dioxide, oxygen, and nutrients concentrations. This will help expand the models, especially at local scales, and thus improve its application in assessing habitable zones on Earth and beyond.”

Studies about the effects of climate change on life are interesting when applied to Earth itself. “The biophysical quantity Standard Primary Habitability (SPH) was defined as a base for comparison of the global surface habitability for primary producers,” Mendez said. “The SPH is always an upper limit for the habitability of a planet but other factors can contribute to lower its value. The current SPH of our planet is close to 0.7, but it has been up to 0.9 during various paleoclimates, such as during the late Cretaceous period when the dinosaurs went extinct. I’m now working on how the SPH could change under global warming.”

The search for habitable environments in the universe is one of the priorities of the NASA Astrobiology Institute and other international organizations. Mendez’s studies also focus on the search for life in the solar system, as well as extrasolar planets.

“This work is important because it provides a quantitative measure for comparing habitability,” said NASA planetary scientists Chris McKay. “It provides an objective way to compare different climate and planetary systems.”

“I was pleased to see Enceladus come out the winner,” McKay said. “I’ve thought for some time that it was the most interesting world for astrobiology in the solar system.”

Mendez presented his results at the Division for Planetary Sciences of the American Astronomical Society meeting earlier this month.

Source: AAS DPS


Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • Lawrence B. Crowell October 18, 2009, 8:35 AM

    It will be interesting to see how it turns out. In some this is an experiment in whether humanity can collectively change its behavior in ways which are rather precedented. It could happen, we have the smarts I think. The question is whether we can control our more reptilian aspects of our brains.

    If we do manage to do this I can imagine the most ardent rightwinged types replaying Charlton Heston at the end of Planet of the Apes, where he is cursing in front of the remains of the Statue of Liberty. :-)


  • Maxwell October 18, 2009, 4:09 PM

    History shows that humanity is more than capable of changing its lifestyle to meet the demands of nature or war. Solution will be found to the environmental crisis, both real and imagined. But there’s no certainty anything will be solved.

    Our mistake is in thinking that right and left wing ideologies encompass the only two possible outcomes when they are simply a compilation of temporary stances on specific issues.
    We are talking about the same humanity that has been able to (on more than one occasion) justify things like genocide as a noble act. That among other seemingly insane chapters from our past makes me think this rabbit hole we’re looking to fall down has no bottom.

    If we aren’t careful as a society, Charlton Heston is likely to have a great deal of company in his mourning.

  • paulc October 18, 2009, 9:18 PM

    The general discussion seems to focus on a few hundred, maybe even a few thousand, humans forming a colony somewhere. Leaves about 6 billion people behind.

    A couple of alternatives.
    1. Some sort of mass genocide or extermination for all but a few thousand.
    2. If conditions on planet earth deteriorate as in the imagination of the contributors, the few hundred (thousand) survivors would fare better here than in some mystical galactic bubble.

  • Paul Eaton-Jones October 19, 2009, 2:09 AM

    The above scenario could well be feasible within the next 500 years given the rate of technological advance of the previous 500. IF the climate- change doomsayers are correct then we’ll all be dead or thrown back to a Stone Age-like level within 150 years and all this is purely academic. The problem is going to be the mass eevacuation of billions of people, should they all want to leave, when we get threatened by giant asteroids, green lizards from the Sirius Cybernetc Corporation or whatever. By the time the sun enters its red giant stage the descendants of humans will be scattered far and wide throughout the galaxy and goodness knows what will be the dominant life form on earth.

  • Maxwell October 19, 2009, 7:04 AM

    Saving humanity is a different goal than saving everyone on earth.
    You only need a few hundred breeding pairs at most to maintain biological diversity. Which means whatever safehold you design doesn’t have to be unrealistically large.

    The only difference between building a survival vault on mars and building one on earth (besides the technical challenge) is the fact uninvited people cant simply walk there.

    So thats fewer machine guns at the door.

  • Lawrence B. Crowell October 19, 2009, 2:29 PM

    I would say your chances of survival are not much better. A major system failure and you are dead. If things go crazy on Earth you might have to fight for survival, but you would stand a chance of making it through.

    Also if things are headed for the deep tank on Earth it is not likely you will get the huge amount of federal dollars required to set up shop on some other planet. Things will probably be pretty shakey in the years leading up to that.


  • Maxwell October 19, 2009, 5:37 PM

    There is not much allure to cash you wont live to spend and which wont have any value in the near future… but Humans do things for many reasons. People are just as apt to rise to the occasion out of a misplaced drive to survival or nationalist urges.

    It depends on the nature of the threat in the end.

    We’ve spent large amounts of money to develop things like seed vaults and underground records storage already. Not to mention we’ve built nuclear bunkers to provide for the continuity of government, housing weapons, and stockpiling supplies.
    This was all in the presumption that after whatever event happens we can pop the lids and come out to start fighting or rebuilding.

    If the threat is a Dinosaur killing sized asteroid or something out of the pages of SCI-FI then what would you surface to… hell on earth?
    Take your chances beyond the moon or down here with fallout the size of cruise ships?

    Some might think it better to be anywhere else. Its not like we weren’t gonna do it anyway given half a chance.
    We would still be depending on equipment either way. Only now your entirely separated from earth and whatever pathogens or people that might be troubling you.

    I think the beauty of living off world is that you don’t have to wait for doomsday to see a return on investment. Its something we’ll do naturally… why not do it in earnest?