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Planetary Habitability Quantified

Habitability is always a matter of definition. Is it a measure of suitability for human life? Or do we take the larger astrobiological view that it’s based on suitability for microbial life, in which case we go from a narrowly defined habitable zone here in our Solar System to one that could potentially stretch from the upper atmosphere of Venus to the suspected subsurface aquifers on some Kuiper Belt objects. But these qualitative definitions have thus far lacked a quantitative counterpart, a method to quantify and compare potentially living worlds.

The matter has drawn the attention of Abel Mendez (University of Puerto Rico, Arecibo), who discussed his quantitative evaluation of planetary habitability at the Division for Planetary Sciences meeting this week in Fajardo, Puerto Rico. One of his results stands out immediately. Using finely tuned planetary models, Mendez found that among Mars, Venus, Europa, Titan and Enceladus, the latter has the highest subsurface habitability. That makes the Saturnian moon a tempting but difficult target, the inaccessibility of its habitable region rendering Mars and Europa better compromises for near-term missions to places where habitability is still unresolved.

Take a look at the habitability functions for various worlds in the image below:


Image: This image 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. 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, it also has the highest mean habitability H, in the Solar System, although too deep for direct exploration. Mars and Europa are the best compromise between potential for life and accessibility. Credit: UPR Arecibo, NASA PhotoJournal.

Quantitative Habitability Theory could be useful if, as advertised, it sets up a baseline for making comparisons with past climate scenarios here on Earth, or examining other planets (including extrasolar ones) in light of a consistent set of principles. Here’s Mendez’ explanation of QH Theory:

“QH Theory is based on two new 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. Both parameters are related to other physiological and environmental variables and can be used to make predictions about the distribution, abundance, and productivity of primary producers, such as plants and phytoplankton, and microbial life in general. Initially, habitability was modeled from the environment’s temperature and humidity because they are easier to measure at planetary scales with ground or orbital instruments. Global habitability and habitations maps were constructed of terrestrial land and ocean areas with data gridded at various spatial and temporal resolutions. Preliminary work shows that the QH Theory is comparable to existing models in predicting terrestrial primary productivity.”

Mendez has established a quantity called Standard Primary Habitability that uses a variety of criteria for establishing the surface habitability on a given world. Interestingly, while the current SPH of Earth is close to 0.7, the figure has been as high as 0.9 in earlier periods, including the late Cretaceous, the time of the dinosaurs’ extinction. Earth achieved, in other words, a higher level of habitability in that era than today, at least as quantified by Mendez, until the events that led to the K/T extinction occurred.

James Kasting (Penn State) finds Mendez’ work useful, saying the methodology “…could also be extended to studies of planets around other stars that may be found during the next two decades.” Further integrating such factors as light, oxygen, carbon dioxide and nutrient concentrations will expand the model for Earth and help scientists extend it to habitable zones beyond.


Comments on this entry are closed.

  • Savino October 7, 2009, 15:28

    Is the little ball in Titan brown for any particular reason?

  • tacitus October 7, 2009, 16:10

    I think in the long run, surface habitability will be by far the more important measure. While it’s cool to attempt to quantify the odds of subsurface life on the icy moons in our solar system, there really isn’t going to be any practical use for it in the hunt for life on exoplanets, at least not in the centuries before we’re capable of routine interstellar travel

    And from a SETI point of view, the surface habitability is the only game in town. Even if there was a way for a subsurface technological civilization to evolve (which seems unlikely, especially underwater) then they would have to have a presence on the surface for us to have any chance of detecting them.

  • Carl October 7, 2009, 18:36

    These are beginnings of new phases of exobiology. The icy moons have received more attention recently than the gas giants as habitable worlds. The Galileo probe sampled the atmosphere and found less water than expected, along with “significantly lower than expected levels of helium, neon, and certain heavy elements, such as carbon, oxygen and sulfur. ” ( http://www.jpl.nasa.gov/news/releases/96/gllprb.html )

    It was but a single instrument suite in a tiny sliver of a huge world. The news wasn’t the best for the case for life, but might Jupiter and Saturn yet offer potential habitability zones high in their atmospheres?

  • Carl October 7, 2009, 19:09

    https://centauri-dreams.org/?p=6308 From an earlier article: “Edwin Salpeter and the Gasbags of Jupiter”. Interesting!

  • Goldstein Hovercraft October 7, 2009, 22:26

    Don’t the geysers of Enceladus make its water more accessible to direct exploration than any of the other worlds under consideration? A probe could directly sample its water by flying through a geyser during a flyby – no orbiting or landing would be necessary. Did the author explain how accessibility was assessed? Also, I wonder whether the author took into account the competing models of what is going on in Enceladus’ interior….

    Is the original paper available? Thanks!

  • Ronald October 8, 2009, 3:10

    I always thought things went downhill after the Cretaceous ;-)

  • Abel October 8, 2009, 8:05

    The sphere of Titan is brown just to show that its habitability is very low.

  • Administrator October 8, 2009, 8:24

    Goldstein Hovercraft wrote:

    Is the original paper available? Thanks!

    Not yet, at least, not to the best of my knowledge. I’ll keep an eye on this to find our where it’s scheduled to appear.

  • philw1776 October 8, 2009, 9:53

    The Cretaceous, like the majority of time since the Cambrian explosion populated the Earth with ‘animals’ was much, much warmer than today. We presently live an an anomolous Icehouse Earth era.


  • ljk October 8, 2009, 10:19

    If the Sol system is left alone (for example, not turned into a Dyson Shell) over
    the next few billion years, when the Sun becomes a red giant it could heat up
    Titan and Europa and a few other favorable outer region places, perhaps enough
    to get the processes for life going on those worlds.

    The whole thing will only last a matter of millions of years before Sol shrinks
    down to a white dwarf, but it would be very interesting to run some calculations
    on what might happen and how far evolution could progress during that far
    future era. We could extrapolate it to other solar systems in their final stages
    as well.

  • Abel October 8, 2009, 14:03

    To clarify, the model for Enceladus is the one proposed by Mitri et al. 2008, but there are more optimistic and pessimistic models. The Enceladus potential habitat zone is too deep for direct exploration, but maybe not too deep for indirect exploration by sampling the jets, assuming the water from the jets come from the potential habitat zone. A paper will be submitted in a few months with the habitat calculations for terrestrial environments. The planetary calculations are part of the initial study of a proposal and will be published much later with more details.

  • Tulse October 8, 2009, 14:53

    Given how difficult (and thus expensive) it would be to directly sample jet material from Enceladus, would it be possible to detect life in Enceladus without directly sampling the jets, via some sort of remote sensing of the jets’ chemical composition? What is the best case one could expect for signatures of life in such cases (for example, would geyers on Earth show any sign of organic life, and if so, how easy would it be to detect remotely)?

  • Administrator October 8, 2009, 15:01

    Abel writes:

    The Enceladus potential habitat zone is too deep for direct exploration, but maybe not too deep for indirect exploration by sampling the jets, assuming the water from the jets come from the potential habitat zone. A paper will be submitted in a few months with the habitat calculations for terrestrial environments. The planetary calculations are part of the initial study of a proposal and will be published much later with more details.

    Thanks for the publication information, as well as for your participation in the discussion, Dr. Mendez. Much appreciated!

  • Goldstein Hovercraft October 8, 2009, 16:17

    Thanks also from me! I look forward to reading your paper. I ‘m curious about what you say about all the worlds you covered, even Venus!

    Back to Enceladus: My understanding is that Mitri wrote of an ice shell that is 50 km (31 miles) thick, or even more than three times that thickness, while Porco wrote of an ice shell that merely 7 meters (23 feet) deep. For interested readers, Mitri’s paper is here (I think):
    and astrobio.net just came out with an easy to read popular article on some of the competing models, including what might be wrong with Porco’s model:

    Tulse, I would love to know the answer to your question – I hope someone here has some insights.

  • ljk October 8, 2009, 20:26

    October 8, 2009

    More Water ‘Out There:’ Ice Found on Asteroid

    Written by Nancy Atkinson

    For the first time, astronomers have confirmed that an asteroid contains frozen water on its surface. Analysis of asteroid 24 Themis shows evidence of water ice along with organic compounds widespread across the surface.

    The scientists say these new findings support the theory that asteroids brought both water and organic compounds to the early Earth, helping lay the foundation for life on the planet.

    Humberto Campins of the University of Central Florida in Orlando and colleagues recorded spectra of 24 Themis over a seven-hour period, and were able to study 84 percent of the rotational period of the spinning rock, Rob Cowen reported in Science News. Using NASA’s Infrared Telescope Facility on Hawaii’s Mauna Kea, the spectra revealed the consistent presence of frozen water as different parts of the asteroid’s surface came into view.

    Analyses of the sunlight reflected off the asteroid also show that organic compounds are widespread on the surface, he added, including polycyclic aromatic hydrocarbons, CH2 and CH3.

    The new finding corroborates earlier observations of the same asteroid by astronomers Andrew S. Rivkin and Joshua Emery who also used the Infrared Telescope Facility. Over several years, Rivkin and Emery had found evidence of frozen water in single spots on 24 Themis but had not studied the asteroid as it made one entire rotation. Together, the two teams’ findings reveal that the asteroid’s entire surface is coated with frozen water, Campins says.

    Full article here:


  • Abel October 8, 2009, 22:29

    The habitat evaluation of the study was a global perspective and does not consider hot spots (we need to characterize them first). Enceladus jets could be due to a near-surface hot spot or due to some much deeper process. In any case, it will be very difficult to use remote sensing to absolutely determinate that any organic material from the jets is of biological origin. Sampling is the best choice. I appreciate your interest in this topic.

  • ljk October 9, 2009, 9:47

    How feasible would it be to conduct a Stardust/Genesis-type mission with a probe flying through the plumes of Enceladus and collect the plume debris with aerogel for return to Earth? Certainly it would be cheaper and easier – though not nearly as desirable – as a lander/penetrator.

    The mission could also be similar to this Europa Ice Clipper proposal from 1997:



    Or are there even easier/cheaper/quicker ways to analyze the moon’s plumes remotely?

  • Tulse October 9, 2009, 14:35

    I would think that a sample return mission would be far too expensive, complex, and risky to be worth pursuing. Presumably if the goal is to do direct sampling, the analysis would have to be done on board the probe.

  • djlactin October 10, 2009, 12:22

    Bosh! “Life” will have a wide variety of ‘styles’. Evaluating ‘habitability’ on the basis of the requirements of water-and-protein based life is too restrictive. For some life forms, molten water might be hell. Like lava is to us.

  • NS October 10, 2009, 19:55

    Re what djlactin said, the first place to look for an alternative form of life is right here on Earth:


  • djlactin October 11, 2009, 0:57

    Thanks NS; fascinating paper!

  • andy October 11, 2009, 6:38

    Regarding the search for extraterrestrial life, the one major habitable zone in the inner solar system that has not been sampled at all is the Venusian cloud layer, which in terms of temperature, pressure and gravity is the most Earthlike extraterrestrial environment in the solar system. Sure it has its problems as a potential life zone, but surely it is worth seeing what’s there. Besides, a balloon mission to Venus could serve as a prototype for similar missions to Titan: Venus is a much easier place to get to!

  • NS October 12, 2009, 0:28

    There already have been balloon missions to Venus, but unfortunately they didn’t do the kind of sampling that might detect life:


  • ljk October 12, 2009, 10:10

    How about looking for life on the Moon? Yes, our natural satellite just 240,000 miles away.

    There have already been proposals to look for terrestrial fossils of creatures that were blasted off our planet from ancient impacts which ended up on the Moon. Some of them could have landed in those permanently shadowed regions at the poles where they suspect a lot of water ice from impacting comets still resides.

    As it has also been shown that microbes can survive the impact blast and trip through deep space inside rock, so let’s go one step further and presume that some of these little astronauts could make it to the Moon and end up in the cometary water ice, where they remain in hibernation awaiting our discovery and revival. If we can imagine similar scenarios for Earth and Mars, surely the much nearer Moon has even better odds.

    In the two biographies on Carl Sagan which came out ten years ago this year, they mention that Sagan was involved with a USAF plan to detonate a nuclear bomb on the Moon. Another spacecraft would fly through the blast debris and search for, among other things, any microbes that might be swelling deep beneath the lunar surface.

    We assumed for a long time that our Moon was drier than the proverbial bone and geologically inactive for eons, which implied that it was biologically dead too. Now we know that world has lots of water, relatively speaking, and not all TLPs were illusions or delusions. It will be very interesting to see what other paradigms are overturned as our explorations increase and improve.

    And you know Mercury has supposedly got water ice from comets in its permanently shadowed polar craters, too, and this is a world with one half that is constantly baking at 700 degrees F. So I’m just saying….

  • andy October 12, 2009, 14:51

    We shouldn’t get carried away with the whole water thing. Just having water there does not necessarily mean that there is life… while ice reservoirs buried in frozen polar craters or beneath the moon’s soil are interesting, there does not seem to be much dynamic chemistry going on on such worlds. And ice is not liquid water, so despite its potential use for space missions it does not seem nearly so good for supporting a biosphere.

    Of the many notable features of the Earth one of the most important is the dynamic environment and complex chemical environment. In the inner solar system, beyond Earth the best example of this is Venus but thanks to the obsession over the fantastically nasty surface conditions the exploration of this fascinating environment has been woefully neglected in favour of large numbers of missions to the frozen dirtball that is Mars.

    (Regarding the picture of Venus as an extreme hellworld, I made the point that at Venus you could find conditions which were a fairly good match to Earth’s temperature/pressure/gravity on a comment thread at Bad Astronomy, without stating that I wasn’t talking about the surface, and got several replies telling me I was nuts because, as everyone knows, the temperature on Venus is higher than Mercury’s daytime and the pressure is immense…)

  • amphiox October 12, 2009, 14:55

    djlactin, you might be right, but from a practical perspective, it makes sense to start with what we know, where we can at least plan and guide our efforts in a rational manner, and have at least some chance of recognizing the stuff we find. To go straight to looking for more exotic life forms would be like trying to go fishing without knowing how big to make our net, how deep to trawl it, what to make it out of, or even where the ocean is.

  • ljk October 15, 2009, 10:55

    Tulse said on October 9, 2009 at 14:35:

    “I would think that a sample return mission [for Enceladus] would be far too expensive, complex, and risky to be worth pursuing. Presumably if the goal is to do direct sampling, the analysis would have to be done on board the probe.”

    Do you have any numbers to go along with this assumption? How much more
    cheaply could an onboard analysis be, assuming it could be done as well as or
    better than in a lab on Earth – assuming we get a probe to Enceladus in the next
    fifty years?

    Maybe the proposed Titan mission could be temporarily diverted to Enceladus
    to sample the plume debris and do the analysis there. At least that will only be
    twenty or so years in the future.

  • ljk October 31, 2009, 1:29

    October 30, 2009

    Bacteria Could Survive in Martian Soil

    Written by Nicholos Wethington

    Multiple missions have been sent to Mars with the hopes of testing the surface of the planet for life – or the conditions that could create life – on the Red Planet. The question of whether life in the form of bacteria (or something even more exotic!) exists on Mars is hotly debated, and still requires a resolute yes or no.

    Experiments done right here on Earth that simulate the conditions on Mars and their effects on terrestrial bacteria show that it is entirely possible for certain strains of bacteria to weather the harsh environment of Mars.

    A team led by Giuseppe Galletta of the Department of Astronomy at the University of Padova simulated the conditions present on Mars, and then introduced several strains of bacteria into the simulator to record their survival rate.

    The simulator – named LISA (Laboratorio Italiano Simulazione Ambienti) – reproduced surface conditions on Mars, with temperatures ranging from +23 to -80 degrees Celsius (73 to -112 Fahrenheit), a 95% CO2 atmosphere at low pressures of 6 to 9 millibars, and very strong ultraviolet radiation.

    The results – some of the strains of bacteria were shown to survive up to 28 hours under these conditions, an amazing feat given that there is nowhere on the surface of the Earth where the temperatures get this low or the ultraviolet radiation is as strong as on Mars

    Full article here:


  • ljk October 31, 2009, 11:58

    The Cyborg Astrobiologist: Testing a Novelty-Detection Algorithm on Two Mobile Exploration Systems at Rivas Vaciamadrid in Spain and at the Mars Desert Research Station in Utah

    Authors: P.C. McGuire, C. Gross, L. Wendt, A. Bonnici, V. Souza-Egipsy, J. Ormo, E. Diaz-Martinez, B.H. Foing, R. Bose, S. Walter, M. Oesker, J. Ontrup, R. Haschke, H. Ritter

    (Submitted on 28 Oct 2009)

    Abstract: (ABRIDGED) In previous work, two platforms have been developed for testing computer-vision algorithms for robotic planetary exploration (McGuire et al. 2004b,2005; Bartolo et al. 2007). The wearable-computer platform has been tested at geological and astrobiological field sites in Spain (Rivas Vaciamadrid and Riba de Santiuste), and the phone-camera has been tested at a geological field site in Malta.

    In this work, we (i) apply a Hopfield neural-network algorithm for novelty detection based upon color, (ii) integrate a field-capable digital microscope on the wearable computer platform, (iii) test this novelty detection with the digital microscope at Rivas Vaciamadrid, (iv) develop a Bluetooth communication mode for the phone-camera platform, in order to allow access to a mobile processing computer at the field sites, and (v) test the novelty detection on the Bluetooth-enabled phone-camera connected to a netbook computer at the Mars Desert Research Station in Utah.

    This systems engineering and field testing have together allowed us to develop a real-time computer-vision system that is capable, for example, of identifying lichens as novel within a series of images acquired in semi-arid desert environments.

    We acquired sequences of images of geologic outcrops in Utah and Spain consisting of various rock types and colors to test this algorithm. The algorithm robustly recognized previously-observed units by their color, while requiring only a single image or a few images to learn colors as familiar, demonstrating its fast learning capability.

    Comments: 28 pages, 12 figures, accepted for publication in the International Journal of Astrobiology

    Subjects: Computer Vision and Pattern Recognition (cs.CV); Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Neural and Evolutionary Computing (cs.NE); Computational Physics (physics.comp-ph); Machine Learning (stat.ML)

    Cite as: arXiv:0910.5454v1 [cs.CV]

    Submission history

    From: Patrick C. McGuire [view email]

    [v1] Wed, 28 Oct 2009 18:26:39 GMT (2387kb)


  • ljk January 28, 2010, 20:12

    How to Find a Habitable Planet

    James Kasting

    Cloth | 2009 | $29.95 / £20.95
    360 pp. | 6 x 9 | 16 color illus. 47 halftones. 25 line illus. 4 tables.

    Ever since Carl Sagan first predicted that extraterrestrial civilizations must number in the millions, the search for life on other planets has gripped our imagination. Is Earth so rare that advanced life forms like us–or even the simplest biological organisms–are unique to the universe?

    How to Find a Habitable Planet describes how scientists are testing Sagan’s prediction, and demonstrates why Earth may not be so rare after all.

    James Kasting has worked closely with NASA in its mission to detect habitable worlds outside our solar system, and in this book he introduces readers to the advanced methodologies being used in this extraordinary quest.

    He addresses the compelling questions that planetary scientists grapple with today: What exactly makes a planet habitable? What are the signatures of life astronomers should look for when they scan the heavens for habitable worlds? In providing answers, Kasting explains why Earth has remained habitable despite a substantial rise in solar luminosity over time, and why our neighbors, Venus and Mars, haven’t.

    If other Earth-sized planets endowed with enough water and carbon are out there, he argues, chances are good that some of those planets sustain life. Kasting describes the efforts under way to find them, and predicts that future discoveries will profoundly alter our view of the universe and our place in it.

    This book is a must-read for anyone who has ever dreamed of finding other planets like ours–and perhaps even life like ours–in the cosmos.

    James Kasting is Distinguished Professor of Geosciences at Pennsylvania State University. He is a renowned expert in planetary atmospheric evolution and is actively involved in the search by NASA for habitable planets outside our solar system. He is the coauthor of the introductory textbook The Earth System.