Habitable Zones: A Moving Target

by Paul Gilster on January 31, 2013

Habitable zones are always easy enough to explain when you invoke the ‘Goldilocks’ principle, but every time I talk about these matters there’s always someone who wants to know how we can speak about places being ‘not too hot, not too cold, but just right.’ After all, we’re a sample of one, and why shouldn’t there be living creatures beneath icy ocean crusts or on worlds hotter than we could tolerate? I always point out that we have to work with what we know, that water and carbon-based life are what we’re likely to be able to detect, and that we need to fund the missions to find it.

The last word on habitable zone models has for years been Kasting, Whitmire and Reynolds on “Habitable Zones around Main Sequence Stars.” Now Ravi Kopparapu (Penn State) has worked with Kasting and a team of researchers to tune-up the older model, giving us new boundaries based on more recent insights into how water and carbon dioxide absorb light. Both models work with well defined boundaries, the inner edge of the habitable zone being determined by a ‘moist greenhouse effect,’ where the stratosphere becomes saturated by water and hydrogen begins to escape into space.

The outer boundary is defined by the ‘maximum greenhouse limit,’ where the greenhouse effect fails as CO2 begins to condense out of the atmosphere and the surface becomes too cold for liquid water. When worked out for our own Solar System in terms of astronomical units, the 1993 model showed the habitable zone parameters extending from 0.95 to 1.67 AU. Earth was thus near the inner edge.

The new model improves the climate model and works out revised estimates for the habitable zones around not just Sun-like G-class stars but F, K and M stars as well. The definition uses atmospheric databases called HITRAN (high-resolution transmission molecular absorption) and HITEMP (high-temperature spectroscopic absorption parameters) that characterize planetary atmospheres in light of how both carbon dioxide and water are absorbed. The revision of these databases allows the authors to move the HZ boundaries further out from their stars than they were before.


Image: An artist’s conception of Kepler-22b, once thought to be positioned in its star’s habitable zone. New work on habitable zones suggests the planet is actually too hot to be habitable. Credit: NASA/Ames/JPL-Caltech.

This looks to be an important revision, one that people like Rory Barnes (University of Washington) are already calling ‘the new gold standard for the habitable zone’ (see Earth and others lose status as Goldilocks worlds) in New Scientist. In Solar System terms, the limits now become 0.99 AU and 1.70 AU. We see that the Earth moves closer to the inner edge of the habitable zone, causing the authors to comment about an important part of their analysis, that it does not factor in the effect of clouds:

…this apparent instability is deceptive, because the calculations do not take into account the likely increase in Earth’s albedo that would be caused by water clouds on a warmer Earth. Furthermore, these calculations assume a fully saturated troposphere that maximizes the greenhouse effect. For both reasons, it is likely that the actual HZ inner edge is closer to the Sun than our moist greenhouse limit indicates. Note that the moist greenhouse in our model occurs at a surface temperature of 340 K. The current average surface temperature of the Earth is only 288 K. Even a modest (5-10 degree) increase in the current surface temperature could have devastating affects on the habitability of Earth from a human standpoint. Consequently, though we identify the moist greenhouse limit as the inner edge of the habitable zone, habitable conditions for humans could disappear well before Earth reaches this limit.

While the small change to the Earth’s position in the habitable zone is getting most of the press attention, I’m more interested in what the new numbers say about M-dwarfs. These small red stars would have habitable zones close enough to the star that the likelihood of a transit increases. The 1993 habitable zone work did not model M-dwarfs with effective temperatures lower than 3700 K whereas the new work takes effective temperatures down to 2600 K. In an article run by NBC News, Abel Mendez (University of Puerto Rico at Arecibo) mentions that Gliese 581d, thought to skirt the outer limits of its star’s habitable zone, may now move toward the habitable zone’s center, increasing the possibility of life emerging there. Other planets catalogued by the Planetary Habitability Laboratory at UPR will be affected as some thought to have been in the habitable zone may move out of it. See A New Habitable Zone for more.

There are other factors to consider about M-dwarfs, especially the fact that planets close enough to these stars to be in the habitable zone are most likely tidally locked, presenting the same face to the star at all times. Neither the 1993 model or this revised one does well at representing a tidally locked world and the authors say they have not tried to explore synchronously rotating planets in different parts of the habitable zone around M-dwarfs. The paper does note that a planet near the outer edge of the HZ with a dense CO2 atmosphere should be more effective at moving heat to the night side, perhaps increasing the chances of habitability.

The overall effect of adjusting our parameters for habitable zones around the various stellar classes will be to improve our accuracy as we look toward producing lists of targets for future space-based observatories. The authors note that the James Webb Space Telescope, for example, is thought to be marginally capable of taking a transit spectrum of an Earth-like planet orbiting an M-dwarf. We’ll need the maximum chance for success before committing resources to specific planets once we get into the business of trying to identify biomarkers on possibly habitable worlds.

The paper is Kopparapu et al., “Habitable Zones Around Main-Sequence Stars: New Estimates,” accepted at the The Astrophysical Journal (preprint). Note that a habitable zone calculator based on this work is available online. The 1993 paper is Kasting, Whitmire and Reynolds, “Habitable Zones around Main Sequence Stars,” Icarus 101 (1993), pp. 108-128 (full text).


coacervate January 31, 2013 at 11:33

W/resp to Aliens living within a HZ, you could say it is a kind of Grey Area.

How cold would Earth be in a Mars orbit?

Ronald January 31, 2013 at 13:08

Well, I think what is really remarkable about this new update of HZ boundaries is the fact that the target has not moved so much: inner and outer boundaries have just shifted outward by just 0.02-0.04 and 0.03 AU resp. This confirms the robustness of the model.
Telling is also the fact that Earth has moved even closer to the inner edge. Even if taking albedo etc. into account, and considering that the limiting (moist greenhouse) surface temperature was generously set at 340 K, there is clearly not a lot of room toward the inside. The last two sentences of the quote are particularly impressive from this perspective, also because “habitable conditions for humans” would probably be habitable conditions for any higher life.
Does this also imply that Earth’s ‘Multicellular (higher) Life Stage’, as discussed recently in a separate post, will end sooner, i.e. not some 0.5 gy from now, but only 0.3 gy? Or 0.1 gy?, Or …?

andy January 31, 2013 at 15:26

Other planets catalogued by the Planetary Habitability Laboratory at UPR will be affected as some thought to have been in the habitable zone may move out of it.

Well the PHL doesn’t even use the 1993 models of the habitable zone. Their model basically works by calculating the effective temperature at some assumed albedo and comparing the result to the average temperature of the Earth.

The “effective temperature looks nice” model of planetary habitability was what led to, among other things, claims that Gliese 581c was a good candidate planet for habitability when it is more strongly-irradiated than Venus…

Adam January 31, 2013 at 16:09

By “habitable zone” the modern definition means “able to have liquid water on the surface” not the earlier meaning of “able to sustain human life”. Over much of the “liquid water zone” an otherwise Earth-like planet would have an atmosphere dominated by carbon dioxide, making it incompatible with human life. An interesting consequence of this new analysis is that cooler stars with higher infrared levels push the habitable zone outwards. The very coolest stars would see their planets undergo a runaway greenhouse at just 80% of the insolation levels Earth receives.

Tarmen January 31, 2013 at 16:44

Plenty of binary systems have M class dwarfs that would have tidally locked planets, but a larger primary star in the the system might shine on the night-side of the M’s locked planet. This would ‘recover’ many of the M class tidally locked planets for consideration in hosting life of some sort. On the other hand, the larger star might ruin the planet completely.

FrankH January 31, 2013 at 17:04

This is a very good paper; if you’re interested in the subject, you should read it. It’s clearly written and I noticed as I read it that any questions I had were well addressed further in the the paper. The only weak point was the evolution of the moons of Jupiter, but that’s just a minor aside in the paper and not really relevant to the main topic.

@andy – I find the PHL is puzzling. All of their “habitable” planets may be habitable for fish (if the planets have water at less than critical temps) , but the crushing surface gravity and probably unhealthy atmospheres would not be very habitable for humans. The “Earth Similarity Index” for all of them should be close to 0. B

Mike January 31, 2013 at 17:30

One way of cross checking the new habitable zone model would be to use Earth meterological climate models and increase the insolation by, say about 2% and run the climate models to observe if any changes support the new HZ model’s predictions. Ofcourse that would apply only to Earth but I think it would provide some testing of the new HZ model.

We all understand that any HZ models can’t help but be a rough guide nevertheless we have to work with the data we got. Who knows what key details we are currently missing. For example, what possible influences a biosphere may have on HZ size and a planet’s duration in it?
Perhaps this lack of modeling for biological features makes the new and maybe the older HZ models somewhat inaccurate for determining Earth’s duration in the HZ with our gradually brightening Sun.

However I still think the new HZ model can be a step forward for exoplanet studies though. Particularily if we can gain a better understanding of HZ sizes around K and M dwarfs.

Astronist January 31, 2013 at 19:56

I do think this obsession with trying to find an exosolar clone of Earth is not a particularly edifying use of time. Yes, we have to work with what we know, as you said, but we also need to keep an open mind towards new possibilities. We are nowhere near the stage of confidently defining one zone of space as “habitable” and the rest of space, by implication, as “uninhabitable”; rather we are, I believe, still at the stage of finding out about the variety contained in the range of the possible.

Supposing, for example, that life is common subsurface on Europa-type worlds. Then life might be more commonly found in the “uninhabitable” zone than in the “habitable” zone. Better, I would suggest, to keep an open mind and a broad interest in specifying the surface and subsurface conditions on all exoplanets and exomoons, not just those that arouse the hope that they might turn out to be clones of Earth.

Further, if we find life on an Earth-analog planet, this will be fascinating, of course, but also unsurprising. But looking for life on analogs of Europa or of Jupiter or of Uranus (or indeed on those bodies themselves) would actually tell us more, because its presence or absence would constrain theories of the origin of life. We already know that life is present on at least one Earth-like world, so finding a second instance of this will not strongly constrain those theories any more than at present.

As Jack Cohen and Ian Stewart wrote in their book “Evolving the Alien”, “the astrobiological concept of a habitable zone is largely useless” (p.10).

Stephen, Oxford, UK

Wojciech J January 31, 2013 at 20:07

PHL announced that they will release a new HEC on February 16 in a public lecture, with the results available online on 18th, Monday.

Abel Méndez January 31, 2013 at 21:19

@andy The PHL uses the Kasting models of the habitable zones as described in Selsis et al. (2007). Equilibrium temperatures are only used for comparisons purposes. It is the NASA Kepler team that uses equilibrium temperatures for their habitable zone criteria.

coolstar January 31, 2013 at 21:19

I saw this paper a bit ago and also found it noteworthy for the minor changes in the original model. Better absorption spectra data (and models) really didn’t change things a lot. I’m also in the group that doesn’t really take PHL work very seriously. Many of the folks who comment here could do as well or better (as could any decent undergrad physics major).

jkittle January 31, 2013 at 21:41

The size of the planet will have an overwhelming influence on the atmospheric composition and surface temperature. Thus heavy planets will overheat up close, and will be more habitable further way than predicted by the model. I have seen no convincing argument that a planet 3 or 5 times earth mass cannot harbor life if it is in the right temperature range.. and it would be habitable outside of the standard Goldilocks region. similarly a small dense planet may be able to have live at a distance closer in. one person above asks, would earth be habitable at the distance of mars? what about a planet 3x earth? Its gravity would only be about 1.5 that of earth, depending on composition.

Eniac January 31, 2013 at 22:41

If the inner edge of the HZ is really defined by the “moist greenhouse effect, where the stratosphere becomes saturated by water and hydrogen begins to escape into space.”, then it would be highly dependent on the mass of the planet, because a higher escape velocity would exponentially inhibit hydrogen from escaping. Does this not completely invalidate the concept of a fixed boundary, much less one determinable to an accuracy of a few percent?

Rob Henry January 31, 2013 at 23:51

Astronist, I also loved Cohen and Stewarts “Evolving the Aliens”, but I think that even they realised that it had its uses.

There may be many solutions that generate HZ zone bands, and I think that validates models take Kepler data and generate surface temperature estimates directly from albedo, then see if water is liquid there at stp, without factoring in whether the atmosphere produced a greenhouse or a anti-greenhouse effect (as on Pluto and Titan).

A very special solution that generates traditional anthropocentric HZ’s takes a Venus/Earth analogues and places them at specific distances from the central star. And to me the major problem here is how poorly understood Venus is. Even at its extreme proximity to the sun, Venus has great difficulty loosing its deuterium. May I be so bold as to suggest that this effect might be retained if there was no lighter gas to compete with. In that case Venus would only need to be 2.8 times heavier (thus its escape velocity root 2 times higher) to retain most of its hydrogen, and, from Kepler data so far, it seems that most planets here are.

So these superEarths might still be susceptible to a moist greenhouse limit, but it might typically be far more gradual than our model before they leave the HZ.

Also, some work really should be done on HZ’s for tidally locked Earth analogues and ocean-worlds. I have a feeling that it will also turn out very different than the traditional HZ.

William Haloupek February 1, 2013 at 2:19

Can anyone tell me what the models predict for the HZ of Sirius B, or any white dwarf?

Mike Prather February 1, 2013 at 2:42

To expand on what Astronist said: if the Earth had formed at the orbit of Jupiter or farther out, and still had the same amount of cometary bombardment, it’s likely that radioactivity in the crust would have still kept enough ocean water liquid and allowed life to flourish where it already does – around deep ocean vents. Makes the current idea of a “habitable zone” woefully anthropomorphic.

I know we need to be looking for future planets to colonize but there definitely needs to be at least two classifications for habitability: one for humans – liquid water, the right ratio of gasses and range of surface gravity, and one that covers the extremes of life in general as we know it.

James Jason Wentworth February 1, 2013 at 4:40

These Habitable Zone studies raise a question: Since stars lose mass as they “burn” their hydrogen fuel, their planets should–however slowly–spiral outward as the gravitational fields of the stars weaken, even if the weakening amounts to only a few percent over stars’ lifetimes. If this is the case, could planets that are (as Earth apparently is) near the inner edges of their Habitable Zones “stay ahead of” the outward-receding inner HZ edges as their stars brighten with age? While this situation probably couldn’t remain that way indefinitely, it might lengthen the time that a given planet could support life, and perhaps even higher forms of life.

Ronald February 1, 2013 at 10:33

@Astronist: with all due respect for the validity of the rest of your comment, but the HZ as defined by Kasting et al. (both old and update) are not necessarily for a ‘clone of Earth’ or an ‘Earth-analog planet’, but purely for liquid surface water on a terrestrial planet. As you can see in the paper, the HZ is not very different even for a super-earth.
So, while what you say may be very true with regard to (subsurface) bacterial life in a broad sense, this present definition of the HZ is probably a good starting point for (water- and carbon-based) higher life, living at the surface and depending on stellar light.

ljk February 1, 2013 at 10:40

Amen to all that, Astronist. All the wonders of the Universe real and potential and we are looking for a copy of us. I am reminded once again of Stanislaw Lem’s comment in his novel Solaris about humans wanting to find mirrors of themselves, not other planets.

And those almost throwaway comments about humanity colonizing an Earthlike alien world without taking into account that a planet which resembles our globe in environment as well as size will do so due to having native life forms. Intelligent or not – and trying to determine that on alien species will be interesting in itself – we should not presume we can just set up a home there without consequences for both parties.

Imagine the reaction if an ETI thought Earth would make a great place to settle. Even if they did this with good intentions and concern for the local flora and fauna, the human response would be anything but coordinated and free of contention.

Ronald February 1, 2013 at 13:50

jkittle, Eniac: from what I understood the mass of the planet (and hence its atmosphere) is of significant influence toward the outside of the HZ, i.e. greater mass will indeed extend the HZ outward. However, toward the inner boundary a runaway effect is achieved very quickly (exponentially) and therefore (greater or smaller) mass will only be of very small benefit to extend the HZ inward.
JJ Wentworth: good question. However, a star like our sun brightens about 10% in a gy. To compensate for that, Earth would have to move outward some 5% during that same gy. I think that it way beyond the stellar weakening gravity effect, but I would like to be corrected here.

andy February 1, 2013 at 14:17

@Abel Méndez: the most visibly promoted habitability metric they use is the Earth Similarity Index (ESI). It is defined here and does in fact use the effective temperature rather than the HZ estimates.

@Astronist: well yes there may be other types of habitable worlds than Earthlike ones, but we must consider how we can detect the presence of life. At least with objects in the solar system we can reasonably consider sending a probe there and drilling through the ice. Finding life under the ice shell of a moon of even the nearest extrasolar cold giant planet (Gliese 832b*) is probably impossible without going there – even finding that a moon exists there is well beyond current observational capabilities. Icy super-Earths which also may be candidates for hosting subsurface oceans remain the province of microlensing observations, with distances measured in kiloparsecs, again not promising for follow-up (the possible exception being the imaging detection of Fomalhaut b, but there we’re seeing a dust cloud rather than the central object itself).

With an Earthlike planet on the other hand we have evidence that such planets can support life, and we have some idea of what to look for and how to design the experiments to detect the chemical signatures of life at long range.

* No I’m not forgetting Epsilon Eridani b: HARPS doesn’t see it.

Alex Tolley February 1, 2013 at 18:08

I don’t understand why the earth is so close the inner HZ. Is the HZ dependent on the age or current luminosity of the sun? We had snowball earth during our history and Kasting comments on a paper in Science v339, 6115 p64-66 concerning hydrogen (in methane) as the solution for why the earth was not frozen during its early history when the sun’s luminosity was lower.

At some point the earth must have been near the outer edge of of a possible HZ.

Is the HZ calculated on some optimum atmospheric composition and density?

Since climate scientists are not even able to model clouds well, how are these incorporated into the HZ models?

Steven Rappolee February 1, 2013 at 20:14

Table two of this paper states that 10 times are planets Co2 levels creates a moist green house effect,(2800 to 3200 ppm)
our increasingly britening star will create this “soon enough” :))
and the UN climate treaty task force states that 30 years from now if we do nothing, we will have 700 PPM
if past 30 years from now we exhaust every carbon molecule on the planet how close could we get to this 3200 ppm?
a great deal of money has been spent on research on climate change and we want to know more about exoplanets, we have a connection here!
we as a species want our remote decendents to do the things we dream of here
we need to leverage these things,
comparative Planetary science study’s partner with earth climate scientists,we do this now but at a low level.
are we sharing models of atmosphere with Venus and Mars in this paper and this to tells us that federal funding for exoplanet research might well tell us our fate?
my thoughts are that the decadel study group may have underrated Venus mission of for the next opportunity Discovery mission but the MAVEN mission to Mars may tell us more about, “early mars”
more cooperation with climate study’s and planetary scientists needed

coolstar February 2, 2013 at 4:51

@JJW the proton-proton cycle has a mass to energy efficiency of only about 0.007. This equates to the sun losing about 5 x10^-14 of it’s mass per year at it’s current luminosity. The orbital radius of a planet is proportional to 1/M(star), to a very good approximation. Over the main sequence lifetime of the sun the earth’s semi-major axis will change very little due to this (or any other) mass loss (it works out to be on the order of 0.0005 AU). Gravitational perturbations due to Jupiter (mostly) already dwarf this.

By the time the sun becomes a white dwarf it’s mass will be around 0.7 of it’s current value, so that clearly will induce substantial changes in the orbits of the surviving planets. Earth will either not survive or be in a very tight orbit around the white dwarf. The data seems to favor not surviving since no total (or nearly so) eclipses of white dwarfs by low mass objects have yet been observed. There’s probably a bit of wiggle room in that last sentence due to the faintness of white dwarfs as well as the lack of continuous photometry on a lot of them. To my knowledge no one has done a rigorous calculation of how large “a bit” is but there is at least one amateur observational campaign underway to look for such eclipses. There’s also been a lot of high time resolution photometry of white dwarfs to look for pulsations with no such eclipses yet observed.

coolstar February 2, 2013 at 5:37

May be substantially more wiggle room than I thought concerning terrestrial sized planets in close orbits around white dwarfs. see http://arxiv.org/pdf/1009.3048v1.pdf, which I had missed. Nice paper, but still didn’t do a rigorous simulation of what fraction of the parameter space had been searched already. They do say that several thousand white dwarfs would need to be monitored, and that hasn’t happened yet. LSST should find them easily, if they exist, and maybe Pan-STARRS also.

coolstar February 2, 2013 at 5:54

My memory must be going! Eric Agol had done a rigorous study of terrestrial planets in the CHZ around white dwarfs here: http://arxiv.org/pdf/1103.2791v2.pdf. And I think this paper had even been discussed on Centauri Dreams (and I certainly remember reading it, NOW). my apologies. Oh well, at least the first paragraph of my original comment is correct.

david spencer February 2, 2013 at 10:42

I wonder, if the early sun was just 70 percent as intense as it is now and even the earth needed a stronger than current greenhouse effect to maintain a liquid ocean then how did mars have free flowing water? It’s “usually” frozen solid now at 590 w/m2, and would have had just 413 w/m2 back when the sun was young.

How massive could the early mars atmosphere been before it was stripped away and how much of a greenhouse effect could it have created? Would the difference in albedo have made much of a difference? What is the minimum size mars would have to be in order to have maintained liquid water to this day? If mars had, say 3x the current mass, would it be inhabitable today, giving rise to a space race to colonize it?

FrankH February 2, 2013 at 16:35

@david spencer – The solar wind can only erode away about 1/3 of the Martian atmosphere; the rest has been lost through escape, impacts, locked up in the soil, frozen out or just wasn’t there to begin with.

Kasting et al 1993 paper (linked above) guesses “several times more massive” for Mars to retain a significant atmosphere.

Using the gas retention plot: (http://astro.unl.edu/naap/atmosphere/animations/gasRetentionPlot.html)

Barring impacts and solar wind effects that would have eroded or heated the atmosphere, a Mars 20% – 25% larger ( or 75% more massive, assuming the same density) might be able to retain water vapor.

jkittle February 2, 2013 at 16:38

Early earth would have certainly had methane and ammonia and these are terrific green house gasses. The O2 we have now is really not so much a consequence of photosynthesis as it is hydrogen loss . This is not to say plants do not create oxygen, it is just that they also decompose back to water and carbon dioxide. The amount of reduced carbon in the crust ( coal etc.) does not account for the tremendous amount of oxygen in the atmosphere. if it were close we would be worried about OXYGEN DEPLETION and not CO2 build up.
Thus a cooler early sun would still work for liquid water somewhere on earth.
In my opinion the discussion of habitable zones is not so much about finding places to live, as to understand how rare earth is in the cosmos. By the time we are bale to travel to these places, out technology and lifestyle should have evolved to the point where space colonies using materials from dwarf planets will be as attractive as being stuck in a deep gravity well of a planet with an atmosphere. For now, I am just excited that We were right/ there are a LOT of planets out there and they are not all around earth- like stars. EVEN ONE of these worlds will have its own value for study and exploitation, and so imagine how much scope for fun, fortune and folly exists in our universe.

Eniac February 3, 2013 at 1:11


The O2 we have now is really not so much a consequence of photosynthesis as it is hydrogen loss .

I don’t think this is correct. The oxygen in the atmosphere turns over in a few million years, and hydrogen loss is not a significant source at all. Carbon burial is.(http://www.ess.uci.edu/~reeburgh/fig2.html)

The amount of reduced carbon in the crust ( coal etc.) does not account for the tremendous amount of oxygen in the atmosphere. if it were close we would be worried about OXYGEN DEPLETION and not CO2 build up.

This logic is faulty. The reason we are not worried about oxygen loss is that the few dozen ppm we are worried about are a substantial fraction of the 300-400 ppm CO2 in the atmosphere, but completely insignificant compared to the 20% oxygen (200,000 ppm).

Also, should not, according to this theory, Venus have tons of oxygen, having lost much more hydrogen than Earth?

I think what we observe is consistent with a primordial atmosphere of CO2 being converted to buried carbon and free oxygen by photosynthesis. Reality, of course, is much more complicated, but I think in first order this makes more sense than the hydrogen loss model.

Rob Henry February 3, 2013 at 4:36

FrankH, I think that your last comment further illustrates how we may be on the verge of a paradigm shift away from the old one you still cling to: namely that a Mars sized body is too small to hold a large atmosphere.

Today spluttering is the best non-thermal mechanism that we can invoke to explain modern Martian loss, and extrapolating this back it comes up far short (as you mentioned). Jeans loss is even less significant over the same period. So where do we build up to the high concentrations of H2 necessary for hydrodynamic escape? I can’t see it, so that leaves comet/meteorite impacts and, after all, Mars would have been in a region of Sol with a very high impact intensity. But here is my problem…

Large impacts strip an entire cap of atmosphere as if it were truncated by a line to the horizon. This means that if a planets diameter divided by its scale height is constant, the same proportion of a planets atmosphere will be striped PER IMPACT. For Mars this ratio is only 2.4x higher than Earth’s, yet Earth has 3.5x the surface area and a far higher gravitation focusing effect to boot. To cut to the chase, its not at all obvious that Earth would have done much better, if at all, given a similar meteorite flux.

Rob Henry February 3, 2013 at 4:58

Jkittle, I fear that you have made the same mistake as me when I first came to this subject. Texts might concentrate on fossil fuels, but it is their precursor, kerogen, that is really all important here.
From that Wikipedia article above there is about 3000x as much carbon locked up as kerogen than as fossil fuel. I do not know its typical oxidation number or the quantity of hydrogen it binds, but if it is similar to coal, this 15 million gigatons of it represents 40 million Gt of O2 released from carbonate or CO2. If it is more similar to oil, this represents 60 million Gt of O2. The current atmospheric inventory is just 11.1 million Gt O2.

Ronald February 3, 2013 at 15:47

jkittle, Eniac: not only is Eniac on the right track with his photosynthetic oxygen turnover statement, it is even much more extreme: present photosynthesis on Earth turns over the entire O2 content of the atmosphere in only about 3000 years!
(quick rationale: the atmosphere weighs about 5*10^15 tons, O2 is about 20% = 10^15 tons, photosynthesis produces about 300 gigatons of O2 per year = 3*10^11 tons, i.e. total turnover in 10^15 / 3*10^11 = about 3000).

And yes, as Rob Henry is also saying, the amount of fossil fuel C in the earth is actually quite tiny (estimates vary from 4000 – 10,000 gigaton of C) in comparison with all organic rock, such as limestone, dolomite, shale (containing kerogen), etc., which is estimated at at least 75 million gigaton of C.

And from what I have understood a planet (of roughly earthlike density) needs to be at least 1/3 Me to retain a significant atmosphere long-term, preferably 1/2 Me. Therefore various researchers take this as a minimum for an earthlike planet.

Rob Henry February 3, 2013 at 19:14

Ronald says “And from what I have understood a planet (of roughly earthlike density) needs to be at least 1/3 Me to retain a significant atmosphere long-term, preferably 1/2 Me.”

And I agree that that is the current thinking. What I disagree with is that it is firmly rooted in theory. Indeed, I’m not entirely sure that theory and fact can be forced to reconcile with this.

Rob Henry February 3, 2013 at 19:27

Ronald, even as FrankH alluded to above, impact erosion is starting to dominate thought as to how Mars lost its atmosphere. What I am looking for is any model or study of it that compares how Earth would compare given similar fluxes. If you know of one I would be grateful, especially since I have the horrible feeling that it has always just been assumed that Earth would do better.

jkittle February 4, 2013 at 9:51

Limestone is oxidized carbon on earth and for Venus we have completely oxidized carbon in the atmosphere… and completely oxidized sulfur. On Venus, It may be that the oxidized carbon is not fixed in crust like it is on earth.
“kerogen” may be the reduced form of carbon left over for photosynthesis, but I am not so sure there is sufficient in the crust to account for the hyge amount of oxygen.
Methane is HIGHLY reduced ( carbon -4) compared to CO2 ( Carbon +4). if we had not lost all that hydrogen even photosynthesis would not be creating partially oxidized carbon like Aromatic hydrocarbons or long chain alkanes , which are more oxidized than methane. and “turning over ” the O2 is not the same as a steady state oxidation level. We also turn over carbon dioxide many time in that same period but it is still accumulating.

andy February 4, 2013 at 15:39

Talking of atmospheric loss, here are some new results which suggest that super-Earths typically retain their primordial hydrogen-rich atmospheres.

Royal Astronomical Society: Are super-Earths really mini-Neptunes?

FrankH February 4, 2013 at 17:04

@Rob Henry – Please quote one paper anywhere that supports your theory that a Mars-mass object well inside a star’s HZ can support a substantial, stable atmosphere over billions of years.
If you read the papers above (including Kasting’s 1993 paper) it’s clear that a Mars sized object is not a good choice for a habitable planet anywhere in the HZ.
Jean’s escape is basically a kindly grandma’s level of atmospheric escape; the 6x factor is just a rule of thumb assuming no other external events beyond isolation heat up or disturb the atmosphere. Even with just the simplest application of Jean’s escape, Mars would have lost significant amounts of water over its history.

UV disassociation could easily break down any remaining water and hydrodynamic escape of hydrogen and other gases can carry away a significant amount of atmosphere. Solar wind sputtering (which was probably much greater in the past than it is now) would remove a significant amount both by heating and mechanically.

Currently, up to half of Mars’ atmosphere is frozen as CO2 in the polar caps.
The pressure cycles from about twice current to almost a hard vacuum over long periods of time as its orbit changes the amount of sunlight that reaches the poles. It’s probably looses significant amounts of atmosphere every cycle.

I only mentioned impacts because that’s one way of heating up an atmosphere locally to the point that the gases reach Mars’ weak escape velocity. I haven’t seen any papers to support this, but impacts do figure in competing theories to the “warm wet” early Mars, although mainly to explain the erosional features… I just extended the concept.

Earth has more than twice the escape velocity of Mars – you really would have to heat up the atmosphere beyond reasonable limits to cause a sizable escape – at least to 900K or more.

Ronald February 4, 2013 at 22:12

Off-topic again, but quite interesting (didn’t andy or Adam say something similar recently?):
Are Super-Earths Actually Mini-Neptunes?

The referred article is: Helmut Lammer, N. V. Erkaev, P. Odert, K. G. Kislyakova, M. Leitzinger, M. L. Khodachenko. Probing the blow-off criteria of hydrogen-rich ‘super-Earths’. Monthly Notices of the Royal Astronomical, 2013.

“These planets are actually surrounded by extended hydrogen-rich envelopes and that they are unlikely to ever become Earth-like. Rather than being super-Earths, these worlds are more like mini-Neptunes”.

“Unlike lower mass Earth-like planets many of these super-Earths may not get rid of their nebula-captured hydrogen-rich atmospheres (…).
Rather than becoming more like Earth, the super-Earths may more closely resemble Neptune (…). Super-Earths further out from their stars in the ‘habitable zone’ (…) would hold on to their atmospheres even more effectively. If that happens, they would be much less likely to be habitable.”

Eniac February 4, 2013 at 23:30

jkittle: It is certainly correct that a low level of hydrogen is needed to support oxygen in the atmosphere. The planet needs to pretty thoroughly oxidized before any O2 can accumulate. If there was H2 or CH4 around, it would, of course, react immediately with any free oxygen. But, the same is true for carbon. It takes removal of both carbon and hydrogen to create free oxygen, and the removal of carbon has been overwhelmingly biological. This means, in all probability, lifeless planets will have either hydrogen and methane rich atmospheres, or ones dominated by CO2. This is what we observe in our system, and it is likely to hold true anywhere else. Unless, perhaps, in systems where there is even more oxygen than here. If oxygen is stoichiometrically in excess of silicon, all other metals, carbon, and hydrogen, all combined, it may exist in free form, I suppose.

Rob Henry February 5, 2013 at 7:23

FrankH asks, “Please quote one paper anywhere that supports your theory that a Mars-mass object well inside a star’s HZ can support a substantial, stable atmosphere over billions of years.” And that is rather reasonable, even though I have never made such a claim.

Firstly I should point out that I feel that the hypothesis that a Mars sized body definably could hold such an atmosphere long under typical HZ circumstances would be every bit as ill-founded as the opposite view given current data. My claim has only ever been that theory allows that possibility, and the facts are consistent with it.

I should also point out that I have previously mentioned that loss due to impacts can only bolster the current paradigm if they are compared with losses an Earth-like planet would experience. Additionally, weathering and sequestration in polar caps has its limits in a HZ planet of low geological activity, and one that had high activity could retain a magnetic field and resist spluttering. Note then that those particular mechanisms for the loss of Martian atmosphere might not be transferable to most other cases.

Thus I will answer that by asking you to point to any paper that has ever felt that it has shown confidence in explaining how Mars could have eroded from a massive atmosphere to 600Pa or gives reference to a HZ Mars-sized planet being unable to hold its atmosphere (rather than the lesser claim that it is currently thought to be unlikely to).

Ronald February 5, 2013 at 10:02

FrankH: I fully agree.
“Currently, up to half of Mars’ atmosphere is frozen as CO2 in the polar caps”.
This also raises the problem where we will get all the necessary N2 from, as a major inert component of an earthlike atmosphere, when terraforming Mars. Is it still present in the (polar) soil as well or has it largely escaped?

Ronald February 5, 2013 at 10:06

And this interesting discussion regarding atmospheric escape, combined with the new publication referred to by andy and myself, again emphasizes the importance of truly terrestrial planets (roughly 0.5 – 2 Me?) in the HZ of (solar type) stars as prime targets for (higher) life.
But I am sure that Paul will do something with that publication soon :-)

jkittle February 5, 2013 at 11:36

How does the temperature of the star influence dissociated hydrogen loss , or even atmosphere loss ( CH4 or O2) . Is it possible that while a Mars-size world is a bit small to be a viable habitat in the “habitable zone” around a yellow sun like ours, perhaps it would retain a thicker atmosphere if it is around a lower temperature sun ( orange or red) . The lower energy spectrum can still heat the atmosphere ( at the right distance) but not provide a lot of high energy photons to “kick” the individual molecules out of the ionosphere. Of course the HZ would be closer in. Conversely , a hotter sun with is higher proportion of UV , may strip the atmosphere from an earth sized planet in its HZ while a 2X super earth orbiting a hot star, may be a better candidate for a truly habitable planet.. Bottom line.. you need an atmosphere, not just the right amount of solar energy input, to be habitable… an atmosphere is even more important then temperature… no liquid water without some air pressure!

Rob Henry February 5, 2013 at 15:24

Frank, I should have started the ball rolling with a quote. Take this one from a Science abstract “the fate of its ancient carbon dioxide atmosphere is one of the biggest puzzles in martian planetology”

From your current position it might seem strange how such an prestigious journal published anything so wrong. It seems that they don’t realise how easy it is to explain atmospheric loss on such a small body. Perhaps you should contact them. (I’m trying to point here to the subtleties you have missed – not being facetious)

Rob Henry February 6, 2013 at 14:53

Jkittle, I agree that it is possible that around some stars Earth sized planets might be too light to hold a HZ zone atmosphere, and that minimum size must vary with star type. Here it is worth noting that Earth’s exobase temperature has been measured to be highly dependant on the phase of the solar cycle. But what is the typical life-bearing case? After all our Sun is an unusually big and hot star.

FrankH February 6, 2013 at 16:45

@Rob Henry

It’s a good paper, since it puts some real data to go with existing theory. The discrepancy between the putative multi-bar atmosphere and what we have now has been known since at least the 90s.

I think this paper is a bigger blow to the “OMG! TEH Sun blew away Mars’ atmosphere because it didn’t have a magnetic field! Think of the children!” meme.
The paper ONLY looked at solar wind erosion, calculated backwards from in-orbit measurements.


“The estimated total losses of carbon dioxide and water are much lower than the few hundred meters of H2O and 1 to 5 bar of CO2 required for the “wet and warm early Mars” model. Therefore, either other escape channels were or are operational, e.g., impact atmospheric removal, photochemical or cold plasma/bulk escape, or water and carbon dioxide are stored in nonidentified reservoirs.

We emphasize that this study considers only one channel for the escape to space. The determination of the escape rates in the form of cold plasma clouds (bulk escape) or via sputtering and photochemical reactions is beyond the scope of the ASPERA-3 experiment capabilities.”

end quote

How to explain the discrepancy?

1 – Include “normal” losses due to Jean’s Escape and hydrodynamic loss. Also take into account the variability of the Martian atmosphere due to its orbital eccentricity. It’s probably not a linear loss with time.

2 – Calculate the atmospheric loss caused by the massive impact in the Northern Hemisphere during the Noachian. It probably lead to significant atmospheric heating (and pretty blunt force mechanical removal). The many, many other impacts the preceded and followed it didn’t help.

As always, the culprit here isn’t just the impacts, it’s Mars’ low escape velocity. Heat up the atmosphere just a bit (and consistently, over millions of years) and you’re going to loose most of that gas to space. It’s very a shallow gravity well compared to the Earth (or Venus).

3 – Other reservoirs for water – Mars has HUGE underground reservoirs of ice as well as a significant reservoir of CO2 in the poles. If the water wasn’t melted or vaporized by impacts, it’s meters to 10s of kms under ground. That ice is not coming out – at least not until the Sun goes into a red giant phase. If any does before then, it will sublimate and eventually, leave.

4 – The “warm wet” Mars is ONE theory to explain the erosional features caused by water, early in the history of Mars. Others (including impact surges – see D.Burt and L Knauth) while not popular, can explain most of the features without invoking a thick, warm atmosphere and plentiful liquid water. “Ground truth” on Mars hasn’t always matched the warm wet theory, since there are vast exposures of ancient rock that would have been modified by a wet environment, yet they’re not.

I think the view is leaning more towards “mostly cold and dry except for some early panic-y periods when all hell broke loose and there were biblical floods everywhere” model, but I think they’re still working on a shorter name for the theory.

What I believe is that even without impacts, Mars would at best have a somewhat thicker atmosphere but probably not thick enough to support liquid water on the surface (the water vapor would escape, anyway). Warm Mars just slightly (by bringing it closer to the Sun) and that atmosphere would be mostly gone.

@Ronald – The ratio of Nitrogen isotopes (N15 to N14, measured by Viking) shows that over 90% must have been lost to space. There is very little Nitrogen anywhere on Mars.
Also, I agree that the so called “super-Earths” are just “mini-Neptunes”. I may be too conservative, but I won’t get excited about an exo-planet until it is in the 0.75 – 1.25 Earth mass range, silicate and in the star’s HZ.

Rob Henry February 7, 2013 at 15:15

FrankH, have you ever seen “The Paper Chase” and heard its memorable line “You come in here with a skull full of mush, and, if you survive, you leave thinking like a lawyer.” Here the point is you can know everything and its immediate meaning, but you still need to see the bigger picture to finally arrive.

You’re right, that whether Mars was ever warm and wet is germane to the question in hand. And your right that this idea is currently in favour. If it was once clement, some models suggest that this could have required up to 300,000Pa of CO2. Even if we dial up all potential loss mechanisms to the maximum limits that theory currently allows to be remotely possible, no one has been able to model how such atmospheric loss might be possible. Mars is just too massive for that. Thus they tend to assume a base (usually in the late Noachian) around 50.000Pa to 100,000Pa. Currently it seems the best they can do.

However, if Mars was never warm and wet, we can start from a piddling 5,000Pa or so. In that case, had we actually started from a low base. Not only would implied loss then become lower, but most of these loss mechanisms are not proportionate to original density. Models would then swing heavily in favour of retention.

If we really did start with 5,000Pa, and carry the implications of this forward, then had actually had 300,000Pa, atmospheric loss would likely be more in the order of that required just to compensate for the tendency of solar insolation to increase with time (or less).

Thus, its not at all obvious that a Mars analog (but with a denser atmosphere) might be a better candidate for the typical life-bearing planet than Earth.

Ronald February 11, 2013 at 11:22

FrankH; thanks,
with regard to N on Mars: this confirms my concerns about terraforming, and the atmospheric retention limitations, of a smallish planet like Mars, the absolute lack of N.
with regard to mass of earthlike planets: maybe you are a bit too pessimistic indeed, about the 0.75 – 1.25 Me mass range, Kepler, HARPS and other researchers usually consider about 0.5 – 2 Me as a safe range for an earthlike planet, or even a bit more, also according to the retention plot mentioned by you.
with regard to “super-Earths” are just “mini-Neptunes”: yes, probably. although another recent study also suggests that there still are two distinct groups of sub-Jupiter planets: DENSITY AND ECCENTRICITY OF KEPLER PLANETS, by Wu and Lithwick.

We separate the planets into two distinct groups, \mid-sized” (those greater than 3Re), and \compact” (those smaller). All mid-sized planets are found to be less dense than water and therefore contain extensive H/He envelopes, likely comparable in mass to that of their cores. We argue that these planets have been significantly sculpted by photoevaporation. Surprisingly, mid-sized planets, a minority among Kepler candidates, are discovered exclusively around stars more massive than 0.8Msol.
The compact planets, on the other hand, are often denser than water. Combining our density measurements with those from radial velocity studies, we find that hotter compact planets tend to be denser, with the hottest ones reaching rock density. Moreover, hotter planets tend to be smaller in sizes. These results can be explained if the compact planets are made of rocky cores overlaid with a small amount of hydrogen,  1% in mass, with water contributing little to their masses or sizes.
Photoevaporation has exposed bare rocky cores in cases of the hottest planets. Our conclusion that these planets are not water-worlds contrasts with some previous studies.
While mid-sized planets most likely accreted their hydrogen envelope from the proto-planetary disks, compact planets could have obtained theirs via either accretion or outgassing. The presence of the two distinct classes suggests that 3Re could be identified as the dividing line between `hot Neptunes’ and `super-Earths.’

So, summarizing, there are the ‘real’ Neptunes with a rocky core and a massive gaseous envelope, and the ‘super-earths’ or rather mini-Neptunes with a rocky core and a much lighter gas envelope. They may (partly) have different formation histories.

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