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traceur
2013-Jun-03, 04:11 AM
This is something that has bugged me for quite sometime:

How far can a Venus-like planet be from the sun and eventually capture enough heat to start forming liquid water? How much further could it be if we added increased volcanic activity into the equation, releasing energy into the heat-trapping atmosphere from within the planet itself? Are there no other heat-releasing chemical reactions that an alien weather system and geology could support?

Within the spectrum of possible (plausible?) planets, it's seems there are many heat generating & trapping mechanisms that could go beyond the goldilock zone into supporting liquid water (I.E Europa).I would like to note that I'm not necessarily making an exolife-optimistic proposition here, the same works in reverse: put any planet that has such mechanisms within the goldilock zone, and wouldn't it become too hot to support liquid water (I.E Venus itself)?

Why then are we insisting on the goldilock zone? What makes earth-like planets more common than Venus-like planets or Europa-like moons?

korjik
2013-Jun-03, 04:53 AM
Isnt the definition of 'Goldilocks Zone' basically on an Earth-like planet liquid surface water can exist? The point seems to me to find planets where humans could live more than where life could live.

Selfsim
2013-Jun-03, 06:51 AM
This is something that has bugged me for quite sometime:

How far can a Venus-like planet be from the sun and eventually capture enough heat to start forming liquid water? How much further could it be if we added increased volcanic activity into the equation, releasing energy into the heat-trapping atmosphere from within the planet itself? Are there no other heat-releasing chemical reactions that an alien weather system and geology could support?

Within the spectrum of possible (plausible?) planets, it's seems there are many heat generating & trapping mechanisms that could go beyond the goldilock zone into supporting liquid water (I.E Europa). Furthermore, put any planet that has such mechanisms within the goldilock zone, and wouldn't it become too hot to support life (I.E Venus itself)?

Why then are we insisting on the goldilock zone? What makes earth-like planets more common than Venus-like planets or Europa-like moons?Good questions ...

The Goldilocks Zone, (Circumstellar Habitable Zone), tends to be bounded (in definitional terms) by the 'Frost Line' (~5AUs), beyond which hydrogen compounds condense into solid ice-grains. The idea here is to separate 'terrestrial-like' from 'Jovian-like', but the 'Frost-Line' and 'Habitability Zone' terms, add little value when we already know that hot Jupiters exist inside the Frost-Line. These 'definitions' are extremely crude, and only 'work' at the broadest of current levels for classifying planets outside our own system. (Where our telescopes are stretched in resolving capability). They are primarily derived from what we currently know of conditions within our own Solar System ... which tells us almost nothing about what other 'systems' might exist elsewhere, or the conditions within their own planetary environments.

As far as I can see, these terms are arbitrarily chosen to inspire, whilst constraining the imaginations of those who seem to dwell over the semantics of other 'definitional' terms such as: 'likely', 'unlikely' and 'the possibilities' of 'exo-life'.

They may also serve as an arbitrarily chosen starting point for counting 'exceptions to the rule', thereby incrementally increasing the boundaries of what is currently known as 'other' information becomes available. But from what I can see, this approach will always be incomplete, (by definition), and will always be subject to unknown uncertainties as observational boundaries increase. The more hopeful might then seek to establish some kind of 'trends' and, (yet again), draw some kind of (flawed) statistical inferences, I suppose.

neilzero
2013-Jun-03, 08:00 AM
Clearly very advanced beings can live almost anywhere. I had not heard as far into the cold as Selfism suggested before, but I agree, moderate amounts of internal heat from the planet can make slightly below the surface habitats a comfortable temperature, perhaps even Europa.
About one million heat releasing chemical reactions are know, but most of them seem improbable for keeping a planet warm.
I'll guess a thousand heat trapping chemicals are known that could be in the atmosphere, but many of them are not very compatible, such as CFCs destroy ozone.
We can only guess what Venus was like billions of years ago and what caused Venus to be like it is.
If we moved present Venus to a circular orbit at 1.3 AU, it would take many centuries to cool to Earth average temperatures, and possibly would not cool further due to the thick carbon dioxide atmosphere.
Possibly it would rain sulfuric acid after the first century, and rain liquid carbon dioxide one million years after the move to 1.3 AU. Large lakes of liquid carbon dioxide are likely, but the trace of present water would be dissolved in the liquid carbon dioxide. Apparently nearly all the water Venus likely had 4.4 billion years ago was lost into space and will return only with the help of terraformers Neil

eburacum45
2013-Jun-03, 11:46 AM
The outer edge of the 'habitable zone' is defined by the outer edge of the liquid water zone, but this is affected by the amount of greenhouse gas in the atmosphere. Unfortunately there are several different estimates of the effect of greenhouse gas (usually CO2) on this liquid water zone. Kasting 1993 calculated the limit at which carbon dioxide would form clouds of dry ice crystals, increasing the albedo and cooling, rather than warming the planet. For a planet orbiting a Sun-like star this limit seems to be 1.37 AU, but he admitted that a planet with a thicker atmosphere could retain liquid water further out.
Habitable Zones around Main-sequence Stars (http://www3.geosc.psu.edu/~jfk4/PersonalPage/Pdf/Icarus_93.pdf)(pdf)

If we consider very large super-earth type planets the outer edge of the habitable zone could be considerably further out. However a super-earth with a very thick CO2 atmosphere and a very high gravity might be (to use Martyn Fogg's term) 'biocompatible' but it would be extremely uncomfortable for humans.

Tidal heating could also move the habitable zone outwards; this effect is probably more significant around red dwarf stars, where the planets orbit very close to the star (and each other). Once again, a planet with high levels of geological stress leading to heating might be biocompatible, but they wouldn't be very comfortable.
Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating (http://arxiv.org/abs/1203.5104/)


Tidal heating should be significant for moons of gas giants far outside the conventional habitable zone.
Habitable Moons around Extrasolar Gas Giants (http://personal.psu.edu/raw25/11-Labs/williams+97.pdf)(pdf)

An icy moon like Europa could have a liquid ocean under the ice, and even support lifeforms using infra-red photosynthesis. See Perez et al.

The potential for photosynthesis in hydrothermal vents: a new avenue for life in the Universe? (http://arxiv.org/abs/1304.6127)

Note that the fact that a planet with a particular mass in a favourable location might contain liquid water, but that doesn't mean it inevitably does. A planet with water might hold life, but we don't know how common life is in the universe; and finally a planet in the habitable zone with liquid water and even with life is not guaranteed to be comfortable for humans (by a long chalk).

Selfsim
2013-Jun-03, 10:58 PM
So the term 'Goldilocks Zone' loses its meaning with each successive post here.

Perhaps the 'Goldilocks' part of the term serves as a reminder of just how much of a fantasy the concept really is(?)

The original concept was used as a first order approximation as a way of narrowing the astronomical part of the search for more of ourselves, elsewhere in the observable universe. But by discarding observed exo-planets which might not fall into the 'Goldilocks Zone', vast numbers of potential habitats may be being discarded as well.

It seems that the Astrobiology 2008 Roadmap (https://astrobiology.nasa.gov/roadmap/) has already considered such issues:

The Astrobiology program seeks to expand our understanding of the most fundamental environmental requirements for habitability. However, in the near term, we must proceed with our current concepts regarding the requirements for habitability.

That is, habitable environments must provide extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism. Habitability is not necessarily associated with a single specific environment; it can embrace a suite of environments that communicate through exchange of materials. The processes by which crucial biologically useful chemicals are carried to a planet and change its level of habitability can be explored through the fields of prebiotic chemistry and chemical evolution.So, if habitability "is not necessarily associated with a single specific environment", what practical use does the concept represent in narrowing the astronomical search for it?

A major long-range goal for astrobiology is to recognize habitability beyond the Solar System, independent of the presence of life, or to recognize habitability by detecting the presence of life (see Goal 7: Biosignatures). Other than colonisation, I cannot see the practical point of "recogniz(ing) habitability beyond the Solar System, independent of the presence of life" (by some undisclosed means). Quite simply, if 'bio-signatures' cannot be detected from a remote exo-planet, (for whatever 'reasons'), then the concept of 'Habitability' of a remote exo-planet, contributes almost zero value to anything of practical use, (due to the inability to retrieve the necessary evidence). And if the bio-signatures are absent from whatever observations are made, then the underlined part is also rendered redundant.

It seems that this entire goal hinges on the hope of detecting some unambiguous bio-signature. And yet, the complexity of doing that unambiguously, locally in a known 'high probability' habitable zone, (like on Mars) is still an enormous stretch. And if one considers the disputed outcome of something like the Viking LR positive results, (which ain't a bad life test), the statements themselves become almost meaningless.

This goal directly addresses the call for searches for Earth-like planets and for habitable environments around other stars that is included in the Vision for Space Exploration .. I'm sorry .. but this is a circular argument of monumental proportions and for it to appear in the introduction to the first goal of NASA's Astrobiology Roadmap .. well, need I say more?

The point is that both habitability ('Goldilocks Zone', etc) is no indication of anything I can see, which is of practical value in the 'hunt' for what 'lives' in some 'zone'. The Astrobiological search for the alternatives (bio-signatures), also seems like 'word-salad'.

Simple Exploration of whatever we can reach, is the only practical way of optimising the search I can see.

ASTRO BOY
2013-Jun-04, 09:22 AM
Isnt the definition of 'Goldilocks Zone' basically on an Earth-like planet liquid surface water can exist? The point seems to me to find planets where humans could live more than where life could live.



Agreed........Life elsewhere could live in many conditions/variations of geology where liquid water does not exist on the surface.......
It's a starting point though, and probably the most familiar where we as a species are able to readily draw the most probable of assumptions from.
Remembering that primitive life forms can also live within solid rock deep within the bowels of the Earth, and even in and around spent nuclear fuel rods and in temperature extremes near that of boiling water.

Paul Wally
2013-Jun-05, 10:06 AM
This is something that has bugged me for quite sometime:

How far can a Venus-like planet be from the sun and eventually capture enough heat to start forming liquid water? How much further could it be if we added increased volcanic activity into the equation, releasing energy into the heat-trapping atmosphere from within the planet itself? Are there no other heat-releasing chemical reactions that an alien weather system and geology could support?

Within the spectrum of possible (plausible?) planets, it's seems there are many heat generating & trapping mechanisms that could go beyond the goldilock zone into supporting liquid water (I.E Europa).I would like to note that I'm not necessarily making an exolife-optimistic proposition here, the same works in reverse: put any planet that has such mechanisms within the goldilock zone, and wouldn't it become too hot to support liquid water (I.E Venus itself)?

Why then are we insisting on the goldilock zone? What makes earth-like planets more common than Venus-like planets or Europa-like moons?

I find the phase diagram of water (http://en.wikipedia.org/wiki/Triple_point) quite a useful tool for understanding and exploring the concept of habitable zone. The concept has more to do with the physical properties of water than it has to do with life, the connection between water and life being only an implication of the idea that liquid water (within a certain temperature range) is sufficient for habitability. I don't think it should be too difficult to calculate a habitable zone for some ideal test planet with clearly defined atmospheric properties, by using the phase diagram of water and solar flux as a function of distance from the sun.

Selfsim
2013-Jun-05, 11:06 AM
I think at the end of the day, the "Goldilocks Zone" concept doesn't really feature in the real planet-hunter's dictionary.
(It features more frequently in media releases and 'outreach' publications).

Terms like: "Earth Similarity Index, Standard Primary Habitability, Habitable Zone Distance, Habitable Zone Composition, Habitable Zone Atmosphere, Planetary Class and Habitable Class", are used to rank exoplanets more formally.

The astronomical parameters (& beyond) used in defining each of these categories, are explained here. (http://en.wikipedia.org/wiki/Standard_Primary_Habitability#Methodology)

Local Fluff
2014-Jan-19, 10:18 AM
HZ is a statistical concept for one factor (the one which can be measured today). It should be modified with whatever model one has for atmosphere, tidal heating et c. about which we know little for any individual exoplanet. "Frost-line" for water would be a better term, I think, but it wouldn't sell as well to the public. I'd be interested in the frost line for methane too, considering Titan like worlds. The liquid methane area should be much larger in any system than the liquid water area, and (KBO type?) objects like Titan seem to be more common than inner rocky planets.

Colin Robinson
2014-Jan-20, 02:31 AM
HZ is a statistical concept for one factor (the one which can be measured today). It should be modified with whatever model one has for atmosphere, tidal heating et c. about which we know little for any individual exoplanet. "Frost-line" for water would be a better term, I think, but it wouldn't sell as well to the public. I'd be interested in the frost line for methane too, considering Titan like worlds.

Yes, the British astrobiologist William Bains has written about the zones for liquid methane and liquid ammonia, if I remember correctly.


The liquid methane area should be much larger in any system than the liquid water area, and (KBO type?) objects like Titan seem to be more common than inner rocky planets.

Maybe so, but in the case of this solar system, there seems to be only one object like Titan, in the sense of having liquid methane on its surface, and that is Titan itself. Atmospheric pressure, as well as temperature, is a factor to consider.

filrabat
2014-Jan-20, 07:29 AM
Is the term "Goldilocks Zone" meaningless? If you ask me, both yes and no.

It does have meaning insofar as a planet conforms to a certain narrow range of characteristics: mass and average density of the solid material, atmospheric mass and composition, the main solvent(s) available, age (internal heat - > volcanism thing), rotation speed (probably), and perhaps a few other things I might have missed. If one of those factors for Earth would have been significantly different, life would either never have taken form or would be radically different from how it actually turned out.

However, it is meaningless insofar that all these traits can be radically different for any one rocky-metallic world, and in fact any change in one trait can very easily affect the quality of the other traits - and this is only when considering carbon/water life! It gets more meaningless if we consider the possibility of other solvents (ammonia's my personal favorite, but perhaps some others as well), which have different boiling/freezing points under certain atmospheric pressures.

For now, I'm not completely ready to give up on the concept of "Goldilocks zone", but the term clearly needs to be refined (i.e. subdivided) in light of not only the different solvents and their properties, but also in terms of the planetary characteristics themselves. In other words, Mars is clearly outside the habitable-zone of a Sun-twin star for a planet of its mass and density, but that doesn't say anything about a super-earth at an analogous distance (in terms of solar energy per square meter per second) from its parent star. Hence, I believe we can indeed use the term Goldilocks Zone IF we use it cautiously (i.e. taking into account all the details mentioned above at minimum, and perhaps more factors if research obligates us to use those other factors as well).


"Frost-line" for water would be a better term, I think, but it wouldn't sell as well to the public.

I disagree. In this sci-fi movie oriented age, I don't think it's too much of a conceptual leap for Joe and Jane Public to see that different volatiles freeze and melt at different temperatures. This certainly is not true of the most educated 1/5 of the general American population (and those of other nations of similar education level). So it shouldn't create too much strain to say "water frost line" "ammonia frost line", etc.

Local Fluff
2014-Jan-20, 01:12 PM
Maybe so, but in the case of this solar system, there seems to be only one object like Titan, in the sense of having liquid methane on its surface, and that is Titan itself. Atmospheric pressure, as well as temperature, is a factor to consider.
Titan's atmosphere doesn't seem to be in very long term equilibrium. As far as I can describe it, I think it has been suggested that Titan might be a KBO type object "recently" captured by Saturn. The relative proximity to the Sun makes its methane melt. Maybe Pluto would become a Titan if it were where Titan is. Maybe Ceres is of the same kind, but since long has boiled off the temporary atmosphere it got when it was captured in the asteroid belt. And although a rocky planet, Mars too has "boiled off" its temporary atmosphere. Maybe large KBOs, icy moons and the two largest asteroids are of basically the same origin, their difference being mostly a function of where they are and have been in relation to the Sun and gas giants? Maybe the distance from the star is more important for the potential for life, than is the intrinsic nature of the (random) mid-sized 500-10000 km radius objects which are in those zones, since in any one zone the same compounds melt and mix.

ravens_cry
2014-Jan-20, 10:16 PM
Well, until we find life on other worlds, it's really hard to say whether the the Goldilocks Zone is a useful tool for weeding out possibilities or a coincidence like Titus-Bode's 'Law'.
About the only thing we can say right now with any level of certainty is life needs a source of energy.

Colin Robinson
2014-Jan-21, 01:05 AM
Titan's atmosphere doesn't seem to be in very long term equilibrium.
As far as I can describe it, I think it has been suggested that Titan might be a KBO type object "recently" captured by Saturn.

There are different theories about the origin and age of Titan's atmosphere, whether or not vulcanism replaces gases lost into space, and a theory about a relatively recent impact event. But I wasn't aware of a suggestion that Titan itself is a recent capture by Saturn.

As I understand it, the mainstream view is that Titan belongs to a class of large moons of Jupiter and Saturn (including e.g. Ganymede and Europa) which have been orbiting those planets for billions of years, since around the time the solar system go started.


The relative proximity to the Sun makes its methane melt. Maybe Pluto would become a Titan if it were where Titan is. Maybe Ceres is of the same kind, but since long has boiled off the temporary atmosphere it got when it was captured in the asteroid belt. And although a rocky planet, Mars too has "boiled off" its temporary atmosphere. Maybe large KBOs, icy moons and the two largest asteroids are of basically the same origin, their difference being mostly a function of where they are and have been in relation to the Sun and gas giants? Maybe the distance from the star is more important for the potential for life, than is the intrinsic nature of the (random) mid-sized 500-10000 km radius objects which are in those zones, since in any one zone the same compounds melt and mix.

Except that smaller objects of similar composition have lower gravity and therefore a lower escape velocity, which means that the atmosphere (if any) tends to boil off more quickly.

If atmospheric pressure is too low, increased temperature will not cause ices to melt into liquid, rather they will sublimate into gas.

Selfsim
2014-Jan-21, 01:08 AM
Well, until we find life on other worlds, it's really hard to say ...The existence of life on other worlds is something imagined.

Any meaning associated with the term: 'Goldilocks Zone', thus originates from only imagined things.

Not much wonder "its really hard to say" anything of scientific value eh?

ravens_cry
2014-Jan-21, 10:44 AM
Except the one example of life we do know. Life, in this solar system, as far as we know, is only in this 'zone'. Is that causation or is that causality? That be the question, eh?

Noclevername
2014-Jan-21, 11:02 AM
Any meaning associated with the term: 'Goldilocks Zone', thus originates from only imagined things.


The term "Goldilocks Zone" originated with reference to liquid water, not life. So there's at least one meaning not associated with imagination, but with physics.

Selfsim
2014-Jan-21, 08:14 PM
The term "Goldilocks Zone" originated with reference to liquid water, not life. So there's at least one meaning not associated with imagination, but with physics.And the presence of liquid water means what exactly?

Selfsim
2014-Jan-21, 08:18 PM
Except the one example of life we do know. Life, in this solar system, as far as we know, is only in this 'zone'. Is that causation or is that causality? ... or correlation?

Colin Robinson
2014-Jan-21, 10:45 PM
The point is that both habitability ('Goldilocks Zone', etc) is no indication of anything I can see, which is of practical value in the 'hunt' for what 'lives' in some 'zone'. The Astrobiological search for the alternatives (bio-signatures), also seems like 'word-salad'.

Simple Exploration of whatever we can reach, is the only practical way of optimising the search I can see.

Whatever questions may be raised about the "Goldilocks Zone" concept, or about NASA's Astrobiology Roadmap, there is no such thing as "simple exploration of whatever we can reach". There are always choices to be made, e.g. where to send the next space probe, what sort of instruments to put on it.

Right now exploration of the Solar System is focused on Mars. This may be partly because of the historical influence of the Goldilocks Zone concept on how scientists and the general public have thought about the Solar System. Could the search be optimised by a "roadmap" which is less Mars-focussed, less Goldilocks-obsessed?

Selfsim
2014-Jan-22, 05:09 AM
Whatever questions may be raised about the "Goldilocks Zone" concept, or about NASA's Astrobiology Roadmap, there is no such thing as "simple exploration of whatever we can reach". There are always choices to be made, e.g. where to send the next space probe, what sort of instruments to put on it.The space probes are clearly designed to achieve the science goals .. which invariably turn out to be based on prior direct observations of the target environment. This has nothing to do with 'Goldilocks Zones'! The instruments always turn out to be various spectrometers and imaging equipment, tuned to inventorying molecules already known to be present in the target area.


Right now exploration of the Solar System is focused on Mars. This may be partly because of the historical influence of the Goldilocks Zone concept on how scientists and the general public have thought about the Solar System. Could the search be optimised by a "roadmap" which is less Mars-focussed, less Goldilocks-obsessed?Mars is the target because it is:

i) way less of an engineering risk than other unexplored planets/moons;
ii) the science information return risks are minimal compared with other targets;
iii) a successful mission builds on already compiled information .. thereby guaranteeing progress towards completing an already substantial knowledgebase.

'Goldilocks' was some fantasy character from some mythical story, no? Why build a scientific exploration strategy based on some fantasy?

Colin Robinson
2014-Jan-22, 07:52 AM
The space probes are clearly designed to achieve the science goals .. which invariably turn out to be based on prior direct observations of the target environment. This has nothing to do with 'Goldilocks Zones'! The instruments always turn out to be various spectrometers and imaging equipment, tuned to inventorying molecules already known to be present in the target area.

I thought one of the specific tasks of Curiosity is to test for and inventory carbon-chain molecules, which were not already known to be present.


Mars is the target because it is:

i) way less of an engineering risk than other unexplored planets/moons;
ii) the science information return risks are minimal compared with other targets;
iii) a successful mission builds on already compiled information .. thereby guaranteeing progress towards completing an already substantial knowledgebase.

All these factors are no doubt considered by the decision makers.


'Goldilocks' was some fantasy character from some mythical story, no? Why build a scientific exploration strategy based on some fantasy?

Well, the same concept has gone by various other names too. You mentioned one of them yourself, in post #3 of this thread: "circumstellar habitability zone". The basic idea of the zone goes back to the days of Alfred Russel Wallace, a biologist contemporary with Charles Darwin: he called it the "temperate zone" of the solar system, in his book Man's Place in the Universe.

Selfsim
2014-Jan-22, 09:04 AM
I thought one of the specific tasks of Curiosity is to test for and inventory carbon-chain molecules, which were not already known to be present.... which was a consequence of what was already known to exist in the area, (from prior measurements/observations) ... all of which had nothing to do with the usual inferences conjured up by the term: "Goldilocks Zone".

Colin Robinson
2014-Jan-23, 08:42 PM
Titan's atmosphere doesn't seem to be in very long term equilibrium. As far as I can describe it, I think it has been suggested that Titan might be a KBO type object "recently" captured by Saturn.

I did a little checking, it is actually not Titan but Triton (biggest moon of Neptune) which is thought to be a recently captured Kuiper Belt Object. This is because of Triton's retrograde orbit and its Pluto-like composition. See the Wikipedia page Triton (https://en.wikipedia.org/wiki/Triton_(moon)).

Paul Wally
2014-Jan-24, 01:55 PM
I think the habitable zone concept needs to be expanded to include the moons of gas giants. The dynamics are quite interesting and complex, because in the case of gas giant moons there is the effect of the sun as well as tidal heating. The liquid water zone is effectively expanded by the presence of a gas giant
or large planet. For instance, if Mars were orbiting a gas giant it would probably have been a more geologically active planet with a thicker atmosphere and liquid water at the surface.

Local Fluff
2014-Jan-25, 01:12 PM
I did a little checking, it is actually not Titan but Triton (biggest moon of Neptune) which is thought to be a recently captured Kuiper Belt Object. This is because of Triton's retrograde orbit and its Pluto-like composition. See the Wikipedia page Triton (https://en.wikipedia.org/wiki/Triton_(moon)).

I'm speculating very loosely. Orbital mechanics do speak against Titan being captured. The general idea would simply be that icy objects in general have a period of active atmosphere and/or subsurface liquid ocean as they come closer to a Sun. Or get tidally massaged by a gas giant which captured them. Maybe such geological/climat activity is not so uncommon, albeit temporary. Maybe Titan is in such a transition phase now. Maybe Ceres has had a history of that too if it has migrated from far away to where it is today.

If Pluto was suddenly put in the asteroid belt, maybe it would generate a Titan-like atmosphere and methane climate for a while. The idea is certainly too simple to be right, but if all those icy middle sized objects are assumed to be identical, then their behavior would be dominated by their distance from the Sun or a gas giant, and by their history of getting there.