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Old 5th March 2014, 01:10 AM   #1
mike3
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Bad news for ET?

Hi.

I saw this:

http://www.astronomy.com/news/2014/0...be-dead-worlds

Quote:
Super-Earths may be dead worlds
Scientists find that planets that form from less massive cores can become benign habitats for life, whereas the larger objects instead end up as “mini-Neptunes” with thick atmospheres and probably stay sterile.
(see link for article)

Basically, it looks that according to the model these scientists came up with, planets with more than 1.5-2x Earth's mass will tend to accrete thick hydrogen atmospheres, akin to those of a gas giant: or more precisely, they will fail to lose those atmospheres. This applies to planets even with less mass than a full-blown gas giant.

They suggest a "sweet spot" of 0.5-1.5 Earth masses for an atmosphere more clement to life. It's interesting that Earth happens to be riiight in the middle of that range...

There appears to be empirical evidence in favor of this:

Quote:
The ongoing discovery of low (average) density super-Earths supports the results of the study. Scientists will need to look even harder to find places where life could be found, setting a challenge for astronomers using the giant telescopes that will come into use in the next decade.
Looks like there are fewer potential homes for ET than we thought...

On the other hand, this makes sense -- it would seem sensible for there to be a transition zone between rocky terrestrial planets and the gas giants, and apparently, the "Super-Earth" mass range is it.
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Old 5th March 2014, 01:30 AM   #2
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Originally Posted by mike3 View Post
They suggest a "sweet spot" of 0.5-1.5 Earth masses for an atmosphere more clement to life. It's interesting that Earth happens to be riiight in the middle of that range...
That would be for life as we know it on Earth. Other possibilities exist, and the "sweet spot" may be much bigger.

Even life as we know it may be able to adjust to denser atmospheres, larger gravity, and so on, if only abiogenesis can be started.
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Old 5th March 2014, 01:49 AM   #3
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Originally Posted by mike3 View Post
Looks like there are fewer potential homes for ET than we thought...
He only needs one.
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Old 5th March 2014, 02:29 AM   #4
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Originally Posted by steenkh View Post
That would be for life as we know it on Earth. Other possibilities exist, and the "sweet spot" may be much bigger.

Even life as we know it may be able to adjust to denser atmospheres, larger gravity, and so on, if only abiogenesis can be started.
An ultradense atmosphere is pretty tough to work with, unfortunately. Thanks to the adiabatic lapse rate, a really thick atmosphere is more or less guaranteed to be ultrahot at the surface. I don't mean "ultrahot" in the sense of "no liquid water", I mean ultrahot in the sense of "no interesting chemistry at all". No hydrocarbons, no proteins, no clays. 1000K, 2000K, 10000K, that sort of thing---you expect the surface to be molten rock and metal. While it's possible to write sci-fi about flying gas-balloon species that live comfortably in the atmosphere, or about fire lizards made of quartz with silicon blood, it's a stretch of the imagination in reality.

The other issue in a super-dense atmosphere is that, deeper in the atmosphere, the thermodynamics are very non-life-like. You can end up in a perfectly black-body thermal bath---1000K gas temperature with 1000K blackbody radiation. I can make a correct (non-creationist-style) 2nd Law of Thermodynamics argument that there's no such thing as photosynthesis if you're swimming in this environment. A sufficiently-hot chlorophyll molecule becomes exactly as good as emitting photons as absorbing them. If some sort of chemical food source is available, it's really difficult to utilize; imagine a set of energy-using reactions where you want to say "this reaction only happens when I get a 4-eV energy input from a glucose molecule", but, well, you frequently enough will get a 4-eV input from random thermal fluctuations.

Earth is nice because there's a very, very, very wide gap between the energy of a typical sunlight photon (~2-3 eV) and the energy of a typical thermal fluctuation (0.02 eV). It's also nice because that thermal scale is well-matched to the energy of flopping/rotating modes of molecules, and the formation/breaking of hydrogen bonds---biomolecules can sample lots of different configurations without waiting for rare thermal fluctuations, or for sunlight, to move them along. The sunlight scale is pretty well matched to the formation/breaking of covalent bonds---one sunlight photon is sufficient to form one not-too-atypical new chemical bond. (That's not a carbon-specific statement at all---the whole periodic table is vaguely in this ballpark. (Covalent bond energies are variations around the Rydberg, 13eV, and molecular vibration/stretch energies are variations around Ry*(electron mass/proton mass) I think.)
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Old 5th March 2014, 06:28 AM   #5
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Darn, ben m! That's a great answer. I had kind of thought heat could be a problem. Just couldn't pin down why (not a physics expert). Thanks for confirming my suspicions. So we're really limited to the < 1.5 Earth-mass region for looking for potentially habitable worlds.
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Old 5th March 2014, 07:42 AM   #6
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Originally Posted by mike3 View Post
Looks like there are fewer potential homes for ET than we thought...
That's your interpretation. The original article does not go into that (as far as I can see).

1) The current methods used for detecting extrasolar planets find uninhabitable planets (heavy ones, and ones too close to the host star) much easier. A lot of methods also favor systems with an ecliptic parallel to our line of sight, meaning that a great many number of systems avoid detection right now. The planets people are most obsessed with (second Earths) are not easily detectable right now.

2) The probability that second Earths exist at all has been gone up dramatically, since we have found that many extrasolar planets. Not only is the probability of systems that look similar to ours higher since we gained that knowledge, we also have found systems that astronomers deemed unlikely to exist beforehand, so the number of possible systems containing second Earths is much wider than thought before.
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Old 5th March 2014, 07:46 AM   #7
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I personally feel that the universe is teeming with life. Moons circling planets like Jupiter may harbor life. I bet Titan has life and Europa etc. Just because a celestial body isn't a planet doesn't mean there isn't life out there. I bet some of the life is intelligent and civilized.

The planet earth exists and that means other similar planets exist also.
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Old 5th March 2014, 07:58 AM   #8
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Originally Posted by mike3 View Post
Looks like there are fewer potential homes for ET than we thought...
That only applies to life as we know it. For all we know it's entirely possible that some forms of life could be living in the atmosphere of Jovian planets, such as Jupiter.
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Old 5th March 2014, 04:38 PM   #9
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Originally Posted by Mudcat View Post
That only applies to life as we know it.
This. If the organism used some liquid other than water as its solvent, or life had evolved to such a degree as to no longer need an atmosphere (lithophiles, for example), or any of a number of other things were true, the idea that the planet needs to be in our Goldilocks Zone becomes false.

Originally Posted by ben m
you expect the surface to be molten rock and metal. While it's possible to write sci-fi about flying gas-balloon species that live comfortably in the atmosphere, or about fire lizards made of quartz with silicon blood, it's a stretch of the imagination in reality.
Why? When you think about it, OUR blood is molten minerals--specifically ice.
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Old 5th March 2014, 05:10 PM   #10
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Originally Posted by ben m View Post
An ultradense atmosphere is pretty tough to work with, unfortunately. Thanks to the adiabatic lapse rate, a really thick atmosphere is more or less guaranteed to be ultrahot at the surface. I don't mean "ultrahot" in the sense of "no liquid water", I mean ultrahot in the sense of "no interesting chemistry at all". No hydrocarbons, no proteins, no clays. 1000K, 2000K, 10000K, that sort of thing---you expect the surface to be molten rock and metal. While it's possible to write sci-fi about flying gas-balloon species that live comfortably in the atmosphere, or about fire lizards made of quartz with silicon blood, it's a stretch of the imagination in reality.
So what happens if a super-Earth with such a thick atmosphere is well away from its sun, so that it's cooler?
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Old 5th March 2014, 05:50 PM   #11
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Originally Posted by Madalch View Post
So what happens if a super-Earth with such a thick atmosphere is well away from its sun, so that it's cooler?
It's a tough balance to strike. Neptune, according to Wikipedia, gets up to Earth temperatures at a pressure of 50 atmospheres.

The adiabatic lapse rate is generated by the gravitational compression of any vertical airflow---you can have the top of the atmosphere as cold as you like, and still find Venus-like temperatures and pressures further down. (The relevant question is: how far does a typical packet of air move vertically in the time it takes to cool radiatively? If the air is moving faster than its radiative-cooling timescale, then it's good at converting gravitational potential energy into heat.) You can have a dense atmosphere that's not superhot only if this process isn't working. If the planet is so cold, or spinning slowly or something, that it doesn't have much vertical airflow, including upslope/downslope winds in mountains. If the atmosphere's composition is just right so that the surface can cool efficiently by radiation (which is probably what keeps Neptune that cool). Etc.

I'm not a particular expert, so that more or less exhausts my thinking on the topic, but I think the above is correct.
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Old 5th March 2014, 09:16 PM   #12
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In terms of ET (an intelligent, tool using lifeform), yes it is unlikely in a super-earth environment. I doubt that simple lifeforms would be put off though. I would be surprised if simple life didn't exist in every possible niche. We just haven't detected any of them yet.

These super-earths may have moons that harbour conditions that may allow more complex lifeforms. Also we are only just able to detect bodies down to just over earth mass, better instrumentation will allow us to detect smaller bodies. There may well be better candidates for ET to live on.
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Old 5th March 2014, 10:51 PM   #13
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Originally Posted by Dinwar View Post
Why? When you think about it, OUR blood is molten minerals--specifically ice.
There's only one periodic table of elements.

There needs to be some sort of material catalytically- and informationally- complex enough to be the machinery of life. At the temperature of molten ice, our periodic table offers at least one option (carbon/hydrogen/oxygen) that fills the bill. Clays offer complex information storage and simple catalysis. Carbon/fluorine/oxygen or carbon/chlorine/oxygen has the same complexity, but distinctly reduced catalytic interest. Replace the carbon with silicon and ... well, there just aren't any nice long-chain molecules to put in the "complex" column.

Raise the temperature to 1000K and this list goes to zero. Molten iron is just molten iron. There are, AFAIK, zero options for "complex chemicals that will float around in molten iron and do things".
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Old 6th March 2014, 12:10 AM   #14
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My quick thought is "BS". Pressure: see deepsea life. Heat: see deepsea vent life. Cold: See Antartica below-ice lakes. Habitable Zone: See above mentioned moons that don't give a ****. (btw, there are estimates now of how many planets on average are in a solar system; are there for moons around planets? Hell, moons could be a much more likely "habitable zone" than planets.)

I think the "habital zone" is extremely conservative and perhaps utterly, completely wrong.

Final thought: Life on Earth began in much different atmospheric and temperature conditions than currently exists now. Despite Earth (unless it's orbit radically changed and I missed that) being always in the "habitable zone". Perhaps a Super-Giant-Earth also has had many such changes and life began at one point within it, then, like every single living thing on this planet, adapted after that and now can live in a very wide range of conditions.
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Old 6th March 2014, 12:16 AM   #15
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Originally Posted by Cainkane1 View Post
I personally feel that the universe is teeming with life.

Almost certainly. Even at only one world per galaxy having life, with some 100 billion galaxies in the universe, that's 100 billion life forms. Sounds like a universe teeming with life to me. (Of course, with only one life form per galaxy, there'd be an awful lot of distance between them.)
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Old 6th March 2014, 06:16 AM   #16
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Originally Posted by Dragoonster View Post
My quick thought is "BS". Pressure: see deepsea life.
Water pressure is different than gas pressure; the adiabatic-compressibility heating only happens with gasses. An air packet driven down of 2-3 psi causes an increase of 9 degrees; an undersea pressure increase of 2-3 psi does not.

Quote:
Heat: see deepsea vent life.
Yep. Deep sea vent life in the range of 350--370K, about 20% higher than human-survival temperatures of 270--300K. (80-100C instead of 0-30C). That tells us nothing at all about whether life can exist, and if so at what chemistry, at 1000C (1300K), 400% higher than human-ish temperatures, at an energy where thermal fluctuations can pretty easily break C-H bonds.

I know one mustn't assume that all life uses the same chemistry as Earth life, but it's all going to use some sort of chemistry. All chemistry comes from the same periodic table and the same value of Boltzmann's constant. It's a non-Earth-specific fact that at 0--100C many chemical bond energies are stronger than thermal fluctuations. It's a non-Earth-specific fact that at 1000C most chemical bond strengths are barely stronger than thermal fluctuations.
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Old 6th March 2014, 05:54 PM   #17
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Originally Posted by ben m
Raise the temperature to 1000K and this list goes to zero. Molten iron is just molten iron. There are, AFAIK, zero options for "complex chemicals that will float around in molten iron and do things".
If we were talking about a molten ball of iron, I'd agree with you. However, minerals are much more complex than that--and therefore mineral melts will be as well. Has anyone ever ran any experiments regarding liquid chemistry in magma? I certainly haven't heard of any.
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Old 6th March 2014, 07:49 PM   #18
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Originally Posted by mike3 View Post
They suggest a "sweet spot" of 0.5-1.5 Earth masses for an atmosphere more clement to life. It's interesting that Earth happens to be riiight in the middle of that range...
It's also interesting that Venus falls right in the middle of that range too. Almost the same mass and surface gravity of earth... but a very different climate.

  Earth Venus
Diameter 12,742 km 12,100 km
Mass 5.9736×1024kg 4.868 x1024 kg
Gravity 9.78 m/s2 8.87m/s2
Atmosphere 100 kPa 9,200 kPa
Avg. Temp. 14 degrees Celsius 462 degrees Celsius
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Old 6th March 2014, 10:18 PM   #19
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Originally Posted by Brian-M View Post
It's also interesting that Venus falls right in the middle of that range too. Almost the same mass and surface gravity of earth... but a very different climate.

  Earth Venus
Diameter 12,742 km 12,100 km
Mass 5.9736×1024kg 4.868 x1024 kg
Gravity 9.78 m/s2 8.87m/s2
Atmosphere 100 kPa 9,200 kPa
Avg. Temp. 14 degrees Celsius 462 degrees Celsius
It's a great terraforming candidate. Once we have a clue how to do that, anyway.
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Old 7th March 2014, 12:03 AM   #20
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Originally Posted by Brian-M View Post
It's also interesting that Venus falls right in the middle of that range too. Almost the same mass and surface gravity of earth... but a very different climate.

  Earth Venus
Diameter 12,742 km 12,100 km
Mass 5.9736×1024kg 4.868 x1024 kg
Gravity 9.78 m/s2 8.87m/s2
Atmosphere 100 kPa 9,200 kPa
Avg. Temp. 14 degrees Celsius 462 degrees Celsius
The interesting thing is, if you go to Venus and descend into the atmosphere to a place where the pressure is about 1 bar (like on Earth), the temperature is about 300K (like Earth). If it so happened that the rocky surface started there, rather than 50km deeper, Venus would be a pretty comfortable planet.
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Old 7th March 2014, 01:57 AM   #21
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Originally Posted by Brian-M View Post
It's also interesting that Venus falls right in the middle of that range too. Almost the same mass and surface gravity of earth... but a very different climate.

  Earth Venus
Diameter 12,742 km 12,100 km
Mass 5.9736×1024kg 4.868 x1024 kg
Gravity 9.78 m/s2 8.87m/s2
Atmosphere 100 kPa 9,200 kPa
Avg. Temp. 14 degrees Celsius 462 degrees Celsius
Yes, but this result is not incompatible with the mass/radius range being a necessary condition. It just shows it is not a sufficient condition.
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Old 7th March 2014, 02:14 AM   #22
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Originally Posted by Brian-M View Post
It's also interesting that Venus falls right in the middle of that range too. Almost the same mass and surface gravity of earth... but a very different climate.

  Earth Venus
Diameter 12,742 km 12,100 km
Mass 5.9736×1024kg 4.868 x1024 kg
Gravity 9.78 m/s2 8.87m/s2
Atmosphere 100 kPa 9,200 kPa
Avg. Temp. 14 degrees Celsius 462 degrees Celsius
Is the density of Venus's atmosphere related to its proximity to the sun?
If it were in our orbit, or Mars's orbit, would it be much different?
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Old 7th March 2014, 04:08 AM   #23
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Originally Posted by Puppycow View Post
Is the density of Venus's atmosphere related to its proximity to the sun?
I doubt it.

Originally Posted by Puppycow View Post
If it were in our orbit, or Mars's orbit, would it be much different?
It wouldn't be quite as hot... so the atmospheric density would be slightly higher, I guess, because the air would contract as it cools, and so more of the atmosphere would be closer to the planet and subject to a stronger gravitational pull.

(ETA: But I could be wrong, I'm certainly no expert in the area.)
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Old 7th March 2014, 08:40 AM   #24
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Originally Posted by ben m View Post
It's a tough balance to strike. Neptune, according to Wikipedia, gets up to Earth temperatures at a pressure of 50 atmospheres.
Keep in mind that Neptune's heat comes mostly from below. Amount of heat at Earth's surface -- or Venus' for that matter, -- which comes from the interior is negligible compared to the amount which comes from the Sun. At Neptune's distance from the Sun Earth's atmosphere would freeze and rain out in a few weeks. I am pretty sure Venus' atmosphere would do same thing in the matter of few years or decades -- still an eyeblink on geological scale. But super-Earths, being halfway between Earth and Neptune, should also have internal heat flow halfway between Earth and Neptune -- in other words, significant. No idea how hot that would keep lower atmosphere.

But I am quite certain it would be pitch black at the surface.
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Old 7th March 2014, 01:59 PM   #25
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Originally Posted by BenBurch View Post
It's a great terraforming candidate. Once we have a clue how to do that, anyway.
Is this myth still being knocked around? You can't terraform either Mars or Venus. It isn't dependent on technology.

Both Venus and Mars are too small of planetary masses. They no longer have strongly convective cores and therefore do not have sufficient magnetospheres. The water on Venus was blown away by the solar wind. The CO2 remains because it is much heavier, more than twice the weight of gaseous water. The carbon dioxide is also heavier than nitrogen or oxygen gas.
CO2 - 44
O2 - 32
N2 - 28
H2O - 18

I'm sorry but the 0.5 Earth mass estimate is not correct for something with a sun like ours. Some have suggested that you could have a red dwarf sun, however that also has problems. As far as I know, solar radiation and solar wind tend to be proportionate. So, if you have less solar radiation then you end up having to move closer and the solar wind gets stronger too. So, it probably is not possible to have life on a planet the size of Mars no matter where it is in the solar system. It's possible that Venus might have supported life in the past if it were farther away from the Sun. However, at the point when the core cooled enough to lose the strong magnetic field, it was only a matter of time before life was not possible. In its current position, it is still doubtful that Venus ever supported life. I've even wondered if the Earth would be a large enough mass without the Moon.

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Old 7th March 2014, 02:10 PM   #26
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Originally Posted by Puppycow View Post
Is the density of Venus's atmosphere related to its proximity to the sun?
No, it's related to volcanism releasing CO2 and not having plant life or shellfish to take carbon out of the atmosphere.

Quote:
If it were in our orbit, or Mars's orbit, would it be much different?
If you get far enough away from the sun, the CO2 is going to form dry ice. However, within the range where the CO2 is a gas, you need living organisms to change the ratio. And, unless you have enough of a magnetosphere to protect your water, you don't have any life.

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Old 7th March 2014, 02:29 PM   #27
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Originally Posted by mike3 View Post
planets with more than 1.5-2x Earth's mass will tend to accrete thick hydrogen atmospheres, akin to those of a gas giant
You've got three issues:
  • Atmospheric insulation
  • Distance from sun
  • Magnetosphere

A higher mass means a thicker atmosphere. So, it should insulate better. That would mean having to move it farther away from the sun to keep the temperature acceptable. Fair enough. However, out by Ceres, you only have about 11% of Earth level solar radiation so it is doubtful to me that you could make up that much with a thicker atmosphere. Perhaps that would work at the orbit of Mars. By the time you get out to Jupiter, the solar wind is so light that it stops blowing away the lighter gases like hydrogen and it builds up. And, of course, with Jupiter's extremely powerful magnetosphere, the gas is well protected. Mars has pretty much nothing in terms of a magnetosphere so the solar wind has scoured the planet. Its atmosphere is only about 2% of what is on Earth even though it has enough gravity to hold much more. Even our Moon has enough gravity to hold a thin atmosphere if not for the solar wind. So, you have to have enough mass to have a strong magnetosphere but not so much mass that the atmosphere is too hot or too dense. You have to be close enough to the sun to be warm but not so close that the solar wind rips away the water. I think the range for this is narrower than some have suggested.
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Old 7th March 2014, 06:01 PM   #28
Brian-M
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Originally Posted by barehl View Post
Both Venus and Mars are too small of planetary masses.
81% of earth's mass is too small?

Then what is sufficient planetary mass?

85% earth mass?
90% earth mass?
100% earth mass?

Originally Posted by barehl View Post
They no longer have strongly convective cores and therefore do not have sufficient magnetospheres.

Wikipedia offers a different explanation...
On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This caused the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead, the heat energy from the core is being used to reheat the crust
Originally Posted by barehl View Post
I've even wondered if the Earth would be a large enough mass without the Moon.
What about a Mars-sized moon orbiting a Jupiter-like planet? Maybe that could work.
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Old 7th March 2014, 06:17 PM   #29
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Well, if we haul Venus out to Mars' orbit, and combine the two bodies, that would have the mass to work, and the core would be molten again....
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Old 7th March 2014, 06:48 PM   #30
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Originally Posted by Madalch View Post
Well, if we haul Venus out to Mars' orbit, and combine the two bodies, that would have the mass to work, and the core would be molten again....
But the towing fees would be exorbitant.
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Old 7th March 2014, 06:54 PM   #31
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Originally Posted by BenBurch View Post
It's a great terraforming candidate. Once we have a clue how to do that, anyway.
I'm sure scientists will get right on it after they come up with a perpetual motion machine.
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Old 7th March 2014, 08:26 PM   #32
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Originally Posted by Brian-M View Post
But the towing fees would be exorbitant.
Covered by Obamacare?
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