[sbgctsa]

are there any circumstances in which a completely lifeless planet/moon could sustain enough atmospheric oxygen to support some humans for some time? (I can't think of any).

---

Well you forgot your beginnings.

You are here.

Humans didn't walk into a lifeless planet with O2 already supplied. We had to wait for the ecosystem to modify the environment before we had a chance to evolve.

[BEyng6hj]

Bit of a science-fiction question: are there any circumstances in which a completely lifeless planet/moon could sustain enough atmospheric oxygen to support some humans for some time? (I can't think of any). Give it water or ice, or whatever elemental weirdness you can think of, as long as humans could conceivably walk around there without vac-suits. But no native life, not even bacteria. There are many circumstances that immediately occur to me:

• There was life there but oxygen production killed it off. To all the original organisms on Earth, oxygen was a deadly poison, so it makes sense that when photosynthesis started oxygen production it killed off everything on the planet.
• There was life there but the planet's crust completely melted either because of internal heat or because of a major impact. This melting killed off everything but left the chemical composition of the atmosphere largely intact.
• The planet began cold, so had plenty of water ice and ammonia around. Ultraviolet radiation from the newly forming sun split the water into hydrogen and oxygen and the hydrogen being less dense escaped into space. The resulting atmosphere would be an oxygen-nitrogen mix. Oxygen is deadly to primitive life so no life would have formed.
• The atmosphere came from de-gassing from the interior, allowing buried oxygen-containing compounds to release oxygen into the environment.

Perhaps the most telling argument is the following. The elements in the universe are, in order of abundance down from highest abundance: Hydrogen, Helium, Oxygen, Carbon, Nitrogen, Silicon, Magnesium, Neon, Sulphur, Argon, Aluminium, Iron. Through photo-dissociation and low density, hydrogen and helium are lost. The atmosphere would then be largely carbon dioxide and nitrogen. Then with heat the carbon dioxide dissociates and the carbon is deposited on the surface and buried leaving an atmosphere dominated by oxygen and nitrogen. No life required.

To me, that seems a far more likely scenario than that of a world with both life and an oxygen atmosphere. I expect life on planets with methane atmospheres to be far more likely than on planets with oxygen atmospheres.

[DrBrightonone]

Well if we look at that situation in reverse, that is we found a planet with Oxygen, we could be confident of it supporting life of some sort.
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If you mean that a planet with 20% O2 would definitly have life already, then I think you are drawing a very low bow.

I did not use the word "definitly"....what I said "Well if we look at that situation in reverse, that is we found a planet with Oxygen, we could be confident of it supporting life of some sort."
But I'm pretty sure, that if an extra solar planet was found with a reasonable Atmospheric Oxygen content, it would start the alarm bells ringing with NASA and other space agencies.

[Nidsstese]

The planet Mercury has an oxygen atmosphere.

Its atmosphere is 52% oxygen, 39% sodium, 8% helium and 1% potassium.
At the surface.

[pBiRXp8u]

The planet Mercury has an oxygen atmosphere.

Its atmosphere is 52% oxygen, 39% sodium, 8% helium and 1% potassium.
At the surface.

That's very surprising - How did it get so much oxygen and keep so much helium?

[km2000]

:-)


Mercury of course has a very tenuous atmosphere so 52$ of a small amount isn't too much.

Also although Mercury's atmosphere is being stripped off the planet by the solar wind, it is also being replenished by radioactive decay.

[arrasleds]

Replenished by outgassing from the surface. I would guess that sunlight is breaking down the oxides in surface rocks releasing oxygen.

On a different world it could be volcanism that is breaking down surface oxides to release oxygen into the atmosphere.

[amelveEnromma]

Science Highlights from Mercury Orbit

December 20, 2011

The MESSENGER Gamma-Ray Spectrometer: A window into the formation and early evolution of Mercury



Figure 1. Schematic illustration of the interior structures of the Earth (left) and Mercury (right). The core of Mercury occupies a much larger fractional volume than does Earth’s core. Figure reproduced from Solomon et al., Space Sci. Rev., 131, 3-39 (2007).
In the mid-1970s, the Mariner 10 spacecraft made three flybys of the planet Mercury. The scientific observations that were made during those encounters formed much of the basis for our knowledge of the innermost planet prior to the arrival of MESSENGER. One of the most important findings was the confirmation that Mercury has the highest uncompressed density of the four terrestrial planets. This high density requires that the planet be made of material that has substantially more metal per unit mass than Venus, Earth, or Mars. In particular, Mercury’s core must occupy a markedly larger fraction of the total mass and volume of the planet than is the case for the other terrestrial planets (Figure 1). Despite Mercury’s metal-rich bulk composition, measurements of the spectral reflectance of the planet by ground-based astronomers indicate that there is very little iron within the silicate minerals of surface rocks and soils.

The conclusion that Mercury is a metal-rich planet with an iron-poor surface presents an important constraint on ideas for how the planet formed. In the three decades between the Mariner 10 and MESSENGER missions, planetary scientists developed a number of theories for the formation of Mercury in attempts to account for these observations. Several physical mechanisms that might have removed much of the outer rocky layers of the planet, leaving a smaller planet with a disproportionately large metallic core behind, were proposed. These mechanisms included evaporation of the outer layers by an early hot solar nebula and removal of most of the rocky shell by one or more giant impacts. Chemical explanations involving the formation of Mercury from different material than that from which Venus, Earth, and Mars accreted were also advanced. Thermal gradients in the solar nebula might have led to the formation of Mercury predominantly from material condensed at high temperatures, for instance, and other scenarios involved formation from primitive solar system material such as that found in certain classes of metal-rich chondritic meteorites. Each of these physical and chemical hypotheses predicts a different composition for the modern surface of Mercury, so measurements made by the MESSENGER spacecraft can be used to discriminate among formation models.



Figure 2. A comparison of the potassium (K) and thorium (Th) abundances on the surface of Mercury (the one-standard-deviation ellipse is indicated in red), the surface of Mars (as measured by the Mars Odyssey Gamma-Ray Spectrometer) Martian meteorites, the surfaces of the Moon (as measured by the Lunar Prospector Gamma-Ray Spectrometer) and Venus (as measured by gamma-ray spectrometers on Soviet Venus landers), and terrestrial oceanic crust. Lines of constant K/Th are indicated at values of 360 and 5500. The lunar values lie on the line with K/Th = 360, whereas the values for the terrestrial planets, including Mercury, cluster near the line with K/Th = 5500. Figure reproduced from Peplowski et al., Science, 333, 1850-1852 (2011).
The MESSENGER Gamma-Ray Spectrometer (GRS) is capable of measuring the surface composition of the planet from orbit by monitoring element-characteristic gamma-ray emissions from the surface (see the Science Highlights article Measuring Mercury’s Surface Composition). Of particular interest are measurements of the abundances of the elements potassium and thorium. These two elements emit gamma rays in the course of natural radioactive decay. In addition, their chemical properties are such that the relative abundances of these elements are little altered by magmatic processes, so the ratio of their abundances on the surface should reflect the ratio within the entire planet. These two elements do have an important difference, however; potassium is a moderately volatile element, whereas thorium is not. As a result, potassium tends to vaporize at much lower temperatures than thorium. As many of the proposed theories for Mercury’s formation involve extreme heating of the planet or its precursor materials, they therefore predict that potassium should be depleted relative to thorium. However, Mercury’s ratio of potassium to thorium as measured by GRS is similar to that of the other terrestrial planets (Figure 2), indicating that Mercury was not subjected to extreme heating during its formation or early evolution. This inference rules out most of the proposed formation mechanisms for the planet. Moreover, the measured ratios of potassium to thorium and thorium to uranium are similar to those of chondritic meteorites, supporting scenarios under which Mercury formed by the accretion of metal-rich chondrites.

Measurements of the abundances of long-lived radioactive elements on the surface of Mercury also have important implications for the thermal history of the planet. The radioactive decay of potassium, thorium, and uranium within the interior of the planet results in the generation of heat. This heat, and that remaining from the formation of the planet, was responsible for driving volcanic activity at Mercury’s surface. From the measured ratios of surface abundances of the radioactive elements and the known half-lives of the unstable isotopes of those elements, there has been a substantial decline in heat production within the planet over the past four billion years. This finding is consistent with the observation from imaging Mercury’s surface that extensive volcanic activity occurred early in the planet’s history and only limited activity has occurred since.





http://www.jhuapl.edu/messenger/soc/...hts122011.html

[Extinimot]

That's very surprising - How did it get so much oxygen and keep so much helium?

Of course as an addition to helium on Mercury being caused by radioactive decay, it could also be temporally captured from the solar wind itself.

[enlinnyGoob]

a completely lifeless planet/moon could sustain enough atmospheric oxygen There is no lifeless planet/rock with free oxygen. LIFE liberated the oxygen on Earth.

One of the most important findings was the confirmation that Mercury has the highest uncompressed density of the four terrestrial planets. This high density requires that the planet be not even slightly surprised.... Mercury was ejected from the Sun a relatively short time ago.....

[royarnekara]

There is no lifeless planet/rock with free oxygen. LIFE liberated the oxygen on Earth.


not even slightly surprised.... Mercury was ejected from the Sun a relatively short time ago.....

You have any references and/or authoritive support for those claims?
The second claim is actually totally wrong.

[triardwonvada]

Originally Posted by Zarkov
There is no lifeless planet/rock with free oxygen. LIFE liberated the oxygen on Earth.


not even slightly surprised.... Mercury was ejected from the Sun a relatively short time ago.....


You have any references and/or authoritive support for those claims?
The second claim is actually totally wrong. At least on Earth I don't think there is any dispute about the first claim is there (other than the choice of capitalisation).

As for the second, if it were true surely Mercury would have a very low density rather than a high one.

[gennickhif]

if it were true surely Mercury would have a very low density rather than a high one. It is under very high pressure

The bubble Universe is a pressure system.. not rubbery mats and fictitious "attraction".

Do the math and you will be surprised.

[AndyColemants]

At least on Earth I don't think there is any dispute about the first claim is there (other than the choice of capitalisation).

As for the second, if it were true surely Mercury would have a very low density rather than a high one.

There is absolutely no evidence for planetary ejection, but we do have observational evidence for the "ACCRETION DISK METHOD" bordering on plentiful.
Planetary migration both inwards and outwards are also part of the accretion disk method.

[bensabath]

There is absolutely no evidence for planetary ejection, Didn't mean to suggest it could be true. just that even as an evidence-free speculation, it doesn't seem to make sense.

[fereupfer]

Didn't mean to suggest it could be true. just that even as an evidence-free speculation, it doesn't seem to make sense.

My fault...I was intending on quoting Zarky, and mistakenly stuck in your post with his quote, consequently actually quoting you...Apologies.

[Japakefrope]

My fault...I was intending on quoting Zarky, and mistakenly stuck in your post with his quote, consequently actually quoting you...Apologies.

None necessary.

[VastDrura]

There is absolutely no evidence for planetary ejection Mercury is a hot planet, Mars is cold
and the sequence of planets is laid out in a BXE quantum sequence


Explain to me how a ring of dust can form a planet with a molten core.... if you appeal to gravity then leave it, because attraction gravity is impossible

[PHOTOSHOPoem]

Explain to me how a ring of dust can form a planet with a molten core.... if you appeal to gravity then leave it, because attraction gravity is impossible

Be a good scientist and perform an experiment - Go to a very tall building and step out of the top window and report back how gravity acts on you.

[Koayrbzh]

Go to a very tall building and step out of the top window and report back how gravity acts on you. You think it pulls (attracts)... so you first