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Astronomy/Planets' Atmospheres

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Question
Dear Courtney,

I have sent this question earlier to Patrick Weiler but he couldn't answer my question and I was advised to ask another expert. If something is written in improper English, please excuse me as English is not my native tongue and I am not living in any English-speaking country

On the Internet one can easily find the information that "Mercury is too small in size (in mass) to have an atmosphere". In other places: "Mercury is too close to the Sun to keep the atmosphere - it was swept away by the Solar wind". So the second thesis says that it originally had an atmosphere, despite being a small planet, only it has lost it in later times. Maybe I am not right
but these two seem contradictory to me.

For ex. Mars also has no atmosphere, and it is also small in size (and mass), yet it is situated much farther from the Sun. The Solar wind is quite weak there, I suppose, at least weaker than near Mercury or even near Earth which has a reasonable atmosphere.

On the other hand Venus is smaller than Earth, and it is loser to the Sun (so I would think that the Solar wind is stronger there than here), yet it has a much thicker, much more dense atmosphere than Earth.

So my question is:
Which is the main reason that Mercury and Mars do not have any atmosphere but Venus and Earth do have it? (What factors decide about it and which factor is the most important one?) E.g. Why Venus's atmosphere is much thicker than the Earth's one, despite that fact that it is both smaller and closer to the Sun?

Thank you for your attention,

Mathew

Answer
The primary reason that Mercury and Mars do not have atmospheres is that they are too small to hold onto one. There is a detailed though incomplete discussion of this on my website at http://cseligman.com/text/planets/retention.htm (The Retention or Loss of Planetary Atmospheres). The discussion there makes things fairly clear, but since it is incomplete and being incomplete was never edited for clarity, the following should help clarify things.

Because Mercury is close to the Sun it is very hot, which causes any atmosphere to expand away from the surface of the planet. Because it is very small, it has a low surface gravity, which also allows the atmosphere to expand away from the surface. Between the two, any atmosphere it once had would be lost in time periods much shorter than the age of the solar system. As a result the only atmosphere it can have consists of gases recently arrived at the planet's surface: gases from the Solar Wind, ongoing outgassing from the interior, and surface rocks vaporized by meteorite impacts (see The 'Atmosphere' of Mercury at http://cseligman.com/text/planets/mercatmos.htm ). These three sources produce an extremely thin atmosphere, in fact too thin to detect without sophisticated instrumentation or satellite measurements. However, any such atmosphere should escape into space over very short periods of time, so the total atmosphere on Mercury would be essentially zero, even ignoring the solar wind's "stripping" effect. The fact that the solar wind does tend to tear gases away from the planet causes the time required to remove any gases to be even shorter; but even in the absence of the solar wind Mercury would have essentially no atmosphere.

Mars, on the other hand, is much further from the Sun, which makes it much colder and compresses the gases toward the surface, and although its surface gravity is the same as Mercury's, that difference in temperature would allow it to retain any early atmosphere for some time. However, "some time" is not much different than the current age of the solar system, so although Mars may have had a much thicker atmosphere in the distant past, its present atmosphere should be much thinner. Over time, what little gas it now has should continue to leak into space, and eventually it should also be airless. In this case the stripping effect of the solar wind might increase the rate of loss of the atmosphere, but even ignoring that Mars would have a much thinner atmosphere than the Earth and Venus. Still, in the current era we would expect it to have a measurable, relatively (though not completely) permanent atmosphere, in contrast to Mercury, whose atmosphere is strictly temporary, and escaping as fast as it is created.

For the Earth and Venus, the substantially higher surface gravity causes the time required to lose their atmospheres to be many times longer than the age of the Solar System, so for all practical purposes their atmospheres should be permanent, ignoring the effects of the solar wind. But though there is some loss of gases in the upper atmosphere of Venus due to the stripping effect of the solar wind, it is very small in comparison with both the amount of air already there and the creation of new gases through volcanic activity at the surface. For the Earth, the presence of a strong magnetic field greatly reduces the effect of the solar wind, though there is still a very small amount of atmospheric loss in the regions that are near the boundary of the magnetosphere. But in both cases the ability to hold onto gases and the very minor effect of the solar wind means that Venus and the Earth should have atmospheres about as thick as now for hundreds or thousands of billions of years.

In other words, although the solar wind can have some effect, it is a minor factor compared to the effects of gravity and surface temperature (as discussed in the web page mentioned at the start of this answer).

Now as for the difference in the amount of air on Venus and the Earth, that is due to the presence of liquid water on the surface of the Earth, and the absence of liquid water on the surface of Venus. As discussed at http://cseligman.com/text/ssevolve/atmosphereevol.htm (The Formation and Evolution of Planetary Atmospheres), the early Earth almost certainly had a much thicker atmosphere than now, and in fact both the Earth and Venus should have had atmospheres much thicker than the present atmosphere of Venus. However, most of those atmospheres should have consisted of hydrogen compounds that would have quickly dissociated and disappeared into space, leaving atmospheres made primarily of very large amounts of carbon dioxide (in other words, essentially identical to the present atmosphere of Venus). In the absence of liquid water, such an atmosphere would be essentially permanent (given the discussion above about the retention or loss of atmospheres), and such is the case for Venus. However, since the Earth is further from the Sun than Venus, it was able to develop oceans of liquid water (which would just boil away into the Venusian atmosphere, and being a hydrogen compound, be quickly lost). Those oceans dissolved the carbon dioxide in our atmosphere, turning it into carbonate rocks of various sorts. What is left after that is a much thinner atmosphere made mostly of nitrogen (oxygen only developed billions of years later, as a waste product of plant metabolism). So Venus has a thick atmosphere because the carbon dioxide left over after it lost its hydrogen compounds is still there. But the Earth's oceans removed most of the carbon dioxide, so we have a much thinner atmosphere. Keep in mind that it is thinner because the carbon dioxide was deposited as rocks. It is not thinner because it was lost into space. As noted above, we can hold onto our present atmosphere for periods that for all practical purposes are infinite.

I'd like to hope that between the discussion here and on the linked web pages, your question has been adequately answered. But if you require clarification of any point, please do not hesitate to contact me again.

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Courtney Seligman

Expertise

I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.

Experience

I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Publications
Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

Education/Credentials
I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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