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In this answer to How is space a vacuum when there are planets, gases, etc? I mention that most of Mars' original atmosphere was swept away by the solar wind after the planet lost its magnetic field and ability to hold the swift charged particles at bay1.

But where did Mars' original atmosphere end up? How far did it get?

1though comments there suggests that's not necessarily a correct and/or complete explanation.

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    $\begingroup$ @uhoh As written, this question is worse than asking where the stars that were born in the same gas cluster as the Sun was born are now. That question is at least worthy of asking because there are some signatures that might help answering that question. But for now, the answer to that question is "who knows?" There are candidates, but some astronomers reject those candidates as not being a solid match. Asking where the gas is now that escaped from Mars long ago is even more of a "who knows" type of question. $\endgroup$
    – David Hammen
    Commented Aug 9, 2021 at 3:26
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    $\begingroup$ I don't think estimates can be made. Small particles are subject not only to gravitation but also to electromagnetism. We don't know how much atmosphere Mars had initially. We don't know when Mars lost the bulk of its atmosphere. The one thing science does know is that it if it did exist, it is now well outside the solar system. $\endgroup$
    – David Hammen
    Commented Aug 9, 2021 at 4:06
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    $\begingroup$ @uhoh Your question is a very basic star system formation question. Once a star ignites, the photons and solar wind from the star "quickly" clears the dust and gas from the protoplanetary disk. The gas spirals out, the dust spirals in. By "quickly" I mean over the course of several hundreds of years to a few hundreds of thousands of years. $\endgroup$
    – David Hammen
    Commented Aug 9, 2021 at 9:31
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    $\begingroup$ @uhoh Mars started losing its atmosphere shortly after Mars first formed. The atmosphere of Mars from a few hundred thousand years ago was similar to its atmosphere today. You're essentially asking about the atmosphere of Mars a few billion years ago, when Mars might have had a substantial atmosphere. That atmosphere is long gone, ejected from the solar system. The thousands to a few hundred thousand years needed to eject gases from the solar system is a short span of time astronomically. $\endgroup$
    – David Hammen
    Commented Aug 9, 2021 at 9:53
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    $\begingroup$ @DavidHammen the question is hard, you don't know the answer and you are not interested in the answer, and for some reason you also posted an answer that doesn't answer it. But these don't add up to the question being objectively "not good". $\endgroup$
    – uhoh
    Commented Aug 9, 2021 at 23:19

2 Answers 2

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Assuming most of the escaping Martian atmosphere is entrained in the solar wind, it will flow outward until it reaches the termination shock, and then slow down in the heliosheath until it reaches the heliopause, currently at a distance of about 120 AU. (The actual location will change, of course, depending on things like the strength of the solar wind and the local conditions of the interstellar medium.) At this point, the former Martian-atmosphere atoms and ions ("Martian particles" for short) will be at rest with respect to the local interstellar medium (ISM) and will gradually mix into it. Since the Sun is moving with respect to the local ISM (currently at about 25 km/s), the Martian particles will gradually separate from the Sun: in a million years, they would nominally be about 25 parsecs away. (They would be diffusing through the ISM as well, but I have no idea what the speed of that would be.)

Beyond that, things start to get very fuzzy. The Sun's motion with respect to the local ISM will be changing, due to the Sun's orbit and the orbits of the ISM clouds. In addition, the ISM gas will be subject to turbulence, strong stellar winds from massive stars, supernova shock waves, and bulk noncircular motions driven by spiral arms and the occasional accreting dwarf galaxy. Differential rotation will also smear out the location of the Martian particles, since even in the absence of all the other ISM motions they will end up following slightly different orbits in the Galaxy. Some of the Martian particles may end up in the halo (e.g., if they are caught up in an expanding superbubble due the combined massive-star stellar winds and supernovae of a large star-formation region). Since stars form out of dense molecular clouds in the ISM, some will end up being incorporated in new stars.

Crudely speaking, most of the Martian particles will probably still be in the disk (some as part of the ISM, some inside stars), in a fuzzy annulus. Where this annulus is will be very hard to determine. This is because we're assuming most of the Martian atmosphere was lost in the first billion years (or less), which means that most of the Martian particles entered the ISM between 3.5 and 4.5 billion years ago -- and we don't really know where the Sun was then. Recent research has shown that stars in spiral-galaxy disks can have the radii of their orbits changed significantly -- moving inward or outward-- by interactions with transient spiral arms (and possibly also by interactions with the bar). This means the Sun might have been formed on an orbit a couple of kiloparsecs closer to the Galactic center than it is now (curent radius of Sun's orbit $\sim 8$ kpc), or a couple of kiloparsecs further away, and then moved (possibly in a series of stages) to its current location sometime between now and then.

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There are three possible fates for small particles:

  1. They fall into the Sun.
  2. They escape the solar system.
  3. They fall into a planet.

Small particles larger than 1 micron across tend to fall into the Sun due to Poynting-Robertson drag. These slowly infalling particles are the source of the zodiacal light. Particles a few nanometers or so across (i.e., molecules) tend to escape the solar system due to interactions with the solar wind.

With regard to option 3, that does happen, but it is rare. The fate of most small particles is to either fall into the Sun or to be ejected from the solar system along with the particles that are ejected from the surface of Sun.

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  • $\begingroup$ "where" and "how far" are asking for a location or at least a distance; but "escape" is a verb. Q: Where did they go? A: They escaped. Q: Yes, but where did they go? How far did they get? While this may be a series of true statements, it doesn't address the question as written. $\endgroup$
    – uhoh
    Commented Aug 9, 2021 at 0:49
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    $\begingroup$ @uhoh: The question as written is nonsensical. Escape implies escape to infinity from the solar gravity well; i.e. the gas that made up Mars' atmosphere might be anywhere in the galaxy by now. No one can tell that. Don't critisize a perfectly good answer by the quality of the question. In fact, the question as written is unanswerable and should be closed. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Aug 9, 2021 at 1:02
  • $\begingroup$ @AtmosphericPrisonEscape post that as an answer and lets see how folks vote on it. "Escape to infinity" is unlikely, "anywhere in the galaxy" will be surprising but if it can be supported properly it will be delightful. But simply not knowing the answer one's self does not a priori make the question nonsense. This is a hard question; not all SE questions can be answered easily. $\endgroup$
    – uhoh
    Commented Aug 9, 2021 at 1:06
  • $\begingroup$ I agree with @AtmosphericPrisonEscape: The question as written is nonsensical. I tried to do something the OP apparently hates me doing with nonsensical questions, which is to read sense into the nonsense. $\endgroup$
    – David Hammen
    Commented Aug 9, 2021 at 3:00
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    $\begingroup$ @DavidHammen nobody likes to have things "read into" what they write. I have a hunch if folks started reading things into your posts you'd go through the roof! If the question is not clear to you or you suspect it might need improvement before you can answer, step 1 is to post a comment asking for clarification or adjustment. $\endgroup$
    – uhoh
    Commented Aug 9, 2021 at 3:49

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