What could possibly save an atmosphere other than a magnetic field? Why can't a magnetic field save the atmosphere in certain cases?

Question

Similar question here.

We know as a fact that the magnetic field protects planets from Solar Wind, a damaging, continuous, atmosphere-stripping wind of charged ions. Thus, a magnetic field protected planet will have a stable atmosphere. Vice versa.

Now let's take a look at some examples that logically fit this theory.

Earth

Earth has a liquid iron outer core which conducts electricity generated by convection to form a huge electromagnet[technically].This magnetic field protects Earth from the Solar Wind stripping away its atmosphere. We can see its effect by looking at Earth currently, not nessesarily affected by the damaging effect of such wind on the atmosphere, only to have minor damage by direct hits of CMEs.

Mars

Mars used to have a magnetic field. In fact, some analyzations of the crustal fields recently made by robots sent to Mars have shown that Mars may have even had a stronger magnetic field than Earth. Sadly, about 4 bya[according to Wikipedia], either repeated bombardment from large celestrial objects that disrupted the interior, or the solidification of its outer core have caused the degration of such. Consequently, Mars' atmosphere is being constantly stripped away by the Solar Wind, and its atmosphere is drastically thinner that what is started with.

These are obviously some pretty straightforward examples that fit very well with the first statement.

Then, there are two rather contradictory planets.

Venus

Venus has no magnetic field. It has a very small and dilute magnetic tail according to Wikipedia, where reconnections and many different events happen. The Solar Wind carries the sun's magnetic field lines which wrap around Venus forming a not very protective induced magnetic field, and according to Wikipedia again the area magnetopause and ionopause is a strong[relatively saying] barrier which prevents the Solar Wind from penetrating deeper into the atmosphere. However, this offers in all little protection the Venus. Thus, naturally, we would expect Venus to act like Mars, with its atmosphere stripped away in a few million/billion years. Yet, this isn't the case. In face, what's more interesting is that Venus orbits closer the Sun than Mars, which means that it gets a higher "dose" of the Solar Wind, which should strip off the atmosphere rather quickly. Even more interesting is that Venus has a thicker atmosphere than Earth.

On the other question I linked, MacUserT said that

...the ionosphere of Venus direct interaction with the solar winds causes an externally induced magnetic field...

Even if this is the case, Venus lacks a intrinsic magnetic field[self generated], which is the "main" protection. Even with this induced field, this protection is substantially not enough, as the Solar Wind [according to space.com] penetrates relatively deep into the planetary exosphere and causes substantial atmosphere loss. This loss, like Earth thru its poles and the tail, would mainly occur on the tail of the induced magnetosphere. The main losses would be hydrogen, helium ions, and according to Wikipedia, also oxygen ions.

So my question is that how is Venus exactly protected by this "induced field" and what is the power of the field? How much atmosphere does Venus lose in one day, and how is this induced field generated?

Now, let's look at another planet with opposite views.

Mercury

Mercury has a large core Mercury also mas a liquid outer core. It is also spinning substantially fast enough so that it can generate a magnetic field stronger that what Mars and Venus have now[It is 300 nT]. However, due to its close orbit, it faces 3 times more pressure from the Solar Wind than that of Venus. This doesn't mean nessesarily that it can't support an atmosphere. My teacher said that Mercury has a very thin atmosphere constantly being stripped away by the Sun made of gasses such as H, He, O, Na, Ca, K, and many other elements from the planetary crust.

However, since it has a relatively speaking, strong magnetic field, doesn't is mean that it should at least have more or less the atmosphere density of Mars? What protection does the magnetic field offer of this isn't the case?

You may be confused so I will I guess put my questions below:

Since Mercury it has a relatively speaking, strong magnetic field, doesn't is mean that it should at least have more or less the atmosphere density of Mars? What protection does the magnetic field offer of this isn't the case?

Answered

So my question is that how is Venus exactly protected by this "induced field" and what is the power of the field? How much atmosphere does Venus lose in one day, and how is this induced field generated?

Answered

Answer 1

usrLTK's answer provides a lot of good details and in particular explains why Mercury wouldn't have much of an atmosphere.

Let me complicate the picture a little by pointed out that some recent research indicates that magnetic fields may not be the guaranteed, automatic atmosphere-protection devices that conventional wisdom suggests. In particular, Gunell et al. (2018) point out that planetary magnetic fields may, in some circumstances, increase atmospheric loss (basically, the cusps and polar caps of the magnetic field can accelerate atmospheric ions out into space). Their calculations suggest, for example, that if Venus had a magnetic field as strong as Earth's, it would lose atmosphere faster than it currently does. Meanwhile, if Earth had only a Venus-style weak, induced magnetic field, it would lose hydrogen a little faster but oxygen more slowly than it does now.

Intriguingly, Venus is currently losing atmosphere at a rate of about 0.5 kg/s, while Earth is losing atmosphere at 1.4 kg/s -- almost three times faster than Venus. (For Mars, the rate is somewhere between 0.7 and 2.1 kg/s.)

Answer 2

If I'm reading your question correctly, your suggestion is that Mercury, having a small magnetic field, should have some atmosphere, perhaps more atmosphere than Mars, with no magnetic field, though Mars does have some surface generated magnetic field regions, it just lacks a field generated by its core.

Solar stripping due to no magnetic field isn't the only factor in maintaining a planet's atmosphere. There's also thermal escape or Jeans escape, which happens when the upper atmosphere's gas molecules move fast enough to escape the planet's gravity. It's a byproduct of temperature and escape velocity.

Additional factors probably play a role, such as outgassing or how much atmosphere there was to begin with, or very large meteor impacts.

Jeans escape can happen whether there's a magnetic field or not. Mercury has the hottest upper atmosphere temperature of the planets (Venus has hotter surface temperature, not upper atmosphere temperature), and it has the lowest escape velocity, so it's the worst of the 8 planets at maintaining an atmosphere. Based on temperature and escape velocity, Mercury loses CO2, which is the heaviest common gas, so, even if Mercury had the strongest magnetic field imaginable, it would still lose its atmosphere.

Mars is much colder with a slightly higher escape velocity so it maintains its atmosphere better, at least against thermal escape. Mars should retain most of the CO2 and perhaps its N2 as well, but a strong solar wind can strip a planet of gas molecules that can't escape thermally, so both methods of atmospheric loss should be considered.

Mars very likely once had an atmosphere. Its polar ice and dried up river beds support that conclusion.

Assuming it had them in the past, Mars may have lost its lighter gasses, such as Ammonia (NH3), Methane (CH4) and Water vapor (H20) as a result of atmospheric escape even before it lost its magnetic field.

Mars may have also lost a share of its atmosphere during a giant impact.

And, while not directly related to your question. Mercury doesn't really have what I'd call an atmosphere, it has an exosphere, which some people call an atmosphere, but I think that's poor terminology. Mercury's exosphere is created by the same process that stripped Mars of it's atmosphere: the solar wind.

In the case Venus, while the induced magnetic field is considerably weaker and reaches less far from the planet than Earth's, it provides some protection of Venus's atmosphere. Stack exchange question on that here. I think the answer could go into more detail, but it gets the gist of it.

Venus actually is losing atmosphere to the solar wind, but it has enough that the rate of loss hasn't stripped the planet, and may not happen for a long time, perhaps when the sun expands a bit . . . but I'm just throwing that out there).

I don't know how precise the data is on this, but Venus's magnetic field offers some protection, so does Venus's gravity relative to Mars's and the vast amount of atmosphere that Venus has also offers some protection. Solar stripping is based on collisions, so a solar particle is likely to only remove one gas molecule. (kinda/sorta). The solar wind is pretty dispersed, so it takes a long time for solar stripping to have an effect. I feel I should add that to my knowledge, that Mars lost its atmosphere to solar stripping is generally agreed with but not 100% certain. I personally like the big meteor impact as playing a role too, but I'm mostly guessing.

To get a better model of atmospheric loss, we'd need to know what the planet's atmospheres were like billions of years ago, and we don't have that information, so there are some unknowns on the rates of loss.

Answer 3

Nitrogen and oxygen have paramagnetic moments (strong) where as carbon dioxide has a diamagnetic moment.

Paramagnetic moments are typically much stronger than diamagnetic moments.

Since carbon dioxide is a heavy trace gas, it rarely leaves the troposphere.

Also, since Venus rotates in the direction opposite to all the other planets in the solar system, there was a conjecture that Venus might have had a collision with a carbon rich object in the distant past.

And since Earth and Mars are down wind from Venus, most of the carbon on Earth and on Mars was a result of that collision.

Mercury has a little carbon but the Sun is a heavy metal star (helium is considered to be a heavy metal) where 1% of the Sun is oxygen - all the elements found on the surface of Mercury can be produced by the Sun from that 1%.