OlympusMons94 t1_j29jrnd wrote on December 30, 2022 at 5:07 PM

Reply to comment by ackillesBAC in How much does the liquid magma of the Earth affect it's surface temperature? by tripperfunster

It doesn't. That's just an outdated, incorrect idea.

Fast moving charged particles from the solar wind colliding with the upper atmosphere can gradually strip away some of the atmosphere through a process called sputtering. Magnetic fields shield from and redirect charged particles, so they can reduce this type of atmospheric loss (but planetary magnetic fields also contribute to atmospheric loss in other ways).

The motion (from convection and rotation) of the electrically conducting molten iron in Earth's outer core generates a magnetic field around the planet. Because this magnetic field is generated within the planet, it is described as an intrinsic magnetic field. The idea was that this is required to prevent the solar wind from stripping away the atmosphere.

However, Venus has a very thick atmosphere, and being closer to the Sun is subjected to a stronger solar wind than Earth. Yet, Venus lacks an intrinsic magnetic field (likely because its core, while molten, is not convecting). Because it lacks an intrinsic magnetic field, the upper atmosphere is exposed to the solar wind and its magnetic field, which induces a weak magnetic field in Venus' upper atmosphere. This induced magnetic field in turn protects the atmosphere from sputtering escape more or less like the intrinsic magnetic field would. The induced magnetosphere is not unique to Venus. Any atmosphere, be it Venus', Mars', or a comet's, exposed to the solar wind will develop an induced magnetic field. As such, atmospheric loss from sputtering is relatively insignificant for not only Earth with its intrinsic magnetic field, but for Venus and Mars as well.

What matters more for the ability to retain an atmosphere is ultimately the balance of a planet's gravity against the motions of gas particles caused by uncharged solar radiation, that is light, which is not shielded by magnetic fields. If the energy from sunlight causes the gas particles to reach escape velocity, they are lost to space. This is thermal escape, and encompasses a number of different processes.

Of particular relevance to Mars, ultraviolet light from the Sun breaks apart CO2 and water vapor molecules, producing ions which move faster than Mars' relatively low escape velocity. Venus and Earth have much higher gravity, so have been more able to hold onto their CO2/oxygen and nitrogen atmospheres. (Although at present, Mars isn't losing its atmosphere much faster than Earth or Venus are. It must have lost atmosphere emuch more rapidly in the past, particularly because the younger Sun would have emitted more UV radiation.)

As it is, though, Venus has lost almost all of its surface/atmospheric water because of solar UV and hydrogen escape. The runaway greenhouse effect it experienced evaporated/boiled any oceans, putting the H2O in the atmosphere where it could be broken up into hydrogen and OH/oxygen. Because hydrogen is so light, it is much more easily lost from the atmosphere to thermal escape than heavier gases like nitrogen, oxygen, or CO2.

ackillesBAC t1_j29uqhl wrote on December 30, 2022 at 6:16 PM

Wonderful answer I thoroughly enjoyed reading that. You must have some sort of background in this stuff?

So going beyond the magnetic shielding concept, what about the idea that an active core and volcanoes are required to release the gases into the atmosphere in the first place?

Edit: scratch that I just seen your other comment answering that question already.

Losing the requirement for having an active core, I would assume, would drastically increase the number possible habitable worlds out there.

This is fascinating stuff to think about, even though it is basically completely irrelevant to day-to-day life.

tripperfunster OP t1_j2a1stu wrote on December 30, 2022 at 7:00 PM

Wow, what a great answer. I think my poor brain needs time to digest all of this.

Thank you for taking the time to be so thorough.

WarpingLasherNoob t1_j2b84oi wrote on December 30, 2022 at 11:41 PM

Thanks for the great answer. I have bit of a follow up question - does a planet need an active core to have a magnetosphere?

OlympusMons94 t1_j2bne1r wrote on December 31, 2022 at 1:32 AM

No. An induced magnetosphere (like Venus, Mars, Europa, Titan, comets, etc. have) doesn't require or have anything to with the core. It just requires the presence of some kind of atmosphere, in which the magnetic field is to be induced.

An intrinsic magnetosphere (like the Sun, Ganymede, Earth, and the other five planets have) is by definition generated in the interior of a planet, and for rocky/terrestrial planets lile Earth and Mercury this would tend to be in the metallic core (as opposed to the rocky mantle). But gas giants and ice giants generate their intrinsic magnetic fields above their core. For example, Jupiter's and Saturn's magnetic fields are generated in their liquid metallic hydrogen mantles.

An "active core" isn't really a scientific term, and can have different meanings in popular discourse. The usual, better meaning is that there is an active dynamo in the core, generating an intrinsic planetary magnetic field. But the absence of an intrinsic magnetic field and the core therefore not being "active" in this way does not imply the core is solid (let alone not rotating; all cores rotate along with the rest of the planet). There needs to be additional forcing to generate a dynamo. (For example in the case of Earth's core, the freezing out of the inner core causes the outer core to convect. Planetary rotation twists this vertical convective motion into spirals and this combined motion drives the dynamo.)

Often, "active core" is instead or additionally taken to indicate or be synonymous with active volcanism or tectonics. But these are driven by processes in the mantle and crust, and not directly related to the core, let alone the magnetic field. So this idea of an "active core" is "not even wrong".