Will a large gas giant always have powerful radiation belts or could a milder radiation environment exist even around super Jovian planets? I know Jupiter’s radiation belts are so strong due to a combination of the strength of its magnetic field, the source of particles from Io, and its own atmosphere. On a large gas giant with no close-orbiting large moon/Io analogue, and a slower rotation, could the magnetic field and radiation belts be more mild? I’m wondering about the potential habitability of a large exomoon orbiting a Super Jovian planet (super Jovian, because a more massive gas giant is more likely to form a more massive moon) in the habitable zone of its star.
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Yes, certainly!. Jupiter has such a strong magnetic field, due to it's rotational speed, and has no connection to it's mass.
How is a magnetosphere generated?
All the planets except Venus and Mars, which have inductive magnetosphere, the magnetic fields are generated via the magnetohydrodynamic processes, which basically means the molten iron in the core generates magnetic field via Amperes's law. As the core spins faster, the magnetic field grows stronger and stronger, so this shows that rotational velocity is proportional to the magnetic field strength/H.
Why Jupiter has a strong magnetic field?
Contrary to popular belief, the secret to Jupiter's strong magnetic field is the rotational speed, with just a day of 10 hours. This gas giant spins at extremely fast speeds, and this makes the core to generate stronger fields. Mass also plays a slight role, as there is more matter inside the core to generate magnetic field from.
Why most gas giants have fast rotational speed?
Gas giants have stronger rotational speeds, due to their mass. Gas giants are usually of greater mass, which makes that accrete more and more continuously so the angular momentum is conserved and added up to the protoplant during it's formation from the protoplanetary disk, so most gas giants have a higher rotational speed, with exceptions like HIP 65426 b. Plus greater mass means greater inertia.
Magnetosphere's relation to radiation like Van Allen Belts
If the magnetic field is stronger then the magnetosphere would be larger, which would capture more particles from the solar wind. However if the magnetohydrodynamic process generates a smaller magnetosphere, the input of charged subatomic particles would decrease, leading to milder radiation belts. HIP 65426 b may have milder belts. However, one extremely crucial factor to consider is the, solar wind activity of the star, so even if it has lesser magnetic field, if it's perihelion is extremely close to the Sun, in the case it would be called hot Jupiter, it would have stronger radiation belts than Jupiter. Even if it were hot, it could provide Jean's escape and ionize the upper atmosphere and again create a stronger radiation belt
How radiation belts influence habitability?
Ionizing radiation is hazardous, as it can penetrate the epidermis. Once penetrated it can damage the DNA and cause mutations, plus it can damage the protein molecules and leptides. These mutations would destroy all of the other creatures which are not resistant to radiation, and pave the path for extremophiles due to the survival of the fittest
In conclusion, if a super-Jovian planet had a very slow rotational speed depending on it's formation scenarios, the radiation belts would be milder, leading to habitability.
Thank you, Hope it helps you!
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$\begingroup$ I would argue that Jupiter's mass plays a key role in its magnetic field, because Jupiter's huge mass creates the pressures necessary to develop the ocean of metallic hydrogen that's largely responsible for generating the field. Spinning fast doesn't do anything if you don't have a huge amount of conductive material to spin. $\endgroup$ Commented Sep 15, 2023 at 19:44
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Jupiter's magnetic field is dangerous only in the radiation belts, the parts of the magnetosphere where charged particles are concentrated. Obviously the parts of the magnetosphere where charged particles are not concentrated wouldn't have any higher density of charged particles than the average volume of interplanetary space at the distance of Jupiter from the Sun. Maybe less if the radiation belts gather all the charged particles.
So a space probe or a manned spaceship could maneuver though the magnetosphere of Jupiter to avoid the radiation belts, and/or have some sort of shielding against charged particles necessary for the long voyage from Earth to Jupiter.
If you are asking about the potential habitability of a large moon of Jupiter or some other gas giant planet, that would depend on a lot of factors.
If the moon orbits outside of the magnetosphere of Jupiter, it will not be protected from charged particles in the solar wind and in cosmic rays by the magnetosphere, anymore than any other object which orbits the Sun at Jupiter's distance from the Sun but outside of Jupiter's magnetosphere.
If the moon orbits inside Jupiter's magnetosphere and also inside one of Jupiter's radiation belts, it will suffer from a higher intensity of charged particles than in interplanetary space.
If the moon orbits inside Jupiter's magnetosphere but not inside any radiation belt, it should be impacted by far fewer charged particles than it would be impacted by interplanetary space.
Location, location, location. The location of a moon's orbit in the magnetosphere of a giant planet is certainly a significant factor in its habitability.
Scientists define a potentially habitable world for liquid water using life in general as one which has bodies of liquid water on its surface. Habitability for humans requires more specific and restricted conditions and is a subcategory of habitability for liquid water using life in general.
Without an atmosphere, water on the surface of of world will boil away into water vapor, so a reasonably dense atmosphere is necessary for liquid water and for potential habitability.
A moon potentially habitable for liquid water using life would have to have a significant atmosphere, and so the atmosphere would protect life on the surface from charged particles. Of course the charged particles would gradually strip away the atmosphere of that moon.
The planet Venus has no magnetic field. Venus is only 0.72 AU from the Sun, while Jupiter is 5.2 AU from the Sun, 7.222 times as far, so the solar wind at Venus is about 52.16 times as strong at Venus as at Jupiter. But Venus still retains an atmosphere many times denser than Earth's after billions of years of the solar wind knocking away atmospheric particles.
So a hypothetical Earth or even Venus sized moon of a giant planet in another star system should have no problem with life being harmed by radiation, even if it orbits in a radiation belt of its planet.
And such a large moon might possibly have its own strong magnetic field to protect it from charged particles.
I note that there are believed to be global oceans under many kilometers of ice in many of the larger objects in the outer solar system.
https://en.wikipedia.org/wiki/List_of_largest_lakes_and_seas_in_the_Solar_System
And it is speculated that that there could be life in those global oceans, and if so it would be protected from radiation by many kilometers of ice.
And you might be interested in the questions about habitable moons in the Worldbuilding Stack Exchange.
And you might be interested in:
https://iopscience.iop.org/article/10.1088/2041-8205/776/2/L33/meta
