<p>Jupiter <em>itself</em> isn't the radiation source that irradiates its moons. There isn't a big chunk of radioactive material in Jupiter that's blasting out ionizing radiation into the system, so the distance to Jupiter doesn't directly mean that much in terms of how radioactive we expect the environment to be. The source of most of the radiation around Jupiter is, in fact, Io itself -- or more specifically, an interaction between Io and Jupiter.</p>
<p>Io spews out enormous amounts of material into its orbital plane. Jupiter's massive magnetic field then ionizes and accelerates those particles. The accelerated particles are what creates the intense radiation belt that fries Io's surface and makes the other moons uninhabitable (at least on the surface; Europa's theoretical subsurface oceans should be quite well protected).</p>
<p>The radiation belts around Jupiter are much like Earth's Van Allen belts, but vastly larger and more powerful, and fed by Io in addition to the sun.  Just as Earth isn't radioactive, but creates the Van Allen radiation belts, Jupiter is only the source of its belts in the sense that its magnetic field is responsible for generating the energy that makes the belt dangerous.</p>
<p>Don't think about radioactive sources. Radiation belts don't work like that. The best way to think about this is  that the moons receive radiation based on how many accelerated particle paths intersect the body as it moves around its orbit. A thicker section of the radiation belt causes more radiation because there are more particles in that section that can collide with a moon, and further out there are fewer relativistic particles and thus less radiation. The intensity of the radiation belt is based on the specific geometry of the magnetic field (which is not round, but sort of &quot;blown back&quot; by the solar wind), the way Io's particles move, the solar wind, the exact orbit the moon is in, and a lot more.</p>
<p>The mechanics behind all this are very complicated and I've enormously simplified them here. If you're interested, research into Jupiter's magnetosphere and radiation belts will lead to more accurate explanations of what's happening.</p>

CC BY-SA 4.0
2024-10-16T15:22:10.200Z
So then why does Europa receive hundreds/thousands times more radiation than Ganymede/Callisto, considering that sometimes Ganymede/Callisto could actually be closer to Io, and even when it's not there not that much further away? Any why do Ganymede/Callisto, which are the same distance (roughly), have so much difference in radiation?
Starship
CC BY-SA 4.0
2024-10-16T15:50:34.393Z
Io is not itself radioactive, it's charging up a radiation *belt* that is most intense close to Io's orbit and less intense further out.  I'm not really following your question.
Darth Pseudonym
CC BY-SA 4.0
2024-10-16T15:51:44.850Z
Yes. So why is it less hundreds of times less intense on Callisto vs. Ganymede when they are roughly the same distance from Io's orbit (and hence the more intense radiation)
Starship
CC BY-SA 4.0
2024-10-16T15:55:17.650Z
See above, I'm an idiot. They are also aren't hundreds of times apart though (or tens)
Starship
CC BY-SA 4.0
2024-10-16T15:59:15.457Z
Well, a radiation field is not a chunk of uranium that sits at a specific orbital distance and outputs radiation in every direction and thus drops off by a simple function of distance from a clearly defined orbital altitude.  The radiation in a radiation belt comes from collisions with relativistic particles, so the radiation a body encounters is based on the density of the radiation field that it's passing through.
Darth Pseudonym
CC BY-SA 4.0
2024-10-16T16:43:08.010Z
Io is certainly a major source of particles, but Jupiter's huge & powerful magnetosphere also captures a lot of particles from the solar wind.
PM 2Ring
CC BY-SA 4.0
2024-10-16T18:24:39.457Z
It does, but Io's particle load is the reason the radiation field is so insanely strong and the reason Io is in particular getting a mega-dose.
Darth Pseudonym
CC BY-SA 4.0
2024-10-16T15:08:28.297Z
2024-10-18T13:52:21.387Z
2024-10-18T13:52:21.387Z
Darth Pseudonym
<blockquote>
<p>Using the Inverse Square Law, which radiation follows ...</p>
</blockquote>
<p>This is not the case with regard to Jovian radiation sources. It's important to remember that &quot;radiation&quot; comes not just from electromagnetic radiation but also from particles that are moving very quickly (and are typically charged).</p>
<p>It turns out that Io itself is the source of the bulk of the charged particles that cause the extremely intense radiation on Io. Io is volcanically active, and because of its smallish size, some of the volcanic eruptions escape Io's weak gravitation. Jupiter's magnetic field acts to dissociate and ionize the erupted gases that are predominantly sulfur dioxide. Jupiter's magnetic field then acts to make these charged particles rotate with Jupiter.</p>
<p>The end result is a radiational torus (&quot;radiational&quot; in terms of radiation exposure), with Io orbiting in the middle of that torus. The torus is not bound; it thins out with distance from Io's orbit. Moons closer to Io's orbit are subject to more of this radiation than are moons further from Io's orbit.</p>
<p>This <a href="https://commons.wikimedia.org/wiki/File:Currents_in_Jovian_Magnetosphere.png" rel="noreferrer">Wikimedia diagram</a> might aid in your understanding. I cannot vouch for the accuracy of this diagram. It is derived from papers by Edwards, T.M.; Bunce, E.J.; Cowley, S.W.H. (2001), and Cowley, S.W.H.; Bunce, E.J. (2001).</p>
<p>From Wikipedia's Magnetosphere of Jupiter / Dynamics / <a href="https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter#Co-rotation_and_radial_currents" rel="noreferrer">Co-rotation and radial currents</a></p>
<p><a href="https://i.sstatic.net/53X9RI4H.png" rel="noreferrer"><img src="https://i.sstatic.net/53X9RI4H.png" alt="Magnetic field of Jupiter" /></a></p>
<blockquote>
<p>The magnetic field of Jupiter and co-rotation enforcing currents</p>
</blockquote>
<p>Europa and Ganymede are continuously assaulted by radiation from particles that escape the Io torus. Callisto orbits outside the teardrop shaped torus-like space that results in the high radiation on Europa and Ganymede.</p>

CC BY-SA 4.0
2024-10-16T15:23:12.030Z
So then why does Europa receive hundreds/thousands times more radiation than Ganymede/Callisto, considering that sometimes Ganymede/Callisto could actually be closer to Io, and even when it's not there not that much further away? Any why do Ganymede/Callisto, which are the same distance (roughly), have so much difference in radiation?
Starship
CC BY-SA 4.0
2024-10-16T15:56:03.757Z
@Starship It's the proximity to Io's orbit rather than to Io itself that drives the high levels of radiation on Europa and the somewhat high levels on Ganymede. Callisto is far removed, so it receives considerably less. The Io torus is a torus with Jupiter at the center rather than Io.
David Hammen
CC BY-SA 4.0
2024-10-16T15:57:34.753Z
Yep, but there is no potential scenario where Callisto is hundreds of tens of times further from Io's orbit than Ganymede, which is what you would need, right?
Starship
CC BY-SA 4.0
2024-10-16T15:59:32.820Z
@Starship That too is wrong. Jupiter's magnetosphere is complex in shape and definitely does not follow the inverse square law. Forget the inverse square law in this case.
David Hammen
CC BY-SA 4.0
2024-10-16T16:00:41.647Z
So what law does account for the hundreds of times difference, exactly?
Starship
CC BY-SA 4.0
2024-10-16T16:01:54.320Z
@Starship Instead of the inverse square law, which does not apply here, think instead of the weirdness of the Earth's Van Allen belts, but only magnitudes weirder in the case of Jupiter. The Earth does not have a close-by moon that is volcanically active. Jupiter does, and Io's ejections supercharge Jupiter's already highly intensive magnetic field.
David Hammen
CC BY-SA 4.0
2024-10-16T16:30:50.617Z
@Starship Callisto is located well outside of Jupiter's main magnetic field. Since it is charged particles rather than electromagnetic radiation that are the dominant cause of the high radiation levels near Jupiter, that Callisto is outside Jupiter's main magnetic field means that trapped particle density is rather low; I suspect it drops exponentially with distance from Jupiter outside the main magnetic field.
David Hammen
CC BY-SA 4.0
2024-10-17T11:22:04.713Z
@PM2Ring Thanks for incorporating my comments into my answer. I was thinking of (but had not acted on) doing just that. You beat me to the punch, and I truly do thank you for that.
David Hammen
CC BY-SA 4.0
2024-10-16T15:19:02.310Z
2024-10-16T23:57:45.353Z
2024-10-16T23:57:45.353Z
David Hammen
<p>There's a wide range of variation in the Galilean moons' exposure to radiation:</p>
<p><a href="https://en.wikipedia.org/wiki/Callisto_(moon)#:%7E:text=The%20radiation%20level%20at%20Callisto%27s,Earth%20Orbit%20or%20on%20Mars." rel="nofollow noreferrer">Callisto</a> receives only 0.01 rem (0.1 mSv) per day, which is about 10 times the amount of background radiation on Earth. It is about 1.88 million kilometers from Jupiter.</p>
<p><a href="https://en.wikipedia.org/wiki/Ganymede_(moon)#Radiation_environment" rel="nofollow noreferrer">Ganymede</a> receives 5-8 rem (50–80 mSv) per day, which is about 500-800 times the amount of radiation on Callisto, despite being about 1.07 million kilometers from Jupiter, nearly 57% of the distance from Callisto to Jupiter.</p>
<p><a href="https://en.wikipedia.org/wiki/Europa_(moon)#:%7E:text=The%20ionizing%20radiation%20level%20at,long%20as%20an%20Earth%20day." rel="nofollow noreferrer">Europa</a> receives 540 rem (5.4 Sv) per day, which is about 67.5-108 times the amount of radiation on Ganymede and 54,000 times the radiation on Callisto, despite being at about 0.671 million kilometers from Jupiter, which is nearly 63% of the distance from Ganymede to Jupiter and nearly 36% of the distance from Callisto.</p>
<p><a href="https://en.wikipedia.org/wiki/Io_(moon)" rel="nofollow noreferrer">Io</a> receives <a href="https://www.astronomy.com/science/weird-object-jupiters-moon-io/" rel="nofollow noreferrer">3600 rem</a> (36 Sv) per day, roughly 6.7 times as much as Europa, 450-720 times as much as Ganymede, and 360,000 times as much as Callisto. It is about 0.422 million kilometers from Jupiter, which is nearly 63% of the distance from Jupiter to Europa, more than 39% of the distance from Jupiter to Ganymede, and more than 22% of the distance from Jupiter to Callisto.</p>
<p>It might seem obvious that some are much further away from the source of the radiation, which is mainly Jupiter, than others - except that the difference in distance doesn't seem like enough to explain the drastic differences in the amount of radiation each moon receives.</p>
<p>Using the <a href="https://openoregon.pressbooks.pub/radsafety130/chapter/inverse-square-law/" rel="nofollow noreferrer">Inverse Square Law</a>, which radiation follows, and knowing that Callisto receives 0.01 rem (0.1 mSv) per day, based on the distance you would predict that Ganymede would receive around 0.03 rem (0.3 mSv) per day, Europa would receive around 0.075 rem (0.75 mSv) per day, and Io would receive around 0.2 rem (2 mSv) per day. If you work off Io's 3600 rem (36 Sv) per day, then you would get about 1364 rem (13.6 Sv) per day for Europa, about 55 rem (0.55 Sv) per day for Ganymede, and about 18 rem (0.18 Sv) per day for Callisto. These values, however, are extremely far off from the actual ones.</p>
<p>As far as I know, there are no other radiation sources that would cause this much variation. So, why do some of the Galilean moons have so much less radiation than others, significantly more than could be explained by their distance from Jupiter?</p>

CC BY-SA 4.0
2024-10-16T16:40:58.807Z
Related: https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter
PM 2Ring
CC BY-SA 4.0
2024-10-17T13:51:06.243Z
It's worth noting that the inverse square law assumes a point source, and it seems likely the the radiation source in this case (the Jupiter and Io sub-system) is too large for that assumption to hold. Close to an area source radiation is approximately constant regardless of distance, then it falls of linearly, then it follows inverse square.
codeMonkey
CC BY-SA 4.0
2024-10-17T14:34:55.740Z
@codeMonkey Ionizing radiation can come from multiple sources. One source, high energy electromagnetic radiation, follows the inverse square law **if** the source is an isotropic point source. Jupiter does not emit much high energy electromagnetic radiation. Another source is high energy particles, which provide the bulk of the ionizing radiation in the vicinity of Jupiter. Those high energy particles do not follow the inverse square law; the source is not Jupiter itself, and the distribution of those particles is not isotropic. The inverse square law simply does not apply here.
David Hammen
<jupiter><natural-satellites><radiation><galilean-moons>
Why do some of the Galilean moons receive so much less radiation than others?
Starship
# Why do some of the Galilean moons receive so much less radiation than others?

(Asked by: username_0 on 2024-10-16)

There's a wide range of variation in the Galilean moons' exposure to radiation:

[Callisto](https://en.wikipedia.org/wiki/Callisto_(moon)#:%7E:text=The%20radiation%20level%20at%20Callisto%27s,Earth%20Orbit%20or%20on%20Mars.) receives only 0.01 rem (0.1 mSv) per day, which is about 10 times the amount of background radiation on Earth. It is about 1.88 million kilometers from Jupiter.

[Ganymede](https://en.wikipedia.org/wiki/Ganymede_(moon)#Radiation_environment) receives 5-8 rem (50–80 mSv) per day, which is about 500-800 times the amount of radiation on Callisto, despite being about 1.07 million kilometers from Jupiter, nearly 57% of the distance from Callisto to Jupiter.

[Europa](https://en.wikipedia.org/wiki/Europa_(moon)#:%7E:text=The%20ionizing%20radiation%20level%20at,long%20as%20an%20Earth%20day.) receives 540 rem (5.4 Sv) per day, which is about 67.5-108 times the amount of radiation on Ganymede and 54,000 times the radiation on Callisto, despite being at about 0.671 million kilometers from Jupiter, which is nearly 63% of the distance from Ganymede to Jupiter and nearly 36% of the distance from Callisto.

[Io](https://en.wikipedia.org/wiki/Io_(moon)) receives [3600 rem](https://www.astronomy.com/science/weird-object-jupiters-moon-io/) (36 Sv) per day, roughly 6.7 times as much as Europa, 450-720 times as much as Ganymede, and 360,000 times as much as Callisto. It is about 0.422 million kilometers from Jupiter, which is nearly 63% of the distance from Jupiter to Europa, more than 39% of the distance from Jupiter to Ganymede, and more than 22% of the distance from Jupiter to Callisto.

It might seem obvious that some are much further away from the source of the radiation, which is mainly Jupiter, than others - except that the difference in distance doesn't seem like enough to explain the drastic differences in the amount of radiation each moon receives.

Using the [Inverse Square Law](https://openoregon.pressbooks.pub/radsafety130/chapter/inverse-square-law/), which radiation follows, and knowing that Callisto receives 0.01 rem (0.1 mSv) per day, based on the distance you would predict that Ganymede would receive around 0.03 rem (0.3 mSv) per day, Europa would receive around 0.075 rem (0.75 mSv) per day, and Io would receive around 0.2 rem (2 mSv) per day. If you work off Io's 3600 rem (36 Sv) per day, then you would get about 1364 rem (13.6 Sv) per day for Europa, about 55 rem (0.55 Sv) per day for Ganymede, and about 18 rem (0.18 Sv) per day for Callisto. These values, however, are extremely far off from the actual ones.

As far as I know, there are no other radiation sources that would cause this much variation. So, why do some of the Galilean moons have so much less radiation than others, significantly more than could be explained by their distance from Jupiter?

**Tags:** jupiter, natural-satellites, radiation, galilean-moons

> **Comments:**
> **username_1**: Related: https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter  
> **username_2**: It's worth noting that the inverse square law assumes a point source, and it seems likely the the radiation source in this case (the Jupiter and Io sub-system) is too large for that assumption to hold. Close to an area source radiation is approximately constant regardless of distance, then it falls of linearly, then it follows inverse square.  
> **username_3**: @username_2 Ionizing radiation can come from multiple sources. One source, high energy electromagnetic radiation, follows the inverse square law **if** the source is an isotropic point source. Jupiter does not emit much high energy electromagnetic radiation. Another source is high energy particles, which provide the bulk of the ionizing radiation in the vicinity of Jupiter. Those high energy particles do not follow the inverse square law; the source is not Jupiter itself, and the distribution of those particles is not isotropic. The inverse square law simply does not apply here.  

---

## 2 answers

### Answer by username_3 on 2024-10-16. Score: 28 | Accepted answer

> Using the Inverse Square Law, which radiation follows ...

This is not the case with regard to Jovian radiation sources. It's important to remember that "radiation" comes not just from electromagnetic radiation but also from particles that are moving very quickly (and are typically charged).

It turns out that Io itself is the source of the bulk of the charged particles that cause the extremely intense radiation on Io. Io is volcanically active, and because of its smallish size, some of the volcanic eruptions escape Io's weak gravitation. Jupiter's magnetic field acts to dissociate and ionize the erupted gases that are predominantly sulfur dioxide. Jupiter's magnetic field then acts to make these charged particles rotate with Jupiter.

The end result is a radiational torus ("radiational" in terms of radiation exposure), with Io orbiting in the middle of that torus. The torus is not bound; it thins out with distance from Io's orbit. Moons closer to Io's orbit are subject to more of this radiation than are moons further from Io's orbit.

This [Wikimedia diagram](https://commons.wikimedia.org/wiki/File:Currents_in_Jovian_Magnetosphere.png) might aid in your understanding. I cannot vouch for the accuracy of this diagram. It is derived from papers by Edwards, T.M.; Bunce, E.J.; Cowley, S.W.H. (2001), and Cowley, S.W.H.; Bunce, E.J. (2001).

From Wikipedia's Magnetosphere of Jupiter / Dynamics / [Co-rotation and radial currents](https://en.wikipedia.org/wiki/Magnetosphere_of_Jupiter#Co-rotation_and_radial_currents)

[![Magnetic field of Jupiter](https://i.sstatic.net/53X9RI4H.png)](https://i.sstatic.net/53X9RI4H.png)

> The magnetic field of Jupiter and co-rotation enforcing currents

Europa and Ganymede are continuously assaulted by radiation from particles that escape the Io torus. Callisto orbits outside the teardrop shaped torus-like space that results in the high radiation on Europa and Ganymede.

> **Comments:**
> **username_0**: So then why does Europa receive hundreds/thousands times more radiation than Ganymede/Callisto, considering that sometimes Ganymede/Callisto could actually be closer to Io, and even when it's not there not that much further away? Any why do Ganymede/Callisto, which are the same distance (roughly), have so much difference in radiation?  
> **username_3**: @username_0 It's the proximity to Io's orbit rather than to Io itself that drives the high levels of radiation on Europa and the somewhat high levels on Ganymede. Callisto is far removed, so it receives considerably less. The Io torus is a torus with Jupiter at the center rather than Io.  
> **username_0**: Yep, but there is no potential scenario where Callisto is hundreds of tens of times further from Io's orbit than Ganymede, which is what you would need, right?  
> **username_3**: @username_0 That too is wrong. Jupiter's magnetosphere is complex in shape and definitely does not follow the inverse square law. Forget the inverse square law in this case.  
> **username_0**: So what law does account for the hundreds of times difference, exactly?  
> **username_3**: @username_0 Instead of the inverse square law, which does not apply here, think instead of the weirdness of the Earth's Van Allen belts, but only magnitudes weirder in the case of Jupiter. The Earth does not have a close-by moon that is volcanically active. Jupiter does, and Io's ejections supercharge Jupiter's already highly intensive magnetic field.  
> **username_3**: @username_0 Callisto is located well outside of Jupiter's main magnetic field. Since it is charged particles rather than electromagnetic radiation that are the dominant cause of the high radiation levels near Jupiter, that Callisto is outside Jupiter's main magnetic field means that trapped particle density is rather low; I suspect it drops exponentially with distance from Jupiter outside the main magnetic field.  
> **username_3**: @username_1 Thanks for incorporating my comments into my answer. I was thinking of (but had not acted on) doing just that. You beat me to the punch, and I truly do thank you for that.  

---

### Answer by username_4 on 2024-10-16. Score: 15

Jupiter *itself* isn't the radiation source that irradiates its moons. There isn't a big chunk of radioactive material in Jupiter that's blasting out ionizing radiation into the system, so the distance to Jupiter doesn't directly mean that much in terms of how radioactive we expect the environment to be. The source of most of the radiation around Jupiter is, in fact, Io itself -- or more specifically, an interaction between Io and Jupiter.

Io spews out enormous amounts of material into its orbital plane. Jupiter's massive magnetic field then ionizes and accelerates those particles. The accelerated particles are what creates the intense radiation belt that fries Io's surface and makes the other moons uninhabitable (at least on the surface; Europa's theoretical subsurface oceans should be quite well protected).

The radiation belts around Jupiter are much like Earth's Van Allen belts, but vastly larger and more powerful, and fed by Io in addition to the sun. Just as Earth isn't radioactive, but creates the Van Allen radiation belts, Jupiter is only the source of its belts in the sense that its magnetic field is responsible for generating the energy that makes the belt dangerous.

Don't think about radioactive sources. Radiation belts don't work like that. The best way to think about this is that the moons receive radiation based on how many accelerated particle paths intersect the body as it moves around its orbit. A thicker section of the radiation belt causes more radiation because there are more particles in that section that can collide with a moon, and further out there are fewer relativistic particles and thus less radiation. The intensity of the radiation belt is based on the specific geometry of the magnetic field (which is not round, but sort of "blown back" by the solar wind), the way Io's particles move, the solar wind, the exact orbit the moon is in, and a lot more.

The mechanics behind all this are very complicated and I've enormously simplified them here. If you're interested, research into Jupiter's magnetosphere and radiation belts will lead to more accurate explanations of what's happening.

> **Comments:**
> **username_0**: So then why does Europa receive hundreds/thousands times more radiation than Ganymede/Callisto, considering that sometimes Ganymede/Callisto could actually be closer to Io, and even when it's not there not that much further away? Any why do Ganymede/Callisto, which are the same distance (roughly), have so much difference in radiation?  
> **username_4**: Io is not itself radioactive, it's charging up a radiation *belt* that is most intense close to Io's orbit and less intense further out.  I'm not really following your question.  
> **username_0**: Yes. So why is it less hundreds of times less intense on Callisto vs. Ganymede when they are roughly the same distance from Io's orbit (and hence the more intense radiation)  
> **username_0**: See above, I'm an idiot. They are also aren't hundreds of times apart though (or tens)  
> **username_4**: Well, a radiation field is not a chunk of uranium that sits at a specific orbital distance and outputs radiation in every direction and thus drops off by a simple function of distance from a clearly defined orbital altitude.  The radiation in a radiation belt comes from collisions with relativistic particles, so the radiation a body encounters is based on the density of the radiation field that it's passing through.  
> **username_1**: Io is certainly a major source of particles, but Jupiter's huge & powerful magnetosphere also captures a lot of particles from the solar wind.  
> **username_4**: It does, but Io's particle load is the reason the radiation field is so insanely strong and the reason Io is in particular getting a mega-dose.  

---

