Earth-sized Jovian Moons

[Apollo7]

On par with my previous post I also wanted to bring up this item. I've read on another forum that Jupiters great magnetic field would make exploration of its moons by human-kind very difficult. I believe the radiation belts around the king of the planets were referred to as particle accelerators and would thus be deadly to any man daring to enter them.

If this is the case, with the great spate of very massive Jovian worlds being found today and the speculation that they may harbor Earth sized or larger moons, how might life develop on such a world?

In Star Wars we have a couple of examples of planets in this situation, namely The Sanctuary Moon of Endor and Yavin IV. Would a planet the size of Earth embeded within the magnetic field of a 4 Jovian mass planet be able to withstand the radiation? Can someone endulge my curiosity as to how such a situation might work? If these jovian dominated systems are as popular as they seem, we may be looking for life on moons and not planets (in a sense anyway).

[Don. Edwards]

I think there is a very good chance one would and could have life. You have to remember that Earth has one thing that the jovian moons don't have. A very strong magnetic field of its own. That magnetic field protects us from the worst possible radiation that can come our way. Namely solar flare radiation. That is many times stronger than even Jupiter can put out.
So if Earth can fend off the strongest radiation thrown at it I am almost sure an Earth size terrestrial moon could do the same as long as it too had a substantial magnetic field. This of course means this moon has to be of a certain mass and made up of material like the Earth. That means with a very hot iron core spinning faster than the surface so it can create the dynamo effect and generate a magnetic field. If you look at most of the planets in our system such as Venus and Mars they have long since lost there dynamo effect and as such they have no measurable magnetic fields anymore. What ever hit Venus and toppled it over on its top must have been enough to slow down the spinning of its core. As for Mars its problem was always size. Mars is half the size of the Earth but its material is not the same. Mars is made up of material that is comparable to pumice. So its lighter and it lost most of its heat early in its life. I really don't think the volcanoes on Mars are active any longer. There is no evidence of and eruption in many millions of years. So I think Mars liquid magma has thickened to the point it has slowed or most likely stopped the core from spinning fast enough to generate a magnetic field. Than again it may have never even had one to begin with. All the outer planets have strong magnetic fields. That’s because there cores spin fast enough to generate the fields. By the time the Sun starts to die away the outer planets may have slowed enough for there magnetic fields to degrade some. Of course I could be wrong on all this and maybe someone with a little more knoledge than me can chime in here. I am just passing along things I have read and see in documentaries over the years.
As I mentioned in your other post the HD 28185 system would be a great candidate to look for and Earth like moon. It’s in the perfect orbit and location. That’s why the add-on I made and updating for Celestia is based on the HD 28185 system. It’s a good place to start looking. It’s to bad its 120 light years away. But hey next time you are outside at night and look up to the foot of Orion and than look a little more down and across to the right. There in the constellation Eridanus is HD 28185. A little G5 star up there has great possibilities.

Don.

[Apollo7]

So what I"m hearing here is a moon comparable in size and composition to Earth would have little difficulty dealing with the magnetic field radiation bands of a super-jovian world. This is a good thing I'm sure. You brought up the situation on Venus and that got me to thinking again. I had forgotten the fact that at one point Venus had an axial tilt rating of 178 degrees. I wonder if the process that knocked Venus over could also have slowed its rotation down such that its day is now longer than its year.

Also you can point to Mercury in explaining the effects of different compositions creating different results in planetary behavior. Despite its small size Mercury is the most dense of all the planets (not accounting for desnity from gravitation) and has a magnetic field of about 1% of Earths. Mars on the other hand lacks such a field and despite being 50% the Diameter of Earth, it is only 10% as massive.

I'm wondering if there is any current research going on as to the theoretical formation of Jovian planets in Terrestrial orbits, in which the moons form mainly out of rocky material rather than a mix of ices and rock?

[Don. Edwards]

I don't know of any research on subject of rocky material moons but it would be interesting to read up on. I am sure someone somewhere is writing a thesis on the subject.
As for Mercury, one of the running theories is that it was larger, possibly Mars size but that it was also involved in a major collision that ripped its crust and outer mantle off. What was left is what we see today. Of course that is just a theory. It's just as likely that Mercury is simply made of more iron rich material than the rest of the planets on a size scale.
Actually Venus is titled at a 178 degrees to this day. That is why it spins retrograde and not prograde. Take any planet that spins in the prograde direction, west to east and turn it over on its pole and you get a planet turning east to west, retrograde. That is Venus.
It's also believed that an Earth sized body struck Uranus and tilted it on its side. Makes you wonder what other big planetoids are out there waiting to be found if they haven't all been ejected completely out of the solar system.

Don.

[Evil Dr Ganymede]

Ganymede has its own internally generated magnetic field, but it's not that strong compared to Earth's. However, the radiation environment on its surface is still quite rapidly deadly. I think Ganymede's magnetic field just takes the charged particles it captures from Jupiter's field and slams 'em into the moon's surface. In fact, I think I recall seeing somewhere that the boundary of the frost caps might be related to where Ganymede's field lines intersect its surface.

That doesn't rule out the possibility that an earth-like magnetic field could be adequate protection though - I don't know the answer to that. If the satellite is far enough from the jovian, it'd be easier to withstand the charged particle barrage. One also has to consider the effect of all this on the satellite's atmosphere, if it has one - there'd be constant and powerful aurorae, that's for sure. The radiation levels on the surface from thee high energy stuff that gets through would be higher than on Earth - if there is any life on the surface it'd probably have a high mutation rate. Life that's underground or under ice sheets would be protected from this though.

[Apollo7]

Great replies so far, lets see.

The idea that Mercury got down-sized is an interesting one, considering its core is something like 80% of its size and mass. (some one who has the actual ratios might want to chime in here). It is also correct that Ganymede has a magnetic field though it is incredibly weak compared to Earth. Of course I shouldn't detract from Ganymede, its the Solar Systems largest moon, and has an extremely tenuous Oxygen atmosphere.

Most of you probably have heard of the DarkStar or Nemesis theory, the unseen companion to the sun. Some observations of long period comets have (under heavy contestment) been said to be the result of a very large (larger than Earth) sized object something like 10,000 AU from the sun. If such objects exist, would they even be observable? Could we ever confirm or deny the existance of large planetesimals on extremely elongated orbits, if said planetesimals have a very large inclination (i>20 deg) and are far out in their orbits? As I recall only a very tiny portion of the sky is imaged on any regular basis, it seems likely that there could be any number of large, unseen bodies that may be in the process of becoming Rogues or are simply in such an orbit that makes their detection difficult.

Is it not true that if you had an Earth-sized object composed primarily of ices that its density would be low and its gravitational effect less than a comparitively sized object consisting of SiO2, Ni, Fe, Ca and such?

if our system is indeed simply one permultation of an infinite number of possibilities I still wonder, why does our particular system lack any close in Jovian mass objects, what processes were in play that preferentially created terrestrial worlds inside 3 AU? If anything my interpretation is the current concepts of Gas-giant formation simply need drastic revision.

Anyway back to the moon question, one thing we have not considered here is the fact that such a world would have a much different cycle of night and day than what we are used to. Involving any number of situations involving passing behind the planets shadow and the lack of any large moon associated with the Earth-like object.

Then theres the issue of increased rate of colissions, as evidenced by Jupiters end-game with comet Shoemaker-Levy 9, a gas giant would have a tendency to draw in debris and the moon would be subject to bombardment beyond the initial period of planet formation. This would have severe ramifications for any life develping on its surface.

So there are clearly many more variables to deal with when your home-planet is a moon of a super-jovian world. More than we deal with here, I must admit the prospect of such planet-sized moons is tantalizing, certainly leaves alot to the imagination too!

Anyway Cheers for now!

[ajtribick]

Hope its ok to still post to this thread...

If the gas giant accreted further out and migrated inwards to its current position, wouldn't the moons be composed predominantly of ice and therefore evaporate in the first few million years of the solar system's formation? I suppose the gas giant could in theory capture planets that accreted in the inner system but wouldn't that probably rip the captured planet apart by tidal forces?

On the point of Earth-sized worlds in the Kuiper Belt... that would make one cool disaster movie... Armageddon II anyone? Like to see them come up with a device that could stop an Earth-mass object...

[Evil Dr Ganymede]

Hope its ok to still post to this thread...

If the gas giant accreted further out and migrated inwards to its current position, wouldn't the moons be composed predominantly of ice and therefore evaporate in the first few million years of the solar system's formation? I suppose the gas giant could in theory capture planets that accreted in the inner system but wouldn't that probably rip the captured planet apart by tidal forces?

'No', and 'probably' to your questions.

The moons wouldn't evaporate - their surfaces might melt, but that's about it. To evaporate them they'd have had to have formed at Pluto's distance entirely out of frozen nitrogen, and moved to where Mercury is! Most moons are made of ice around a rocky core anyway so at worst you'd end up with a deep ocean above a rocky seafloor.

On the point of Earth-sized worlds in the Kuiper Belt... that would make one cool disaster movie... Armageddon II anyone? Like to see them come up with a device that could stop an Earth-mass object...

Easy. Just bury this one nuke 800ft below the surface and it'll blow teh object up. It worked in Armageddon for something the size of Texas, after all

[Don. Edwards]

That movie was so stupid.
#1. A body that size would because of gravity formed almost nearly as a spherical object and not that mase of jagged rock they gave us. But a nice Ceres size asteroid thats round like the moon isn't scarry enough I guess. As if something the size of the moon comming at the Earth isn't a scarry thought.

Don.