How does mars have a magnetic field, with a cooled core?

[Ethan Howell]

I have been studying astronomy and astrophysics for about 2 years now, though on my own time and by my self. A recent question came into my mind, Can arctic planets exist close to their star? Now, in my understanding of planetary science, in order for a planet to be cool and close to the sun, it would need a pretty thick atmosphere, and a strong magnetic field in order to protect that atmosphere from the star's solar winds. Instantly, mercury came to mind, so I tried to find out why mercury doesn't have a thick atmosphere, and came out understanding that mercury's magnetic field is not very strong, and the sun's solar winds blow away it's atmosphere, but the magnetic field does protect it from some solar winds. I can understand why mercury has a weak magnetic field, because it is the smallest planet and has the least mass. So I directed my study to the cores of other planets, I know venus's magnetic field is significantly weaker than earth's, because the effect of magnets weakens the hotter it gets, and on venus, where it is hot enough to melt lead, it would have a pretty weak magnetic field. I just can't understand how it holds onto it's atmosphere, with strong solar winds and a weak magnetic field. To make matters worse, I tried to find out why Mars still has a magnetic field. I was taught that a magnetic field generates when a planet has a solid inner core and a liquid outer core, because the flow of liquid iron creates a magnetic field. Mar's core has been cooled since the creation of our solar system, but yet it still generates a magnetic field, though being geologically inactive. How? Though the title only holds one of the questions I had, may you answer the ones I couldn't fit in the title?

 

[rootone]

Mars doesn't have a significant global magnetic field, there are local areas with magnetic fields but they're not organised in a systematic way.
It probably means that Mars did once have a substantial global magnetic field which produced these local magnetic areas, but the global field is no longer being generated, or as at least it has much weakened.

 

[Ethan Howell]

Mars doesn't have a significant global magnetic field, there are local areas with magnetic fields but they're not organised in a systematic way.
It probably means that Mars did once have a global magnetic field which produced these local magnetic areas, but the global field is no longer being generated,

I understand that, but what I am having trouble grasping is how those local areas are still being generated.

 

[rootone]

The best idea I have heard of is that they may be areas where some minerals exist which can naturally turn into permanent magnets given enough time.
Like lodestone on Earth
https://en.wikipedia.org/wiki/Lodestone

 

[mfb]

in order for a planet to be cool and close to the sun, it would need a pretty thick atmosphere

Are you sure? At least the atmospheres in our solar system increase the temperature, and they always lead to more uniform temperatures.

mercury came to mind, so I tried to find out why mercury doesn't have a thick atmosphere

That's reversing the logic.

because the effect of magnets weakens the hotter it gets, and on venus, where it is hot enough to melt lead, it would have a pretty weak magnetic field.

The magnetism of Venus and Earth is not driven by permanent magnets that would lose their field by heating, and the temperature on the surface is irrelevant because the field is produced much deeper down.

How Venus keeps its atmosphere is still an open question, and the answer is probably complex, involving magnetic fields induced by the interaction of solar wind and atmosphere, and similar things.

I tried to find out why Mars still has a magnetic field.

Uh well, it is extremely weak. Moon still has some field as well, mainly locally from remaining magnetized material in it.

 

[Ethan Howell]

Are you sure? At least the atmospheres in our solar system increase the temperature, and they always lead to more uniform temperatures.That's reversing the logic.The magnetism of Venus and Earth is not driven by permanent magnets that would lose their field by heating, and the temperature on the surface is irrelevant because the field is produced much deeper down.

How Venus keeps its atmosphere is still an open question, and the answer is probably complex, involving magnetic fields induced by the interaction of solar wind and atmosphere, and similar things.Uh well, it is extremely weak. Moon still has some field as well, mainly locally from remaining magnetized material in it.

The reason why I chose to find out why mercury doesn't have a thick atmosphere is because it is close to the sun, and still has a molten outer core. Therefore it should have a significant magnetic sphere, should it not? The answer to my initial question does lie with mercury. Why mercury doesn't have a large atmosphere is simply because it's magnetic sphere is not strong enough to hold onto the atmosphere with the strong solar winds. Also, under the right conditions, I think that a planet close to it's star could be cold. It would have to have an atmosphere that does not absorb much heat.

 

[Ethan Howell]

The best idea I have heard of is that they may be areas where some minerals exist which can naturally turn into permanent magnets given enough time.
Like lodestone on Earth
https://en.wikipedia.org/wiki/Lodestone

That makes sense, thanks!

 

[Hornbein]

I know venus's magnetic field is significantly weaker than earth's, because the effect of magnets weakens the hotter it gets, and on venus, where it is hot enough to melt lead, it would have a pretty weak magnetic field.?

It doesn't work that way. The Sun has a strong magnetic field.

 

[mfb]

Mercury is simply too hot and too small to hold an atmosphere over longer timescales. Even with a magnetic field it would have lost any initial atmosphere by now.

 

[Chronos]

Venus has an induced magnetosphere, probably due to its extensive atmosphere as noted by mfb. See http://sci.esa.int/venus-express/50246-a-magnetic-surprise-for-venus-express/ for discussion.

 

[D H]

The reason why I chose to find out why mercury doesn't have a thick atmosphere is because it is close to the sun, and still has a molten outer core. Therefore it should have a significant magnetic sphere, should it not? The answer to my initial question does lie with mercury.

Venus, Mars, and the Moon all have a molten outer cores, yet none has a substantial internally-generated magnetic field. Mercury does have a global magnetic field, but it's rather puny compared to that of the Earth. We still don't quite know why the Earth's magnetic field is so old (over 4 billion years old) and so persistent. While dynamo theory goes a long way, the numbers just don't seem to add up. At least one of a ridiculously high heat flux across the core-mantle boundary, a ridiculously hot early Earth, or a ridiculously young inner core is needed.

A very recently published paper, https://www.researchgate.net/profile/D_Andrault/publication/299551629_The_deep_Earth_may_not_be_cooling_down/links/57022d5708ae1408e15eb144.pdf , proposes an interesting solution: The Moon. But it's too early to tell whether this too fails to explain the mystery of the Earth's rather unique magnetic field.