# Tag Info

42

Yes, a tectonically inactive planet can retain a long-term atmosphere. You make the connection that a lack of plate tectonics on a planet indicates a "dead" core and thus said planet has no magnetosphere. As such, I'm going to interpret your question as, can a planet without a magnetosphere retain an atmosphere long-term? As proof, I offer up Venus. Venus' ...

36

It depends on what object it's acting on. There are many objects, including stars, that have magnetic fields where Lorentz forces on charged particles like electrons and protons are stronger than the gravitational force on them. Also remember that the strength of the Lorentz force depends on the speed of the particle moving through it, so a fast enough ...

31

Venus has a strong ionosphere that protects it against violent solar winds. So, even though Venus has no intrisic magnetic field, it has an effective, induced magnetic field due to the interaction between the solar winds and the atmosphere, that protects it against solar winds. Venus atmosphere is thick enough to have a consequent ionosphere, that would be ...

17

Let's look at the proper magnetic force (as opposed to the Lorentz force on a moving, charged object described in @KenG's answer) on a specimen $S$ of magnetized material with mass $M_S$ as a way to try to compare. Let's arbitrarily assume it has a fixed, permanent magnetic moment $m_S$. We can't use iron because it will saturate too easily. Then let's ...

16

Our own magnetic field is generated by convection currents in Earth's liquid outer core. A useful summary from Physics.org: Differences in temperature, pressure and composition within the outer core cause convection currents in the molten metal as cool, dense matter sinks whilst warm, less dense matter rises. This flow of liquid iron generates ...

15

Nothing "escapes" a BH - in the sense that a signal originating inside the event horizon remains forever inside. If something is observed moving away from the BH, then it was generated outside the event horizon. If it was generated inside, it would never be observed at all, forever and ever. Gravity itself does not "escape" a BH - and neither does "not ...

14

There is an interesting article on the magnetosphere of Venus on the ESA Science and Technology site. You can find the article here and it will probably answer your question. The article states, like you did, that there are planets, like Earth, Mercury, Jupiter and saturn, have magnetic fields interland induced by there iron core. These magnetic fields ...

13

So would the Moon's magnetic field affect the Earth's magnetic field, just as its gravitational pull affects Earth's gravitational pull for oceans? Yes, but only slightly. Firstly, magnetic fields can superimpose, so the field at any point is the sum of the field due to the Earth and the field due to the moon. However, the moon is rather far away (and has ...

9

No, the galactic magnetic field is very weak, about 0.1nT. It is able to bend the trajectory of highly-energetic charged particles and also to align dust grains across the magnetic field. However, is too weak to affect the rotation of a galaxy. Although the origin of galactic magnetic field is not clear yet, the supermassive black holes do not ...

9

Indeed conservation of angular momentum dictates that in a single star like the the Sun, rotation should be much slower when it becomes a red giant. This is because at the present time the Sun does not rotate at vastly different rates with depth, thus when it expands, the moment of inertia increases drastically and convection in the outer envelope will ...

9

It isn't impossible, but the short answer is "no". A gravitational field will accelerate all matter and energy equally while a magnetic field will only accelerate moving electric charges (other magnets). The force due to gravity is proportional to the inverse square of the distance, and the force due to magnetism asymptotically approaches the inverse cube ...

8

There are other ways to lose atmosphere. For example Jean's Escape. If average velocity of a gas molecule exceeds escape velocity, the planet will lose atmosphere. Venus' atmopshere is mostly $CO_2$ which has a higher molecular weight than the $0_2$ and $N_2$ of our atmosphere. So, for a given temperature and pressure, the carbon dioxide molecules have a ...

8

The Sun is not a point object, in fact it occupies a huge swath of space. Nor is it solid, rather the Sun is mostly fluid. Therefore, there is not one single "Sun's magnetic field" but rather the aggregate of hundreds, thousands, or perhaps trillions of magnetic fields, each of which changes in complex ways and are affected by (and affect themselves) nearby ...

8

There are three approaches with which people have looked for this, and not yet been too successful: A transiting planet planet with a strong magnetic dipole and/or very strong host star winds might produce a visible signal when the magnetospheric bow-shock passes in front of the star as well. The idea is that at the bow-shock the streaming hydrogen would ...

8

Can any magnet (or even one much stronger) of the Earth divert the cosmic rays as Earth's? No, because even if the strength of the field may be higher, the size of magnetic field is too small. Although the force that your local fridge magnet exerts may be larger than the one of the Earth, it does so only for a much smaller area. Even if you brought your ...

7

Actually, the core has two parts. The outer core is liquid, while the inner core is solid. As explained in the Wikipedia article about Earth's magnetic field: The Earth's magnetic field is mostly caused by electric currents in the liquid outer core, which is composed of highly conductive molten iron.

7

The Roche limit is defined in classical physics, dealing with materials that behave classically. That said, of course people have tried to estimate the counterpart in relativistic conditions. It turns out that it behaves roughly like the classical case when considering a liquid body orbiting a black hole. For the neutron star case and a more massive stellar ...

7

The Sun's magnetic field is way too weak to have any measurable effect on the Earth's orbit. The sunspot cycle does produce a small but detectable signature in the global average temperature (about $0.2\sideset{^{\circ}}{}{\mathrm{C}}$). Over a billion-year timescale the Sun is getting hotter as helium builds up in its core.

6

When plasma moves in a magnetic field then the charged particles follow helical paths around the field lines, due to the $q\vec{v}\times \vec{B}$ Lorentz force. If the radius of gyration is small, then the plasma is effectively tied (or frozen) to the field lines. However to decide whether it is the motion of the magnetic field that enforces motion of the ...

5

How would you get a magnetar close to Mars, within the solar system? OK, let's leave that to one side, but the Earth's magnetic field is of order $10^{-4}$ Tesla at strongest. As the dipole field of a magnetar diminishes as $r^{-3}$ and could be as high as $10^{11}$ Teslas at the surface of the magnetar, at a radius of 10 km, then the magnetar woud need to ...

5

Perhaps one way of answering this is to note that the (approximately) dipole magnetic fields of a magnetar fall off radially as something like $r^{-3}$. So at the magnetar surface the fields could be of order $10^{11}$ Teslas (at a maximum), but these are reduced by a factor of $\sim 8$ at 2 magnetar radii, a factor of $\sim 27$ at 3 magnetar radii etc. Now ...

5

In theory, a charged and rotating black hole can generate its own magnetic field. The magnetic (and electric) field can exist and can be measured outside the event horizon of the black hole. I completely agree with both existing answers that magnetic field does not "escape" from black holes, however I would argue that it is extremely unlikely that any real ...

5

I'll give a slightly different answer: The escape velocity at the surface of our Moon is about 2.38 km/s. Derived from this paper, as a rule of thumb, an atmosphere can survive 4.5 billion years, if it's average molecule velocity is below 1/6 of the escape velocity of the planet/moon. Applied to our Moon, it's 2380 m/s / 6 = 396.67 m/s. Carbon dioxide has a ...

5

There are such evidence in the case of Mars. Observations from Mars Global Surveyor show evidences of crustal magnetization. In particular, this magnetization has extensive, east-west trending linear features in Terra Cimmeria and Terra Sirenum. These are probably reminiscent of magnetic features associated with a reversing dipole. There are no such ...

5

Magnetars (and neutron stars in general) don't need a dynamo to create their magnetic fields. Their magnetic fields are "frozen in" at the time of their formation. To really see why this is, you have to understand a lot about electromagnetism, but I can boil it down to the basics. Keep in mind that neutron stars are highly mysterious objects and we don't ...

5

There are, in general, two classes of explanations for neutron star magnetic fields: fossilized magnetic fields and active magnetic fields (see here for an early overview on some of the internal battery models). The "fossilized" field theory - which is well-accepted, as far as I know - states that neutron star magnetic fields are leftovers from the magnetic ...

5

This was a coronal mass ejection. Those 1973 astronomers weren't looking at the picture correctly. They didn't have the tools at that time to look at the picture correctly. Coronal mass ejections (the term used now) were only discovered a couple of years prior to that picture taken from Skylab, via the Orbiting Solar Observatory 7 satellite. Those early ...

5

Mars and Titan differ markedly in distance from the Sun, composition, and possibly geological activity. Titan is about 6.3 times as remote from the Sun as is Mars, which means Titan receives about 1/40th solar radiation that Mars does, with about the same reduction in solar wind. Mars' small size coupled with its much closer proximity to the Sun allowed the ...

5

usrLTK's answer provides a lot of good details and in particular explains why Mercury wouldn't have much of an atmosphere. Let me complicate the picture a little by pointed out that some recent research indicates that magnetic fields may not be the guaranteed, automatic atmosphere-protection devices that conventional wisdom suggests. In particular, Gunell ...

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