Printable Version of Topic

Posted by: dvandorn Dec 27 2005, 01:33 PM

This question has been bothering me for some time:

A lot of data suggests that Mars lost its magnetic field a *long* time ago -- like, in Noachian times. Something like three and a half billion years ago. More than anything else, the pattern of atmospheric depletion suggests this strongly.

There is also significant evidence that Mars has undergone volcanism for almost its entire history -- some lava flows have been dated via crater counts at only 10 million years or so.

The "accepted view" is that Mars lost its magnetic field because its core solidifed. Now, how does it logically make sense that Mars' core cooled so much that it congealed 3.5 billion years ago, but that enough heat was retained in the mantle to drive remarkably extensive volcanic activity for almost the entire remainder (to date) of the history of the planet?

Now, perhaps I am simply uninformed about the process of planetary cooling; it would make sense to me that a planet would cool from the outside in, not from the inside out. If that "common-sense" perception of planetary cooling is wrong, please, someone explain it to me...

In absence of better data about planetary cooling, though, it occurs to me that perhaps what needs to be questioned is not how Mars could be so volcanically active with a cold, congealed core. The appropriate question is whether or not a planet can spin rapidly (one turn in only a few tens of hours), have a molten core, and *not* generate a magnetic field.

After all, we only *theorize* that Earth's magnetic field is generated solely by the rapid rotation of its molten nickel-iron core. We have precious little data about the core/mantle boundary -- it seems possible to me that it is the rotation of the Earth's core/mantle *boundary* layer, and not the rotation of the core itself, which generates the magnetic field.

That would open up the possibility that Mars could *still* have a small molten core which is still driving mantle convection of some form or another. If a change in state, composition or other nature of Mars' core/mantle boundary is what killed its magnetic field, *not* the solidification of its core, that would mean the same thing could possibly happen some day on Earth.

And an Earth without a magnetic field is, in the long run, pretty much an uninhabitable planet.

I'll also toss into this discussion a morsel I read in the past year. Based on motion measurements using the Apollo laser retro-reflectors, one peer-reviewd paper insists that the Moon's observed motions can only be explained if its core is in fact still molten. Not only molten, but rotating at a slightly different rate, and around a slightly offset axis, from the rest of the Moon. (There does seem to be a thick layer of undifferentiated chondritic material overlying the core and a pretty thin layer of mantle; all of the volcanism we see on the lunar surface, it would seem, was the result of both immediate and stored accretion heating.)

And, of course, we all know the Moon has no intrinsic, global magnetic field.

So, once again, the question is begged: how sure are we that a fast-spinning, rocky planet which lacks a global magnetic field *must* have a cold, congealed core?

-the other Doug

Posted by: Bill Harris Dec 27 2005, 02:07 PM

Good questions, Doug.

To add to the mix of confusion, Venus is Earth-sized, is presumed to have a large iron core and has observed regional volcanism and presumed tectonics.

And no magnetic field.

Here are a couple of background papers:

http://www-spc.igpp.ucla.edu/personnel/russell/papers/venus_mag/

http://www-spc.igpp.ucla.edu/personnel/russell/papers/mars_mag/


My feeling is that we owe a lot to our Moon. The formation of the Moon (favoring the grazing-collision theory) jump-started the geothermal engine and the gravitational flexing of the Moon keeps the pot stirred. A magnetic field and plate tectonics make the Earth a comfy place to stay.

--Bill

Posted by: Richard Trigaux Dec 27 2005, 02:08 PM

Interesting questions.

It seems (as far as I know) that a planet like Earth solidifies BOTH inwards from the crust, and outwards from the core.

Earth has a molten iron core, which is still in the process of solidifying into a solid seed into the very center. This process of iron solidification is still producing many heat, maybe twice as much as the radioactivity. And this heat produces hot spots into the stony mantle. And the movement of the liquid iron produces the magnetic field. Reversely, the stony mantle is solidifying from the surface, but, with convection, it stills keeps a relatively homogenous temperature, while the core cooling ensures that the mantle temperature is relatively constant over the ages.

What is going on on other planets maybe follow the same sheme with a different time scale, but there are discrepancies which origin is not very clear yet. My favourite opinion is that Mars iron core was completelly soidified from long ago, stopping any magnetic field (except for some magnetic remnants in the cold crust). But there may still be some weaker mantellic convection from radioactive heating (remember that the mantle is not liquid, the pressure maintains it in a solid state with only slow creeping movements. But when mantellic rocks approach the surface, they melt and produce volcanism). Eventually this mantellic convection has a simple Benard cells pattern, producing the Tharsis dome and related volcanism. This convection movement is weaker than on Earth, with no hot spots, and it does not involve the crust (no plate tectonics, or only in very ancient times). And the mantle cannot produce a magnetic field, because: 1)it is an electric insulator 2)its movements are very slow (creeping solid).

If we extrapolate to the Moon, it would be completelly solidified from long ago, after producing a last volcanic episode 3 billions years ago (The "seas"). To explain a still molten stony core (and a solidified iron core, if any) needs a different model. Maybe the very slow heat leak rate through the very thick solid layers allow a higher temperature building, explaining it is liquid, but this is speculative. Convection in such a liquid stony core would produce a weak rotation movement.

Venus would be similar to Earth, with a hot spot volcanism, but with no plate tectonics. This discrepency perhaps arises from the fact that there is no water on Venus. Some say that on Earth water is very efficient to cool thick layers of oceanic crust, and it would be these thick layers which would drive plate tectonics when they fall toward the deep mantle in the slow overal convection pattern. The fact that Earth has a magnetic field and Venus has not, is not understood today. Perhaps the iron core is completelly solidified now.

Mercury should be intermediary between Mars and the Moon, but it does not fit with this sheme (as far as I know no volcanism never took place on Mercury, when the smaller Moon produced large lava flows 3 billion years ago). This perhaps arises from the fact that Mercury has a much larger iron core, but such a core should have driven a strong volcanism when it was still liquid.

Posted by: BruceMoomaw Dec 27 2005, 10:23 PM

There's a much better case than that: nobody seriously questions that Venus has a molten core, but it also has absolutely no magnetic field. There is currently a knock-down debate over why this is so, centering on the possibility that its thick lithosphere prevents the planet's internal heat from escaping fast enough to drive the convection currents in the core's metal that are necesary to generate a magnetic field -- EXCEPT during its hypothesized periodic spasms of crustal recycling and violent heat escape, during which it may briefly switch on a strong magnetic field before cooling off and regrowing that thick crust.

This can't be the explanation with Mars, though, because Mars definitely does NOT undergo periodic spasms of surface overturning and violent heat release -- even through the Tharsis Bulge. There has been quite a lot of very complex theorizing about how its relatively cool and solid core can be reconciled with a fair amount of continuing Martian volcanism, much of which you can read in the LPI's invaluable central file of abstracts at http://www.lpi.usra.edu/publications/abstracts.shtml

Posted by: lyford Dec 28 2005, 01:44 AM

Hi Bill - do you have any links for one (me) to read up on this idea? Thanks

Posted by: Bill Harris Dec 28 2005, 02:48 AM

Lyford, I don't know of any references to this; this is a idea that I've kicked around for years on my own. Many of the unique properties of Planet Earth are the result of our unique companion. Let me see if I can find some links to support my idea.

One big enigma on Mars is the Tharis Bulge. Here we have a large area uplifted some 10 km with, so to speak, no visible means of support. For an area to remain uplifted for eons something has to hold it up, otherwise it will slowly but surely sink down to base level. Two ways of doing this: the area can be underlain by a thick, light granitic crust overlying a denser basaltic mantle. This is how the "cores" of the continental plates stay elevated on Earth. Or there can be a hot, bouyant mantle plume underlying the uplifted area and keeping it elevated. Problem is, there don't seem to be light grantic plates on Mars, especially under the Tharsis Bulge. So with Mars, the mechanism has to be a rising plume. Given that there is has been major volcanism in Tharsis for a long time, this idea is favorable, but current thought is that Mars is cold and inactive.

An Earthly analog to the Tharsis bulge is the Mongolian Plateau. Similar in size and uplift and with evidence of volcanism, the Mongolian Plateau was uplifted when the Indian plate was subducted under the Asian plate (Himalayas) creating a thick, bouyant continental (granitic) crust. In this respect, dissimilar to Tharsis.

Anyway, here is a MOLA elevation image of the Tharsis hemisphere and a bouyant uplift image for cogitating.

More later...

--Bill

Posted by: CosmicRocker Dec 28 2005, 06:18 AM

I'm sure I am not up to date on the latest thinking with respect to planetary magnetic fields, but it should be noted that we do know earth's magnetic field cyclically reverses, and has periods where it is essentially zero. It would be unlikely though, that the other planets thought to have molten cores but no magnetic field are all in that temporary state.

I find it curious, too. Surely elemental abundances in the original rotating and condensing globule that formed our solar system varied radially. Judging by average densities of the planets, earth may have had a larger proportion of iron. Perhaps it also had a larger proportion of radioactive elements that ended up in it's mantle, heating it from the inside out. Just guessing...

Posted by: Richard Trigaux Dec 28 2005, 07:44 AM

Bill Harris, the Tharsis bulge is surrounded by an extensive network of faults and folds, and this rather supports the idea of a tectonic origin (a plume) rather that an isostatic origin (continent-like granite core). The mongolian plateau (rather known as the tibetan plateau, as Mongolia is a bit northermost) formed by the superposition of two continent cores (India getting under Asia) and thus it is a buoyant double layer of granite. (To be exact I think it is also a dynamic structure, as the India plate cannot go so far under Asia, it is just under the Himalayas. Should the India plate movement stop, most of the tibetan plateau would subside rapidly).


CosmicRocker, the Earth magnetic field is more complex than just a dipole. It seems that there are several vortexes in the iron core, each generating its own field, normal or reverse-oriented. From the surface, we see the average field (which is mandatoritly a dipole) but depending of the number of vortexes and the orientation of each, this average field can switch from north to south and vice-versa. When this occurs, there is not really a moment with a zero magnetic field, but rather a movement of the magnetic poles, and eventially a more complex structure than just a dipole. This can happen in some years, as it was recorded in some lava flows. Today there is a slow weakening, and a movement of the magnetic pole from Canada toward Siberia. This is due to the emergence of a new reverse-oriented vortex under souh Atlantic (if I remember well). Wether or not it will result into an inversion of the dipolar field is completelly unknown today, but I rather guess not.


BruceMoomaw, the idea of Venus experiencing spasms of crustal recycling, although not proven today, is interesting. What I guess is that planets which don't have a plate tectonics behave diferently, and evacuate heat with different processes. So it is earth which would be the exception. If it is true that it is water which produces the plate tectonics, so it is the presence of water which produces a smooth escape of internal heat and the overal mantellic convection. But at a moment there must be more heat to evacuate to maintain such a functionning.

The problem with Venus would be:
1) its smaller core is already solidified. This event probably produced an extensive volcanism episode, followed by a general weakening of volcanism, in a Mars-style. (remember that the solidifying of the iron core produces more heat than just radioactivity. So the end of the solidification may result in decrease to a much lower level than before).
2) the slow rotation rate of the planet is unable to drive the core dynamo. It is generaly recognized that the quick rotation of Earth is a necessary ingredient to its magnetic field.

Posted by: edstrick Dec 28 2005, 09:50 AM

I do not know the current "community best opinions" on Venus' core, but a decade ago, discussion included the fact that the temp at the surface of the core was likely hotter than in Earth (approximate convective equilibrium of mantle with a hotter surface), and the central pressure distinctly lower than in Earth (modestly lower mass). The combined results would be that on Venus, there may be no solid inner core, as the temp's too high and the pressure too low for it to have started to form. This would eliminate what may be the major heat source in Earth's core, the phase-change of liquid outer core to form the slowly crystallizing inner core. (Amounts of potassium, uranium and thorium dissolved in the core on Earth and other planets was and is a matter of *MAJOR* debate, so that heat source is uncertain.)

On Mars, there's some reason to think that the planet may be somewhat sulfer rich compared with Earth (I don't recall the exact reasons why), and the lower melting point of a iron/iron-sulfide core melt could keep part of the Martian core still molton. We just don't know and won't know till we get enough good seismic data on the silicate planets.

Posted by: dvandorn Dec 28 2005, 11:32 AM

Forgive me for stating the obvious, but this discussion just highlights the need for both a seismic network and a network of heat-flow measurements across the globe of Mars. And Venus. And Mercury, for that matter.

Seems like a somewhat esoteric dataset, I know. But considering the vast implications of an imperfect understanding of the heat-release dynamics of our own planet, I'm beginning to think that *these* are the highest-priority measurements awaiting us...

-the other Doug

Posted by: edstrick Dec 28 2005, 12:27 PM

The other very very useful measurement from a geophysical network is magnetic fields. The induced magnetic fields and their variability as the planet's externally imposed (solar wind and magnetosheath) fields blow back and froth tell you a *LOT* about internal electrical conductivity etc. Witness the flyby data at Europa etc. proving there's a highly conductive layer below the visible ice.

It's by far the best for these measurements to have a small orbiter up-wind of the magnetosphere/magnetosheath to monitor the external fields.

Posted by: Bill Harris Dec 28 2005, 03:00 PM

Here are a couple of background info links to Venus and Mars magnetic fields, FYI.

http://www-spc.igpp.ucla.edu/personnel/russell/papers/venus_mag/

http://www-spc.igpp.ucla.edu/personnel/russell/papers/mars_mag/

--Bill

Posted by: RNeuhaus Dec 28 2005, 03:29 PM

Re-edited.

The Tharsis bugle might be of the result of lack of Mars tectonic movement. Since the plate has remained static for billions of years and it has helped to build up the biggest volcans of our solar system.

The other point is that there is a theory about the Scars of Mars.

VI. Location of the Bulge Region on Mars

Figure 7 illustrates the Opposite Hemisphere of Mars. Three kinds of phenomena are brought to attention:

1. Bulging

2. Rifting

3. Volcanism

I Bulging of Mars

Most planets are not perfect spheres. Earth, for instance, is an oblate spheroid - fatter around the equator and flattened at the poles; this is due to a compromise between centrifugal force and gravity. The Moon has a lump or bulge on one side. Mars has both, for Mars also rotates like the Earth, and at a similar rate (one day of rotation is 24 hours 37 minutes). Mars is an oblate spheroid with a bulge region. The bulge region is known as the Tharsis Bulge.

The Tharsis Bulge is a large shield or uplift area in the Opposite Hemisphere. It is approximately 6 1/2 miles high in the center relative to the general crust and on that small planet it is about 3000 miles in diameter; it crosses 90 degrees of latitude. The central core area of the Tharsis Bulge is very close to the equator and around 105 degrees West Longitude.

The Hellas Crater is centered around 295 degrees West Longitude and 45 degrees South Latitude. Tharsis, a bulge zone, is almost 180 degrees opposite to Hellas Planitia, the lava region where Hellas hit Mars. To be precise, it is 190 degrees to the west (or 170 degrees to the east). We suggest that the massive Hellas strike on one side of Mars caused the bulge on the opposite side. The Hellas impact asteroid, the core of Astra, may have been between 850 and 900 miles in diameter, and it probably struck Mars at a speed of 6000 miles per hour, which is 100 miles per minute.

Based on the fact that the center of the Hemisphere of Craters is at 45 degrees South Latitude, it is concluded that Astra approached Mars to the south of the ecliptic plane, and perhaps up to 1000 miles to the south at the moment of fragmentation. Also, from other data yet to be presented, it would seem that Astra approached Mars on the sunward side (of Mars); probably Astra was moving toward aphelion and Mars probably had just passed aphelion about 30 days previously and was beginning its motion toward perihelion.

The angle formed between Hellas Planitia, the smooth, magma-covered region engulfing the Hellas Crater, and the Tharsis Bulge suggests the trajectory of Hellas as it approached Mars. Since Hellas is about 10 degrees east of the center of the Hemisphere of Craters, it is logical to conclude that Hellas hit Mars about 20 degrees, or 700 miles off center. And center is described s the subpoint under the fragmentation. Astra and the fragment Hellas careened toward Mars slightly from the south of the ecliptic (orbital) plane of Mars. Hence the line between Hellas and Tharsis proceeds in a northerly trajectory from if Hellas to Tharsis; this is the direction of the pressure waves inside Mars.

Thirty minutes prior to the impact of the Impact Asteroids (and at the moment of fragmentation), Mars must have been writhing in a sudden tidal surge. This tidal surge continued without significant abatement during the 30 minutes that the Impact Asteroids (and to a lesser extent the Current Asteroids) were approaching. The three largest asteroids Hellas, Isidis, and Argyre broke open the crust of Mars so wide that massive "bleeding" extrusions of magma resulted.
Thus we find in the Opposite Hemisphere three kinds of relief from the Hellas asteroid impact. These are bulging and volcanism - both rapid responses to the need for relief. And there is also rifting, a slower response. In addition to the Tharsis Bulge opposite the Hellas impact, note is also taken of the Elysian Bulge opposite the Isidis impact area.

And within a 25-minute period, in addition to the massive, surging tides of internal magma, Mars received scattershot blasts by about 2800 impact asteroids which were over 15 miles in diameter, plus many more of smaller diameter. But above all, the impact of Hellas, approximately 38 percent of Astra's original mass, must have devastated the innards of Mars. So immense were the pressure waves, we believe, that the Tharsis Bulge resulted as a single mechanism to relieve the sudden thrust. Its uplift required just a few minutes.

The pressure waves from the Hellas impact are estimated to have traveled within Mars at a rate of 150 miles per minute. The diameter of Mars was about 4200 miles (and growing by the accretion of Astra materials). In about 28 or 29 minutes the Hellas pressure waves were reaching the opposite side of Mars on the inside of its crust. We propose that these pressure waves caused the uplift in the Opposite Hemisphere, in the equatorial region about 170 degrees from the Hellas impact.

Thus we propose that the time lapse between Astra's fragmentation and the uplifting of the Tharsis Bulge was no less than 55 minutes and no more than 75 minutes. From fragmentation it took the Hellas Impact Asteroid about 30 minutes to arrive at Mars' crust and it required another 28 minutes for shock waves to arrive on the opposite side. Perhaps another 10 to 15 minutes were needed for the uplifting process hence 75 minutes in all from fragmentation to Tharsis.

There is another bulge zone on Mars. The Elysian Bulge is located in the region 230 degrees West Longitude and 40 degrees North Latitude. This region seems to be at the opposite end of the trajectory formed by the Isidis Impact Asteroid, which body came within a hundred miles of missing Mars. Isidis is on the rim of the Hemisphere of Craters. The Elysian Bulge is just opposite that impact zone.

A prominent opinion of the relationships between the Tharsis and Elysian bulges, and the Valles Marineris (see Section VII, below) and the general geological history of Mars comes from Moore and Hunt: [7]

Extending eastward from Tharsis is an immense canyon system, Valles Marineris, while radiating outward from it are large arrays of tensional faults (graben); both are presumably related to the formation of Tharsis itself. North of the canyons are numerous fascinating outflow channels
which appear to have been produced during a period of catastrophic flooding, between 3,500 and 3,000 million years ago. . .

While Mars' early history is not well understood, it is likely that the resurfacing of the northern hemisphere took place very early on, perhaps as long as 4,000,000 million years ago. Many scientists believed this was in some way connected with the formation of Mars' inner core region
[ Emphasis added.]

In contrast with what "many scientists believed," the conclusion of this essay is that the later period of catastrophic flooding on Mars was more like 5000 than 5,000,000,000 years ago. And the timing of the Astra fragmentation and the Hellas Impact Asteroid event was between 5000 and 15,000 years ago not more. Contradicting or substantiating facts, such as dust drift accumulations in Mars' craters and other data, will undoubtedly clarify the matter in due time.

Mars' general scene prior to the fragmenting of Astra. It is suspected, however, that Mars had a small, icy or Frosty" satellite (not shown).

After Astra's fragmentation, Mars acquired two small asteroid-like satellites, Deimos and Phobos. And perhaps Mars acquired, temporarily, a ring of lesser asteroidal debris, judging by the pitlets on Deimos and Phobos.

At the time of Astra's fragmentation, it is suspected that both the surface of Mars and "Frosty" were enriched with iridium, assuming iridium to have been abundant as a trace element in Astra's composition. A further catastrophe ensued later (not discussed in this paper), wherein Frosty also fragmented, depositing ice and water upon the surface of Mars, among other places. [8]

II. Location of the Rifting Region on Mars

Noctis Labyrinthus: Between the Tharsis volcanoes and the Valles Marineris is a complex system of fractures resulting from an extension of the crust. The area has the highest elevation of the region and is believed to be the apex of uplift. The smoother Noctis Lacus area has wind sculptured features, to the west of which is a complex system of criss crossing fractures. [9]

Chaotic terrain exists on Mars just to the east of the Tharsis Bulge. It includes a maze of short, interconnected canyons which has the name Noctis Labyrinthus. This region has what appear to be a series of uplifted blocks or sections, and this also indicates the impact of the Hellas impact Asteroid on the opposite side: [10]

Visible even on long-distance images of Mars is the great canyon system, which straddles the globe just south of the equator between longitudes 30° and 110° W. Called Valles Marineris, this 4,000 km-long network begins on the east side of the Tharsis Bulge and ends in an immense region of chaotic terrain between Chruse Planitia and Margaritifer Sinus. At its deepest it is some 7 km deep and individual canyons are up to 200 km in width. In the impressive central section, where there are three roughly parallel, interconnecting rifts, the total width is 700 km.

In this thesis Mars acquired about 70 percent of Astra's mass as it gathered that much of the volume of asteroidal debris. Hellas alone was as much as 38 percent of the mass. Mars new mass is between 1 and 1 1/2 percent greater than its former mass: for that kind of a sudden growth the thin crust needed to find some accommodation for expansion. The crust, parallel to he equator, split open and expanded, just as a tear in a pantleg will occur, due to a new, sudden, nternal stress such as the abruptly added weight of the wearer.

Our calculations suggest that, over the next century or two, the diameter of Mars increased from about 4199 to 4222 miles, the Valles Marineris being one example of how the planet relieved its newly developed internal stress. The Valles Marineris was a slow and gradual type of relief originating with new isostatic pressures on the crust's inside. The Tharsis Bulge was an example of Mars' relieving pressure in a sudden thrust or uplift. Thus both gradual and slow and sudden or quick responses to this event are seen on the surface of Mars' Opposite Hemisphere .

III. Location of Major Volcanic Sites on Mars

Thrusting, rifting, and volcanism are all methods for relief of internal stress, or isostatic pressures, building up on the inside of the crust. Olympus Mons, is the largest of the Martian volcanoes. Olympus, like Ascraeus, Pavonis, Arsis, Tharsis Tholus, Uranus Patera, and Uranus Tholus, is a volcano on the edge of the Tharsis Bulge. Olympus is on the northwest periphery The two Uranuses are on the northeast periphery of Tharsis. Ascraeus Mons is in the north central section of Tharsis, like Pavonis Mons. Arsis Mons is on the westerly edge. Labyrinth Noctis and Valles Marineris are on the easterly periphery. All of these features are expressions of catastrophism-and especially the Hellas Impact Asteroid.

The size of Olympus Mons (24 km) almost three times as high as Mauna Loa (8 km from peak to the bottom sea), Hawaii, measured from its Pacific Ocean base. [ 11 ] And the volcanic base of Olympus Mons is about 180 miles in diameter-about the distance from North Seattle to South Portland. Compared to Olympus, Mount Baker, Glacier Peak, Mount Ranier, Mount St. Helens, Mount Adams, and Mount Hood are six volcanic pimples.

There is no question that Olympus Mons suddenly erupted simultaneously with the uplifting of Tharsis. Its formation likely began within 90 minutes after Astra's fragmentation, within 60 minutes of Hellas' impact, and within 30 minutes of the internal pressure and shock waves reaching the Tharsis region.

Thus we find in the Opposite Hemisphere three kinds of relief from the Hellas asteroid impact. These are bulging and volcanism - both rapid responses to the need for relief. And there is also rifting, a slower response. In addition to the Tharsis Bulge opposite the Hellas impact, note is also taken of the Elysian Bulge opposite the Isidis impact area.

We suspect that a line drawn between each of the following features will point toward the fragmentation location, some 3000 miles in space near the Southern Hemisphere of Mars:

1. From subpoint at 319 degrees West and 45 degrees South to Mars' center.

2. From Olympus Mons to the center of Hellas Planitia.

3. From Arsia Mons to the center of Argyre Planitia.

4. From the Tharsis Bulge to Hellas Planitia.

5. From the Elysian Bulge to Isidis Planitia.

If this analysis is valid, then further investigation should reveal that a difference exists in the steepness of the walls of various craters, with the steeper side opposite the subpoint, and the shallower side on the nearer side facing the subpoint.

http://www.thule.org/mars/mars2.html]Source of the information, click here.[/URL]

P.D.
I know that Jupiter has very strong magnetic field and it is composed mainly of Helium and Hydrogen in the core. Is the magnetic field the product of rubbed iron?

Rodolfo

Posted by: Richard Trigaux Dec 28 2005, 09:30 PM

The core of large gaseous planets is very hot (perhaps more than 30000°C at the center of Jupiter) so that many phases become conductive. Especially it is expected to find metallic hydrogen, which is expected to be a good conductor.

Posted by: Richard Trigaux Dec 28 2005, 09:34 PM

There was recently a NASA probe to measure Earth heat budget (with the purpose of measuring climate change). Would such a probe be sensitive enough to measure geothermic heat flow? If yes it would be enough to send replicas of this probe around these planets.

Posted by: Bob Shaw Dec 29 2005, 01:23 AM

One of the few things we can say with certainty about all the rocky planets of the Inner Solar System is that they have as many differences as similarities.

The presence of Earth's Moon, and it's putative 'Big Whack' origin makes our own world seem pretty much atypical, though I'm bound to say that the 'Goldilocks Zone' of liquid water seems helluva interesting. Terrestrial plate tectonics seem to be mediated (at the very least) by water in the mantle; other planets don't have the water we have; none of them have active plate tectonics. Venus turns inside-out every now and again, Mercury seems to do not a lot except shrink, and Mars builds up giant shield volcanoes but not much else. But Earth is saturated with water, and it simply squirms...

...perhaps we should treat Earth as a small 'Ice Giant' rather than a member of the rocky worlds!

Bob Shaw

Posted by: BruceMoomaw Dec 30 2005, 02:42 AM

Heat-flow measurements from orbit MAY be practical with a microwave radiometer -- it's always been one of the instruments highly ranked for a US lunar polar orbiter, had we ever flown a full-scale one, and it may yet fly at some point. But the atmosphere of Mars -- let alone the thick one of Venus -- hopelessly fouls up such measurements for those worlds. (I imagine you could do it for Mercury. I'm getting contradictory news on whether it's possible for airless ice-covered worlds, because of the different thermal properties of ice.)

One interesting footnote: an abstract from this year's LPSC ( http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1454.pdf ) suggests that you COULD use a thin plate just lying on the surface to measure Venus' heat flow, rather than having to drill a hole and stick a chain of temperature sensors down it -- because the surface temperature of Venus, thanks to its thick air, changes very slowly compared to that of the Moon and Mars. But several days would still needed for the measurement.

Posted by: The Messenger Dec 30 2005, 05:37 AM

Is there really compelling evidence that core properties dominate the Earth's magnetic field? I think a good case can be made that the oceans are a major player: They are conductive, tidal flow and barriers are both aligned in general East-West directions, and there is certainly galvanic action. If there is a net East-West galvanic moment, we could have tremendous continent to continent current flow.

It is an interesting hypothesis, because it implies dumping corrosive metals in the ocean has the pontential for disrupting the Earth's magnetic field. Is it just a coincidence that the Atlantic magnitosphere anomally overlays one of the most complex oceanic currents?

Posted by: BruceMoomaw Dec 30 2005, 08:00 AM

Messenger, I think you're off in scientific La-La Land again. If the oceans were generating substantial magnetic fields, it would be instantly apparent by comparing measured magnetic field strength in the middle of a continent as compared to that by a shoreline or in the middle of the ocean.

Posted by: Richard Trigaux Dec 30 2005, 08:29 AM

Further reflexion lead me to think in a similar way. On the Monn and on Mercury there are places which don't see any Sun heat for days or month, and even never. In such places the radiated heat must be equal to the geothermal flow.
But on Mars and Venus, can a radiometre measure the geothermal flow from space? The probe I was speaking about (I still don't remember the name) was measuring the heat received and emitted by the Earth with enough sensitivity to detect the difference, Earth absorbing heat because of the temperature increase by greenhouse effect. So we could also measure an excess due to geothermal activity. I just don't know the order of magnitude of the two effects. On Venus we can expect that there is no going on increase of greenhouse effect (no climate change) but on Mars this is less sure, we observed changes in ice cap from year to year.
And also for a global measure in geothermal heat, we must account with volvanoes. A giant eruption every 50 million years can evacuate more heat than the constant heat leak.

Posted by: Rob Pinnegar Dec 30 2005, 05:03 PM

Yeah. The odd ton or two of battery acid will definitely be able to overcome the effect of all that salt. I'd stick to discussing the environmental impact.

This is reminiscent of people worrying that gravity boosting of spacecraft could throw Jupiter out of its orbit.

Posted by: Rob Pinnegar Dec 30 2005, 05:11 PM

In principle, maybe, but the problem is that the amount of power delivered by the Sun to the Earth is something like ten thousand times larger than the amount of geothermal energy that the Earth produces. This would be similar for Venus, and not too much different for Mars.

You'd need to know the albedoes of Venus and Mars to four or five significant digits to pick up the geothermal flow, and it would also be necessary to chart the solar constant to a high degree of accuracy.

It's a nice idea, Richard, but I don't see it being implementable in the near future.

Posted by: Richard Trigaux Dec 30 2005, 05:30 PM

As a people informed in environment issues, I can say that dumping toxics in the ocean can expectably arise problems of... toxicity, but certainly not of disturbing the magnetic field, ask this to Greenpeace and others, they will confirm (and certainly laugh). This is NOT AN ENVIRONMENT ISSUE. There are already enough real environment issues without adding imaginary ones.

Earth magnetic field is precisely tracked by satellites today, and is known down to the core (we cannot "see" into the core because it is conductive) and if there was magnetic effects in ocean currents, they would be noticed for long ago. The only mysteries are into the core, which dynamic is not really understood, and with some "magnetic anomalies" into the continent, which are very probably metallic deposits.

Posted by: Richard Trigaux Dec 30 2005, 05:36 PM

However this probe was able to pick a minute difference, what Earth is currently absorbing due to the increase of temperature. This implied to measure the albedos and solar constant with the highest possible accuracy available today. The question is here: is the geothermal flow also measurable with this technology? If the outward geothermal flow is much weaker than the inward greenhouse heat absorbtion, you are right. They did not mentioned the geothermal flow about the probe and its result, so likely it could not be measured.

Posted by: The Messenger Dec 30 2005, 06:30 PM

If we are talking purely about alignment and origin, I still think the oceans are worthly of a good head scratch - Magnetic moments originating in the oceans could be selectively magnitizing the underlying core materials - a small dynamo driving the otherwise chaotic ramblings of a much larger one. This is consistent with metal deposits in the continental crusts piling up in anomalies, since they don't get their daily dose of order.

Posted by: BruceMoomaw Dec 31 2005, 03:08 AM

The heat-balance satellite Richard is thinking of probably ERBS, put in orbit by Challenger in October 1984. It's an indication of how difficult it is to study global-warming questions WITHOUT satellites that it took a couple of years of measurements by ERBS to answer as fundamental a question as whether Earth's current cloud cover is, on balance, heating or cooling the planet. (It turns out to be substantially cooling us -- at least as it exists now, dominated by cumulus rather than cirrus clouds). ERBS, like UARS, is about to be shut down at last; but the newer big Earth Observing System satellites (especially "Aura") have greatly improved instruments to look into the same matter.

By the way, "Triana" -- the satellite which Al Gore devised in a misguided brainstorm to hang at the L-1 Earth-Sun point just to beam back continuous pictures of Earth -- was equipped, at the insistence of the National Academy of Sciences, with similar energy-balance instruments that might actually have made it worthwhile. But while the satellite was constructed during the Clinton Administration, it's now sitting in a warehouse because the current GOP government refuses to launch it and thereby give Gore favorable publicity.

As for orbital heat-flow measurements: any atmosphere whatsoever simply complicates the transfer of geothermal heat energy into space so much that there is no conceivable workable way to measure it from space -- except, of course, for flat-out local volcanism such as Mars Odyssey and Venus Express are looking for.

Posted by: RNeuhaus Dec 31 2005, 10:36 PM

http://www.windows.ucar.edu/tour/link=/physical_science/magnetism/generating_magnetism.htmlhttp://www.windows.ucar.edu/tour/link=/physical_science/magnetism/generating_magnetism.html

Scientists think, although not certain, there are two essential ingredients for generating a magnetic field. Those two ingredients are

* magnetic material
* currents

A bar of iron can be made into a magnet by wrapping it with wires and running a current through the wires.

It is believed a planet, or a star, can generate a magnetic field if it has both of the two ingredients above. It must have enough magnetic material, and it must have currents moving inside the magnetic material. If a planet does not have enough of either of these two ingredients, it will not have a magnetic field. Planets which do not have magnetic fields include Venus (moves very slowly), and Mars (most the iron is on the surface, and not molten).


Venus lacks of magnetic field (Richard has told us) due to very slow rotation.
Mars lacks of magnetic field due to it lacks molten iron.

Rodolfo

Posted by: JRehling Jan 1 2006, 04:11 AM

There's certainly more to it than that, for the simple reason that Mercury has a magnetic field, even though it also rotates very slowly.

The complete story is unknown, but I suspect that some kind of rotational flux is required: variation in rotational period as a function of distance from the axis within some magnetic medium. If a world had an iron core and rotated extremely rapidly, but lacked that flux, I can't see why that would create much of a magnetic field. So I think we'll find that Earth and Mercury have portions of their cores that rotate at different rates (this is already known to be true of Earth), while Venus does not.

Posted by: gpurcell Jan 1 2006, 04:57 PM

As I recall, Triana also has to go up on Shuttle, which has posed a problem on the manifest even before Columbia....

Posted by: BruceMoomaw Jan 1 2006, 09:23 PM

Triana can be easily launched on an expendable rocket -- once it gets past the GOP. At a minimum, if they're not going to fly it, its energy-balance instruments should be reinstalled on some of the solar-astronomy satellites headed for the L-1 point. But then this White House isn't wildly enthusiastic about global-warming research either, as you may have noticed (although the GOP Congress did force refunding of the Orbiting Carbon Observatory, which is a step in the right direction).

Posted by: Bill Harris Jan 1 2006, 09:55 PM

I wonder if Mercury's resonant rotation (59 days/88 day 'year'), a 2:3 ratio, has anything to do with this. The rotation rate does not change, although the apparent motion of the Sun does this celestial tapdance.

A primer on the rotation:

http://cseligman.com/text/planets/mercuryrot.htm

--Bill

Posted by: Tom Tamlyn Apr 11 2021, 06:07 PM

Fascinating article in The Atlantic about using the Geldingadalur volcanic eruption in Iceland as a proxy for studying ancient Martian vulcanism.
https://www.theatlantic.com/science/archive/2021/04/iceland-volcano-eruption-volcano/618567



[Mods: I'm reviving this thread because the subject matter seemed to fit. Please move if you disagree.]