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Thread: Mercury and the Winged Messenger

  1. #1 Mercury and the Winged Messenger 
    Universal Mind John Galt's Avatar
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    For those of you who are not aware of it, NASA’a NASA's MESSENGER spacecraft flew by Mercury on January 14th. It will have further flybys later this year and 2009, before going into orbit on March 18th 2011.

    Principle results of this flyby, the first since Mariner 10 visited in the 1970s are:
    1.The spider - one hundred narrow, flat-floored troughs spreading out from a complex central region occupied by an impact crater.
    2.A full view of the Caloris basin, one of the largest impact basins in the solar system. (This has led to an increase in its estimated size, to 960 mile diameter.)
    3.Distinct differences in the magnetosphere from what was observed by Mariner
    4.uv-emissions of Na, Ca and H in Mercury’s exosphere
    5. Over 1,200 pictures of the surface, including many areas not previously seen.
    JPL report that MESSENGER saw an internal magnetic field that is well described by the field from a dipole nearly aligned with the planet’s spin axis (dipole tilt ~ 10°). …The dipolar field is consistent with an active electrical dynamo in which the magnetic field is produced by electrical currents flowing in an outer core of molten metal.
    Sources:
    http://www.spacedaily.com/reports/Su...rcury_999.html
    http://messenger.jhuapl.edu/


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  3. #2  
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    that's some pretty cool stuff...can't wait to find out more....come on 2011...get here faster...


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  4. #3  
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    Good shtuff.

    Nice animation on the Messenger site.
    Wolf
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    Administrator KALSTER's Avatar
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    A full view of the Caloris basin, one of the largest impact basins in the solar system.
    One thing they noticed about this impact crater, is that its basin is not dark as thought.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    WYSIWYG Moderator marnixR's Avatar
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    is there a specific reason why craters should be dark ?
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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  7. #6  
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    is there a specific reason why craters should be dark
    Ophi would be able to tell you exactly, but I think they expected it to be like some of the moon's craters where the basin is black as a result of lava flow after the impact event. I think...
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  8. #7  
    Universal Mind John Galt's Avatar
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    You are too kind. I shall have to do some research to answer precisely. You are correct that lava filled impact basins (the norm on the moon) would be dark, since typically the lavas will be basalts, in which dark minerals predominate. If the infill has been subjected to significant later bombardment, making the surface rougher, this would also tend to increase its reflectivity and brightness. If that is the explanation it would point to an early eruption of lavas into the impact basin. this would be in contrast with happedn on the moon with the Mare basalts and might therefore provide new insights into planetary formation and earlier history.
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    I have done some reading on this point. Kalster, I am slightly puzzled by your original statement: it seems to imply that it is the Messenger probe that has revealed the Caloris basin infill to be lighter than the equivalent Mare basalts on the moon. This is not the case. That observation was made after the Mariner 10 encounters back in the 1970s.

    My speculative explanation above was generally correct (an aging effect related to aspects of regolith formation), but specifically completely wrong (nothing whatsoever to do with regolith structure..)

    This quotation, relating to the lunar Mare basalts, explains it quite clearly

    Within the regolith are large numbers of agglutinates: small complex glass bonded particles produced when micrometeorite particles melt and weld together rock and mineral fragments. As a rule of thumb, such agglutinates accumulate in proportion to the time the regolith has been exposed to impacts. It has been found that albedo is controlled largely by the composition of these particles (which is related to the chemistry of the underlying rocks) and by the length of time they have been exposed at the lunar surface. …….. regoliths enriched in Fe and Ti are the darkest…..Where regolith has been disturbed, as in a major impact, the fresher material ejected by the impact will be of a higher albedo than the undisturbed regolith, but will gradually darken with time.”

    Source: Cattermole, P Planetary Volcanism: a Study of Volcanic Activity in the Solar System 2nd Edition Published by John Wiley and Sons 1996 ISBN: 0-471-96051-9 page 201

    Thus the lighter infill of the Caloris basin may be explained by
    1)A more recent impact than is the case for the lunar Mare (impacts dated around 3.8 Gy).
    2)A lower proportion of iron or titanium in Mercurian basalts compared with those of the moon.
    3)A combination of the two effects.

    It may be relevant that the lunar basalts are generally of a higher titanium content than the terrestrial ones. Thus Mercurian basalts may be closer in composition to those of the Earth. This could shed light on aspects of planetary formation (for example, might the difference be related to the proposed impact origin for the moon?)

    An alternative explanation would be that the Caloris basin infill and similar smooth plains of high albedo are made up of impact ejecta, rather than eruptive basalts. According to Cattermole (cited above), several authors (Strom 1977, Trask and Guest 1975, Murray et al 1975) argue for a volcanic origin, but others (Wilhelms 1976, Overbeck et al 1977) think the light plains are indeed basin impact ejecta. (Similar features are to be found on the moon, but their origin is also disputed.)

    Now, a couple thoughts occurred to me while considering these points. Both are highly speculative.

    Many of you will be aware that the Mariner 10 survey of the planet mapped less than half of the planet (45%). This included just under half of the Caloris Basin. The bulk of the smooth light plains that have been mapped are roughly centred on Caloris. If the Messenger studies show this pattern is repeated in the previously unmapped area it will be highly suggestive. In that instance I would suggest that the Caloris infill will turn out to be volcanic, but the outer ring of smooth light plains will be composed largely of impact ejecta. If that is the case I’ll go even further out on a limb and suggest, based on asymmetry in the smooth light plains distribution that the impacting body causing the basin came in from the south west (or the NE if I have my maps upside down!)

    My second speculation relates to the magnetic field. Mercury was expected, on account of its small size, to have cooled to the point where there was no longer any molten core and hence no way of generating a magnetic field such as the Earth’s. If the Caloris Basin impact was comparatively recent (which would explain the high albedo of the infill basalts) might it have imparted sufficient thermal energy to the core to maintain it in a partially molten condition? I haven’t even done a back of the envelope calculation to check out if this is even plausible, but I do find it interesting.

    Once Messenger gets into orbit its array of instruments will answer many of these questions. We may be confident it will raise many more. Mercury will remain an elusive and interesting planet for some time to come.
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  10. #9  
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    Quote Originally Posted by Ophiolite
    I have done some reading on this point. Kalster, I am slightly puzzled by your original statement: it seems to imply that it is the Messenger probe that has revealed the Caloris basin infill to be lighter than the equivalent Mare basalts on the moon. This is not the case. That observation was made after the Mariner 10 encounters back in the 1970s.
    .
    I read the article/post on hypography. Here. Here are some of what I was quoting:
    Quote Originally Posted by On Hypography they
    The cratered expanse pictured above was mostly in darkness 30 years ago when NASA's Mariner 10 spacecraft made the first and (until now) only flybys of Mercury. Last week, MESSENGER caught the terrain in sunlight for the first time. In the mid-1970s, Mariner 10 caught a tantalizing glimpse of the basin's edge, a ring of shadowed mountains thrown up long ago by some catastrophic impact. A comet or asteroid had smashed Mercury and gouged a crater bigger than the state of Texas. What was inside? No one could say.

    MESSENGER snapped the picture that geologists had long wanted: Caloris in its entirety, a top-down view in broad daylight—and the results were surprising. Many experts expected the interior of Caloris Basin to be dark, like the dark 'seas' of hardened lava that fill major impact basins on the Moon and give anthropomorphic form to the "Man in the Moon." Instead, Caloris is bright inside and pitted with regions of interesting color.
    I can see how my poor wording gave the wrong impression. Anyway, this is interesting!
    Quote Originally Posted by Ophiolite also
    Mercury was expected, on account of its small size, to have cooled to the point where there was no longer any molten core and hence no way of generating a magnetic field such as the Earth’s
    Is there any reason why they rule out nuclear activity as the cause of a molten mantle on Mercury (If in fact they rule it out)?
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  11. #10  
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    Quote Originally Posted by KALSTER
    I can see how my poor wording gave the wrong impression.
    Actually you gave the right impression of the article. It was when I did some checking I rapidly found the clear belief that the Caloris basin contained lighter material than corresponding basins on the moon.
    Now part of the problem here is that terms like light and dark are all relative. Is the linked article trying to say that they are much lighter than previously imagined? I don't know.

    The more I read into the issues the more intriguing it becomes. (I have read relatively little about Mercury in the past.) For example this passage "Spectra of areas dominated by Caloris Basin with the encompassing smooth plains may show Fe2+ abundances in the soil comparable to lunar highlands soil." From Wendell W. Mendell The dependence of reflectance spectra of Mercury on surface terrain Icarus Volume 59 1984
    The lunar highlands contain extensive ejecta and are therefore correspondingly light. So again we have the data pointing one way, then another. Roll on orbital insertion.

    I was not sure what you meant by nuclear reactions. The role of radioactive decay as a means of generating heat will be fully taken into account in any models for the cooling of the planet. Were you thinking in terms of the radical ideas proposed for the Earth in which there is, in essence, a nuclear reactor at its centre?

    This paper addresses some of the issues.

    Leake,M.A. The chronology of Mercury's geological and geophysical evolution: The vulcanoid hypothesis Icarus Vol 71 1987

    Previous discussions of Mercury's evolution have assumed that its cratering chronology is tied to that of the Moon, i.e., with Caloris forming about 3.9 Gyr ago as part of a late heavy bombardment that affected all of the terrestrial planets. That assumption requires that Mercury's core formed very early, because associated expansion features are not visible, and must have been erased before the cratering flux declined. Moreover, the modest amount of global shrinkage inferred from visible compressional features on Mercury's surface implies that the core is either largely molten at present, or had largely solidified before the end of the bombardment. The former interpretation requires a significant volatile content or implausibly large internal heat sources, while the latter raises questions about how to generate the planet's magnetic field. We have investigated whether constraints on Mercury's chronology could be relaxed by effects of a Mercury-specific bombarding population of planetesimals interior to its orbit, encountering the planet only occasionally due to secular perturbations. Such “vulcanoids” could have been a significant source of early cratering. However, those in orbits that can cross Mercury's are depleted by mutual collisions in 1 Gyr, and can provide at most a modest extension of the period of heavy bombardment. Further inside Mercury's orbit, lower collisional velocities might allow survival of vulcanoids to the present. We report on a search for such bodies and on observational limits to such a population. We also review evidence that Mercury's intercrater plains are of volcanic origin and mainly predate Caloris, and that scarp formation (and global contraction) mainly postdates Caloris and has continued to recent times. If global lineaments are the product of tidal despinning, they constrain core formation to the first half of the planet's lifetime. While some questions and inconsistencies remain, the preponderance of evidence suggests that Mercury differentiated early, and at least half of its core volume is presently molten, probably due to a significant content of some light element such as sulfur.

    I've quite abandoned my speculation about the Caloris impact generating the thermal energy to maintain the core in a liquid state. The energy release is just too little, by at least an order of magnitude.

    Thank you for engaging in dialogue on this one: it is forcing me to dig into references and I am learning a lot along the way.
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  12. #11  
    Forum Ph.D. Wolf's Avatar
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    Wow...actual scientific discussion....For a moment I thought I was in the wrong place...
    Wolf
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  13. #12  
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    Were you thinking in terms of the radical ideas proposed for the Earth in which there is, in essence, a nuclear reactor at its centre?
    Well, yes. You call it radical and I thought that it was the current model. I think I read some old books!

    But then I read this on Wikipedia?:
    The internal heat of the planet (Earth)is most likely produced by the radioactive decay of potassium-40, uranium-238 and thorium-232 isotopes.
    Your blue reference describes the possibility of the presence of "volatile content" (radioactive?)as a reason for the molten core as well as the possible prevalence of lighter elements such as sulfur mixed in with the iron majority, while on wiki they proposed (perhaps the more updated) idea of tidal forces generating the heat. Maybe I should check some other sources than Wiki, just in case they got it wrong.

    PS: You are learning about Mercury and I am learning geology in general! I like.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  14. #13  
    WYSIWYG Moderator marnixR's Avatar
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    i think Mercury could have a partly molten core for the same reason that Io and Europa are not the inert ice balls they were originally thought to be : tidal effects
    "Reality is that which, when you stop believing in it, doesn't go away." (Philip K. Dick)
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  15. #14  
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    Quote Originally Posted by KALSTER
    Were you thinking in terms of the radical ideas proposed for the Earth in which there is, in essence, a nuclear reactor at its centre?
    Well, yes. You call it radical and I thought that it was the current model.
    The conventional view is that uranium and the other common radioactive elements are lithophiles. i.e. they are attacted to the rockforming minerals. Therefore they are concentrated in the crust and the mantle.
    Herndon has proposed that uranium combined with sulfur to produce a mineral denser than iron which then sank to the heart of the Earth where it formed a breeder reactor, heating the Earth internally and providing the energy to power the magnetic field.
    There is a nice review of the concept here:
    http://geology.about.com/od/wildgeot...uclearcore.htm
    Or you can visit Hendron's own site for many more details:
    http://nuclearplanet.com/
    Your blue reference describes the possibility of the presence of "volatile content"
    If the volatile content is high this would keep the core molten at comparatively lower temperatures. The water, or other volatiles, lowers the melting point of the mix.
    Quote Originally Posted by marnix
    i think Mercury could have a partly molten core for the same reason that Io and Europa are not the inert ice balls they were originally thought to be : tidal effects
    and
    Quote Originally Posted by kalster
    on wiki they proposed (perhaps the more updated) idea of tidal forces generating the heat
    Now according to this paper van Hoolst, T., Jacob,C. Mercury's tides and Interior Structure Journal of Geophysical Research, Volume 108 2003, "Due to the 3:2 spin-orbit resonance, the tides on Mercury cannot be divided into the three classically known tidal frequency bands for the Earth: the diurnal, semidiurnal, and long-period tides. Instead, the tides all have periods of the order of one Mercury day, or, equivalently, one Mercury year, and their amplitudes have been calculated with an accuracy of 10-5 m2s-2."
    My initial impression is that tides with a frequency of one Mercurial day are not going to be generating sufficient internal frictional heat to have a significant effect. I shall try to find out more.
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  16. #15  
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    From Wiki
    Orbital simulations indicate that the eccentricity of Mercury’s orbit varies chaotically from 0 (circular) to a very high 0.47 over millions of years. This is thought to explain Mercury’s 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.
    And:
    A mechanism that has been suggested for keeping it liquid are particularly strong tidal effects during periods of high orbital eccentricity.
    They seem to suggest that the chaotic behaviour of Mercury's orbit could add to the tidal effects, as it is constantly moving between periods of high and low eccentricity (currently at 0.21). Since it varies in as little as millions of years, I’d guess that it is not enough time for it to cool down after a period of frictional heating when eccentricity is high?

    Oh, I did not know what volatiles were, I do now. :wink:
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  17. #16  
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    Have you given up on this, Ophiolite?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  18. #17  
    Universal Mind John Galt's Avatar
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    Quote Originally Posted by KALSTER
    Have you given up on this, Ophiolite?
    No. In relation to the molten core you mentioned wikipedia's observation that "A mechanism that has been suggested for keeping it liquid are particularly strong tidal effects during periods of high orbital eccentricity".
    Unfortunately they do not give any reference for this; I can find nothing online; my own library has only meagre material on Mercury, in standard astronomy texts.
    It does not sound impossible: I hesitate to go as far as to say that it sounds plausible.
    I keep returning to the thought that had the Caloris Basin impactor been a late event it perhaps could have input sufficient thermal energy to the planet to partially melt the core. (I need to calculate that, but my physics is poor.) Anyway, that simply does not gel with the (universally) accepted idea that the impact event occured during the Heavy Bombardment phase. Ages of portions of planetary surfaces can be computed from a crater count - there seems no way that Caloris could be a 'recent' event.

    I noticed in the wikipedia article that they have three explanations for why Mercury has such a large iron core:
    1) Mercury was originally more than double its present size and was struck by a very large planetesimal (in the same way that it is thought the early Earth was struck by a Mars sized body, the result of which was the formation of the moon.) In the case of Mercury this impact stripped away the proto-crust and most of the mantle.
    2) When the proto-sun 'switched on' it was intense enough to 'burn' away much of the mantle material.
    3) Drag effects within the accreting solar nebula led to the loss of lighter particles.

    I am surprised they make no mention of a fourth possibility. The kinds of material that could condense at any time/point within the accretion disc depend upon its temperature. The temperature generally drops of away from the sun. Thus beyond the 'ice line' out where the gas giants and the ice giants formed, water and other ices could condense. Close in, where Mercury was forming, the first materials to condense would likely have been refractory materials such as iron. It therefore does not seem strange to me that it could have such a large iron core.
    I do not know if the wiki author has merely overlooked this option, or if it is rejected on some other grounds. I make it from general principle, not from having read of it anywhere.
    I shall think about that some more, but it may be a couple of years before the thoughts become tangible. :wink: We just need many more data, which Messenger will provide in a few years.
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