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Unmanned Spaceflight.com > Inner Solar System and the Sun > Mercury
AlexBlackwell
The embargo won't be lifted for a few more hours, but note that the May 4, 2007, issue of Science will have an interesting paper Margot et al. (and accompanying Perspectives piece by Sean Solomon) regarding Mercury and a possible molten core, a paper that also makes the cover.
AlexBlackwell
Mercury has molten core, Cornell researcher shows
By Lauren Gold
Cornell University Chronicle Online
May 3, 2007

Surprise Slosh! Mercury's Core is Liquid
By Ker Than
Staff Writer, Space.com
posted: 03 May 2007
2:00 pm ET
AlexBlackwell
NASA Antenna Cuts Mercury to Core, Solves 30 Year Mystery
NASA/JPL
May 3, 2007
nprev
Hmm...sulfur enrichment needed...Mercury migrated inward? Wonder if some planet we know might be missing a moon after all... huh.gif
AlexBlackwell
For those who don't have access to Science, or who cannot comprehend the paper, check out Emily's blog entry on the subject, which is a fairly decent summary, and without all the messy numbers.
AlexBlackwell
MESSENGER PI Discusses Significance of News That Mercury Has Molten Core
MESSENGER Mission News
May 4, 2007
nprev
What I find most mysterious is how Mercury could have such a disproportionately large and at least partially still liquid core yet have a thick crust with no evident prior surface activity and apparently little in the way of a mantle.

Does this perhaps suggest that Mercury did indeed form further out, cooled rapidly, yet had a rapid rotation rate and was re-heated during its orbital migration inward by dynamic tidal braking as it settled into the 3/2 spin resonance and significantly eccentric orbit while its core "re-melted"? Sort of like a thick-skinned Io in some ways...

Another point to consider is Venus' rotational resonance with Earth. Something very interesting and not at all obvious may have happened to all three planets in the early days.
mchan
QUOTE (AlexBlackwell @ May 4 2007, 02:35 PM) *

Kind of sucks the links here are for a pay to view only. At least they could have linked the NASA press release.
edstrick
When they discovered Mercury's mag field from Mariner 10, besides the absolute magnetic moment (something like field strength*volume) of the field, the other major observation was that the field was off center, tilted and distorted in ways that magnetosphere pressures on the field couldn't explain. Similar things are seen at Uranus and Neptune.

The inference was that the field was being generated in a rather shallow shell, rather than deep inside, far from the surface. In Mercury's case, plausible modles had the core frozen to maybe 2/3 of it's radius with a solid inner core and a convecting outer core.

The question was how could a core of a small and geologically inactive world stay partially molten over 4.5 billion years. A generally plausible model was that refined calculations of lower thermal conductivity in a heavily fractured megaregolith (like the moon's) and the even lower thermal conductivity of a deep, old regolith, could significantly reduce global heat flow and extend the life of a molton core. My recollection is that that was marginal, and models of some additional heating than just that from inner core freez-out would help keep enough core molton to provide a dynamo.
Jeff7
Could the heating be a result of tidal forces? As I recall, Mercury has a fairly elliptical orbit. It might not be nearly as severe as what Io experiences, but it still might be enough to liquefy the core.
AlexBlackwell
QUOTE (Jeff7 @ May 5 2007, 05:42 AM) *
Could the heating be a result of tidal forces? As I recall, Mercury has a fairly elliptical orbit. It might not be nearly as severe as what Io experiences, but it still might be enough to liquefy the core.

Bruce Bills had an LPSC abstract on this a few years back. See also Correia and Laskar [2004].
Rob Pinnegar
QUOTE (edstrick @ May 5 2007, 12:52 AM) *
The question was how could a core of a small and geologically inactive world stay partially molten over 4.5 billion years.


I haven't read the new papers, but off the top of my head, a growing inner core might well be made of solid iron which would tend to enrich the sulfur concentration of the remaining outer mantle. This could help keep it molten.
edstrick
"Could the heating be a result of tidal forces? As I recall, Mercury has a fairly elliptical orbit."

Tidal heating involves dissipation of energy. The result is that the orbit would circularize over time. Io and Enceladus are in resonances that keep re-ellipticizing <new word?> their orbits. I don't know the timescale for plausible orbital ellipticity evolution at Mercury, but that it hasn't gone circular over 4.5 billion years argues that the crust is pretty rigid and does not dissipate energy. A fluid core is so fluid that it doesn't dissipate tidal energy efficiently.
Marz
Looks like more clues to a molten core:

http://www.nasa.gov/home/hqnews/2008/oct/H...er_Mercury.html

with this AP article:

http://news.yahoo.com/s/ap/20081029/ap_on_sc/sci_mercury
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