The embargo won't be lifted for a few more hours, but note that the May 4, 2007, issue of Science will have an http://www.sciencemag.org/content/vol316/issue5825/twis.dtl#316/5825/657c regarding Mercury and a possible molten core, a paper that also makes the http://www.sciencemag.org/content/vol316/issue5825/cover.dtl.
http://www.news.cornell.edu/stories/May07/margot.mercury.html
By Lauren Gold
Cornell University Chronicle Online
May 3, 2007
http://www.space.com/scienceastronomy/070503_mercury_core.html
By Ker Than
Staff Writer, Space.com
posted: 03 May 2007
2:00 pm ET
http://www.jpl.nasa.gov/news/news.cfm?release=2007-050
NASA/JPL
May 3, 2007
Hmm...sulfur enrichment needed...Mercury migrated inward? Wonder if some planet we know might be missing a moon after all...
For those who don't have access to Science, or who cannot comprehend the paper, check out http://www.planetary.org/blog/article/00000961/ on the subject, which is a fairly decent summary, and without all the messy numbers.
http://messenger.jhuapl.edu/news_room/status_report_05_04_07.html
MESSENGER Mission News
May 4, 2007
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.
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.
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.
"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.
Looks like more clues to a molten core:
http://www.nasa.gov/home/hqnews/2008/oct/HQ_08-275_Messenger_Mercury.html
with this AP article:
http://news.yahoo.com/s/ap/20081029/ap_on_sc/sci_mercury
Powered by Invision Power Board (http://www.invisionboard.com)
© Invision Power Services (http://www.invisionpower.com)