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In Brief May 2010

Active Pluto


Geoscientist 20.5 May 2010


PlutoPluto, radius ~1142km, downgraded to the status of a “dwarf planet”, was thought to be a ball of ice and rock with little contrast and little activity. New images taken by the Hubble telescope show a completely different picture1. Trying to resolve the detail on such a small and distant object is like trying to see the markings on a soccer ball 60km away – but Hubble has revealed a mottled planet, with dark orange areas and charcoal black areas, and a mysterious central bright white spot rich in carbon monoxide frost.

The changes are probably due to surface ice, sublimating on the sunlit pole and then refreezing on the other pole. The seasonal cycle takes 248 years. The overall colour is believed to be caused by ultraviolet radiation from the distant sun breaking up methane on the surface, leaving behind a dark, orange-red, carbon-rich residue. Pluto is an active world, with both its orbit and axial tilt driving seasonal changes, unlike the Earth where the axial tilt is the main driver. The seasons are asymmetric because the orbit is elliptical. The New Horizons spacecraft is due in 2015 to fly by Pluto and obtain more detailed images.
  1. www.nasa.gov/mission_pages/hubble/science/pluto-20100204.html 

Lunar meteorites


My attention was taken by a description of the petrography, mineralogy and trace element geochemistry of lunar meteorite Dhofar 1180, from Oman1. This is an unusual polymict regolith breccia with both highland feldspathic and Fe –rich mare components, and with lithic clasts representing highly fractionated rocks or their residual melts. It is a mingled breccia. It is similar to Apollo 16 soils, but there are differences. Isotopic evidence points to a lunar source.

Out of >1000 meteorites discovered in the Dhofar desert of Oman, it appears that no less than 50 came from the Moon1. Even allowing for multiple finds from the same fall, this is an astounding figure, and reflects the modern, highly organised searches of desert regions, which really commenced with the realisation that the Nullarbor limestone desert in Western Australia was ‘littered with meteorites’2. This many finds within a small area of desert indicates that the Earth must have been continuously showered with meteorites spalled off the Moon. Finds in North Africa, Oman and Antarctica relate to quite recent impacts – not more than a million or so years ago. None relates to the so-called ‘Great Bombardment’ of the Moon, which ended about 3900 million years ago.

I have long been worried by two aspects of this hypothesised bombardment. Firstly, there is not a trace of it in Earth’s geological record, and the Archaean-Hadean boundary at 4000 million years, erected by Preston Cloud3, can only realistically be defined by the appearance of life on Earth after it4, which remains a very uncertainly date5. Geological evidence from this far back is scanty indeed5, and the lack of evidence in the geological record is by no means fatal to the hypothesis. What is astonishing is the second anomaly that, whereas asteroidal meteoroids collide frequently with one another to form mixed breccias, there is no trace of lunar material in any asteroidal meteorites.

Planetologists seem to have neglected the problem of throw-out from the Moon during the ‘Great Bombardment’. If Mare Imbrium is really a huge impact scar 1250km in diameter, the mind boggles at the volume of target material that must have been thrown out, and, though such a huge impact might have vaporised all the target material, just think how much rocky material, ejected from the smaller impacts, would have reached us when we consider how much reaches us today! The total lack of mixed asteroidal/lunar meteorites on Earth and in space is surely a major unexplained anomaly. So - did the ‘Great Bombardment’ ever happen? Questions remain to be answered.

Clearly, if Dhofar is so productive, what about the Kavir and Jaz Murian deserts in Iran and the Mashkel Depression in Pakistan? They might be the ‘wrong kind of desert’ for meteorite finding, but I am familiar with the Jaz Murian and I think they are all worth a look!

References

  1. Zhang, A., Hsu, W. 2010, Petrography, mineralogy and trace-element Geochemistry of lunar meteorite Dhofar 1180. Meteoritics and Planetary Science 44(9); 1265-1286.McCall, G.J.H. 1967. The progress of meteoritics in Western Australia. In: Moore , P., ed., “1968 Yearbook of Astronomy”, Eyre & Spottiswoode, London; 146-155.
  2. Cloud, P. E. 1973. A working model for the primitive Earth. American Journal of Science 272; 537-548.
  3. Dineley, D.L. 2000. Hadean. In: Hancock, PL. & Skinner, B.J. “Oxford Companion to the Earth”, Oxford University Press, Oxford; p 490.
  4. McCall, G.J.H. 2010. New paradigm for the Early Earth; did plate tectonics as we know it not operate until the end of the Archaean? Australian Journal of Earth Sciences 57; 1-7.
  5. McCall, G.J.H. 2010. New paradigm for the Early Earth; did plate tectonics as we know it not operate until the end of the Archaean? Australian Journal of Earth Sciences 57; 1-7.