Geoscientist Online

Snows of yesteryear

Do ice meteorites exist? And how can we find them if they do? Joe McCall investigates…

If we accept the chemical condensation model of the Solar System^1,2 ice meteorites should exist. But they would melt on impact, in any temperature zone except Antarctica, where they could be recovered³. But how could we distinguish them from terrestrial ice? Such ‘dirty ice balls’ could be detected by studying their radioactive isotope content or organic chemicals. Acoustic and electromagnetic techniques (using long wavelengths, 10-40Hz) are likely to be useful and one would need a refrigerated transport system.

Hegyi et al.⁴ suggest using a Husar rover, equipped with suitable instruments. Ammonia and methane, they suggest, may be the clue to detection. They would drill to 3cm and place a gas sensor in the hole. Conductivity would be measured. Any sample would be recovered using a robotic arm, and GPS would position the recovery site precisely. They envisage three rovers working in parallel to cover a large area.

Fodor³ envisages extending the search to the Moon, ‘using the perpendicular
suspended LIDAR technique on the well known optical package which works as laser optical localisators’. This is an entirely new proposition and covers untried
methods. Scientists in China recovered in 1995 what they believed to be chunks of meteoritic ice which ‘plummeted to Earth’ in Zhejiang Province (just south of Shanghai)5. An amateur geologist, Zhong Gongpei, was nearby when farmers saw three large chunks of ice crash into paddy fields of Yaodou village. A cloudy streak and a ‘whooing sound’ were reported, before they crashed down in three fields one kilometre apart. One fist-sized piece left a crater one metre in diameter. A frozen food company kept them from melting. Meteorite expert Wang Sichao of the Purple Mountain Observatory, Jiangsu Province, said that they were probably ice meteorites, but further analysis was needed to confirm this, including analysis for isotopes and cosmic dust. The ice was apparently white, transparent, of irregular shape with air bubbles (?) on the surface and within. It was light and did not show the familiar layered structure of hailstones.

Was this a tall story? No mention of the analytical results is available 13 years later, and one must assume that the results were negative. One alternative possibility is that the lumps fell off the wing of an aircraft. There is also the question of friction in atmospheric entry. Ice meteorites would be likely to melt much further in - or entirely - on atmospheric entry: perhaps only quite large ice bodies would survive to land, even in Antarctica.

It is apparent that interest in ice meteorites has been revived and there will be searches in Antarctica and on the Moon, in the near future backed up my ‘state of the art’ technology. Mars, however, is too dusty to be a suitable location for such research3.

References

1.  
2. Bérczi, Sz, Lukács, B 1994 Icy meteorites on Antarctica 19th NIPR Symposium on Antarctic meteorites, p 19.
3. Földi, T, Bérczi, Sz, Lukács, B 1995 Search for icy meteorites in Antractica. 20th NIPR Symposium on Antarctic meteorites, p68.
4. Fodor, E 2008 Technological methods in the search for icy meteorites on cold regions. Meteoritics and Planetary Science 43 Supplement, A181.
5. Hegyi, S, Kovács, B, Hudoba, Gy, Istenes, Z, Bérczi, Sz 2008. Search for ice meteorite in Antacrtica by Husar Rover. Meteoritics and Planetary Science 43 Supplement, A183.
6. http://www2.jpl.nasa.gov/s19/news56.html : (1995 report by J Parker)
   
   
   

Jacking up the yak

Joe McCall reports on the surprisingly late timing of the Tibetan Plateau's uplift

The Himalayan-Tibetan Plateau, the World’s largest highland, plays an important role in driving the Asian Monsoons and global climate. However, the timing of its uplift remains a matter of debate. Researchers Yang Wang et al.¹ suggest a rethink is in order.

A late Pliocene fauna has lately been discovered in the Kunlung Pass Basin - an asymmetric pull-apart. Bones and teeth of ancient herbivores were recovered from the Lower Qiangtang Formation, in and around two layers of organic rich mudstone - the age of which has been derived from palaeomagnetic studies as 2.5-2.0Ma (i.e., the lower part of the Matuyama division, Late Pliocene). The herbivore fossils include hipparionine horse, bovids, rhinoceros and other unidentified mammalian herbivores.

The authors compare this fauna with modern herbivores living around the Kunlung Pass – wild Tibetan asses, yak and five small mammals. The δ13C values from tooth enamel show that the diets of the fossil herbivores were very different from those of the present population. The δ18O values for the same tooth enamel also showed a significant change in the local meteoric water The δ18O values of fossil fish teeth and bones as well as gastropod shells, clearly indicate that in Late Pliocene time the Kunlung Pass Basin was occupied by a freshwater lake. The climate was warmer and wetter, hospitable to animal and plant life – very different from the rock desert and cold steppe of today. The authors conclude that the basin has been uplifted by ~2700 (+ or – 1600) metres since ~2-3 Ma, after the Late Pliocene, and later than commonly supposed.

1.   Yang Wang, Xiaoming Wang, Yingfeng Yu, Chunfu Zhang, Qiang Li, Zhijie Jack Tseng, Gary Takeuchi, Tao Deng 2008. Stable isotopes in fossil mammals, fish and shells from Kunlun Pass Basin, Tibetan Plateau: Palaeo-climatic and palaeo-elevation implications. Earth and Planetary Science Letters 270; 73-85.

 

Mars giant impact

Joe McCall finds room for doubt

The possible discovery of a huge crater on Mars is reported by Jeffrey Andrews-Hanna and colleagues from MIT¹. This revives an old suggestion that the Borealis Basin is an impact structure. At 8200km across this would be the largest impact structure in the Solar System. Hitherto, the multi-ringed Valhalla Structure on Jupiter’s satellite Callisto held the crown, at ~4000 km diameter. There is no certainty, of course, that Valhalla is impact-generated but this multi-ringed form (one can count at least 13) is unknown among terrestrial impact structures. Andrews-Hanna admit that they have not proved impact origin, but they believe that they have shifted the tide.

The researchers also believe that the basin formerly held an ocean before Mars lost so much of its atmosphere, and its water either sublimated away or froze beneath the surface. Certainly there was more water circulating, probably much of it in hydrothermal systems rather than long lasting rivers, lakes and oceans, earlier in Mars’s history; but the evidence for such an ocean does not appear weighty.

It is said that the impact (?~4 billion years ago) was not so big as to melt everything; though Margaret Marinova² (California Institute of Technology) believes that it was big enough to blast off half the martian crust. So where did it go? We have some small Mars-derived meteorites, but they are small objects. However there is not the slightest evidence, in the thousands of asteroid-sourced meteorites that have been examined (something like 30,000 in all) that there are an asteroidal/lunar and asteroidal/Martian composites. This is a quite amazing absence: I have drawn attention to it before, but it seems that scientists just do not want to know about it. I remain sceptical about such enormous impact structures, and wonder why Marinova believes this postulated giant impact would not have caused large-scale melting: there is much melt-rock produced by shock in much smaller terrestrial impact structures.

References

1. Andrews-Hanna, J C et al., 2008: The Borealis Basin and the origin of the martian crustal dichotomy. Nature 26 June 2008, p1212-1215;
3. Marinova, M M et al., 2008: Mega-impact formation and the Mars hemispheric dichotomy. Nature 26 June 2008, p1216-1219.

 

Transantarctic Mountains Again

I recently reported on the finds of Australasian strewn field microtektites trapped in local detritus and weathering pits, joints and fractures in the Transantarctic Mountains (Geoscientist 18.9 pp4). Alongside them a great number of micrometeorites have now also been recovered (tektites are micro-ejecta from large terrestrial impact structures; micrometeorites are microscopic extra-terrestrial particles).

Van Ginneken et al.¹ describe six out of hundreds of 400-1100 μm-sized micrometorites recovered from the summits of Frontier Mountain and Miller Butte. There are literally thousands of smaller micrometorites with them, as well as cosmic spherules. The six specimens examined are of chondritic structure and mineralogy (olivine-pyroxene) and can be classified with the common H4 and L6 classes of common chondrite stony meteorites. When fresh they have a scoriaceous magnetite crust; but most are weathered, and covered with iron oxides and sulphate encrustations. Various degrees of weathering reflect different terrestrial residence ages; the fresher ones have a fusion crust, formed by surface melting on passage through the Earth’s atmosphere, just like megascopic meteorites.

This discovery, which is related to peculiar climatic conditions on this icy continent, opens up a whole field of research on the flux of nanoparticles to Earth.

Reference

1. Van Ginneken, M, Folco, L, Rochette,P 2008. Micrometeorites in the 400-1100μm size range from the Transantarctic Mountains. Meteoritics and Planetary Science 43 Supplement, A160.
   
   
   

And finally… Victorian greenhouse

The State of Victoria, with approximately five million inhabitants, four million of whom live in metropolitan Melbourne, relies for industrial and domestic energy on the brown coal (lignite) deposits of the Latrobe Valley, 150km east of Melbourne - making Victoria one of the highest per capita emitters of carbon dioxide in the world¹.

Australia is committed to significant reductions on greenhouse gas production by 2050. Obviously, replacing the brown-coal fired energy generation must be considered. In the short term, replacing the coal-fired energy generation by natural gas, from the Bass Strait, irrespective of the benefits of waste reduction and use of renewable resources, is an option. Nuclear energy generation might be a long term solution, with no CO₂ footprint; but although Australia has enormous uranium ore reserves, it has no downstream processing facilities, and only a single pilot scale nuclear plant, restricted to producing medical isotopes.

Any action will be taken to replace the Latrobe brown coal will depend on political factors, as any change is likely to be hugely unpopular in many quarters.

1. Walton, M. 2008. Greenhouse debate hots up in Victoria. Imperial Engineer, Issue 8; 21-22.