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Remote sensing in space

The fireball of Villalbeto de la Peña, Spain

Joe McCall reports on an important conference on those starry messengers, the meteorites... and other stuff


Geoscientist 17.9 September2007


“In Nature’s infinite book of secrecy, a little I can read”

The Meteoritical Society held its latest meeting, MetSoc69, at ETH in Zurich. Nowadays, research in these areas is largely a matter of remote sensing, hence largely instrumental and of ever increasing diversity and accuracy. However the deductions drawn about the asteroids and planets are to a degree, inevitably more speculative than they might have been in days gone by. The meeting however displayed the amazing range of this fascinating field, from which I have selected a few highlights.

Yorkshire terror

In 2005 a rare pallasite stony-iron was discovered in Yorkshire 1 (Figure 1). was unusually sulphur rich, the high troilite content adding to our knowledge of core formation in the pallasite parent body. The metal-rich regions are consistent with the view of Scott 2 that pallasites formed by injection of metallic liquid into dunite, composed largely of olivine.

Meteorites and asteroids

A major concern in meteoritic research is to match the various types of meteorite to visible asteroids. One method is to work from petrological, mineralogical and other instrumental studies in the laboratory and so build up a picture of the asteroidal parent body - which may not now exist.

A second, quite different approach is to use cameras and reliable observer reports to define the orbital characteristics of fireballs from which meteoroids are recovered – and hence to find the source of the particular meteorite type. Until quite recently only three or four such orbits had been determined but a fall of an L6 type ordinary chondrite in January 2000(Villalbeto de la Peña, Spain - Figures 2, 3) became the ninth such case 4. All nine orbits determined indicate a source in the Main Asteroid Belt and the meteorites include both chondrites and achondrites. Phil Bland of Imperial College and associates 5 have set up a camera network and search-and-recovery parties on the Nullarbor Plain, Western Australia and already have defined one fireball orbit, though without recovering any material (it may have been of very small mass).

Gaffey 6 recounted how asteroid spectral studies had been initiated in the early 1970s to identify specific parent bodies of meteorites, in order to place the compositional, temporal and genetic data from meteorite studies into a spatial context within the early solar system. Naively, it was then supposed that asteroid-meteorite linkages would quickly reveal themselves. The identification of 4-Vesta as the likely parent body of the HED (howardite, eucrite, diogenite) achondrite differentiated meteorites encouraged this belief. However, the failure to identify other matches, and of several proposed matches under close scrutiny, have squashed this unfounded optimism. Three decades later only four likely parent bodies have been identified – for HEDs, H-chondrites, IIE irons and aubrite achondrites. Advances in meteoritic science have since revealed that the early models that ruled our thinking in those days were over simplistic.

Shot stars

In 2003 Birger Schmitz described the find of fossil L-Chondrite meteorites embedded in Middle Ordovician (~ 470 Ma) limestones in southern Sweden7. The flux of meteorites to Earth was two orders higher then than now, a fact attributed to a collision of parent bodies somewhere in space. Schmitz updated the story 8 , reporting on the discovery of extraterrestrial chromite [with higher TiO2, Cr/(Cr+Al) and a narrow range of V2O5] in the limestone. This has also been recognised in the resurge deposits of the Lockne impact structure in Central Sweden, with which this event is now coupled.

• Figure 1. The Hambleton, Yorkshire pallasite find. The dark angular enclaves are olivine; the white areas are FeNi metal (picture supplied by Diane Johnson PSSRI and Open University).

Chondrules rule

In a recently published work I reviewed theories on the origin of chondrules 9; concluding like John Wood10 that despite years of research this key question is still not resolved. At MetSoc69, an early nebular origin was generally favoured, and E R D Scott11 considered the isotopic evidence – concluding that chondrule formation-ages (determined using long- and short-lived isotopes) demonstrated that chondrites accreted from diverse batches of chondrules in the nebula over a 4 Myr period, beginning c. 1 Myr after the formation of CAIs (Calcium-Aluminium rich refractory Inclusions). Iron meteorites and achondrites derived from bodies that accreted earlier, when 26Al was abundant.

Two different scenarios have been advanced to explain the long period of chondrule formation: one involving accretion on the surface of an existing parent body, and another suggesting that differentiated and chondritic parent bodies formed at different locations, at different times. The latter is more consistent with meteoritical evidence, but requires that existing dynamic models for planet formation be modified to allow five million years for planetesimals to form.

Palme, in his Presidential address 12, noted the observed contrast between the narrow range of (solar-like) chemical and isotopic compositions found in centimetre-sized chondrite samples, and millimetre-sized components (chondrules, inclusions, fragments and matrix). Chondrules in ordinary and carbonaceous chondrites show a wide range of chemical composition, including varying degrees of oxidation and isotopic ratios.

The chaotic mixture produces a very simple bulk composition when scaled to average solar system abundances. He concluded that the accretion of chondrules had involved chemically fractionated materials, heterogeneously distributed in the nebula.

Meteoritic microfossils

The Pliocene (2.5 Ma) Eltanin impact structure (~25km diameter) 13 is unique in being the only impact structure situated in the deep ocean (5 km). The structure displays a chaotic mass of breccia, including shock-melted asteroidal material and minute unmelted meteorite fragments (mesosiderite, howardite). The impact disrupted sediments on the ocean floor, which were in turn overlain by a pulse of meteoritic ejecta.

New cores from the Freeden Seamounts reveal that the topmost sediments are bioturbated. At 2.7km depth, calcareous agglutinated tubular foraminifera rapidly colonised the sediments after the event, and in deeper water (5km, where calcite dissolves) such tubes are siliceous - or the sediments barren. The unicellular organisms living in the 7mm by 3mm tubes constructed their shells from mineral grains and from their known habit of colonising environments devastated by catastrophic events are known to ecologists as ‘disaster’ species. The second most abundant grains on the Freeden Seamounts (after fragments of other calcaereous microfossils) are meteoritic fragments. One point-count revealed 70 % ejecta, (mainly vesicular melt rock, with pyroxenes and anorthite from the Eltanin meteorite).

Figure 2. A recovered fusion crust coated and oriented mass of Villalbeto de la Peña Spain.


Attention at MetSoc69 was focused on the final results from the preliminary examination team 14 on the returned samples from the Stardust mission to Comet Wild-2, which had already revealed a surprisingly cratered surface (Figure 4). Dust samples from the coma, released from the nucleus hours before, were trapped in aerogel and in minute impact craters on aluminium foil. The nature of the coma dust suggests that Wild-2 is not a reservoir of presolar matter, and implies large-scale radial transport within the solar nebula.

Antarctic meteorites

Yangting Lin 15 reported on 5354 meteorites from the Grove Mountains, Antarctica. The meteorites were stranded in blue ice fields on the east side of 64 nunataks stretching for 50km on the “downstream” side of the ridge. They were very much smaller than in the American and Japanese finds. The overlap of successive collecting sites indicates that meteorites are still coming up.

Extraterrestrial planets

Whereas the Stardust mission actually got beyond “remote sensing” to “direct sampling”, the search for extraterrestrial planets16 involves the extremes of remote sensing. In the past 11 years 196 such planets have been recognised, mainly by Doppler Spectroscopy, which look for variation in stellar velocity induced by gravitational influence of planetary companions. A dozen have been recognised by means of transits or microlensing effects.

The most exceptional case is a system of three Neptune-like planets - the two inner believed to be mostly rocky - orbiting a sun-like star with an asteroid belt. Planets as small as a few Earth masses have been detected in this way, but models suggest that c. 90% of extraterrestrial planets are not detectable because they do not accrete enough gas to become giants 17.

Figure 4. The ~5 km diameter nucleus of Comet-Wild 2 imaged by the Stardust Mission (NASA image, supplied by Alan Bowden, Liverpool Museum).

The Moon

A number of presentations focused on lunar-sourced meteorites now appearing in the Antarctic and hot deserts. These descriptions are important, because the meteorites, impacted off the Moon in geologically recent times, sample regions of the lunar surface not touched by either Apollo or Luna - including the far side. They also carry inclusions within the breccias that can be referred to regions below the regolith (about which we really know very little).


Mars can always be counted on to turn up the interesting and unexpected – including the remarkable find of a stony meteorite on the planet’s surface. An excellent poster presentation18 on a Nakhlite from Antarctica (MIL03346) described the seventh Nakhlite, which like the others had a formation age of ~1.3Ga. What interested me was that whereas previously described Nakhlites had been holocrystalline (with the texture of intrusive rocks) this specimen had areas of glassy mesostasis in enclaves between the coarsely crystallised minerals.

This texture is very rare on Earth, though I have described it from nodules carried up in the Katenmening Basalt of Silali caldera volcano, Kenya 19. They clearly represent crystallisations of ‘gabbro’ in a shallow magma chamber. This suggests that the impact which detached the Nakhlites from Mars excavated such a subvolcanic magma chamber. However, the most astonishing fact about the Nakhlites is that none is in any way shocked. How this can be, when they have been blown off Mars by an impact powerful to propel them to Earth, is incomprehensible - and the most extraordinary fact that I discovered at MetSoc69.

Further information



1. Johnson, D , Hutchison, R & Grady, M M 2006 Textural evidence for melt processes on the pallasite parent body Meteoritics & Planetary Science (M&PS) 41 [Supplement(S)]; A88
2. Scott, E R D 1977 Formation ofolivine metal textures in pallasite meteorites Geochimica et Cosmochimica Acta 41; 693-710
3. Downes, H & Milttlefehldt, D W 2006 Identificatiojn of a common R-chondrite impactor on the ureilite parent body M&PS 41S; A46
4. Trigo-Rodriguez, J M , Borovicka, P , Spurny, J L et al 2006 The Villalbeto de la Peña meteorite fall: determinations of atmospheric trajectory and orbit M & PS 41; 505-517
5. Bland, P A , Spurny, P , Bevan, A W R and others 2006 First light for the desert fireball network M&PS 41S A23
6. Gaffey, M J 2006 Spectroscopy and the asteroid-meteorite link A perspective after three decades M&PS 41S ; A60
7. Schmitz, B 2003 Shot stars Geoscientist 13(5); 4-7
8. Schmitz, B 2006 The disruption of the L-chondrite parent body and the great Ordovician biodiversity event M&PS 41S; A159
9. McCall, G J H 2006 Chondrules and calcium-aluminium rich inclusions (CAIs) In McCall, G J H , Bowden, A J & Howarth, R J , eds , Geological Society Special Publication 256; 345-361
10. Wood, J A 2006 The message in meteorites Ibid 498-500
11. Scott, E R D 2006 Meteoritical and dynamical constraints on formational mechanisms for asteroids and planets M&PS 41S; A162
12. Palme, H 2006 The chondrite paradox M& PS 41S; A138
13. Kyte, F T , Gersonde, R & Kuhn,G 2006 Meteoritic microfossils in the Eltanin impact deposits M&PS 41S; A103
14. McKeegan, K , Aleon, J , Alexander, C et al 2006 Isotopic composition of cometary matter returned by the stardust mission M&PS 41S A119
15. Yangtin Lin, Yitai Ju, Xiaxin Xu et al 2006 Recovery of 5354 meteorite in Grove Mountains, Antarctica by 22nd Chinese Research Expedition M&PS 41S; A107
16. Mayor, M 2006 From gaseous giants to rocky planets M&PS 41S; A118
17. Benz, W 2006 Thr formation of terrestrial planets: what do the extra-solar planets tell us? M&PS 41S; A20
18. Anand, M , Burgess, R, Fernandes, V & Grady, M M 2006 Ar-Ar age and halogen characteristics of Nakhlite MIL 03346, records of crustal processeson Mars
19. McCall, G J H 1970 Gabbroic and ultramafic nodules: high-level intracrustal nodular occurrences in alkalic basalts and associated volcanic rocks from Kenya, described and compared with those of Hawaii In: Ringwood, A E & Green, D H , eds “Phase Transformation and The Earth’s Interior”, Phys Earth Planet Interiors (North Holland Publishing Company) 3, 255-272