Meteorite mystery: Amino acids from same asteroid are derived from different sources

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  • Published: Nov 21, 2011
  • Author: Steve Down
  • Channels: HPLC
thumbnail image: Meteorite mystery: Amino acids from same asteroid are derived from different sources

Unique astronomical event signals research flurry

On October 6, 2008, an observatory in Arizona spotted an asteroid hurtling through the sky. Nothing special in that, you might think, but this one was on a direct collision course with Earth. If fragments could be collected after impact, it would allow the study of meteorite fragments that are directly linked to a specific body in space. That would be a first.

Fortunately, the asteroid's path took it over the Nubian Desert in Northern Sudan. This remote impact spot prevented any human casualties or structural damage but also left a clear area to search for fragments, should any have survived the flight though the atmosphere and the force of the impact itself.

Some observers thought that the whole object, weighing an estimated 80 tons, would have completely disintegrated but scientists and student volunteers combed the area and struck it rich. Meteoritic fragments had survived and more than 600 were collected, aided by the difference in colour between the local brown sand and stones and the black space material. They weighed a total of 10.7 kg.

These pieces were given the collective name Almahata Sitta, an Arabic term meaning Station Six, the name of a railway stop in the desert near which witnesses had seen the fireball as it came to ground.

The meteorites were made available to the international community for examination through a formal collaboration between the University of Khartoum and the SETI Institute in California, who had teamed together in the search. A special issue of Meteoritics & Planetary Science in 2010 published much of the research to date, including the petrography, elemental compositions, cosmogenic nuclides, radioisotopes and spectral properties.

The data suggested that the parent asteroid, named 2008 TC3, contained an unexpected mixture of several types of materials originating from primitive and evolved types of asteroids. It had been produced by a succession of break ups and collision-based enlargements, had encountered temperatures greater than 1300°C during its formation, and had not been affected by water.

Some of the studies examined the presence of extraterrestrial amino acids and polycyclic aromatic hydrocarbons. The amino acids were found in a ureilite-type fragment (fragment #4), one of the rarer forms of meteoritic material, but their discovery was surprising, given the thermal history of the asteroid. Amino acids are unstable over temperatures of 500°C.


Amino acid examination

Further investigation of amino acids in Almahata Sitta has now been carried out by scientists from the Sudan and the USA, to try and gain a clearer picture of their origins and distribution. Aaron Burton and colleagues from the NASA Goddard Space Flight Center, MD, the Carl Sagan Center at the SETI Institute, and the University of Khartoum examined two more meteorite fragments.

The amino acid compositions of a second ureilite (fragment #27) and an H5 ordinary chondrite (#25) were analysed, along with a portion of desert sand adjacent to #25. The amino acids were extracted with boiling water in sealed ampoules to obtain the free amino acids.

The extract was spilt in half and one portion was subjected to acid hydrolysis to release primary amino acids that were in bound form, for example as carboxylic acids containing primary amines, to give a total amino acids fraction.

The amino acids in both fractions were converted to fluorescent derivatives for analysis by HPLC. They were separated on a C18 column with a mobile phase gradient of methanol in aqueous ammonium formate and analysed by combined fluorescence and mass spectrometric detection, using electrospray ionisation.


Multiple sources of amino acids within asteroid

All three meteorite fragments contained 180-270 ppb amino acids but their distributions differed from that of the local desert sand, which contained almost 2000 ppb. The sand contained mainly L-alpha-amino acids such as L-aspartic acid, L-glutamic acid, L-alanine, L-serine and L-valine.

Some of these were also present in the meteorites, but they also contained several amino acids that were not related to proteins, which must be of extraterrestrial origin. They included beta-, gamma- and delta-amino acids.

The amino acid compositions of fragments #25 and #27 were similar but they differed from that of #4. This was surprising, given that #4 and #27 are both ureilites from the same asteroid. Fragments #4 and #27 also had different oxygen isotopic signatures, so it appears that they came from different parts of the parent ureilite.

The question remains as to how the amino acids were formed on the asteroid. It seems likely that those from #4 are from a different source than #25 and #27 but they were all formed after the asteroid had cooled to lower temperatures.

The researchers speculated that those in #25 and #27 might have formed at an interface between ureilite and H5 chondrite pieces within the asteroid, with #4 being formed at a ureilite interface. Alternatively, those in #25 and #27 might have arrived on other bodies that collided with the parent H5 chondrite.

They also suggested that the Fischer-Tropsch/Haber-Bosch reaction might have been involved, the amino acids being formed from carbon monoxide, hydrogen and ammonia on catalytic iron or nickel surfaces within the asteroid. This path could be investigated by lab studies.

So, the amino acid distributions mirror the heterogeneous composition of 2008 TC3. It remains to be seen whether their production in this "hot" asteroid is unique or commonplace in space, which the analysis of other ureilites will help to ascertain.

Their wider detection in ureilites will add another type of meteorite to those which could have imported organic matter and contributed to the early prebiotic brew on Earth.


The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

A group of meteorites which fell to Earth in the Sudanese desert contain amino acids that came from different extraterrestrial sources, even though they all originated from the same parent asteroid

One of the meteorites of Almahata Sitta, about 4 cm in diameter
Image: Peter Jenniskens (SETI Institute/NASA Ames)

 

A group of meteorites which fell to Earth in the Sudanese desert contain amino acids that came from different extraterrestrial sources, even though they all originated from the same parent asteroid

The students preparing to begin the search for meteorite fragments

 

 

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