Tag: Pre-solar grains

Asteroid Bennu: a ‘lucky dip’ for NASA and planetary science

Published on by zooks777Leave a comment

I must have been about ten years old when I last saw a ‘lucky dip’ or ‘bran tub’ at a Christmas fair.  You paid two shillings (now £0.1) to rootle around in the bran for 30 seconds and grab the first sizeable wrapped object that came to hand:. In my case that would be a cheap toy or trinket, but you never knew your luck as regards the top prize. There is a small asteroid called 101955 Bennu, about half a kilometre across, whose orbit around the Sun crosses that of the Earth. So it’s a bit scary, being predicted to pass within 750,000 km of Earth in September 2060 and has a 1 in 1,880 chance of colliding with us between 2178 and 2290 CE. Because Earth-crossing asteroids are a cheaper target than those in the Asteroid Belt, in 2016 NASA launched a mission named OSIRIS-REx to intercept Bennu, image it in great detail, snaffle a sample and ultimately return the sample to Earth for analysis. This wasn’t a shot in the dark, as a lot of effort and funds were expended to target and then visit Bennu. But unlike me at the fair ground, NASA will be very happy with the outcome.

The asteroid Bennu, showing its oblate spheroidal shape, due to rotation, and its rubbly structure. Source: NASA/Goddard/University of Arizona via Wikimedia Commons

Bennu is a product of what might be regarded as ‘space sedimentation’, indeed a kind of conglomerate, being made up of boulders up to 58 m across set in gravelly and finer debris or ‘regolith’. High-resolution images revealed veins of carbonate minerals in the boulders. They suggest hydrothermal activity in a much larger parent body – one of many proto-planets accreted from interstellar gas and dust as the Solar System first began to form over 4.5 billion years ago. Its collision with another sizeable body knocked off debris to send a particulate cloud towards the Sun, subsequently to clump together as Bennu by mutual gravitational attraction. The carbonate veins can only have formed by circulation of water inside Benno’s  parent.

The ‘REx’ in the mission’s name is an acronym for ‘Regolith Explorer’. Sampling was accomplished on 20 October 2020 by a soft landing that drove a sample into a capsule, and then OSIRIS-REx ‘pogo-sticked’ off with the booty. The capsule was dropped off by parachute after the mission’s return on 24 September 2023, in the manner of an Amazon delivery by drone to a happy customer. So, you can understand my ‘lucky dip’ metaphor. And NASA certainly was ‘lucky’ as the contents turned out to be astonishing, as related two years later by the analytical team in the US, led by NASA’s Angel Mojarro (Mojarro, A. et al. 2025.Prebiotic organic compounds in samples of asteroid Bennu indicate heterogeneous aqueous alteration. Proceedings of the National Academy of Science, v. 122, article e2512461122; DOI: 10.1073/pnas.2512461122).

The rock itself is made from bits of carbonaceous chondrite, the most primitive matter orbiting the Sun. It contains fifteen amino acids, including all five nucleobases that make up RNA and DNA – adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U) – as in AUGC and AGCT. Benno’s complement of amino acids included 14 of the 20 used by life on Earth to synthesise proteins. The fifteenth, tryptophan, has never confidently been seen in extraterrestrial material before. Alkylated polycyclic aromatic hydrocarbons, also found in Bennu, are seen in abundance in interstellar gas clouds and comets by detecting their characteristic fluorescence when illuminated by mid-infrared radiation from hot stars using data from the Spitzer and James Webb Space Telescopes. These prebiotic organic compounds have been suggested to have played a role in the origin of life, but exposure to many produced by human activities are implicated in many cancers and cardiovascular issues.  A second paper by Japanese biochemists and colleagues from the US was also published in early December 2025 (Furukawa, Y. and 13 others 2025. Bio-essential sugars in samples from asteroid Bennu. Nature Geoscience, v. 12, online article; DOI: 10.1038/s41561-025-01838). The authors identified several kinds of sugars in a sample from Bennu, including ribose – essential for building RNA – and glucose. Bennu also contains formaldehyde – a precursor of sugars – perhaps originally in the same brines in which the amino acids formed.

Yet another publication coinciding with the aforementioned two focuses on products of the oldest event in the formation of Bennu: its content of pre-solar grains (Nguyen, A.N. et al. 2025. Abundant supernova dust and heterogeneous aqueous alteration revealed by stardust in two lithologies of asteroid Bennu. Nature Astronomy, v. 9, p. 1812-1820; DOI: 10.1038/s41550-025-02688-3).  In 1969 a 2 tonne carbonaceous chondrite fell near Allende in Mexico. The largest of this class ever found, it contained tiny, pale inclusions that eight years of research revealed to represent materials completely alien to the Solar System. They are characterised by proportions of isotopes of many elements that are very different from those in terrestrial materials. The anomalies could only have formed by decay of extremely short-lived isotopes that highly energetic cosmic rays produce in a manner analogous to neutron bombardment: they are products of nuclear transmutation. It is possible to estimate when the parent isotopes produced the anomalous ‘daughter’ products. One study found ages ranging from 4.6 to 7.5 Ga: up to three billion years before the Solar System began to form. It is likely that the grains are literally ‘star dust’ formed during supernovae in nearby parts of the Milky Way galaxy. Bennu samples contain six-times more presolar grains than any other chondritic meteorites. Nguyen et al. geochemically teased out grains with different nucleosynthetic origins. These ancient relics point to Bennu’s formation in a region of the presolar cloud that preceded the protoplanetary disk and was a mix of products from several stellar settings.

The results from asteroid Bennu support the key idea that that amino acid building blocks for all proteins and the nucleobases of the genetic code, together with other biologically vital compounds arose together in a primitive asteroid.  Its evolution provided the physical conditions, especially the trapping of water, for the interaction of simpler components manufactured in interstellar clouds. Such ‘fertile’ planetesimals and debris from them almost certainly accreted to form planets and endowed them with the potential for life. What astonishes me is that Bennu contains the five nucleobases used in terrestrial genetics and 70% of the amino acids from which all known proteins are assembled by terrestrial life. But, as I try to explain in my book Stepping Stones: The Making of Our Home World, life as we know it arose, survived and evolved through a hugely complex concatenation of physical and chemical events lasting more than 4.5 billion years. The major events and the sequences in which they manifested themselves may indeed have been unique. Earth is a product of luck and so are we!

See also: Tabor, A. et al. 2025. Sugars, ‘Gum,’ Stardust Found in NASA’s Asteroid Bennu Samples. NASA article 2 December 2025. Glavin, D.P. and 61 others 2025. Abundant ammonia and nitrogen-rich soluble organic matter in samples from asteroid (101955) Bennu. Nature Astronomy, v. 9, p. 199-210; DOI: 10.1038/s41550-024-02472-9

A cometary air-burst over South America 12 thousand years ago

Published on by zooks777Leave a comment

Earth-logs has previously covered quite a few hypotheses involving catastrophic astronomical events of the past, often returning to them as new data and ideas emerge. They range from giant impacts, exemplified in the mass extinction at the K-Pg boundary to smaller-scale events that may have coincided with important changes in climate, such as the sudden onset of the Younger Dryas, and a few that have been suggested as agencies affecting local human populations such as the demise of Sodom by a cosmogenic air-burst. Some of the papers that spurred the Earth-pages posts have been widely regarded in the geoscience community. Yet there have been others that many have doubted, and even condemned. For instance, data used by the consortium that suggested an extraterrestrial event triggered the frigid millennium of the Younger Dryas (YD) have been seriously and widely questioned. A sizeable number of the team that were under close scrutiny in 2008 joined others in 2019 to back the YD air-burst hypothesis again, using similarly ‘persuasive’ data from Chile. Members of the original consortium of academics also contributed to the widely disputed notion of a cosmic air-burst having destroyed a Bronze Age urban centre in Jordan that may, or may not, have been the site of the Biblical Sodom. Again, they cited almost the ‘full monty’ of data for high-energy astronomical events, but again no crater or substantial melt glass, apart from tiny spherules. Now another paper on much the same theme, but none of whose authors contributed to those based on possibly ‘dodgy’ data, has appeared in Geology (Schultz, P.H. et al. 2021. Widespread glasses generated by cometary fireballs during the Late Pleistocene in the Atacama Desert, Chile. Geology, published online November 2, 2021; doi: 10.1130/G49426.1).

Peter Schultz of Brown University, USA and colleagues from the US and Chile make no dramatic claims for death and destruction or climate destabilisation, and simply report a fascinating discovery. In 2012 one of the authors, Nicolas Blanco of the Universidad Santo Tomás in Santiago, Chile, found slabs made of glassy material up to half a metre across. They occurred in several 1 to 3 km2 patches over a wide area of the Atacama Desert. Resting on Pleistocene glacio-fluvial sediments, they had been exposed by wind erosion of active sand dunes. The glass is dark green to brown and had been folded while still molten. For the glass slabs to be volcanic bombs presupposes a nearby volcano, but although Chile does have volcanoes none of the active vents are close enough to have flung such large lumps of lava into the glass-strewn area. The glassy material also contains traces of vegetation, and varies a great deal in colour (brown to green). Its bulk chemical composition suggests melting of a wide variety of surface materials: quite unlike volcanic glasses.

Chilean glass occurrence: panorama of large glass fragments in the Atacama Desert; a specimen of the glass; thin section of glass showing bubbles and dusty particles (Credit: Schultz et al. 2021; Figs 1B, 2D and 2C)

Microscopic examination of thin sections of the glasses also reveals nothing resembling lava, except for gas bubbles. The slabs are full of exotic fragments, some of which closely resemble mineral assemblages found in meteorites, including nickel-rich sulfides embedded in ultramafic material. Others are calcium-, aluminium- and titanium-rich inclusions, such as corundum (Al2O3) and perovskite (CaTiO3), thought to have originated as very-high temperature condensates from the pre-solar nebula: like the celebrated ‘white inclusions’ in the Allende meteorite. Some minute grains resemble dust particles recovered by the NASA Stardust mission to Comet 81P/Wild-2 which returned samples to Earth in 2006. Zircon grains in the glasses, presumed to be locally derived, have been decomposed to zirconium oxide (baddeleyite), suggesting melting temperatures greater than 1670°C: far above the highest temperature found in lavas (~1200°C). Interestingly, the green-yellow silica glass strewn over the Sahara Desert around the southern Egypt-Libya border also contains baddeleyite and cometary dusts, together with anomalously high platinum-group elements and nanodiamonds that are not reported from the Chilean glass. Much prized by the elite of pharaonic Egypt and earlier makers of stone tools, the Saharan glass is ascribed to shock heating of the desert surface by a cometary nucleus that exploded over the Sahara. Unsurprisingly, Schultz et al. come to the same conclusion.

Any object entering the Earth’s atmosphere does so at speeds in excess of our planet’s escape velocity (11.2 km s-1). Not only does that result in heating by friction with the air, but much of the kinetic energy of hypersonic entry goes into compressing air through shock waves, especially with objects larger than a few tens of metres. Such adiabatic compression can produce temperatures >>10 thousand °C. Hence the ‘fireballs’ associated with large meteorites. With very large air-bursts the flash of radiant energy would be sufficient to completely melt surface materials in microseconds, though rugged topography could protect areas shadowed from the air-burst by mountains, perhaps explaining the patchy nature of the glass occurrences.  (Note: the aforementioned papers on the YD and Sodom ‘air-bursts’ do not mention large glass fragments, whereas some surface melting would be expected). Some of the Chilean glass contains carbonised remnants of vegetation. Radiocarbon dating of four samples show that the glass formed at some time between 16.3 to 12.1 ka. Yes, that does include the age of the start of the YD (12.9 ka) and human migrants had established themselves in northern Chile and coastal Peru after 14.2 ka. Yet the authors, perhaps wisely, do no more than mention the coincidence, as well as that with the disappearance of South American Pleistocene megafaunas – more severe than on any other continent. With a very distinctive product, probably spanning a far larger area of South America, and attractive to humans as an ornament or a resource for sharp tools, expect follow-up articles in the future.

See also: http://www.sci-news.com/space/atacama-desert-comet-10247.html, Science News, 8 November 2021; Vast patches of glassy rock in Chilean desert likely created by ancient exploding comet, Eureka Alert, 2 November 2021.