Elements Covers

CosmoELEMENTS

The Enduring Mystery of Australasian Tektites

Molten glass rained down from the sky over parts of Southeast Asia, Australia, Antarctica, and into the neighbouring ocean basins during the Pleistocene, about 790,000 years ago. These glass occurrences, long recognized to be remnants of melt formed during meteorite impact, are known as the Australasian tektites. Their distribution defines the largest of at least four known strewn fields across the globe, strewn fields being regions over which tektite glass are scattered from what are thought to be single-impact events. The three other big tektite strewn fields are associated with known source craters, including the Bosumtwi (1.07 Ma, Ghana), Ries (15 Ma, Germany), and Chesapeake Bay (35.5 Ma, USA) impact structures. At only 790,000 years old, the Australasian tektite strewn field is both the youngest and the largest known. Despite much effort, the source crater has yet to be discovered. The search to locate it represents something akin to a “holy grail” in impact cratering studies.

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Asteroid 16 Psyche: NASA’s 14th Discovery Mission

When our Solar System was just an infant, thousands of small early planets formed in just a few million years (Scherstén et al. 2006). Some grew to hundreds of kilometers in diameter as they swept up pebbles, dust, and gas within the swirling solar nebula. Heat from the decay of short-lived radioactive isotope 26Al was trapped and, in some cases, melted the planetesimal interiors. The molten interiors quickly differentiated: denser material settled to their centers, leaving lighter silicates to cool into thick mantles that surrounded metal cores (e.g. Weiss and Elkins-Tanton 2013).

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Search (and Discovery) of New Impact Craters on Earth

When looking at other terrestrial planetary bodies of the Solar System, such as our Moon, Mars, Mercury or the asteroids, it is obvious that impact craters are the dominant geological features to be seen on their surfaces. On Earth, however, impact craters are not so obvious and, in most cases, they are hard to spot. Our planet is geologically active. Its surface is constantly altered by plate tectonics and erosion and is largely covered by oceans and (densely) vegetated areas, making the identification of impact craters difficult. In addition, on Earth, an impact crater cannot be recognized, like on other planetary bodies, based only on its morphological characteristics because circular features can be formed by a variety of completely different geological processes (e.g. volcanism, salt diapirism, etc.)

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OSIRIS-REX: The Journey to Asteroid Bennu and Back

In May 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third of its New Frontiers program missions. The previous, yet ongoing, two New Frontiers missions are New Horizons—which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on 1 January 2019—and Juno—an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The OSIRIS-REx spacecraft departed for near-Earth asteroid (101955) Bennu aboard a United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. eastern daylight time (EDT) on 8 September 2016 for a seven-year journey to return samples from Bennu. Bennu is an Earth-crossing asteroid that has an orbital semi-major axis of 1.1264 AU, which is greater than that of the Earth, but a perihelion distance of 0.89689 AU, less than the Earth’s aphelion distance.

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Carbonaceous Chondrite Impact Melts

Collisions between planetary bodies (such as asteroids colliding with one another or with planets) have played a role in the geologic evolution of our Solar System since the formation of planetesimals, the earliest kilometer-scale bodies. Shock damage from collisional impacts leaves evidence on surviving planetary materials that range in scale from kilometer-sized craters to nanometer-sized mineral structural defects.

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Aqueous Alteration and Accretion of Chondrite Parent Bodies: When and Where

Events such as the Shoemaker−Levy 9 comet impact into Jupiter (July 1994) and the Chelyabinsk meteorite impact in Russia (February 2013) are reminders of the dynamic processes that were part of the formation of our Solar System from a protosolar molecular cloud of interstellar and circumstellar dust and gas. High-temperature (up to 2000 K) transient heating events (e.g. shock waves, current sheets, lightning, etc.) resulted in thermal processing (evaporation, condensation, and melting) of the primordial molecular cloud matter. In general, however, the ambient temperature of the disk decreased radially from the proto-Sun. When temperatures fell below 160 K, water vapor condensed directly into water ice, forming a front known as the “snow line”. The snow line likely did not reside at a single location in the disk, but rather migrated as the luminosity of the proto-Sun, mass accretion rate, and disk opacity all evolved with time. Some models suggest that the snow line could be located at about 5 astronomical units (1 AU = average distance between Earth and the Sun) early in disk evolution, which is not far from Jupiter’s current orbit, but is likely to have been present at 2−3 AU when the disk was just 2−4 My old (Ciesla and Cuzzi 2005).

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NASA’s Cosmic Dust Program: Collecting Dust Since 1981

Cosmic dust refers to particles that originate from the interplanetary medium and that have a diameter of ~100 µm or smaller (Brownlee 1985). This material, also known as interplanetary dust particles (IDPs), can be collected directly from Earth’s atmosphere. NASA has collected particles in the stratosphere for nearly three decades.

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Qarabawi’s Charm: Looking Beyond the Science

If you have been reading CosmoELEMENTS over the last few years, you will have realized that much of our understanding of Solar System history, including its earliest events, is derived from the study of meteorites. Scientifically, they are exceptionally valuable samples and, as such, it is sometimes hard to take a step back and realize that some meteorites have a cultural and historical value that is even greater. Back before the cosmic origins of meteorites were accepted, it is understandable that any material that fell from the heavens was the subject of some debate; historically, meteoritic material was often viewed as especially rare and valuable, as a gift from the Gods, or even cursed.

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