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Cosmic-ray-produced-nuclides in meteorite Dishchii'bikoh

April,  2020

Cosmic-ray-produced-nuclides in meteorite Dishchii'bikoh

Dust cloud of the Dishchii'bikoh meteorite fall photographed from Phoenix, Arizona, at 10:58 UTC, June 2, 2016. Photo: Mike Lerch, Phoenix.

On June 2, 2016 a large fireball was observed throughout the southwestern US. This fireball was produced by a meteor entering Earth’s atmosphere. Frictional heating between the meteor and the compressed atmosphere in front of the meteor heat the meteor, causing it to glow. Many meteors collide with Earth’s atmosphere and produce fireballs. Most are destroyed in the atmosphere, but a few survive and land on Earth’s surface. As these meteors traverse the atmosphere the outer layer of the meteor is ablated. The picture shows the dust trail resulting from this ablation in the early morning sun. Most meteors traversing Earth’s atmosphere completely burn up, leaving no traces of the material that was on a collision course with Earth. Occasionally, fragments of the meteor survive the atmospheric heating and land on terra firma, and are then properly called meteorites. Doppler radar data indicated the possibility of surviving fragments and their possible location: the White Mountain Apache tribal lands. A team of researchers was given permission to hunt for meteorites. Fifteen small meteorites were found, the aggregate mass was about 80 g. A full description of this find is given on it's Meteoritical Bulletin entry webpage and more information, including a few pictures on Arizona State University's Dishchii'bikoh webpage.

Some of these samples were subsequently allocated to an international team for scientific study. The PRIME Lab at Purdue University participated in this consortium. The results of this study were published in the April 14 issue of Meteoritics and Planetary Science.

PRIME Lab measured the rare radionuclides 10Be (t1/2 = 1.36 Myr; 1 Myr = 106 yr), 26Al (t1/2 = .705 Myr), and 36Cl (t1/2 = .301 Myr) in Dishkii'bikoh using a technique known as Accelerator Mass Spectrometry (AMS). These radionuclides are produced by cosmic ray bombardment of the meteoroid while it is in space, before its collision with Earth. Cosmic rays, primarily protons produced in high energy events in our galaxy - super nova explosions - collide with the atomic matter (oxygen, silicon, magnesium, iron, and others) within the minerals present in the meteor. These collisions, called spallation reactions, break apart the nucleus of the atoms, leaving lighter fragments: nuclei, protons, and neutrons. Among the lighter nuclides are 10Be, 26Al, and 36Cl. We know the production rates of these radionuclides from studies of lunar surface cores and materials irradiated with proton and neutron beams produced in accelerators. Measurements of these radionuclides can shed light on the exposure conditions of the meteor (PDF).

Our measurements of 10Be in Dishkii’bikoh indicate that it was exposed to cosmic rays for at least 7 Myr. We know this because the 10Be activity is saturated.  Frames A and B of the figure show some possibilities for the pre-atmospheric size of Dishii’kiboh. The y-axis shows the measured activity in decays/min-kg. The x-axis shows the depth within the meteor in cm. The different curves represent radii. The grey area is the activity of 10Be and 26Al we measured in Dishii’kiboh. We can estimate both the radius of the meteor and the location of our specific sample within that body by simultaneously satisfying the 10Be and 26Al measured activities. For the meteor from which Dishkii’bikoh is derived, the radius is somewhere between 105 and 175 g/cm3 and that the specific location of Dishkii’bikoh is about 50 g/cm2 from the surface.  These may seem odd units for a diameter, and they are, but the radionuclide measurements only indicate how much material the cosmic rays penetrated, not its density. Given the data in hand, we can’t rigorously determine the density, but even with these uncertainties we know that this object was approximately 80 cm. in diameter when it collided with Earth. Using this information allowed other researchers to determine, using cosmic-ray-produced 21Ne, that the total cosmic ray exposure age was ~ 11 Myr.

What do we know about Dishkii’bikoh? From chemical analyses we know it is similar to many other meteorites that have been found on Earth; specifically it is an LL7, a type of ordinary chondrite. We also know that Dishchii’kiboh is ancient; U-Pb dating places the age of Dishchii’bikoh at 4471 ± 6 Myr. For most of the history of the solar system, the parent body of Dishkii’bikoh remained intact, until ~ 11 Myr ago, when a collision in the asteroid belt – perhaps the inner asteroid belt - between objects liberated many fragments from an LL7 parent body. These fragments drifted across the asteroid belt and ultimately were scattered into an Earth-crossing orbit via gravitational interactions with Jupiter. Once the object was in an Earth-crossing orbit it was just a matter of time. The press release provides additional details.