When the Chelyabinsk meteor exploded in the morning skies of the southern Urals in February 2013 it prompted a flurry of interest in the vast population of small bodies that share our orbital turf around the Sun.
Now a study by a large group of scientists, published in the journal of Meteoritics & Planetary Science by Righter et al., suggests that the original Chelyabinsk object was already no stranger to cosmic collisions.
The researchers have analyzed and mapped the detailed properties of 3 pieces of the meteor that were recovered with minimal terrestrial contamination. That's a somewhat forgiving criterion: Two of these bits came from a sharp-eyed forester spotting holes in the snow and plucking out what are charmingly termed 'ice carrots' - the refrozen spikes after the meteor fragments ploughed into the surface. Nonetheless, combined with the analyses of other fragments, a dataset has been generated that offers a unique set of insights to the meteor's earlier history.
The new work is a tour de force of highly technical methods, deploying optical microscopy, electron microscopy, Raman spectroscopy, and the analysis of isotopic compositions with a range of sophisticated tools. Critically, the measurements reveal an extremely complex history for the Chelyabinsk material - recorded in isotopic ratios and mineralogical alterations.
In fact, some of the isotopic 'clocks' typically used to date rocky solar system material (such as Rubidium and Strontium ratios, and Argon isotope ratios) appear to have been partially or totally 'reset' at various points in the past. These resets can be caused by strong thermal changes altering isotopic ratios, and variations in exposure to cosmic radiation.
The conclusion is that before the Chelyabinsk body fell on the Earth it experienced at least 8 significant impacts, starting around 4.53 billion years ago, and again at 4.45 billion years, 3.73, 2.81, and 1.46 billion years ago, followed by 852 million years ago, 312, and most recently 27 million years ago.
Those impacts happened out in the wilds of interplanetary space, as asteroidal material collided with other asteroidal material in a great game of cosmic pinball. We can still see this kind of jostling taking place today. In 2010 the Hubble Space Telescope caught the aftermath of just such an impact, some 90 million miles from the Earth, and visible in this image:
Indeed, one other tantalizing result from the new study suggests that there could have been a collision event within the past million years or so (based on the rock's cosmic ray exposure). This indicates that what hit the Russian skies may have come from the quite recent breakup of a Near Earth Object.
The Chelyabinsk meteor carried all of this complicated and messy history with it. Those earlier impacts led to thermal changes (metamorphism) and fragmentation. And that checquered past is even consistent with the trajectory of the meteor's fireball and the way in which it broke up in Earth's atmosphere - a last gasp before leaving its ancient secrets for humans to decipher.