Detailed study shows the Novato meteorite had a battered past, preserving evidence of multiple impacts during its existence.
The skies over San Francisco suddenly lit up during mid-evening on October 17, 2012. The cause was a meteor roughly 35cm (1 foot) across entering the Earth’s atmosphere. As it fell, the bolide spectacularly disintegrated as it reached a brightness rivaling the full Moon. Moments after disappearing, a series of sonic booms were heard throughout the Bay Area. On the 19th, Novato resident Lisa Webber found the first fragment of the meteorite after remembering hearing a series of thuds on her garage the night of the event. Over the next month five more fragments were found by meteorite hunters. Although the Novato meteorite had a mass of approximately 80 kg (176 pounds) before it entered the atmosphere, only 363g (12 oz) of the meteorite was found.
The Novato fall was classified as an L6 chondrite. Chondrites contain glassy mineral beads called chondrules leftover from the very earliest days of the Solar System. The L denotes which family of chondrite the meteorite was related to. Most chondrites fall into one of three families, H (High-iron), L (Low-iron), and LL (Low-iron, Low-metal). The families are descended from separate main belt asteroids that experienced cataclysmic impacts early in their history. The number 6 denotes that the chondrules within this meteorite have been nearly destroyed by heating.
A glance at the meteorites found in the Novato fall hints at the rough history they have experienced. The meteorite was a breccia, or broken fragments of rock barely held together. The individual mineral grains of the meteorite are crushed and melted along fractures, both signs that the rock had been hammered by impacts But the full history of the meteorite is buried deeper still, at the elemental level. In a study appearing in this month’s issue of Meteoritics & Planetary Science, a group of researchers Qing-Zhu Yin (University of California – Davis) attempted to piece together some of this hidden history.
Chondrites contain other minerals in addition to chondrules. One of these is the phosphate mineral apatite, and the Novato meteorite is particularly rich in it. Apatite is a useful mineral for radioactive dating techniques because uranium tends to associate with phosphorus. Once incorporated into the mineral’s structure, uranium and its daughter products tend to stay put unless the rock is heated to ~800C. When scientists examined the the apatite grains they found that they clustered around two ages – 4.45 billion years ago and 470 million years ago. These ages probably represent partial resets of the radioactive clock in the intense heat of an impact.
Support for an impact at the 470 million year date can be found in a growing body of evidence. Almost all L chondrites that have been dated through radioactive dating give a cluster of ages around 470 million years ago. Although debris would have initially been confined to the asteroid belt, gravitational interactions with Jupiter would have quickly thrown the debris into an Earth-crossing orbit. Soon after the breakup of the L chondrite parent body the impact rate on Earth spiked to an estimated 40-100 times the normal rate. Eight impact craters dating to between 470-450 million years old have been found to date. Fossil L chondrite falls from the time period have also been found in Sweden and China, and L chondrite mineral grains can be found in sediments worldwide.
But the older event, 4.45 billion years ago, is more enigmatic. Yin and his team propose that this event is actually fossil evidence of the massive impact that formed the Moon. Earlier this year, a group lead by William Bottke (Southwest Research Institute) calculated that debris from the impact would have eventually spread into the asteroid belt, causing widespread damage. One problem: the Moon impact is generally thought to have occurred 4.36 billion years ago. But Yin’s group suggests that age could be wrong. Other meteorites, not just L chondrites, show a reset of their radioactive clocks between 4.53 and 4.45 billion years ago, while almost none show resets between 4.4 and 4.1 billion years ago. To Yin’s group, that’s a clue that the commonly accepted timing is wrong.
That suggestion is likely to be controversial and it will be years before the question is settled. Regardless, the Novato fall has proven to be an extremely interesting event, and has set the stage for further investigations into the deep history of the Solar System.
Featured image: The Novato fireball, captured in a series of images as it fragmented over the San Francisco area. Credt: Robert Moreno, Jr./Jenniskens et. al. 2014.