Around 775, Earth’s atmosphere experienced a jolt in carbon levels. Scientists proposed that the extra carbon-14, which occurs naturally in trace amounts in the atmosphere, could have come from an outburst of energetic particles from the sun or other stars.
A team of scientists suggested January 16 in Scientific Reports that the increased carbon could have come from a comet impact. But new calculations of the size and mass of such a comet show that the space rock would have been 100 kilometers across and 100 billion to 1,000 billion tons.
An impact of a rock of that size would have been disastrous for the planet and would have left more evidence in geological and written records, argue the authors of a paper posted January 23 on arXiv.org.