Three recent studies improve our understanding of environmental conditions on early Earth—important not just for reconstructing the history of our own planet, but for assessing the habitability of planetary bodies in general.
The first of these studies was led by John Tarduno from the University of Rochester and reported in Proceedings of the National Academy of Sciences. The authors present evidence of a strong magnetic field around Earth, from about 4.1 billion to 4 billion years ago. Their conclusion is based on magnetite inclusions in certain minerals (zircons), and thus appears to be very reliable. A strong magnetic field would have been critical for life to originate on Earth, because it would have protected the surface from the solar wind. Stars like our Sun are known to expel large amounts of harmful radiation when they are still young, and without a magnetic field it is doubtful life on Earth’s surface would have been able to survive the barrage.
What was Earth’s atmosphere like at that time? Based on modeling work reported by Owen Lehmer from the University of Washington and colleagues in Science Advances, it appears to have consisted of at least 70 percent carbon dioxide. We already knew from previous research that the early Earth atmosphere was very low in oxygen. Lehmer et al. claim that a carbon dioxide content of 70 percent or more could explain the observed oxidation of iron found within 2.7 billion-year-old micrometeorites. It’s likely that these high concentrations extended back to the beginning of the Archean time period about 4 billion years ago, meaning that life may have originated under this kind of atmosphere.
Another key environmental factor affecting early Earth was bombardment by meteorites—not just micrometeorites but also larger impacts, both of which were more common than today. Unfortunately we don’t have a complete record of those, because rocks older than about four billion years are very rare, having been erased by geological activity. We only can obtain estimates based on the cratering rate on the Moon—in places where we can still see ancient craters—and extrapolate that rate to Earth.
Despite these limitations, a team led by Timmons Erickson from the Astromaterials Research and Exploration Science Division at the NASA Johnson Space Center recently reported in Nature Communications the oldest meteorite crater yet found on Earth. It was identified from minerals that were altered and shocked during an impact in what is now western Australia about 2.2 billion years ago.
That impact may have had enormous consequences for Earth’s climate, because it happens to have occurred when Earth had just emerged from a period of glaciation. Just prior to that our planet was fully or almost fully covered by ice—a scenario usually referred to as Snowball Earth. The impact was so strong it would have turned ice directly to water vapor, a potent greenhouse gas, warming the planet enough to end the ice age. Earth’s biosphere, which was at that time only microbial, would have been able to multiply and diversify.
Whether the Australian meteorite was really the cause for the warming, or whether the timing of the strike was only coincidental, we don’t know. But the research shows us the power extraterrestrial events have to change our climate. And it gives us insight—along with the other recent studies—into what conditions on early Earth were really like.