The greatest challenge facing astrobiologists is that there is only one planet known to us that has life. Of all the bodies of the Solar System, only Earth has a dense atmosphere, liquid water on its surface, and the organic chemistry that supports life. However, these conditions did not exist billions of years ago when Earth was still young. While the nebula from which the planets formed was rich in volatile elements, the high temperatures in the inner Solar System largely prevented them from condensing, leaving them mostly in a gaseous state.
As a result, these elements were not incorporated into the solid rocky materials from which the inner planets formed. Only celestial bodies that formed farther from the Sun retained the substances essential to life, which raises questions about how and when they were introduced to Earth. In a new study, researchers from the University of Bern showed for the first time how the chemical composition of primordial Earth was complete three million years after it formed (ca. 4.5 billion years ago). Their results imply that the ingredients for life (water, carbon compounds, sulfur, etc.) were introduced later, likely by an impact.
The study was conducted by Pascal Maurice Kruttasch and Klaus Mesger, a postdoctoral researcher and a Professor Emeritus of Geochemistry (respectively) with the Institute of Geological Sciences (GEO) at the University of Bern. Mesger is also a member of the scientific committee that oversees Bern’s Center for Space and Habitability (CSH). Their study, which was part of Kruttasch’s dissertation at GEO, was published on August 1st in Science Advances.
The team focused on two isotopes, Manganese 53 (53Mn) and Chromium 53 (-53Cr), in meteorites and terrestrial rock samples. They then used model calculations to set limits on how long it would take for Earth’s chemical composition to develop. This allowed them to determine the ages of these elements and deduce the chemical signature (the unique pattern of chemicals that make it up) of primordial Earth. Their results showed that Earth’s composition was complete less than three million years after the formation of the Solar System, providing the first empirical data on the original composition of primordial Earth. As Kruttasch explained in a University of Bern release:
A high-precision time measurement system based on the radioactive decay of manganese-53 was used to determine the precise age. This isotope was present in the early Solar System and decayed to chromium-53 with a half-life of around 3.8 million years. Our Solar System formed around 4,568 million years ago. Considering that it only took up to 3 million years to determine the chemical properties of the Earth, this is surprisingly fast.
These results support the Giant Impact Hypothesis, which states that the Earth-Moon system formed due to a massive impact ca. 4.5 billion years ago between primordial Earth and a Mars-sized object (Theia). It is further theorized that Theia formed farther out in the Solar System, and its composition would include more volatile elements, including water. In effect, the team’s analysis indicates that primordial Earth was a dry, rocky planet, and its collision with Theia introduced all of the elements that made life possible here.
Their findings also contribute significantly to our understanding of the processes at work in the early Solar System and provide clues about how and when life emerged. They could also be significant in the search for life beyond Earth (astrobiology) and determining whether rocky planets orbiting closer to their Suns could possess the ingredients necessary for life. The next step, says Kruttasch, is to investigate the collision event in more detail, which will likely involve computer modelling and simulations:
The Earth does not owe its current life-friendliness to a continuous development, but probably to a chance event – the late impact of a foreign, water-rich body. This makes it clear that life-friendliness in the universe is anything but a matter of course. So far, this collision event is insufficiently understood. Models are needed that can fully explain not only the physical properties of the Earth and Moon, but also their chemical composition and isotope signatures.
Further Reading: University of Bern, Science Advances