The young Earth may have experienced a period particularly favorable to the chemistry preceding life around 4.33 billion years ago.
At that time, our planet looked very different from today. It was still experiencing numerous asteroid and comet impacts. The most violent could heat the surface intensely, vaporize water, and destroy fragile molecules already present.
Artistic reconstruction of a very young Earth during the Hadean, shown with a surface still intensely heated.
Credit: Jeancarlosptino / Wikimedia Commons — CC BY-SA 4.0.
Researchers reconstructed this period using a three-dimensional computer model. Their simulation covers one billion years, between 4.5 and 3.5 billion years ago. It calculates, among other things, how impacts heated the Earth's crust and how long this heat persisted.
The team then compared these temperatures with those that several molecules associated with life can withstand. Among them is RNA, a molecule capable of carrying genetic information. One hypothesis proposes that before the first modern cells, RNA-based systems preceded DNA and today's proteins.
According to the model, large impacts regularly made vast regions uninhabitable until around 4.4 billion years ago. The situation then changed. Collisions became less destructive, and sufficiently cool areas could persist near the surface. The molecules needed for prebiotic chemistry could then remain intact for longer.
Artistic reconstruction of the early Earth subjected to impacts and intense volcanic activity.
Paradoxically, impacts could also create interesting environments for this chemistry. Water circulating through hot rocks produces hydrothermal systems. These bring together water, heat, and chemical energy. Around 4.33 billion years ago, such environments may have been numerous, at a time when the planet was no longer regularly sterilized by large collisions.
The researchers' results indicate that at that time, physical conditions may have allowed chemical reactions linked to the origins of life to persist over long periods.
This period predates estimates for the emergence of the common ancestor of today's forms of life by more than 100 million years. We therefore have a scenario that allows a significant window of favorable chemical conditions before life emerged.
The researchers now want to better connect these thermal models with the chemical reactions that may have occurred in ancient hydrothermal systems.