Life on Earth Needed “Unmelted Asteroids”

The word “volatile” is commonly used in the space exploration community, but it has a different meaning than when used otherwise. In space exploration, volatiles are defined as the six most common elements in living organisms, plus water. Earth had enough volatiles for life to start here, but it might not have been that way. Researchers from the University of Cambridge and Imperial College London now think they have a reason why Earth received as many volatiles as it did – and thereby allowed it to develop life in the first place.

One characteristic of volatiles that makes them both difficult to deal with but easy to transport is that they vaporize at relatively low temperatures. Granted, a relatively low temperature could be 950°C for zinc, the volatile the researchers chose to look at. 

They chose zinc because it has a unique composition when captured in meteorites, allowing researchers to identify its source based on that composition. Previously, some of the same researchers had found that the zinc found on Earth had come from different parts of our solar system. About half had originated out past Jupiter, while half came from closer to home.

Most originating sources were objects called “planetesimals” – essentially proto-planets that had not yet had time to form. Planetesimals were common in the early solar system but became less so as they began to form into what we think of today as the major planets. However, many of the ones that existed early in the solar system were subjected to something that younger ones weren’t – harsh radiation.

Radiation was everywhere in the early solar system, and many planetesimals that formed during this period were subjected to it. Notably, the heat from these radiation sources caused the planetesimals’ volatiles to vaporize and be lost to space. So, the researchers at Cambridge and ICL thought they might be able to differentiate the age of the source of some of those volatiles – particularly zinc.

It turns out that they could. They measured the zinc concentration in many meteorites whose originating planetesimal was known. They then modeled where the Earth received its zinc from. Since zinc is one of the vital volatiles thought to be essential to the development of life, this model could help understand how life might (or might not) develop on other worlds.

They found that the vast majority (about 90%) of the Earth’s zinc was contributed by planetesimals that weren’t subjected to the high radiation levels of the early solar system. In essence, they were the ones whose volatiles weren’t vaporized, allowing them to contribute more of these valuable, life-giving materials despite only contributing 30% of the Earth’s overall mass.

Additional work is needed to study whether similar heating effects affected the amount of other volatiles delivered to the early Earth. And even more work is required to model how that volatile delivery model might work for other planets, such as Mars, or even exoplanets further afield.

But for now, this is another piece of the puzzle that answers an important question about the early solar system. And, maybe more importantly, it shows how many things have to go right for life to develop in the first place.

Learn More:
University of Cambridge – How did the building blocks of life arrive on Earth?
Martins et al. – Primitive asteroids as a major source of terrestrial volatiles
UT – The Building Blocks of Earth Could Have Come From Farther out in the Solar System
UT – Citizen Scientists Find Fifteen “Active Asteroids”

Lead Image:
An iron meteorite from the core of a melted planetesimal (left) and a chondrite meteorite, derived from a ‘primitive’, unmelted planetesimal (right).
Credit: Rayssa Martins/Ross Findlay