Martian and lunar meteorites rewrite the conditions for a habitable planet

A study of Mars’s zinc reveals that primitive asteroids were the key source of volatile elements essential for habitability, reframing what we know about how planets acquire life’s components.

Led by the Department of Earth Science and Engineering (ESE) at Imperial College London, the research analysed Martian meteorites (the parts that break away from asteroids or planets) to determine the Red Planet acquired a key volatile element, zinc, almost exclusively from primitive, ancient asteroids that formed nearby.

This is in contrast to Earth, and a challenge to the notion that trawling the far reaches of the Solar System is a necessary step for a volatile-rich planet – and hence potential habitability. Instead, it’s the type of material accreted by a planet, not the origin, that determines whether it will have enough volatiles to be habitable.

“We know that Earth collected material from both the inner and outer Solar System to build its inventory of volatile elements, but it raises the question of whether the delivery of more exotic outer Solar System material was an essential contributor,” said Dr Rayssa Martins, now at the University of Cambridge, who led the work as part of her PhD at ESE.

“Our research shows that the crucial factors to a planet’s habitability are that primitive materials are preserved and incorporated by terrestrial planets – a finding that could help guide the search for habitable worlds beyond our Solar System.”

Our research shows that the crucial factors to a planet’s habitability are that primitive materials are preserved and incorporated by terrestrial planets – a finding that could help guide the search for habitable worlds beyond our Solar System. Dr Rayssa Martins Postdoctoral Research Associate, University of Cambridge; formerly at the Department of Earth Science and Engineering, Imperial

Early conditions for life

Most of the chemical elements that are essential for life as we know it are also volatile – that is, they easily vaporise and, with examples including carbon, oxygen and zinc.

Tracing the origin and abundance of volatiles essentially takes an inventory of the raw materials that were available early on in a planet’s history to define whether it had the necessary conditions to potentially seed life. Through previous studies that took direct measurements from rovers and spacecraft, and analysed Martian meteorites that have hit Earth, we already know that Mars has had a rich supply of volatile elements from the very beginning.

For decades, scientists have debated where Earth and its neighbouring planets acquired these elements, which could shed new light on the formation of planets and their potential habitability.

Tracing origins

To understand where these volatiles may have originated, the study, published in Scientific Reports, employed a powerful forensic tool in the form of tiny signatures called nucleosynthetic isotope anomalies, found in meteorites and other planetary bodies. These signatures are a means to match planetary building blocks back to the regions in which they formed in the early Solar System, since different meteorite groups from either inner or outer space have different signatures.

Zinc was used for this purpose because, as a moderately volatile element, it serves as a tracer for the origin of other similarly volatile elements essential for habitability. By analysing the zinc composition of six Martian meteorites, the team determined that Mars’s zinc (and by extension, other volatiles) came overwhelmingly from so-called non-carbonaceous meteorites originating from the inner Solar System, which also confirmed findings from earlier studies.

Whereas for Earth, the team’s previous work had showed that about half its zinc was supplied by materials that formed nearby, while the other half was delivered by materials formed much further out, beyond the orbit of Jupiter.

“Mars is proof that a planet doesn’t need to go far to build its zinc inventory and can be volatile-rich without input from the outer Solar System,” said Dr Martins.

Primitive asteroids: the key to habitability

Instead, the critical factor for acquiring volatile elements appears to be the nature of the accreting asteroids. Taking their work a step further, the researchers incorporated their zinc data into a comprehensive model that distinguished between primitive, unmelted asteroids and asteroids that had experienced early melting and loss of volatiles.

The results revealed that while primitive material constituted only about 30% of Earth’s mass, it delivered a significant 90% of its zinc. A similar case was found for Mars, although the pattern was even more pronounced, with primitive asteroids contributing about half of the planet’s mass and, similarly, supplying 90% of its zinc.

Evidently, the primitive asteroids are the real treasure trove for volatile elements and are the single most important factor that determines whether a rocky planet becomes volatile-rich and potentially habitable. Professor Mark Rehkämper Professor of Isotope Geochemistry, Department of Earth Science and Engineering, Imperial

“Evidently, the primitive asteroids are the real treasure trove for volatile elements and are the single most important factor that determines whether a rocky planet becomes volatile-rich and potentially habitable,” said senior author Professor Mark Rehkämper, Professor of Isotope Geochemistry in ESE.

“Previously, the focus had been on a planet’s location and whether it could capture volatiles from the outer Solar System via comets or carbonaceous asteroids, but we’ve proved that while that can happen (as with Earth), it isn’t necessary.”

Meanwhile, recent geological findings from NASA's Perseverance rover, in work also involving ESE, similarly prompted a rethink of what we know about Mars, by building an unexpected picture of a once-watery and potentially habitable environment in its Jezero Crater.

Lunar insights

The study also provided new insights into the Moon’s formation. Analysing five Lunar samples, the team attempted to see if the Moon’s zinc signature differed from Earth’s to test the prevailing giant impact hypothesis: that the Moon formed from debris after a Mars-sized planet, Theia, struck the early Earth.

Surprisingly, for every element tested, it has been well established that the Moon and Earth have identical isotopic compositions, and now, the team’s results suggest this is also the case for zinc. This raises a fundamental question about the standard hypothesis, which predicts that the Moon should be composed primarily of material from Theia and have a distinct isotopic composition different from Earth’s.

“The fact that their compositions are identical for so many elements (and now likely for zinc too) means we might need to go back to the drawing board to see whether there are other mechanisms that could have been invoked to explain the giant impact hypothesis,” said Dr Martins.

“While we can’t yet confirm nor rule out that Earth and the Moon have different zinc with confidence, our work adds weight to the idea of extensive mixing after the giant impact.”

Searching for candidate worlds

Ultimately, the study has provided a new paradigm for what makes a planet volatile-rich, highlighting that a planet can be a candidate for life simply by accumulating a portion of primitive asteroids during its construction.

“Having enough primitive material in the mix may be a key requirement for building volatile-rich planets – but we still don’t know how common that is in other planetary systems. This remains an intriguing question for future research,” said Dr Martins.