First published in issue 2 of SOLVE magazine, 2020
Here on Earth, strung along the Orion Arm of the Milky Way galaxy, we are bit by bit coming to know the cosmic order that underlies planetary life.
Our solar system is thought to have formed from gravity-driven contraction of interstellar gas and dust into the Sun and planets.
There are, however, periods in this chronology that are murkier than others. Particularly vexing for planetary scientists is a gap covering the first billion years of Earth’s history (3.5 to 4.5 billion years ago), when it was evolving the ability to sustain life.
Dr James Darling, who is a Reader in Earth and Planetary Materials at the University of Portsmouth, studies this time period. He says there are answers to be found, but they are hidden within the microscopic features of ancient – and therefore exceptionally rare – rock.
These features can be ‘read’ using advanced analytical methods to reveal the forces at play during the rock’s formation, including the occurrence of massive impact events, the eruption of volcanoes or the movement of tectonic plates.
These time-machine-like glimpses into a different era are lacking on Earth, unfortunately, when it comes to the period when life first formed. Dr Darling explains why:
“The Earth has been resurfaced and reworked by plate tectonics and erosion throughout its history, so the surface we have today is relatively young,” Dr Darling says. “In fact, there are very few places on Earth where it’s possible to find rocks that are billions of years old.”
Fortunately, this scarcity does not hold true beyond our planet, as researchers learned following the birth of the space age.
“When the Apollo missions to the Moon brought back rock samples, we started to realise those lunar craters and surfaces were very ancient – billions of years old,” Dr Darling says. “So, we can look to other planets – and to planetary fragments in the form of asteroids – to understand the forces that created them and learn a lot about the early Earth.”
That is precisely Dr Darling’s speciality: he works up laboratory methods to analyse billion-year-old fragments of rocks sourced from the Moon, asteroids and Mars. These methods typically involve using high-energy light, electron and ion beams to probe mineral structures and chemistry down to the atomic scale. He then combines the results with observational data about the geological features of these extraterrestrial bodies.
Given the relevance of this work to issues around the origins of life, Dr Darling also looks for signatures that can weigh in on another vexing question: whether Earth was alone in being able to evolve a biosphere and living organisms.
He undertakes this work in collaboration with space exploration missions or the museums and agencies that curate extraterrestrial samples, including the Apollo materials (whose analysis is overseen by NASA) and meteorites from the Royal Ontario Museum.
The lunar samples continue to reveal the relevance of space exploration to understanding Earth’s history, with Dr Darling’s group recently discovering that massive impact melt sheets were responsible for forming large portions of the Moon’s crust. His team is revving up its efforts and collaborations, with much of the focus shifting to Mars. This is due to the number of missions underway that are exploiting Mars’ current orbital proximity to Earth.
At its closest, Mars is about 57.6 million kilometres away, which most recently occurred on 6 October 2020. That proximity was exploited by three separate robotic missions by NASA, the United Arab Emirates and China.As well as leading ongoing projects studying Martian meteorites, Dr Darling is involved with the ExoMars mission, which is due to be launched in 2022 during Mars’ next approach by the European Space Agency (ESA) in partnership with Russia’s Roscosmos. The ExoMars payload is expected to include a drill designed to collect soil samples to a depth of two metres below the surface to search for signs of life – past or present – with Dr Darling part of the geology team guiding targeting and priorities.
While all these missions will bring about many discoveries – and awe viewers back on Earth – Dr Darling will be standing by, looking to mine the data for clues about Mars’ deep past.
The questions that intrigue him most deal with Martian geology that speaks of a past topography that closely resembled Earth, including the presence of an atmosphere, oceans, rivers and volcanoes.
One key question is why Mars’s evolution diverged so drastically from Earth’s and what drove that divergence. Another is whether past similarities were sufficient for Mars to have started down the pathway of birthing and sustaining life.
“Looking to the immediate future, I think we can better constrain the geological timeline for Mars and put some real radiometric age constraints on when things happened on Mars – when volcanism and impacts happened, when surface water disappeared and intertwining that with the search for life,” Dr Darling says.
“This is my main Martian project at the moment – a £350k STFC [Science and Technology Facilities Council] award to radiometrically date Martian magmatism.
“Because Earth and Mars may have started out similarly, but went down very different paths, this analysis stands to bring about a much better understanding of our own planet’s early history.”