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Cosmic Hotspots for Life

asteroid

Crater cooling over at least half a million years could have given primitive bacteria enough time to evolve.

By Martin Schmieder & Fred Jourdan

New evidence reveals that large meteorite impacts took long enough to cool for microbial life to emerge and thrive in the wet and warm conditions of the impact crater.

Similar to our pockmarked Moon, Earth was once covered with numerous meteorite impact scars. In contrast to our living planet, the Moon has been stone-dead for a long time. Even today, giant impact basins several hundred kilometres across, preserved over about 4 billion years, are exposed on the lunar surface. These large craters are, in turn, superimposed by thousands of smaller and younger impact craters struck since that early time of intense cosmic bombardment. Many of the impact basins are filled with dark volcanic basalts visible as the black fillings of the lunar maria that together form the “Man in the Moon”.

But we don’t have to go that far; 188 “fossil” meteorite impact craters are currently known on Earth – 29 in Australia, 22 in Asia, 48 in Europe, 19 in Africa, 60 in North America and 10 in South America. Earth is a geologically active planet on which Mother Nature knows a multitude of ways to recycle rocks and reshape the outer layer of crust on which we live.

Various processes associated with plate tectonics can erase impact craters from the Earth’s surface over time. Erosion is a seemingly never-ending process that, slowly but surely, grinds down every impact crater. Some deeply eroded craters are barely visible as such, and can only be recognised by the study of rocks that were crushed, shocked and even melted as indirect geologic evidence for impact.

Many subsurface impact craters are only known because of exploration-motivated oil and ore drilling. It has become clear that they can contain loads of oil, gas and rich mineral deposits that were generated and trapped when the initially hot, fresh craters cooled down from temperatures that reached several thousand degrees. Such hydrothermal deposits develop during the circulation of hot fluids inside a cooling impact crater. We’ll come back to this aspect later.

The Lappajärvi Impact

One very interesting impact crater lies in Finland, almost opposite Australia on the globe. The Lappajärvi crater, about 23 km in diameter, has long been considered a terrestrial analogue for impact craters on the Moon. Because of its eroded state it is usually referred to as an “impact structure” rather than an impact crater.

At this size, impact structures tend to retain a complex morphology with an uplifted central peak. Such a peak forms much like water rebounding when you drop a stone into a pond; the major difference is that this impact-rebound occurs in solid rock!

The Lappajärvi impact structure is today largely covered with the waters of the lake it was named after; only parts of the original crater rocks are surficially exposed and accessible in the field. Black impact melt rocks – rocks almost completely melted and recrystallised upon impact – occur on Kärnä Island inside Lake Lappajärvi, where they are part of a 150-metre-thick solidified impact melt sheet that fills the central parts of the crater.

The Lappajärvi impact caused extreme shock pressures and temperatures high enough to reset the natural radioactive clock of impact-molten rocks and minerals. This enables us to date the impact event using the argon–argon (40Ar/39Ar) dating technique, which is not only sensitive enough to determine the exact timing of the meteorite impact, but also to measure the duration of impact crater cooling to more tolerable temperatures in the aftermath of the catastrophe.

Left: Lappajärvi impact crater in Finland, and argon-argon dating materials. Clockwise from top left: False-colour satellite image of Lappajärvi with an outline of the crater’s diameter (dashed line); black impact melt rock from Kärnä Island; impact-melted crater basement granite with yellow-white foamy K-feldspars; high-resolution scanning electron image of K-feldspar crystallised in a crater-hosted hydrothermal system. Sample courtesy: P. Bockstaller

We investigated two different rock and mineral samples from Lappajärvi crater. Under the microscope we meticulously handpicked particles of black impact melt rock from Kärnä Island and yellow-white potassium feldspar minerals (K-feldspar) separated from an impact-melted crater basement granite. The pale K-feldspar particles exhibit a foamy-vesicular appearance and, in places, odd-shaped crystal aggregates – eye-catching evidence for melting and degassing at high temperatures.

Argon dating of the melt rock and granite K-feldspars revealed that the cosmic Lappajärvi strike occurred 76.2 million ± 290,000 years ago in the Cretaceous, when the dinosaurs still lived. While the melt rock yielded perfectly reproducible ages in all six individual analyses, the dating results for the feldspars produced a series of cooling ages ranging from 75–76 million years ago.

The time gap between the crystallisation of the impact melt and the K-feldspar in the granite is the measurable expression of hydrothermal fluid circulation that precipitated the feldspar minerals deep inside the crater.

In a very simplified way, the cosmic impact scars on the Earth’s crust can be compared to the scars on our human body: while the uppermost skin usually clots and cools within a few minutes, the deeper parts of the wound stay inflamed for longer and often take days to heal. At Lappajärvi crater, this “healing” lasted at least 600,000 years – and possibly longer than a million years.

Enough Time for Life

Why are these results from a far-away Finnish impact crater so important? The direct dating of hydrothermal circulation at Lappajärvi reveals, for the very first time in absolute numbers, how long the cooling of a medium-sized impact crater on Earth really takes. Only theoretical calculations and rough estimates had been available previously for such craters, and suggested cooling times lasting between thousands and tens of thousands of years.

Our new results for Lappajärvi, which have been published in Geochimica et Cosmochimica Acta, demonstrate that crater cooling lasted an order of magnitude longer. This suggests that even comparatively small impact craters like Lappajärvi can drive long-lived hot fluid systems that can sustain temperatures supporting microbial life for an extended period of time.

Our findings are crucial as the hypothesis that early life forms evolved in impact craters ran into problems because the time necessary for life to develop and evolve in a sufficiently warm host crater was generally considered too short. Crater cooling over at least half a million years, as our measurements suggest, offers a much more convincing scenario in which primitive bacteria have enough time to evolve.

Supportive evidence for this hypothesis was recovered back in the late 1970s through the exploration of the mid-oceanic ridges that rise from the sea floor, where microbial life thrives in submarine volcanic chimneys that emit hot hydrothermal fluids.

The upper temperature limit for life on Earth lies around 120°C for extremophile bacteria, while thermophiles seem to flourish between 50°C and 80°C. A long cooling duration from a few hundred degrees down to ambient temperature inside an impact crater means that several parts of the crater sit at these preferable temperatures at any given time.

The temperature conditions that reign within an impact crater are therefore ideal for simple organisms like bacteria.

Life Brought by Comets and Asteroids

Life-essential elements such as carbon are commonplace in the Earth’s crust, but they also occur in carbonaceous chondrites, one of the oldest known types of meteorites delivered from space. In fact, these meteorites contain various organic molecules and amino acids, which are the precursors to protein molecules. Some researchers even proposed that asteroid and comet impacts could have delivered these “bricks of life” to Earth.

Phosphorous is abundant in the Earth’s crust, but it occurs in an oxidised form that cannot be incorporated into molecules such as RNA and DNA. Only the reduced (non-oxidised) form of phosphorus can be incorporated into these molecules.

Pressure and temperature conditions during large meteorite impacts are extreme enough for the complete reduction of a series of elements, including phosphorus. In other words, impacts provide the right kind of phosphorus to be part of RNA and DNA.

Once combined, organic molecules (e.g. sugars), amino acids, reduced phosphorus, water and residual heat from the impact together represent a primordial cocktail under ideal conditions for simple microbial life forms to bloom.

Lappajärvi-sized impacts occur about every 500,000 years on Earth. Compared with impact basins more than 200 km across (such as the Earth’s oldest and largest impact structure, Vredefort in South Africa, and the Sudbury Basin in Canada) that are formed every 150 million years or so, medium-sized impact craters are about 300 times more common. Their sheer frequency makes this population of impact craters efficient natural laboratories and an important habitat for the evolution of microbial life on early Earth.

One can even speculate that these conditions might have been just right not only for bacteria to thrive but also for life itself to emerge. Arguably, this is one of the oldest and most fundamental questions in science. Being realistic regarding the amount of knowledge we currently possess about the early Earth, and acknowledging that other places such as volcanoes or hydrothermal vents can also provide life-friendly conditions, the search for the origin of life continues.

Past Life on Mars?

These findings have major implications for life – not only on early Earth, but also on other planetary bodies such as Mars. The suspicion of ancient life on early Mars is such a hot topic now that NASA has sent a series of multi-billion dollar probes to the surface of the red planet. The plan is to unravel whether Mars could have harboured bacterial life forms in its distant past. If the impact–life connection is valid we may expect to find fossils of bacteria or other evidence for primitive life forms in impact crater rocks preserved at the Martian surface.

The last Mars robotic mission – Curiosity – was especially designed to explore Gale Crater, a 155 km-diameter crater thought to be 3.6 billion years old. Satellite images tell us that its central peak, Mount Sharp, consists of two geologic parts: a central domain of older, crushed rebound rocks and an overlying succession of younger sediment layers that, like an open book, contain invaluable information about the Martian climate history. At the moment, NASA scientists are particularly interested in this pile of sediments that could reveal climatic variations and the presence of significant amounts of water, which would in turn signify a decent chance to detect signs of early life on Mars. According to our study of Lappajärvi crater, it would be worth having a closer look at the central peak itself, where evidence of fossil bacteria might lie in cracks and voids of impact-shocked rocks or right at the interface between the central peak and the oldest sediment layers.

The Curiosity rover is still on its way to Mount Sharp and should reach this destination soon. This is very exciting in the context of finding clues for past life on another planet!

Impact Craters as Biodiversity Hotspots

Meteorite impacts and life is always a story about the good and the bad. The large Chicxulub impact in Mexico’s Yucatán Peninsula caused a magnitude 12 earthquake and mega-tsunami, ejecta fall-out worldwide, and left behind a 190 km-diameter impact crater. The majority of researchers believe that this impact significantly contributed to the mass extinction that killed the dinosaurs at the Cretaceous/Paleogene boundary 66 million years ago. Eventually, this very asteroid impact probably paved the way for the rise of the mammals and mankind. Much earlier, Australia’s giant Acraman impact seems to coincide with a sudden diversification of microbes in the Ediacaran time period some 580 million years ago.

From what we know now from Lappajärvi, even relatively small impact craters may have acted as oases for early organisms. All this argues for a tight connection between impacts and life, although not as bad as people used to think. We are currently planning future studies to investigate additional impact craters in Australia and worldwide to investigate their role as biodiversity hotspots involved in the origin and evolution of life.

The simplest forms of microbial life on Earth emerged more than 3.5 billion years ago. We still don’t conclusively know where the first terrestrial life forms ultimately originated from. Maybe every one of us had our ultimate common ancestral microbe fostered in a warm, cosy impact crater. Think about it the next time you visit one of the magnificent Australian impact sites on your outback trip.

Martin Schmieder is a Research Associate at The University of Western Australia’s School of Earth and Environment, and a Curtin University Associate. Fred Jourdan is a Senior Research Fellow at Curtin University’s Department of Applied Geology.