A habitable planet

Life has existed on Earth for billions of years. Stabilising mechanisms have helped our planet remain habitable to this day.
The solar system emerged some 4.5 billion years ago. Earth and the other rocky planets formed around the nascent Sun. (Photograph: Keystone / Detlev van Ravenswaay)

It all began 4.5 billion years ago, when the debris from previous generations of stars formed a large molecular cloud. A part of this collapsed under its own gravity to create a new star – our Sun. The remaining gas and dust flattened into a spinning disc around this newly formed star. Over time, small particles of dust clumped together into kilometre-­sized bodies called planetesimals, which eventually accreted into Earth and the other rocky planets. A final catastrophic collision between our newly formed planet and another large object ejected debris into Earth’s orbit, which gradually coalesced to form the Moon.

Yet this early inner solar system still lacked key elements required for the formation of life. "The Sun was extremely hot in those early stages," says Maria Schönbächler, a cosmochemist and ETH professor at the Institute of Geochemistry and Pet­rology. As a result, the original building blocks in Earth’s orbit were almost completely devoid of vola­tile elements such as hydrogen, carbon, oxygen and nitrogen that would subsequently play an essential role in the emergence of life. "But our research shows that, as the proto-Earth grew, it was also receiving material from more distant parts of the solar system, where temperatures were cooler and volatile substances were able to condense into solid bodies," she says.

Rocks and dust

The formation of the early Earth generated huge quantities of heat that melted its initial materials. Molten metals sank to the centre of the planet to form a mostly iron core. This was enveloped by a surface of molten rock, a magma ocean in which volatile compounds dissolved. Over the course of millions of years, the molten Earth cooled and the magma gradually solidified. "As the magma ocean crystallised, incompatible volatiles such as water and carbon dioxide were released," says Schönbächler. The outgassing of these substances formed the Earth’s early atmosphere.

"The early solar system was probably fairly chaotic, with all sorts of rocks and dust particles flying around," she says. "The Earth swept such material into its orbit, acquiring even more of the elem­ents required for life – though we now know that the bulk of this material had already been accumulated in the initial phase before the Moon was formed."

Removing CO₂

Earth’s first atmosphere was primarily composed of water vapour and carbon dioxide. "Most models indicate that the atmospheric CO₂ concentration was half a million times higher than today," says Derek Vance, who also works as an ETH professor at the Institute of Geochemistry and Petrology. This created a massive greenhouse effect that was hostile to life, with temperatures probably exceeding 100 degrees Celsius. "Earth had to rid itself of this excess of atmospheric carbon before it could become habitable," he says. Scientists have yet to offer a compelling theory as to how this might have happened.

"This was easy once the Earth got older," says Vance. "In fact, our planet has removed excess carbon from its atmosphere a number of times in its history. But this process takes millions of years, so it’s definitely not a solution to our current problems with greenhouse gases!" As part of the natural carbon cycle, atmospheric carbon dioxide dissolves in raindrops to form carbonic acid. Once this reaches the Earth’s surface, it dissolves rocks through a complex process of chemical weathering. The products of this weathering are flushed into the ocean via rivers and groundwater and accumulate on the seabed. "In simple terms, you take CO₂ from the air and deposit it in the ocean as calcium carbonate," says Vance. "Then you have to push that rock deeper into the mantle and bring up new rock to repeat the same cycle – and that’s exactly what Earth does through tectonic processes."

The key to this cycle is negative feedback: when the Earth’s surface gets warmer, rock wea­thering increases. "In other words," says Vance, "the more carbon you pump into the atmosphere, the faster it’s removed." This negative feedback helped create the stable conditions that allowed life on Earth to evolve over the course of billions of years.

The question of when the Earth’s crust formed and when it acquired an ocean of liquid water is still a matter of dispute, however; though tiny grains of the mineral zircon dating back 4.4 billion years may be evidence that the Earth cooled relatively quickly. Equally contentious is the question of the origin of life. Did it emerge in the depths of the ocean or 
closer to the surface? And when exactly did it begin? "Biologists suspect that the first microorganisms emerged around 4 billion years ago," says Vance. "But the oldest fossils are 3.5 billion years old, and I think they provide the clearest evidence of when life began." What we do know for certain is that there were living organisms on Earth 3 billion years ago.

At that time, the atmosphere was rich in nitrogen but lacked oxygen. What changed Earth’s atmosphere and created the conditions for the evolution of new life forms was actually life itself – all thanks to photosynthesis. Algae used the Sun’s energy to convert water and carbon dioxide into 
sugars and oxygen. As a result, molecular oxygen (O2) began accumulating in the atmosphere around 2.5 billion years ago. At high altitudes, ozone (O3) absorbed harmful UV radiation. "All forms of contin­ental animal life need oxygen to breathe plus a protective ozone layer," says Vance. "But we wouldn’t even have any oxygen without plants." Earth’s magnetic field, which is generated by molten iron and nickel in Earth’s outer core, also shields us from cosmic radiation.

"So many things could have gone catastrophically wrong for life on Earth," says Vance. Meteor impacts and giant volcanic eruptions have caused sharp rises in atmospheric greenhouse gases a number of times in Earth’s history, leading to global warming. One such event, 252 million years ago, resulted in the extinction of 70 to 80 percent of all living species. Earth has also experienced cold periods that some scientists believe turned the planet into a ball of ice. Yet the Earth survived, says Vance: "Over very long periods of time, our planet is able to repair itself by means of the stabilising effects of negative feedback."

Further information

Maria Schönbächler is Professor of Cosmochemistry in the Department of Earth Sciences at ETH Zurich. 

Derek Vance is Professor of Geochemistry in the Department of Earth Sciences at ETH Zurich.

This text appeared in the 22/04 issue of the ETH magazine Globe.