Asteroid barrage warmed young Earth and encouraged life
Modelling suggests giant impacts topped up the planet's atmosphere to keep it warm under a weak sun. Belinda Smith reports.
A 300-million-year barrage of comets and asteroids doesn't sound like a nurturing environment – but instead of stifling life on Earth, the bombardment may have given it a leg up.
Scientists in the US, led by Simone Marchi from the Southwest Research Institute in Boulder, Colorado, modelled the effects of giant impacts – that is, meteorites more than 100 kilometres wide – during the planet's youth. They found the immense impacts rumbled through the Earth's mantle, causing dissolved gases to hiss out over millions of years and sustain an atmosphere capable of retaining heat and liquid water – considered necessary for the emergence of life.
The work, published in Earth and Planetary Science Letters, provides a possible solution to the "faint young sun paradox" – and may explain why our neighbour Venus is so inhospitable today.
Our sun wasn't always a brilliant fiery ball. As an infant, around four billion years ago, it was around 30% fainter in visible wavelengths. That amount of radiation, planetary scientists calculated, could not have been enough to keep water on Earth in a liquid state.
So why do ancient crystals from this period show Earth did have liquid oceans, at least sporadically? The baby planet must have released greenhouse gases, such as carbon dioxide and methane, to shroud it and keep it warm. But how?
Theories include volcanic outgassing, where eruptions spat vast quantities of gases into the atmosphere like shaken soft drink, to rock-vapourising meteorite impacts.
But modelling suggests volcanic activity at the time was only a few times higher than present-day levels. To maintain an atmosphere capable of retaining liquid water, volcanoes would have to have pumped out carbon dioxide at more than 100 times today's rate. And being peppered by small asteroids and comets, while producing local heat and atmosphere, would dissipate within years.
So Marchi and colleagues proposed another mechanism – giant space rocks.
During the Late Heavy Bombardment, around four billion years ago, the solar system was awash with bits of leftover material. The inner rocky planets were certainly not spared from the inundation. The moon was born when a developing planet slammed into Earth during this period.
outgassing may have lasted 100 million years at a time – sufficient to keep the atmosphere thick enough for the planet to harbour liquid water
So Marchi's team wondered: what were the long-term effects of impacts from meteorites hundreds of kilometres wide?
As the Earth was so young, they reasoned, it hadn't had time to develop an enclosing crust like we have today, so any gases would have come from the thick layer of hot rock called the mantle.
The problem is no one knows for sure how much – and types of – gas was dissolved in Earth's mantle at the time. There's very little material existing today from that era.
"We tried to be a little bit conservative in our approach," Marchi says. He and his team worked backwards from current-day measurements of dissolved gases in the mantle and subtracted what they thought was delivered to the Earth over the past four billion years.
When large meteorites were added to the simulations, the initial blast of heat and gases from vapourised mantle at the surface crater caused a temporary uptick in heat, cooling within 1,000 years or so. But the long-term benefits came from delayed rumblings below.
As an impact sent waves down through the mantle, the compression and stretching heated it. Well after the impact's initial effects settled, that deep hot mantle worked its way to the surface, releasing gases and topping up the atmosphere.
The simulations show this outgassing may have lasted 100 million years at a time – sufficient to keep the atmosphere thick enough for the planet to harbour liquid water.
And this random battering may explain why Venus, which is similar in size to Earth, has a hot atmosphere full of carbon dioxide but little water. "After doing the work for Earth and realising that large collisions can really have a huge effect on the climate, then it was obvious to start thinking about Venus," Marchi says.
He and his colleagues hypothesise that as Venus – which lies 260 million kilometres closer to the sun as Earth – was pummelled and degassed, water vapour split into hydrogen and oxygen by the sun's strong ultraviolet and X-ray radiation to be whisked out of the atmosphere, leaving the dense fog we see today.