Four and a half billion years ago, Earth was going through a rather peculiar phase. Its atmosphere was thick and, by today’s standards, quite toxic. The Moon, having just formed, appeared much larger in the sky than it does now and emitted a faint glow from the residual heat of its own creation. The planet’s surface, meanwhile, was literally a sea of lava. Everywhere.
For a long time, scientists believed this molten state of Earth was a relatively brief period. However, a new preprint study, available on arXiv and authored by researchers from the Kapteyn Astronomical Institute, suggests this molten phase could have persisted for over half a billion years.
But why would it last so long? Eventually, wouldn’t the magma comprising Earth’s surface cool down and solidify? In the long run, yes. However, this cooling process could have been significantly prolonged by two competing forces: the tidal forces exerted by the newly formed Moon and the greenhouse effect generated by Earth’s primeval atmosphere.
We’re familiar with the Moon’s influence on ocean tides today. But when it was first forming, and much closer to Earth than it is now, its gravitational pull had a far more dramatic effect on our planet’s physical structure. Since gravitational forces are distance-dependent, this proximity meant the Moon was essentially kneading Earth’s interior like dough. This process generated “tidal heating,” which produced immense amounts of internal heat, fueling the magma oceans from within.
On the flip side, Earth possesses the ability to radiate this heat out into space, eventually leading to cooling and the formation of a solid surface. This cooling would be significantly hampered, though, if the atmosphere blocked its heat-releasing capability. The magma itself contributed to this atmospheric shielding. It released gases, creating a potent greenhouse effect far more intense than anything we experience today.
To simulate the interplay between the Moon’s internal heating and the atmosphere’s greenhouse effect, the researchers utilized a planetary evolution model called PROTEUS. Through this model, they identified several periods during Earth’s early development when the planet reached a state of global radiative equilibrium. In essence, it was radiating heat into space at roughly the same rate it was being heated by the Moon’s tidal forces. During these intervals, the magma wouldn’t solidify; rather, the process of Earth’s solidification would come to a standstill. According to the study, these periods of stalled solidification could have lasted anywhere from 2 to a staggering 320 million years.
This considerable variability is attributed to a specific aspect of Earth’s early chemistry: oxygen fugacity, essentially the degree to which its mantle was oxidized or reduced. If the mantle was in a more oxidized state, water would have been retained until much later stages of magma ocean crystallization. Once water was eventually released as vapor, it would have triggered a significant greenhouse effect, consequently keeping the surface molten for an extended period compared to other scenarios.
Conversely, if the mantle was in a reduced state (meaning hydrogen and methane were dominant), the planet would have degassed its greenhouse gases earlier, resulting in a less pronounced greenhouse effect. In such a case, the surface could only have remained molten for a longer duration if the lunar tides were considerably stronger than initially theorized.
Such a magma world might sound like a terrifying playground for children playing “The floor is lava,” but it also presents a challenging environment for the very genesis of life. However, the study proposes that this prolonged magma ocean phase might have been precisely what life needed to eventually emerge. The conditions present during this extended magma ocean period, particularly concerning the oxygen fugacity at the surface near the iron-wüstite buffer, would have led to a methane to carbon dioxide ratio in the atmosphere of approximately 0.1. While this number might seem highly specific, it’s crucial for the photochemical production of hydrogen cyanide.
Today, hydrogen cyanide is lethal to most known life forms. But during the nascent stages of life, astrobiologists consider it a vital precursor molecule for the creation of RNA and proteins – the fundamental building blocks of life. Therefore, Earth’s magma ocean phase, sustained by our much closer Moon, could have provided the necessary time for the accumulation of prebiotic chemical compounds that ultimately paved the way for life’s origins later in the planet’s history.
While we cannot assert this with absolute certainty, it represents an intriguing hypothesis supported by logical chemical reasoning. The question remains whether Earth’s initial conditions – a large, nearby moon and a lava-covered surface – were unique. However, the more thoroughly we understand the early Earth conditions that led to the emergence of life, the better our chances of recognizing it when we eventually detect it elsewhere in the galaxy.
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