In the late stages of the formation of Earth, the planet was a brutally hot, rough place. But perhaps not precisely in the way you might imagine.
Most renderings of that time show red-hot lava flowing around craggy rocks, with meteorites falling and volcanoes erupting. But according to those who study the time, the reality was rather different.
There was most likely no land much of the time, the medium to large meteorites arrived every few thousand years , and the surface was the consistency of a kind of room-temperature oil. Of course it was not oil, since this was a pre-organic time. Rather, it was mostly molten silicates and iron that covered the Earth in a “magma ocean.”
At its most extreme, the magma ocean may have been as deep in places as the radius of Mars. And it would have created thick atmospheres of carbon dioxide, silica dust, other toxic gases and later water vapor.
While meteor impacts did play a major role in those earliest days, the dynamics of the magma ocean were more determined by the convection currents of the super-hot magma (2000 degrees F and more), the high winds blowing above the surface, the steam atmosphere it often created and ultimately by the cooling that over hundreds of million of years led to the formation of a solid crust.
There is a burgeoning scientific interest in the magma ocean, which is expected to be part of the formation of any terrestrial planet and some lunar formations. The research focuses on the gaining an understanding of the characteristics and diversity of magma oceans, and increasingly on the potentially significant role it plays in the origin of life on Earth, and perhaps elsewhere.
The reason why is pretty simple: life (i.e., biochemistry) emerged on Earth from geochemistry (i.e., rocks and sediment.) Some of the earliest geochemistry occurred in the magma ocean, and so it makes sense to learn as much as possible about the very earliest conditions that ultimately led to the advent of biology.
What’s more, scientists believe that magma oceans created the conditions that allowed molten iron to drop down to form the planet’s core (necessary for creating magnetic fields), that resulted in the formation of more complex and thick atmospheres, and that produced water cycles. All are essential for the formation of a habitable planet and for the emergence of life.
The magma ocean is a central focus of the unusual origins-of -life institute that I’ve visited in recent weeks, the Earth-Life Science Institute (ELSI) in Tokyo. While individual researchers around the world work on problems related to the magma ocean, ELSI has put together a kind of critical mass of international scientists of varied backgrounds to take on the subject. That team includes ELSI vice director John Hernlund and his wife, seismologist Christine Houser.
Geophysicist Hernlund said that “essentially, magma oceans are the answer to the question of where we came from.” And how the planetary evolution that led to life began.
He likened those vast expanses of liquefied metal and rock to a kitchen where meals are cooked from a collection of ingredients.
“If you put some vegetables and meat into a pot of cold water or just let them sit, you’re not going to get anything particularly interesting,” he said.
“You need the heat, and that’s what the magma ocean provides big-time,” added his wife and ELSI colleague, seismologist Christine Houser.
These molten oceans consisted primarily of metals and silicates, along with gases including CO2, methane and water vapor, and other elements (spices?) that crashed into the Earth from space. The magma ocean sometimes covered the entire globe, sometimes only parts, and in time it cooled enough on to crystallized and form the first crust of the planet.
It should be noted, however, that while there is some agreement among geoscientists about the presence and basic features of an early magma ocean, there is little concrete evidence that proves their conclusions. There are no direct remnants of the magma ocean, only some chemical signatures carry evidence of its long-ago presence. Not surprisingly, there are scientists who dispute that a magma ocean ever existed.
But there are physical realities that scientists such as Hernlund and Houser say required a magma ocean. The first and foremost is that large mostly iron core at the center of the planet, the presence of which is not easy to explain without a magma ocean.
Iron is a heavy metal that is thought to have arrived on the proto Earth often mixed with silicon and silicates. Without great heat to melt those elements, the iron would have stayed where it was — mixed among the silicates and other compounds of early Earth.
To deep earth geoscientists like Hernlund and others at ELSI, logic points to a super-hot magma ocean that melted the rocks and metals and allowed the heavier liquid iron to sink down to the center. Something similar is known to have happened on our moon.
On Earth, enough of iron sank down to the center to form a core that in turn became the crucial heart of the planet’s protective magnetic field.
While a magma ocean is a particular and identifiable phenomena, it by no means exists, behaves and solidifies the same way on all planets and moons.
Another ELSI research scientist, planetary systems specialist Keiko Hamano, published at paper in Nature that compared the likely magma ocean episodes on two quite similar planets, Earth and Venus. Actually, she also made broader exo-planetary conclusions based on a planet’s location in relation to its host star, the size and the chemical makeup of the planets.
Planets beyond a certain critical distance from their host stars, she found, are expected to have much shorter periods with magma oceans — along the lines of several million years. But those within that critical distance can have magma oceans for 100 million years and longer.
Models showed that a striking result of the differing magma ocean regimes is that, however similar the planets may otherwise be, the planets and their atmospheres will have different fates. The ones with shorter-lasting magma oceans are likely to retain whatever water vapor is present in the magma and gradually recycle it to form a water ocean.
The closer-in planets with the longer-lasting magma oceans, however, are likely to lose whatever water they might have initially had as the water molecules are broken apart and the lighter hydrogen floats into the high atmosphere and space. The end result is that the planet becomes desiccated, while remaining a super hothouse because of released water vapor and greenhouse gases in the atmosphere.
This long-ago presence of magma oceans may well explain, or help explain, why Earth is temperate and supports life while otherwise quite similar Venus is bone dry and has surface temperatures of 860 degrees F.
“Atmosphere folks generally don’t care of about the magma ocean itself, and researchers in magma oceans don’t know a lot about the early atmosphere,” Hamano told me. “I want to connect the two fields because you really can’t understand either unless you begin to understand both.”
Here’s additional intriguing possibility from Hamano: An enduring puzzle about Venus is that its surface is largely smooth and un-cratered. In planetary science terms, that would suggest it is a young planet. But it is not; it was formed at the same time as Earth.
Hamano suggests that the smooth surface may be a function of those connections between the magma ocean and the atmosphere. Unable to lose its heat, the Venusian atmosphere may have kept the planet so hot that the magma ocean survived for as long as 3.5 billion years. And when a meteorite falls into a magma ocean, it leaves no craters behind.
There is also variability in how magma oceans come to be.
Perhaps the most common is formation by via incoming planetesimals, asteroids or, in the case of our moon, a planet nearly the size of Mars. The impact produces enormous heat, which then radiates outward and perhaps around the entire planet and deep into it.
The early inner solar system had many more flying objects than are found now, and a planet like Earth could have had multiple magma ocean periods, said Shigeru Ida, a planetary formation specialist and vice director at ELSI.
But its magma ocean could also have been formed by an intense greenhouse effect, one created by the release and collection of high-pressure hydrogen in the atmosphere. That process has been proposed as an alternative, or corollary, to the impact theory — a greenhouse effect so intense that it makes rocks melt.
Ida explained that magma oceans can be formed as well on smaller objects due to the radioactive decay of aluminum-26, an isotope mainly produced in supernovae but prevalent in the early solar system. The heat produced by the radioactivity is believed to be strong enough to have melted rock and separated iron from silicate on small bodies like the the asteroids Ceres and Vesta, and some protoplanets.
“We have our ideas about what caused the magma ocean on Earth, but nobody has proof,” Ida said. “We know there was great heating and melting and separating of iron and silicates, but in truth we don’t know even on Earth if it was from a giant impact or the greenhouse.”
I also listened to a talk by Ida about “pebble accretion,” a relatively new theory that small rocks (inches to yards in diameter) from the outer solar system migrated into the inner solar system during planet formation and played role in the accretion process. Ida’s approach to the subject was in line with ELSI research — it examined what kind of magma ocean the “pebbles” would form. The answer was that models showed it would be much smaller and more shallow than a magma ocean caused by a giant impact.
Swimming in the magma ocean field need not be limited to our solar system.
Both Hamano (in an Astrophysical Journal paper) and Hernlund say that magma oceans are still being formed in the galaxies all the time, and that planets with those oceans can become compelling targets for future direct imaging of exoplanets. The trick would be to look for young stars and the young planets that might orbit them.
The discovery of an exoplanet magma ocean — accomplished through the detection of certain chemical signatures — could provide important insights into the formation of planets today and a most intriguing look into our distant past as well.
Many Worlds 