Asteroids reveal Earth’s structure
The birth of the Solar System left three distinct asteroid types as remnants from planet formation. Each asteroid type allows us insight into part of Earth’s structure.
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Earth’s iron core is 2885km beneath the surface, impossible to examine directly. A mission to the asteroid Psyche is now delayed, but when launched could provide close-up information on the core of a small planet destroyed in a collision.
In Jules Verne’s A Journey to the Centre of the Earth, the half-crazy Professor Lidenbrock travels with guide Hans and reluctant nephew Axel to the Earth’s centre via an Icelandic volcano. And after many dangerous adventures the trio returns to Earth’s surface again via a volcano in Italy.
More than a century and a half has passed since Jules Verne wrote his famous science-fiction novel, but we are nowhere near reaching the centre of the Earth. We have drilled only 12km deep, and will probably never access the core of iron and nickel so that geologists can examine first-hand how the core formed inside the young Earth.
So instead, scientists are looking the other way – towards space, where asteroids still include all the building blocks that formed the rocky planets of Mercury, Venus, Mars – and Earth. Astronomers have already launched probes into orbit around stony asteroids similar in make-up to Earth’s mantle. Probes have also visited carbonaceous asteroids similar to those which probably provided Earth with water and possibly life.
So far, we have never visited a metallic asteroid. But NASA now has plans to do so, sending the Psyche probe on a 2.4 billion kilometre space mission to an asteroid of the same name.
The Psyche asteroid consists primarily of metal, and astronomers believe that it could be the core of a planetesimal – a small planet – that was exposed when the planetesimal collided with another one.
As we cannot travel to Earth’s interior like Professor Lidenbrock, the mission to Psyche will give us the best opportunity yet to study an iron core at close range.
Solar System: a shooting booth
Psyche is shaped like a giant potato. The metallic asteroid is 277km long and averagely 223km wide, and it is so heavy that it includes 1% of all the matter of the Asteroid Belt between Mars and Jupiter – and that’s more than a million asteroids.
The asteroids are remnants from the formation of the Solar System, and to understand how the different types of asteroids now relate to Earth’s interior, we must travel back in time to the formation of the Solar System.
The Sun was formed 4.6 billion years ago, from a cloud of dust and gas that collapsed into a star. The remains of the
cloud ended up as a disc rotating around the Sun’s equator. There the dust began to collect together into first pebbles, then boulders, and subsequently into planetesimals the size of the Moon.
Close to the Sun, these planetesimals were heated from the outside by sunlight and also from the inside by radioactive decay. This caused the small planets to melt, aloowing heavy metals such as iron and nickel to travel to the centre, forming a liquid iron core surrounded by a mantle of lighter, molten rock.
Further towards the outskirts of the Solar System, the planetesimals could lose their radioactive heat into the cold of space. So those did not melt, and the carbonaceous asteroids, which date back to those planetesimals, include the rocky planets’ most original building blocks: loosely cemented pebbles and rocks.
In the beginning, the planetesimals of the inner Solar System collided over and over again, and the biggest planetesimals grew further by attracting the remnants of the collisions. By 4.56 billion years ago, the four inner rocky planets had formed, and had just about cleared their orbits of smaller planetesimals and their remains in the shape of asteroids that were either flung into the Sun or out to the Asteroid Belt between Mars and Jupiter.
In the closest part of the Asteroid
Belt, stony asteroids from the mantles of destroyed planetesimals are the most common objects to be encountered, and they were the first to be paid a visit by a probe from Earth.
In 2005, the Japanese Hayabusa probe entered orbit around the stony asteroid Itokawa, and five years later the probe returned to Earth with a small sample. The analyses confirmed that stony asteroids are made up of the same minerals as Earth’s mantle.
In 2020, the Hayabusa-2 probe dropped a capsule with 5.4g of asteroid dust collected from deeper inside the Asteroid Belt from the carbonaceous asteroid of Ryugu. (The capsule landed in Woomera, South Australia.) Next year, a US probe, OSIRIS-REx, should return with 400g of dust from another carbonaceous asteroid known as Bennu.
Observations made during the visits to the asteroids indicated that the planetesimals, of which carbonaceous asteroids are the remains, include significant quantities of water, and Bennu’s surface in particular is rich in organic substances. This supports those scientists that believe Earth’s water and perhaps the seeds of life on Earth could have come from carbonaceous asteroid impacts.
By studying these asteroids, scientists are gaining knowledge about the original building blocks that formed Earth’s crust and mantle.
Earth’s core is inaccessible
But no probe has yet visited a metallic asteroid, the type which will better reflect the composition of Earth’s inner iron core, which begins at a depth of 2885km and continues to the very centre of the Earth, which lies at an average of 6371km from Earth’s surface. The outer core is mainly liquid iron and nickel which, via their rotating motions, generate Earth’s protective magnetic field. The inner core is solid metal, due to the intense pressure, although recent findings indicate it might exist in some kind of super-ionic state (see Science Update last issue, p10).
So far it has only been possible to examine the core indirectly via seismic data. So planet researchers are extremely interested in taking a closer look at the large metallic asteroid Psyche. The asteroid could be an iron core from a destroyed planetesimal and so provide a treasure trove of information about how Earth and the other rocky planets – Mercury, Venus, and Mars – got their iron cores in the young Solar System.
According to the dominant theory, this would have happened in the same way as in planetesimals – by the rocky planets’ interiors melting, after which the heavy metals travelled into the core. The metallic asteroid may be able to reveal details about the process that we are unable to study directly down here on Earth.
Psyche gets more mysterious
By the time that NASA gave the green light for the Psyche mission in 2017, planet researchers had been convinced for decades that the big metallic asteroid was a bare metal core from a destroyed planetesimal. They believed that 90% of the asteroid’s matter was pure metal.
There were good arguments in favour of this. First and foremost, Psyche’s surface reflects radar waves far more strongly than either stony asteroids or carbonaceous asteroids, indicating that the surface is loaded with iron and nickel, corresponding to Earth’s core.
But last year this simplistic concept developed cracks, when Lauri Siltala from the University of Helsinki in Finland, and Mikael Granvik from the Luleå University of Technology in Sweden published data on 10 small asteroids that had came near to the big metallic asteroid between 1974 and 2019. Their encounters with the heavyweight changed the orbits of the small asteroids – but not so much as should have been the case if Psyche is indeed a massive lump of iron.
When the two planetary researchers calculated the asteroid’s density, the result of 3.9g per cm corresponded to only half the density of iron. This indicates that only some 30-60% of Psyche’s matter is actually iron and nickel.
So scientists have now tweaked their theories about Psyche’s origin and development. The dominant theory now involves the metal core from a destroyed planetesimal attracting rock and gravel from the shattered mantle, thus getting a thin crust of rock and metal that quickly hardened. Iron volcanoes from the still red-hot metal core would subsequently have ejected flows of liquid iron and nickel through cracks in the crust. Molten metal is easy-flowing, so the metals spread across large parts of the surface. The cold of space then cooled the outside of the metallic asteroid, while the liquid metal core also eventually hardened.
The theory about iron volcanoes could explain why major areas on the asteroid’s surface are covered in pure metal. But the explanation is speculative. Scientists still don’t know for sure whether such ferrovolcanism even exists.
Probe to solve Psyche’s mystery
The 2.6-tonne Psyche probe was planned to launch on 1 August 2022 from Kennedy Space Center in Florida. But late delivery of the spacecraft’s flight software and testing equipment meant the project couldn’t make the proposed launch date. A review board is currently discussing options and will report in September.
There are launch opportunities in 2023 and 2024, but the relative positions of Earth and Psyche are then less favourable, making the journey 6 years instead of 3½.
Testing was crucial because the probe has a new type of ion thruster known as Hall, which makes extremely efficient use of its fuel, the noble gas xenon. A storage tank of only 922kg of the gas should suffice for the probe’s entire journey and subsequent orbits around the asteroid.
A traditional ion thruster would have required five times more fuel, with a cost five times higher. Such a pricey project might never have got off the ground.
The mysteries of Psyche’s origin and composition stand a good chance of being solved when the probe does finally enter orbit. The probe carries three instruments, and can use its own radio communication with Earth to calculate the distribution of matter in the asteroid. This gravimetric calculation uses the fact that the distribution of matter in Psyche’s interior will influence the probe’s path, and this will be determined from the radio signals.
The data will reveal whether or not scientists are right in predicting a heavy concentrated core of iron and nickel, surrounded by lighter rock.
The probe’s magnetometer will also check whether Psyche is magnetised. If so, it would prove that the asteroid formed as an iron core in a former planetesimal, because if liquid iron circulated in the core as a result of the small planet’s rotation and generated a magnetic field, the field would have been frozen in magnetic particles as the asteroid hardened.
If so, we will know that the planetesimal included a small version of Earth’s core.
The probe’s camera will photograph Psyche at close range, but will also record ultraviolet and near-infrared radiation from the surface. If, after the destruction of the planetesimal, the bare metal core attracted rock and gravel from the former mantle and got a thin crust of rock, this data will reveal the type of rock minerals.
These observations will further reveal whether iron volcanoes did subsequently cover parts of the surface in liquid metal.
The preliminary orbit begins at an altitude of 700km around Psyche, but moves in ever closer during its planned duration of 21 months, ending only 85km above the surface, allowing scientists to get closer to something like Earth’s core than they have ever been, and far closer than the nearly 2900km to Earth’s actual core deep below.
Such close-up exploration from the Psyche probe should revolutionise our knowledge about how Earth and the other rocky planets formed and got their metal cores in the young Solar System.
So even though we may never be able to travel deep down into Earth’s interior like Jules Verne’s Professor Lidenbrock, NASA’s scientists still hope to study Earth’s core by having their Psyche probe take what will now be an even longer journey in the opposite direction.