A massive asteroid with a surface area approximately twice the size of the Chicago metropolitan area impacted Ganymede, the largest moon in the solar system, around four billion years ago, a new study claims.
The impact of this asteroid, if it actually occurred, would have drastically impacted the moon's environment in ways scientists can only begin to fathom. Luckily, the means of proving, or disproving, this hypothesis and its implications may already be available.
The findings of this study were published in the peer-reviewed academic journal Scientific Reports.
What is Ganymede and what is so special about it?
Ganymede is the largest of the 100-plus moons orbiting Jupiter, and is also the largest moon in the Solar System. It is so big, that it is larger than the planet Mercury.
Just like the Earth's moon, Ganymede is tidally locked, meaning only one side of it ever actually faces Jupiter.
Ganymede has long been fascinating for scientists due to its composition, with considerable evidence pointing to it having a huge subterranean ocean - one so vast that it could have more water than all of the Earth's oceans combined.
The fact that it has an ocean is very significant because it means something is heating the moon up enough for water to take on a liquid state. Some observations have even spotted evidence of water vapor on the surface.
But another interesting feature of Ganymede is its terrain, much of which shows signs of major asteroid impacts. This is something researchers have long been aware of, and there is some evidence that backs this up.
These impacts were believed to have occurred around four billion years ago. At this time, some scientists believe that a huge swarm of asteroids and comets struck planets and moons in the solar system. This hypothesized event is known as the Late Heavy Bombardment, and while whether it actually happened as well as a possible cause are still debated among academics, there is still evidence that it occurred.
What is especially interesting about Ganymede's impact sites, though, is the relationship it seems to have with its axis.
Did an asteroid impact change the rotation of Jupiter's moon?
The tidal axis of a moon is what determines what side of a tidally locked object faces the thing it rotates. In this case, it is what connects Jupiter with the center of the side of Ganymede that always faces it.
And according to study author Dr. Naoyuki Hirata of Kobe University, that axis isn't where it always was. Four billion years ago, a gigantic asteroid struck Ganymede and forced it to spin around, making it so that the site of this massive impact is now always facing away from Jupiter.
Supporting this is something few have paid attention to: That the center of the largest asteroid impact site on Ganymede - one of the largest in the outer solar system - is directly opposite the tidal axis.
Hirata ran computer simulations to determine this. What he concluded was that it was specifically an asteroid with a radius of 150 kilometer, slamming into Ganymede at an angle between 60 and 90 degrees.
Now, let's put the size of that asteroid into perspective.
The study did not provide the diameter of the asteroid, rather it provided a radius, which is a line from the center of a circle to the edge. In other words, it's about half the diameter.
If its diameter was 150 kilometers, it would already be absurdly large. The asteroid impact credited with wiping out the dinosaurs was only 10 kilometers wide. But this is far larger than that.
Using the radius, we can calculate the surface area of the asteroid, and while not perfect because it was almost certainly not a perfect circle, it is still a way to estimate the sheer magnitude of the asteroid.
With a radius of 150, the circumference - that is to say, the surface area - of the asteroid is over 70,685 square kilometers. To put that in perspective, consider Chicago, one of the largest cities in the US, and the center of one of the largest metropolitan areas in the US, known as Chicagoland.
Chicagoland itself has an area of 28,120 square kilometers, meaning this asteroid that struck Ganymede would have been well over twice the size of Chicago.
Further supporting the claim of an asteroid impact shifting the tidal axis is that it may have happened before, specifically on Pluto. There, a huge impact event is theorized to have caused the planet's polar orientation to shift.
But just what did this asteroid impact do aside from alter the tidal axis?
It turns out, simply moving the axis is far more significant than one might expect, and requires an absurd amount of power.
On Earth, an impact from an asteroid over a kilometer in diameter would be a global cataclysm and possible apocalyptic scenario. An asteroid this big would be far beyond that.
According to the outlet New Scientist, University of Illinois Chicago Prof. Andrew J. Dombard referred to such a hypothetical impact scenario like this on Earth as "a global sterilizing event."
Ganymede, which is significantly smaller, did not fair much better.
The massive asteroid would have pushed through Ganymede's surface, destroying everything in its path and around it, and essentially flinging so much material from underground all around the moon that the surface would be permanently altered.
The impact would likely have gone as far as to make it into the underground ocean below, potentially severely impacting it as a result.
The moon itself would have spun around until in its new tidal axis, and when the dust eventually settled, the result would be a gravitational anomaly - a spot on the moon where gravity is more intense due to the greater amount of mass.
This also is thought to have occurred on Pluto, where the aforementioned major impact event formed a gravity anomaly at the impact site, known as Sputnik Planitia.
But that's still just a hypothesis. Ultimately, while we know an impact certainly occurred on Ganymede at the indicated site, we don't truly know how large it must have been because we don't yet have enough data on Ganymede's gravity and topography. And the fact that Ganymede's composition so so unique, being composed almost equally of rock and water - the later of which includes a large amount of ice - could change the variables in this study.
More research is needed, but luckily, we may have a way of studying Ganymede further in the near future.
In 2023, the European Space Agency (ESA) launched its biggest mission ever, JUICE (Jupiter Icy Moons Explorer). At the time of writing, this space probe in the middle of its 900-million-kilometer journey towards Jupiter and its moons for a four year mission.
Set to arrive in 2031, JUICE will see researchers able to study four main targets, the first being Jupiter itself and the rest being three of its moons: Callisto, Europa, and Ganymede.
From the start, JUICE was already equipped with a host of scientific instruments, and a major focus was always going to be studying the topography and composition of moons like Ganymede for signs of water and energy sources. This makes it well suited to gather information about this hypothetical asteroid impact, and how it may have changed Ganymede inside and outside.