Science
The discovery of 12 additional moons orbiting Jupiter has much to teach us about the evolution of our solar system.
By Karlie Noon.The many moons of Jupiter
Call it serendipity. Over the past couple of years, while most of us on Earth grappled with a pandemic and lockdowns, astronomers across Chile and Hawaii were looking for Planet X, a hypothetical ninth planet of our solar system. Its existence is suggested only by mathematical modelling and some strange orbits of smaller objects beyond Neptune. Despite its potentially gigantic proportions – a mass estimated at roughly 10 times that of Earth – this mysterious body is challenging to observe, given what’s thought to be an exceptionally elongated orbit around the sun of 10,000-20,000 years.
As their telescopes scoured the outer solar system, these astronomers realised their observations included Jupiter. But they weren’t just any run-of-the-mill observations. They were the best-resolution images ever captured of the stunningly stormy gas giant and its surroundings. So astronomers Scott Sheppard, of the Carnegie Institution for Science, David Tholen, of the University of Hawaii, and Chad Trujillo, of the Northern Arizona University, investigated the planet’s surroundings. By going deeper than any observations previously made of Jupiter, they discovered 12 new moons.
The Minor Planet Center – the official body for observing and reporting on astronomical objects that aren’t planets, based in Cambridge, Massachusetts – confirmed the detections and orbits in January, giving the fifth planet from the sun a total of 92 moons. That’s the most of any in our solar system, overtaking Saturn’s 83.
This isn’t the only happy accident that’s led astronomers to discoveries – some of which have literally changed the way we perceive our universe. Karl Jansky was working as an engineer in 1932 when he took on a project to identify and remove interference from radio-wave telecommunications. He noticed that the disruptions were coming from the sky, and changed as the antenna swept across it, indicating a variety of sources. His discovery quickly became the foundations for the field of radio astronomy. Two decades earlier, Harvard astronomer Henrietta Swan Leavitt found a special type of star that pulsates. The luminosity of these stars and the period of their pulse was later correlated to distance by Edwin Hubble, and became a tool to help astronomers measure our universe. It’s these stars that alerted Hubble to the existence of other galaxies and the stunning revelation that Earth was not the centre of the universe.
So how important are Jupiter’s additional moons?
Historically, moon discoveries have provided great insight into our solar system. Galileo discovered Jupiter’s first four moons about 1609, and collectively we now know them as the Galilean moons: Io, Europa, Ganymede and Callisto. Each of these is distinct, with its own particular compositions, motions and environments. Io is the closest to Jupiter, and this proximity causes Io’s geology to rise and fall – not unlike the swelling and contracting sea tides of Earth. As Io orbits either closer to or farther away from Jupiter, its geology remains in a state of flux. This movement induces friction, which generates subterranean heat that erupts as volcanoes. Io is the most volcanically active body in our solar system.
If Io might be defined by its heat and fire, researchers believe Europa could have twice the amount of Earth’s water, which could conceivably harbour extraterrestrial life forms. Then there’s Ganymede, which, with a diameter of more than 5000 kilometres, is the largest moon in our solar system, and the only one with its own magnetic field.
Finally, there’s Callisto, which is largely composed of rock and ice. It is one of the oldest bodies in the solar system, providing insight into its early composition, back when the planets were still forming. Similarly, researchers believe Earth’s moon was born in the early solar system when a massive collision occurred between Earth and a Mars-sized world. The debris from the epic crash coalesced over billions of years to create the moon we see today. Studying the composition of Earth’s moon acts as a window into the early solar system more than 4.5 billion years ago.
Jupiter’s new moons may have more to tell us about the evolution of our solar system, as Scott Sheppard explains: “These outer moons are important to understand because they are the last remnants of the population of objects that formed in the giant planet region as the rest of the material was incorporating into the planets.”
The 12 moons have a variety of sizes, orbits and positions in relation to the gas giant. Five of them are at least eight kilometres wide, with the others ranging between one and three kilometres. All have large orbits, taking more than 340 days to complete an entire cycle, with the nine most distant taking more than 550 days. These nine moons all have retrograde motions: they circle Jupiter in the opposite direction to the planet’s rotation. This backward motion suggests that their origins are different – that Jupiter’s immense gravitational pull captured these nine retrograde satellites, meaning they did not form around the planet like other moons.
None of the newly discovered moons has been named yet, and that’s a trickier task than it seems. Size really does matter, as we saw in the past with Pluto, which lost its qualification as a planet in 2006 because the International Astronomical Union (IAU) – the Paris-based non-governmental organisation that promotes astronomy globally – found it wasn’t large enough for its orbit to dominate other objects and debris. It was reclassified as a dwarf planet.
The new high-resolution surveys of our celestial environment in the past decade have made so many smaller objects discoverable that the IAU has stopped naming moons less than 1.5 kilometres wide. And there are other restrictions on what to call Jupiter’s moons: all must be named from Greek and Roman mythology and, if the moon also happens to have a retrograde orbit, its name must end with an “e”.
Looking into the past informs us of the possibilities of the present and future. Across the next decade, at least three different space missions are intending to visit Jupiter. One of these missions is the Jupiter Icy Moons Explorer spacecraft, or JUICE, which the European Space Agency has scheduled to visit the gas giant in April this year. The mission seeks to obtain even higher resolution images of Jupiter and three of its largest moons: Ganymede, Callisto and Europa. The mission expects to gain insight into the environmental conditions on each of the moons, which will help determine if they’re potentially habitable.
Similarly, NASA hopes to launch the Europa Clipper mission in October 2024. The mission’s spacecraft would take five-and-a-half years to reach the planet, which it would then orbit until it passes Europa about 50 times, giving it enough opportunity to capture images and conduct analysis of the moon. Despite the surface of Europa being an icy crust, researchers believe there are vast salty oceans under its surface, potentially carrying life that has formed completely independent from life on Earth. The spacecraft will conduct several tests using cameras, radars and chemical analysis.
China has also announced a mission to Jupiter scheduled for 2029, which will reach its destination in 2036. If these missions do discover the existence of life on the moons of Jupiter, it will profoundly impact how we view our solar neighbourhood, our entire universe and even ourselves as the only known space-farers in existence.
If life is possible on other planetary bodies within our solar system, the future of life on Earth and how we proceed with space travel could look remarkably different within our lifetimes.
This article was first published in the print edition of The Saturday Paper on March 18, 2023 as "The many moons of Jupiter".
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