The Planet That Pulls on Worlds
A Note Before I Begin
As I did with The Part That Won’t Go Away, I leaned on Chat a little more than usual for this one.
I know enough astronomy to understand most of what I am reading when something like this catches my attention. I know the broad mechanics. I know what tidal forces are. I know why moons get locked into resonances and why gravity becomes more complicated when several large bodies start tugging on one another.
But, I am not an astronomer.
So, I used Chat partly as a translator and partly as a guardrail. I wanted to understand the details without accidentally building an essay around some oversimplification I picked up twenty years ago.
It corrected one of those, actually.
And, then a few days after I started thinking about this piece, I found another article about Jupiter that made the whole thing stranger.
More on both in a minute.
A Tide Made of Rock
I have reasons for liking basically every planet.
Mercury is this scorched little thing clinging absurdly close to the Sun. Venus is almost Earth-sized and completely hostile. Mars is Mars, close enough to feel imaginable. Saturn has somehow been allowed to exist with those rings without anyone accusing the universe of showing off. Uranus is tipped nearly on its side. Neptune is blue, distant, violent, and barely seems real.
And, then there is Jupiter.
Jupiter stands, or I suppose floats, alone.
Being the largest planet obviously helps. You could fit more than a thousand Earths inside it by volume. It is made mostly of hydrogen and helium and does not have anything we would recognize as a normal solid surface. Its storms can swallow planets. Its magnetic field is enormous.
Then there are the moons.
Jupiter currently has 115 officially recognized moons.
One hundred fifteen.
The four I usually think about are the Galilean moons Galileo saw in 1610: Io, Europa, Ganymede, and Callisto. They are extraordinary enough that it is easy to forget they represent only a small fraction of the things orbiting Jupiter.
A recent Space Daily piece gave me one detail I have not been able to stop thinking about.
Io does not hold its shape.
As Io completes its roughly forty-two-and-a-half-hour orbit, Jupiter’s gravity pulls on it unevenly. The moon stretches and relaxes. Its solid ground can move by as much as about one hundred meters.
Not water.
Rock.
A tide in the actual body of a moon, rising and falling by roughly the height of a thirty-story building.
The number is model-based rather than a measurement of one particular patch of ground moving exactly one hundred meters up and down, but the underlying phenomenon is very real. Io is being continuously deformed.
And, that deformation is why it is the most volcanically active world we know.
Rock bends. Rock resists. Some of that mechanical energy becomes heat. Io is effectively being worked over from the inside by gravity.
Even stranger, Jupiter is not doing this alone. Europa and Ganymede help keep Io’s orbit slightly eccentric through a repeating gravitational resonance. Without those nudges, tidal friction would gradually circularize Io’s orbit and the great squeezing would diminish.
Instead, the system keeps going.
Jupiter pulls Io.
Io moves.
Europa and Ganymede keep the geometry from settling down.
The interior heats.
The volcanoes erupt.
Gravity becomes geology.
The Moons Remember
Then I found another Space Daily piece, and it made me realize that the relationship works in the other direction too.
Jupiter changes its moons.
But, the moons also tell us about Jupiter.
This time the important ones are not the famous four. They are Amalthea and Thebe, two small, irregular inner moons that would be easy to leave out of a casual list of interesting things orbiting Jupiter.
Amalthea is only about 84 kilometers in mean radius. Thebe is smaller still, about 49.
Both have slightly tilted orbits.
Slightly really does mean slightly here: fractions of a degree. But, apparently, that can be enough.
Researchers Konstantin Batygin and Fred C. Adams modeled those tiny orbital inclinations as possible leftovers from resonances that swept through the inner Jovian system billions of years ago as Io migrated outward.
The chain of reasoning gets complicated quickly, which is another place I was happy to have Chat beside me.
Those little tilts help constrain where Io used to orbit. That helps reconstruct the inner edge of the disk of gas and material that once surrounded young Jupiter. That, combined with models of Jupiter’s rotation and contraction, gives scientists a way to work backward toward the planet itself.
The result is astonishing.
Roughly 3.8 million years after the first solids in the solar system formed, Jupiter may still have been somewhere around two to two-and-a-half times its present radius.
Not mass.
Radius.
It was a much more swollen world, still hot from formation and slowly contracting toward the Jupiter we know now.
The researchers also infer that its surface magnetic field may have been roughly fifty times stronger than its present characteristic field.
Again, these are reconstructions, not measurements anyone somehow took four and a half billion years ago. Amalthea and Thebe are not little hard drives with Jupiter’s baby pictures on them.
But, in a way, their orbits are records.
A fraction of a degree survives for billions of years, and from it scientists can begin reconstructing a planet that no longer exists in that form.
I love that.
The moons have been pulled around by Jupiter for almost the entire history of the solar system.
And, in the way they move now, some trace of that history remains.
The Planet That Does Things
I think this gets at why Jupiter fascinates me differently from the other planets.
Jupiter does not merely have interesting features.
It does things to everything around it.
Its moons almost feel like a small planetary system of their own.
Io is volcanic beyond anything on Earth.
Europa appears to hide a global ocean beneath its ice, with tidal heating helping keep that interior from simply freezing solid.
Ganymede is larger than Mercury and produces its own magnetic field.
Callisto looks like something the early solar system beat nearly to death and then left alone.
Amalthea and Thebe, meanwhile, are small enough to seem almost incidental beside them, yet the geometry of their orbits may preserve information about what Jupiter looked like when the entire solar system was young.
And, those are six moons.
Out of 115.
There are dozens upon dozens of other little worlds and captured AVVobjects moving through this enormous gravitational system, plus rings, dust, and countless smaller bodies that do not even make it into the official moon count.
All of them are orbiting this striped thing that never becomes solid beneath the clouds.
And, Jupiter’s influence extends much farther than its moons.
For years, I had absorbed the familiar idea that Jupiter helped make life on Earth possible because it acted as a kind of gravitational shield, throwing dangerous comets and asteroids away before they could reach us.
That turns out to be too simple.
This was one of the places where Chat stopped me.
Jupiter certainly can eject objects from the solar system or redirect them away from Earth. But, its gravity can also send objects inward. It may sometimes reduce impacts on the inner planets and sometimes contribute to them.
So, “shield” gives Jupiter too clean a job description.
Gravitational architect is probably closer.
Its formation and enormous mass helped sculpt the solar system we inherited. It scattered material. Captured objects. Altered orbits. Rearranged trajectories.
Sometimes that may have reduced danger.
Sometimes it may have created it.
Either way, Earth did not develop in a solar system where Jupiter was merely sitting in the background.
Its gravity was part of the story.
An Appetite for Scale
I have written elsewhere on Open Doors about my decidedly non-expert interest in astronomy and the larger universe.
I do not want to become an astronomer. I never did.
What I seem to want is enough understanding to get close to the strangeness.
Enough to know what the numbers mean before they stop meaning anything.
A hundred meters is comprehensible.
A thirty-story building is comprehensible.
So, imagine standing on solid ground and having the entire landscape slowly rise and fall through that distance because a planet hanging in the sky is pulling the world beneath your feet out of shape.
Two times Jupiter’s present radius is also comprehensible, at least mathematically.
Then you remember how large Jupiter already is.
And, suddenly the young planet becomes difficult to picture again.
That is usually the point where astronomy gets me.
It gives me a handhold and then takes it away.
A hundred meters.
A fraction of a degree.
A moon eighty-four kilometers in radius.
Numbers small enough to understand.
Then those numbers become volcanoes covering a world, orbital scars surviving billions of years, a vanished disk reconstructed from the tilt of a battered little moon, and a young Jupiter swollen to more than twice the radius of the giant planet we already struggle to comprehend.
Maybe that is what separates Jupiter for me.
Gravity is normally invisible. We know it through consequence. A dropped object. An orbit. A tide.
Around Jupiter, the consequences become enormous enough to see.
Moons lock together in rhythms.
Rock flexes.
Interiors heat.
Volcanoes erupt.
Orbits migrate.
Tiny inclinations preserve ancient events.
Comets change course.
A whole family of worlds carries the history of the thing they orbit.
And, somehow, the same gravity that is bending Io today also helps scientists look backward into a Jupiter that disappeared billions of years ago.
Jupiter does not have to move toward anything.
It just has to be there.
Everything else has to reckon with its weight.
And, apparently, some of those things remember.
It’s fascinating
Enormous and always pullijg
A system all its own
