คำบรรยาย (154)
0:00Astronomers are on a scavenger hunt
that started almost 100 years ago,
0:04searching for something
right in front of their eyes.
0:06They know something’s out there,
0:07holding galaxies together and sculpting the
0:09observable universe into this awesome shape.
0:12And they’ve given it a name: dark matter.
0:14The only problem is…no one has
any idea what dark matter is.
0:18It barely interacts with
anything except through gravity.
0:20It’s invisible, and all attempts
to detect it directly have failed.
0:24But maybe… just maybe, a
giant moon orbiting Jupiter
0:28has been collecting evidence
of dark matter all along.
0:34The problem with looking for dark matter is that
0:36we don’t even really know what we’re looking for.
0:38It’s like trying to go on an Easter egg
hunt if you’ve never seen an egg before.
0:41Also, the eggs are invisible.
0:43And impossible to touch.
0:44But over the past several decades,
scientists have come up with
0:46all sorts of ideas about what
dark matter might be like.
0:49These days, one particularly popular hypothesis
is that it’s a bunch of tiny particles
0:53known as WIMPs that barely ever
interact with light and regular matter.
0:57But very, very occasionally, a
WIMP could collide with an atom
1:00and release a little jolt of energy.
1:02Assuming they actually exist,
and this actually happens,
1:05we could detect those jolts.
1:07So around the world, scientists have built massive
1:09underground detectors full
of heavy atoms like xenon.
1:12But after years of searching, they’ve had no luck.
1:14And that’s possibly because they’re
looking for the wrong thing entirely.
1:18Some physicists have proposed
that dark matter is actually
1:21made up of more massive clumps of stuff,
which they call macroscopic dark matter.
1:25Unlike WIMPs, this stuff is supposed to be able to
1:27interact with regular matter, even violently.
1:30But these clumps would also be incredibly rare.
1:32Like, one might hit the Earth every 100,000 years.
1:36That’s a thousand times
longer than the time humanity
1:38has even been thinking about dark matter,
let alone actively hunting for it.
1:42And in-between those hypothetical collisions,
1:44scientists have used process of elimination to
1:46figure out what we might be looking
for, and how we might find it.
1:49For one thing, we know roughly how
massive a typical clump would need to be.
1:53And by “roughly”, I mean if
it were less than an apple,
1:56or more than a large asteroid,
scientists would have already
1:59detected macroscopic dark
matter in past experiments.
2:02That leaves a pretty big range in the
middle, but it’s better than nothing!
2:05And in a 2025 paper, one scientist
came up with a new plan to look for it.
2:09The paper is still a preprint, meaning it hasn’t
2:12been peer-reviewed yet,
but this is the basic idea.
2:14If macroscopic dark matter is real,
surely it’s collided with objects
2:18in our solar system a bunch of
times in the last 4.6 billion years.
2:22Earth’s surface changes too much
to hold onto evidence like that.
2:25But there are objects in our
solar system that barely change,
2:28recording every strike to
their surface across the eons.
2:31And we already have two space
missions headed to one of them.
2:34Ganymede isn’t just Jupiter’s biggest moon,
2:36it’s the biggest moon in the solar system.
2:38In fact, it’s bigger than the planet Mercury.
2:41And its surface is old.
2:42A third of it is nearly as old
as the solar system itself,
2:45and the rest is thought to be
around two billion years old.
2:48So Ganymede is covered in the scars of
everything that’s ever hit it in that time,
2:53including… just maybe… macroscopic dark matter.
2:56And the author of this 2025 preprint
realized that if macroscopic dark matter
3:00does exist, it would leave
a really distinctive scar.
3:03But before we get into more detail,
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4:10When a typical asteroid hits
a rocky body like Ganymede,
4:14it usually shatters and leaves
behind a shallow crater.
4:16But according to models, macroscopic dark matter
4:19would have to be enormously/ dense.
4:21I’m talking a billion times denser than water.
4:24The only place you can find stuff that dense
4:26in the known universe is in
the cores of collapsed stars.
4:29But that’s not the only extreme
property this stuff has got.
4:32The macroscopic dark matter streaming through
4:34our solar system probably wouldn’t be local.
4:37It’d be sailing in from
somewhere else in the galaxy,
4:39traveling as fast as 270 kilometers per second.
4:43For comparison, your typical asteroid would crash
4:45into Earth going about 18 kilometers per second.
4:48So a clump of macroscopic dark matter wouldn’t
4:50just burst into pieces when it
hit the surface of Ganymede.
4:53It would punch into the moon like a bullet…
4:55and maybe even come out the other side!!!
4:57So what kind of evidence would this
cosmic hit and run leave behind?
5:00Our 2025 paper gives us an idea by simulating
5:03an impact from a 2-meter-wide dark matter bullet.
5:06Based on the exact numbers the author used,
5:08the impactor would bore a hole
roughly 5300 kilometers deep.
5:12But Ganymede’s average diameter
is just a bit smaller than that,
5:15hence the potential for an exit wound.
5:17But Ganymede is also chock full of
ice, both on and beneath its surface.
5:21So while the dark matter blob
blazes through the moon’s insides,
5:25it vaporizes the ice around it and sends a
5:27shock wave racing outward from the borehole.
5:29Thanks to some thermodynamic happenings
we don’t have time to get into today,
5:33this winds up ejecting material
in the impactor’s path,
5:36which we will be coming back to, later.
5:37Next, pressure drops in the borehole
as energy from the shock wave
5:41radiates through the moon for a couple seconds,
5:43until it loses enough energy and peters out.
5:45It leaves behind a massive cavity,
5:47which at its widest measures
a kilometer and a half across!
5:50But the destruction isn’t over.
5:52Almost immediately, debris starts falling inward,
5:54starting wherever the pressure is
highest and the hole is narrowest.
5:57But instead of just refilling the
cavity, some of the infalling debris
6:01also gets blasted upward in
what’s called a Worthington jet.
6:04You know what happens when someone
cannonballs into a pool and
6:06a plume of water shoots up above them?
6:08It’s basically the same thing as that.
6:10By the end, the impact event
leaves behind not just a crater,
6:12but a bunch of debris from deep
underground scattered in and around it.
6:16If the composition of that debris
differs from what Ganymede is normally
6:20rocking on its surface, astronomers
might be able to distinguish
6:23normal impact craters from those
caused by macroscopic dark matter.
6:26And if they get really lucky, they might
even be able to match these craters
6:30with exit wounds, where the dark
matter flew out the other side.
6:33Now before anyone gets too
hyped about this hypothesis,
6:35we can’t do any of this crater searching just yet.
6:38It’s not a job for a regular telescope to tackle.
6:40But in the early 2030s,
NASA’s Europa Clipper and the
6:43European Space Agency’s Juice,
will both reach Jupiter.
6:47And both will give us a clear
look at Ganymede’s surface
6:49at some point during their mission.
6:51If dark matter scars do exist,
the probes should spot them.
6:54With all that said, we still
don’t have any amazing evidence
6:56that even suggests dark
matter really is macroscopic,
6:59let alone that it left scars on Ganymede.
7:02But some of our most important
theories started out as wild ideas.
7:05And when we’re dealing with one of
the biggest mysteries in science,
7:08sometimes it takes a wild idea to get somewhere.
7:10Or at the very least, tell us where not to look.
