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It’s been six years since Juno began orbing Jupiter. During that time, the probe has been
busy gathering incredible images and data from the gaseous giant. Veterans of this channel may
remember videos I released on the mission’s first and third anniversaries, but a lot has
happened since then. In 2021, Juno began its extended mission beyond the initially planned
37 orbits to include flybys of Jupiter’s icy inner moons Ganymede, Europa and eventually Io.
It’s been over two decades since Galileo visited these moons, and by studying changes in them,
NASA hopes to understand what might be going on beneath their icy surfaces
(and without letting the cat out of the bag too soon, that appears to be a lot).
Also, thanks to the probe’s powerful cameras, JunoCam and its Stellar Reference Unit navigation
camera, Juno is taking some of the very best images we’ve had of these icy moons.
So, with the long-running mission’s end on the horizon (NASA plans to decommission the probe in
September 2025) now seems like a good time to ask: what has Juno seen? I’m Alex McColgan, and you
are watching Astrum. Join me today as we view the stunning highlights of Juno’s extended mission and
unpack the surprising discoveries that are already changing our understanding of the Jovian system.
Let’s start with a quick recap. Juno launched from Cape Canaveral, Florida, in August 2011.
After travelling for five years and 2.8 billion kilometres, the space probe began orbiting Jupiter
on July 5, 2016. After completing most of its primary objectives, NASA extended Juno’s mission
and set its sights on some of Jupiter’s largest moons. In total, the extended mission will add
42 orbits, or Perijoves, by 2025, at the end of which, NASA will deorbit the probe by burning it
up in Jupiter’s atmosphere to avoid potentially contaminating those moons. This procedure is in
accordance with NASA’s planetary protection guidelines, but it’s of special importance
in Juno’s case. Scientists have long wondered whether there could be life on Jupiter’s inner
moons, and NASA doesn’t want to muddy up any future discoveries that could be made there.
So, on June 7, 2021, Juno performed its close flyby of Ganymede, using a gravity assist to
bring its orbital period from 53 days down to 43 days. Juno had already performed a distant flyby
of Ganymede in 2019, when it reached a proximity of 97,000 kilometres, but this one brought the
probe within just 1,000 kilometres of the moon’s surface. In this photograph captured by JunoCam,
you can see nearly half of Ganymede’s total surface in breath-taking detail.
For reference, the resolution is about 1 kilometre per pixel. I love the crisp rendering
of Ganymede’s unique structural features: you can practically feel its texture with your fingertips.
Notice how the icy surface is littered with craters, light and dark patches,
and long striations. The darker areas show older terrain, which is heavily cratered, whereas the
lighter areas are comparatively younger and less cratered. But what about those long striations?
We know, based on previous data, that Ganymede contains at least one vast salty subsurface
ocean beneath its icy outer layer, which some speculate could be suitable for life.
Scientists think these long structural features might reveal faults produced by tectonic movements
generated by heat from the moon’s iron-nickel core. But this is a subject of ongoing research.
This image, taken from the same flyby on June 7th, shows Ganymede’s Tros Crater in greater
detail. The reason it appears so bright is that it’s covered in ice. Here is another image from
June 7th I find remarkable. It’s a photograph of Ganymede’s dark side, and it was taken by
Juno’s Stellar Reference Unit navigation camera. The Stellar Reference Unit was designed to keep
Juno on course using starlight, which makes it ideal for photographing low-light conditions.
For scale, the resolution is between 600 and 900 meters per pixel. Again,
notice the moon’s long striations and craters, some of which are stacked on top of each other.
I’ve seen a lot of photos of Ganymede, many of which I’ve covered previously, but I’m floored
by the level of detail Juno was able to capture in such low light conditions. Interestingly, one of
the most surprising discoveries from the mission came not from the orbiter itself but from Hubble.
Yes, that Hubble. Allow me to explain. You see, to support Juno’s exploration of the Jovian moons,
NASA asked Hubble to monitor Ganymede’s ultraviolet signals. Ganymede has an atmosphere
(albeit a very thin one), and one of NASA’s goals was to find traces of oxygen they long suspected
was hiding there. Nearly everyone expected to find plenty of oxygen based on previous analyses of the
moon’s ultraviolet emissions. But much to our surprise, Ganymede’s atmosphere had very little
oxygen in it. What they found instead was a whole lot of water vapour. This discovery has led to a
significant revision to our model of Ganymede’s atmosphere, and could suggest that water vapour
might even be present in the atmospheres of icy bodies throughout the solar system.
After visiting Ganymede, Juno made its flyby of Europa on September 29, 2022,
which gave the probe a gravity assist that shortened its orbit to 38 days.
Here is a dramatic image from the flyby, taken from a distance of 351 kilometres.
It shows a portion of Europa just north of the equator. What I find extraordinary here is the
day-night boundary known as the terminator. The deep shadows really accentuate the ridges, troughs
and craters that riddle the moon’s surface, showing it in almost 3-dimensional relief.
Scientists think that Europa, like Ganymede, harbours a vast ocean beneath its icy exterior.
But new findings suggest there could be a lot more going on than the simple ocean/crust model would
have you believe. What do I mean by this? Well, hold that thought as we study this next image…
This photograph was taken by Juno’s Stellar Reference Unit camera and shows a zoomed-in
portion of Europa’s surface. It was taken during the moon’s night from a distance of
412 kilometres. With a resolution of 256 meters per pixel, this is the highest-resolution image
that Juno took of Europa. At a glance, your first thought might be: what am I looking at?
Well, what you’re seeing is a heavily fractured region of Europa’s surface
crisscrossed with grooves and ice ridges. And that oddly shaped dark patch just to the right
of the centre could yield important clues as to what’s behind this geological complexity.
Scientists believe this darkened area shows an eruption occurring below the surface ice.
The thing is: Europa’s icy shell is over 10 kilometres thick. Imagine the pressure needed
to penetrate so much ice! However, a new paper published by the American Astronomical Society
proposes a ground-breaking new theory (pardon the pun) with a different explanation. According to
the paper’s authors, Europa’s cryovolcanic activity shows evidence of brine reservoirs
trapped within the icy outer crust. This would mean that, far above the moon’s subsurface ocean,
there could be lakes of high salinity brine that slush about, expanding the surrounding ice like
a balloon and bursting through on occasion. It’s a compelling theory, but to prove it, we may have
to wait for the European Space Agency’s Clipper Mission, which will reach Europa sometime in 2030.
I know that’s eight years away, but it’s not too early to mark your calendars!
Let’s head back to Jupiter one more time. You see, while Juno was paying visits to Ganymede
and Europa, it also solved a big mystery, one that goes back more than 40 years.
When the Voyager Mission first visited Jupiter in 1979, the probes detected mysterious flashes
in Jupiter’s atmosphere. Scientists assumed these were lightning flashes similar to those
we see on Earth, meaning water would have to be present in all its states, solid, liquid and gas.
Given what we know of Jupiter’s atmosphere, this would put the storms at least 45 kilometres below
the visible cloud tops. However, Juno disproved this theory – and the truth is stranger than we
imagined. Juno’s Stellar Reference Unit camera saw something weird – night-time
flashes at a far higher altitude than we thought possible. Given the incredibly cold temperatures
at this altitude (reaching below minus 88 degrees Celsius), scientists deduced that the storms were
made of ammonia-water-based clouds, a phenomenon that doesn’t exist here on Earth. In these strange
and violent thunderstorms, the ammonia acts like an antifreeze, forming a solution of two parts
water-one part ammonia that remains liquid despite unimaginably cold conditions. And apparently,
as these thunderstorms produce powerful lightning strikes, they are raining down massive hail made
of water-ammonia slush – an unusual precipitation that scientists are calling “mushballs.”
Jupiter is a strange planet, and the more we learn, the stranger it gets. So, there we have it:
some of the best highlights of the trailblazing Juno Mission since we last checked in. The next
stop for Juno will be Io sometime in 2023, and personally, I can’t wait to see what’s in store.
Juno has vastly exceeded my expectations every step of the way, and if Ganymede and Europa are
any indication, our understanding of Io is about to get a whole lot more interesting.
Juno has returned some crisp, amazing images. If you’re like me, one of the things you love about
space is the incredible beauty of the vistas we find out there. Our solar system is filled with
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Thanks for watching. If you enjoy recaps of spacecraft missions, be sure to check out
this playlist here for a lot more. And thanks to my patrons and members for your support. If
you want your name added to this list, check the links below. All the best and see you next time.