Thanks to Surfshark for sponsoring today’s
video. In 2012, the Dawn spacecraft completed a 14-month
survey around the giant 500km wide asteroid, Vesta. But its mission wasn’t finished yet, the
second and perhaps most interesting leg of its mission was coming up, a detailed survey
around the dwarf planet Ceres. You see, Dawn was to be the first mission
ever to go into orbit around two different extra-terrestrial objects. It could do this because it was a rather unusual
spacecraft, equipped with three ion engines. These engines don’t produce a lot of thrust
in one go like a rocket, but rather, they can very efficiently run for years at a time. Over the course of Dawn’s mission, these
thrusters provided a velocity change of 11.5km/s, meaning Dawn could enter into quite a tight
orbit around Vesta, do detailed surveys there, escape Vesta’s gravity and head on over
to Ceres. The anticipation within the scientific community
to reach Ceres was palpable. Even with the aid of the Hubble Space Telescope,
the best image we had of Ceres was this. It was still a mysterious body. What was lying in wait there? Would Dawn hold up to years in the unforgiving
environment of space? And what would Ceres reveal about our own
solar system? I’m Alex McColgan, and you’re watching
Astrum, and together we will explore the surface of Ceres, and find out what the Dawn spacecraft
saw and discovered there. After 2 years in transit between the two bodies,
Dawn finally began the approach to Ceres. As days passed, the resolution of Ceres got
better and better. Details like craters could finally be resolved,
and most interestingly of all, bright white dots could be seen. I remember at the time that as these images
were coming in speculation was rife about what they could be. As higher resolution images were received,
it looked like the brightest spot was actually two separate spots, and then the increased
resolution revealed it was in fact several different spots. Dawn also observed the scarred nature of Ceres
up close, with craters littering the surface. Although, there aren’t as many craters here
as previously expected. Taking this into consideration and the bright
spots I already mentioned, it became clear that Ceres is not as inactive and inert as
we may have previously thought. Before we delve deeper into that, let’s
first give you some context. Ceres is a very unusual body, seemingly out
of place in the asteroid belt. Most asteroids are mainly composed of non-volatile
substances, mainly rocks and metals. Ceres, on the other hand, has a similar composition
to that of a comet, it is, in other words, an icy world. However, being this close to the Sun, any
ice directly on the surface sublimates. This means that the surface crust is rocky,
yet porous, with water locked into the gaps, with a ratio of about 90% rocks and 10% water. Beneath the surface, there is believed to
be a muddy mantle and a large core of hydrated rocks, such as clays, where rock and brine
are mixed together at a 50:50 ratio, although this has been hard to confirm. Other models suggest the core could be a lot
drier and smaller, with a greater ratio of water to be found in the mantle. Either way, water is definitely present in
Ceres in large quantities, making up perhaps 50% of its total volume due to Ceres’ low
density. And it’s this water that perhaps renews
the surface of Ceres, albeit over extremely long timescales. You see, these bright spots are what is known
as cryovolcanoes. Unlike regular volcanoes, which spew lava
out from the mantle, cryovolcanoes erupt water. This is a 3D model based on Dawn data of the
biggest bright spot on Ceres. Water on Ceres is packed full of salt, meaning
that when a cryovolcano on Ceres erupts, the water sublimates, and the salt is left behind. This was directly observed over the brightest
patch on Ceres, known as Spot 5, as a haze was periodically seen over this area, indicating
that water there had sublimated. These bright spots darken over time from exposure
to the Sun through space weathering, so it’s likely that many old cryovolcanoes exist on
the surface of Ceres, although we can now only see the most active and most recent ones. However, the ones we can see aren’t just
limited to just the bright spots we’ve looked at so far, there are many of them dotted around
the dwarf planet. Water was discovered in other regions around
Ceres too. Due to Ceres’ very minimal axial tilt of
only 4°, some craters at its poles are in perpetual darkness. The bottom of these craters never see direct
sunlight, meaning water ice can exist here without sublimating. This is very similar to our Moon, where water
ice is also thought to have been trapped at the bottom of perpetually dark craters for
billions of years. What else did Dawn spot on the surface of
Ceres? As I mentioned, it saw plenty of craters,
each with unique characteristics. Some were very round and defined, others had
scarps along the crater floor. There were plenty of examples of complex craters,
with tall peaks in the centre. There was even evidence of crater rims collapsing,
for example, this rock having fallen away from the crater walls. Smaller rocks were also spotted having fallen
down crater walls, evidenced by the trails they left behind. I also love these photos taken at an angle,
pointed along the limb of the dwarf planet. To me, these give a much better idea of the
scale of the features we are looking at, although without an atmosphere to provide a sense of
depth, its hard to judge sizes in images like these. Fractures were also spotted all over Ceres,
indicating there have been stresses in the crust. Some of them are relatively young, perhaps
only a few hundred million years old. Some of them come in the form of grooves and
troughs, where the crust has been stretched, and other fractures can be seen in the form
of rows of mountains, where the crust has been compressed. One particularly unusual feature was spotted
on Ceres, called Ahuna Mons. It’s a mountain about 20km wide and 5km
high, but what’s unusual about it is how it just sticks up from the surrounding area,
with no apparent cause. It would be less unusual if there were other
features like this on Ceres, because then it could be said that it’s a global phenomenon,
but it’s the only thing like it on the entire world. The best bet we currently have is that it
is an old cryovolcano, formed because of a large impact directly on the other side of
the dwarf planet. Seismic waves from large impact can propagate
through the crust of a planet, and where the waves meet again on the opposite side is known
as the antipode. Known antipodal regions around the solar system
tend to have some kind of weird terrain, like that found on Mercury. Ahuna Mons could fit this description, with
seismic waves from an impact on the opposite side of Ceres triggering volcanic activity
here. In 2017, Dawn concluded its mission, having
been a tremendous success. It finally ran out of propellent, which means
it can no longer stay pointed at Earth to send back data or receive commands. It’s been left in a stable orbit around
Ceres, a monument in space that will remain there for at least another 20 years. Perhaps more missions will utilise ion engines
going forward after the success of this one, but in the meantime, Dawn’s discoveries
and data will be at the heart of asteroid research for many years yet. Thanks to Surfshark for sponsoring today’s
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so there’s no risk to try it out for yourself. Link is in the description below. Thanks for watching! If you liked this video, check out the other
one I made about Vesta during the first leg of this mission. Want to support Astrum? Check the links in the description to become
a Patron or a member. All the best! And see you next time.
