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.
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Thanks for watching!
If you liked this video, check out the other one I made about Vesta during the first leg
of this mission.
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Check the links in the description to become a Patron or a member.
All the best!
And see you next time.