The NASA Dawn Probe's stunning findings around the dwarf planet Ceres and the protoplanet 4 Vesta. A complete recap of the Dawn mission.Discover our full back catalogue of hundreds of videos on YouTube: https://www.youtube.com/@astrumspaceFor early access videos, bonus content, and to support the channel, join us on Patreon: https://astrumspace.info/4ayJJuZ
Chapter 1: What is the Dawn spacecraft and its mission?
Let's start with Dawn itself. Its trip to Vesta took four years, utilising a slingshot from Mars' gravity along the way. And you'll be forgiven for not seeing anything special about this trip from this perspective. However, zooming in on the Dawn spacecraft itself reveals its very special feature, an ion engine.
Ion engines had been tested already on NASA's Deep Space One, however the way Dawn utilised it pushed this technology to a whole new level. Thanks to its ion engine, it was the first ever spacecraft to go into orbit around two separate extraterrestrial bodies. You see, while ion thrusters aren't very powerful, they are extremely efficient, and so can remain on for extended periods of time.
Unlike chemical thrusters, which rely on reactions causing heat and pressure to push gas away from the rocket, ion thrusters simply ionize neutral xenon gas with electricity to create acceleration. These ionized gas particles rush out of the engine at 150,000 km per hour, which pushes the spacecraft in the opposite direction. As the ionized gas is expended slowly,
Dawn can only create so much electric charge after all, the acceleration is really slow. It would take Dawn four days to accelerate from 0 to 100 km per hour. But over extended periods, it really adds up. Dawn was firing its thrusters for 85% of the time during the transit to Mars, expending only 72 kg of xenon propellant and gaining over 1.8 km per second velocity.
Upon reaching Vesta in July 2011, the images it started returning wowed the science community. It really was not what they were expecting. After being captured by Vesta's gravity, Dawn lowered its orbit to get a closer look at this unique asteroid. So, what made Vesta an unexpectedly nice surprise? The first thing you'll notice about Vesta is that it has an unusual shape.
It kind of looks like a squashed ball. There are two reasons for this. The first is that it is not very big. Yes, these asteroids, although big for asteroids, are pretty tiny on astronomical scales. Vesta is not quite big enough for it to be in hydrostatic equilibrium, or in other words, to be rounded by its own gravity, as it is only about 500km in diameter.
This gives it the surface area of Pakistan, about 800,000 square kilometres. You'll see how small it is if you compare it to our Moon, although it should be noted that even at this size, it still contributes towards 9% of the total mass of the asteroid belt, which can help you appreciate just how dispersed the asteroid belt really is.
The second reason for this unusual shape are the two giant impacts it experienced in its past. Estimated to have occurred over 1 billion years ago, Vesta was impacted not once, but twice around its south pole with planetary-scale objects. These impacts produced craters so large, they penetrated all the way to the mantle of the asteroid.
The crust has since cooled off and solidified, leaving a complex crater called Rhea Silvia. As these craters have overlapped, Rhea Silvia is the most recent and thus the most prominent crater that remains.
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Chapter 2: What did Dawn discover about Vesta?
Their structure and composition reveal some clues about how Vesta was formed, and the Dawn mission attempted to broaden that understanding. From a combination of all the data collected, it has been revealed that Vesta is very unique in our solar system. It is the only remaining rocky protoplanet, or in other words, it is a planetary embryo that never finished forming
The theory goes that as the solar system was forming, dust from the early protoplanetary disk coalesced into thousands of different planetesimals. These planetesimals collided with each other over time, building up into the large planets we see today. Our moon is thought to have formed from such an impact, with a large planetesimal impact in Earth, called Theia.
The debris from the collision coalescing in Earth's orbit, and over time, rounding under its own gravity to form the moon we know today. Vesta obviously got started on its way to becoming a planet. It experienced plenty of large impacts with planetesimals, As a result of all these impacts, the heat generated by them meant that at one point Vesta had an active mantle under the surface.
Even today, it is believed to still have a core of iron about 220km in diameter, a core similar to the other terrestrial worlds like Mercury, Venus, Earth and Mars. However, Vesta's interior has since cooled off, meaning the interior has solidified. But because of this differentiated interior, it likely would be called a dwarf planet today if not for those two collisions we talked about earlier.
Chapter 3: How does Vesta's shape differ from other celestial bodies?
One of the criteria for a dwarf planet is that it is rounded under its own gravity. But as the collisions happened roughly 1 billion years ago, a few billion years after Vesta formed, Vesta had already cooled off too much for it to be elastic enough to return to a shape in hydrostatic equilibrium. And the reason Vesta never became a planet?
Fingers are currently being pointed at Jupiter, which stole mass that would have otherwise formed Vesta, or at least disturbed enough of it to stop Vesta from ever getting going. Now, to the naked eye, Vesta does appear quite bland. This is a true colour image of Vesta, appearing as you would see it.
However, if you have a camera that can see in a wide variety of wavelengths of light, suddenly Vesta's true variety becomes apparent. In this composite image, the black material is likely ejecta brought by a large meteor impact. The red material is likely also from an impact, but is material that melted before solidifying again. Dawn also made some unexpected discoveries on Vesta's surface.
Vesta is thought to be very dry, with little to no volatiles found in its crust. Why then has evidence of past flowing been observed? In this false colour image, you can see a crater a couple of kilometres across, with a flow channel coming out of it, the different colours indicating it consists of a different material to the surrounding area.
The exact origin of this material is unknown, but perhaps it was brought by the impactor and melted on collision. Another fascinating discovery was found in one of Vesta's young craters, Marcia, Near the bottom of the crater, Dawn observed something called pitted terrain. Why pitted terrain was found on Vesta is a bit of a mystery, as we have only seen it on Mars before that.
However, scientists believe hydrated mineral rocks on the surface may have been rapidly heated, perhaps by another impact, releasing the water in the rocks, which exploded as the water degassed into space, leaving these craters you see here. And I would be remiss to mention that this Marcia crater is part of a chain of craters which makes up the famous snowman found on Vesta.
What is interesting though, is if you notice the terrain is relatively smooth around these craters. This is believed to be because a blanket of ejecta covered the region from the impacts, smoothing it over. Dawn was only around Vesta for a year before it left Vesta's orbit and moved on to the second leg of its journey towards the actual dwarf planet, Ceres.
Using its ion engine, it was able to build up speed until it was finally fast enough to escape Vesta's gravity altogether, and then begin its journey towards 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? After two 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.
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Chapter 4: What are the impacts that shaped Vesta?
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.
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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 recent ones. However, the ones we see aren't just limited to the bright spots we've looked at so far, there are many of them dotted around the dwarf planet.
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Chapter 5: Why is Vesta considered unique in our solar system?
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 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 a 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. Ahunamons could fit this description, with seismic waves from an impact on the opposite side of Ceres triggering volcanic activity here. If it is a cryovolcano, it actually has some analogues around the solar system.
It would be classified as a dome volcano, similar to Mount St Helens, or to some domes seen on Mars. In 2018, Dawn concluded its mission, having been a tremendous success. It finally ran out of propellant, which meant it can no longer stay pointed at Earth to send back data or receive commands.
It's been left in a stable, derelict orbit around Ceres, a monument in space that will remain there for at least another 20 years. Dawn's discoveries and data will be at the heart of asteroid research for many years yet, but it also leaves behind another legacy. Its ion engines were the key to its success, and there are now many other missions that currently use them.
SpaceX's Starlink satellites have ion thrusters on board, as well as China's Tiangong space station. ESA's BepiColombo mission will be using them to get to Mercury, and NASA's DART mission is using them to get to the binary asteroid system Didymos and Dimorphos. If you want to know more about that, in my opinion, intriguing mission, check out my video I made about it here. Thanks for watching.
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