A new study by Curtin University researchers has shed light on the early days of Vesta, the second largest asteroid of the main belt and the fourth such object to be discovered.
As NASA’s Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Dawn studied Vesta from July 2011 to September 2012. The towering mountain at the south pole — more than twice the height of Mount Everest — is visible at the bottom of the image. The set of three craters known as the ‘snowman’ can be seen at the top left. Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA.
Discovered by Heinrich Wilhelm Olbers on March 29, 1807, Vesta is the only main-belt asteroid visible to the unaided eye.
It rotates once in 5.34 hours and orbits the Sun in 3.63 years. It has an ellipsoidal shape with radial dimensions of 286 x 279 x 223 km.
Due to its large size, Vesta is believed to be a differentiated body with a core and a mantle just like our own planet.
Collisions between asteroids in the belt enable them to leave their orbits and travel great distances in our Solar System, potentially colliding with other planetary bodies.
“Vesta is of tremendous interest to scientists trying to understand more about what planets are made of, and how they evolved,” said Professor Fred Jourdan, lead author of the study.
“Vesta is the only largely intact asteroid which shows complete differentiation with a metallic core, a silicate mantle and a thin basaltic crust, and it’s also very small.”
“In a sense it’s like a baby planet, and therefore it is easier for scientists to understand it than say, a fully developed, large, rocky planet.”
Vesta was visited by NASA’s Dawn spacecraft in 2011, when it was observed that the asteroid had a more complex geological history than previously thought.
With the aim of hoping to understand more about the asteroid, Professor Jourdan and colleagues analyzed well-preserved samples of volcanic meteorites found in Antarctica that were identified as having fallen to Earth from Vesta.
“Using an argon-argon dating technique, we obtained a series of very precise ages for the meteorites, which gave us four very important pieces of new information about timelines on Vesta,” Professor Jourdan explained.
“Firstly, the data showed that Vesta was volcanically active for at least 30 million years after its original formation, which happened 4.565 million years ago. While this may seem short, it is in fact significantly longer than what most other numerical models predicted, and was unexpected for such a small asteroid.”
“Considering that all the heat-providing radioactive elements such as aluminium-26 would have completely decayed by that time, our research suggests pockets of magmas must have survived on Vesta, and were potentially related to a slow-cooling partial magma ocean located inside the asteroid’s crust.”
“The research also showed the timeframes when very large impacts from asteroids striking Vesta were carving out craters of ten or more kilometers deep from the asteroid’s volcanically active crust,” said Dr. Trudi Kennedy, co-author of the study.
“To put this into perspective, imagine a large asteroid smashing into the main volcanic island of Hawaii and excavating a crater 15 kilometers deep — that gives you an idea of what tumultuous activity was happening on Vesta in the early days of our Solar System.”
Scientists further explored the data to understand what was happening deeper in the asteroid by calculating how long it took for Vesta’s deep crustal layer to cool down.
Some of these rocks were located too deep in the crust to be affected by asteroid impacts, and yet, being relatively close to the mantle, they were strongly affected by the natural heat gradient of the protoplanet and were metamorphosed as a result.
“What makes this interesting is that our data further confirms the suggestion that the first flows of erupted lava on Vesta were buried deep into its crust by more recent lava flows, essentially layering them on top of each other. They were then cooked by the heat of the protoplanet’s mantle, modifying the rocks,” Dr. Kennedy said.
The team also concluded that the meteorites they analyzed were excavated from Vesta during a large impact, possibly 3.5 billion years ago, and were agglomerated deep into a rubble pile asteroid, where they were protected from any subsequent impacts.
A rubble pile asteroid is formed when a group of ejected rocks assemble under their own gravity, creating an asteroid that is essentially a pile of rocks clumped together.
“This is very exciting for us because our new data brings lots of new information about the first 50 million years or so of Vesta’s early history, which any future models will now have to take in to account,” Dr. Kennedy said.
“It also raises the point that if volcanism could last longer than previously thought on the protoplanet, then maybe volcanism on the early Earth itself might have been more energetic than we currently think.”
The findings were published in the March 2020 issue of the journal Geochimica et Cosmochimica Acta.
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F. Jourdan et al. 2020. Timing of the magmatic activity and upper crustal cooling of differentiated asteroid 4 Vesta. Geochimica et Cosmochimica Acta 273: 205-225; doi: 10.1016/j.gca.2020.01.036
