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Description

Here is 4 Vesta, the second largest planet in the Asteroid Belt between Mars & Jupiter. In this sketch, based on a photo imaged by the Dawn spacecraft in July 2011, we’re looking down on the Rheasilvia basin on Vesta’s south pole. An impact event one billion years ago blasted out one of the Solar System’s largest craters, and the rebound from the blast produced perhaps the Solar System’s largest mountain--slightly taller than Olympus Mons on Mars. The debris from that impact produced 15,000 or so smaller asteroids which share a similar orbit, and many stony meteorites are thought to trace back to this impact event as well.

Can’t see the basin? Because it’s so large, it’s quite subtle and shallow. At 500 kilometers across, its diameter is 87% that of Vesta itself. Almost hilariously, Rheasilvia is covering up another basin which is nearly as large, the Veneneia basin. Over 50% of Veneneia is covered by Rheasilvia. Veneneia is between 2 and 4.2 Billion years old, is 395 kilometers across, 70% the diameter of Vesta, and can be barely seen slightly below Rheasilvia in this image.

You’ll notice at the beginning of this post I called Vesta a planet. Perhaps the term “Planetoid,” “Asteroid,” or “Minor Planet” would be more suitable, but the truth is that for the most part planetary scientists do not differentiate between planet, dwarf planet, asteroid, satellite, etc.* Vesta has a lot of planet-like properties--it’s got an iron-nickel core, a rocky mantle, and a distinct crust. It even seems to have tectonic activity, like the Moon and Mercury. Only about four or five asteroids are internally differentiated this way. Vesta would count as a “dwarf planet” at least, were it not for the Rheasilvia basin having been dredged out of it, preventing it from remaining rounded.

*spicy planet opinions under the cut.
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*The number of planets in the solar system is basically arbitrary, and the notion of only 8 or 9 is much more recent than you’d think. After all, why are the satellite planets (as moons have long been called) not counted? Why are the gas giants and the terrestrial worlds grouped into the same collection? Depending upon how you define planet, the solar system could have four, eight, thirteen, 33-ish, several dozen, hundreds, or millions. Planet shouldn’t be a rigidly defined term, and as far as I can tell, planetary scientists generally do not agree with or use the 2006 IAU definition of a planet.

Description

Here is 4 Vesta, the second largest planet in the Asteroid Belt between Mars & Jupiter. In this sketch, based on a photo imaged by the Dawn spacecraft in July 2011, we’re looking down on the Rheasilvia basin on Vesta’s south pole. An impact event one billion years ago blasted out one of the Solar System’s largest craters, and the rebound from the blast produced perhaps the Solar System’s largest mountain--slightly taller than Olympus Mons on Mars. The debris from that impact produced 15,000 or so smaller asteroids which share a similar orbit, and many stony meteorites are thought to trace back to this impact event as well.

Can’t see the basin? Because it’s so large, it’s quite subtle and shallow. At 500 kilometers across, its diameter is 87% that of Vesta itself. Almost hilariously, Rheasilvia is covering up another basin which is nearly as large, the Veneneia basin. Over 50% of Veneneia is covered by Rheasilvia. Veneneia is between 2 and 4.2 Billion years old, is 395 kilometers across, 70% the diameter of Vesta, and can be barely seen slightly below Rheasilvia in this image.

You’ll notice at the beginning of this post I called Vesta a planet. Perhaps the term “Planetoid,” “Asteroid,” or “Minor Planet” would be more suitable, but the truth is that for the most part planetary scientists do not differentiate between planet, dwarf planet, asteroid, satellite, etc.* Vesta has a lot of planet-like properties--it’s got an iron-nickel core, a rocky mantle, and a distinct crust. It even seems to have tectonic activity, like the Moon and Mercury. Only about four or five asteroids are internally differentiated this way. Vesta would count as a “dwarf planet” at least, were it not for the Rheasilvia basin having been dredged out of it, preventing it from remaining rounded.

*spicy planet opinions under the cut.
--------------------------------------

*The number of planets in the solar system is basically arbitrary, and the notion of only 8 or 9 is much more recent than you’d think. After all, why are the satellite planets (as moons have long been called) not counted? Why are the gas giants and the terrestrial worlds grouped into the same collection? Depending upon how you define planet, the solar system could have four, eight, thirteen, 33-ish, several dozen, hundreds, or millions. Planet shouldn’t be a rigidly defined term, and as far as I can tell, planetary scientists generally do not agree with or use the 2006 IAU definition of a planet.