While looking up information about the recently discovered Tamu Massif in the Pacific Ocean, I was surprised to learn that Olympus Mons is not the tallest Mountain in the solar system. I was even more surprised to learn that the mountain was not even located on a planet.

Vesta, one of the three largest asteroids in the asteroid belt between Mars and Jupiter, is smaller than any of the planets. It is second to Ceres, the dwarf planet that is also found in the asteroid belt, and more massive than the other two asteroids that contain the bulk of the mass of the belt itself, Pallas and Hygeia. It is named after the Roman goddess of hearth and home. So let talk, a little, about Vesta.

Is Vesta Actually a Planet Posing as an Asteroid? The asteroid Vesta was first discovered two centuries ago, but until NASA’s Dawn spacecraft arrived there and began beaming back images and data, Vesta was seen as just another blurry, rocky satellite out there orbiting in the asteroid belt.

Now, with Dawn’s instruments giving researchers their first really good look at Vesta’s composition and surface features, some astronomers are wondering if perhaps they haven’t discovered a small terrestrial planet rather than an asteroid.

The distinction between “dwarf planet” and “asteroid” is a bit nebulous and ultimately up to the International Astronomical Union. But in a post here we noted that the composition of Vesta is actually very similar to Earth’s–with separated layers like a core, mantle, and crust–suggesting that at one point in Vesta’s past it had a hot molten core that caused heavier materials (like iron) to sink inward while lighter materials flowed to the top.

This process is called differentiation, and it occurred on all of the terrestrial planets (Mercury, Venus, Earth, and Mars) in our solar system. Topographic characteristics like huge mountains and valleys, ridges, craters, and plains are lending further credence to this analysis. All told, Dawn researchers believe Vesta formed more like a terrestrial planet than the average asteroid.

That formation would’ve happened about 4.5 billion years ago at the same time the rest of the planets were forming. As Jupiter gathered mass, the gravity it exerted on the asteroid belt interfered with the coalescing of material there. Vesta was probably on its way to becoming a terrestrial planet before its growth was stunted. Still, it differentiated in its early life just like a planet, and the Dawn team is now looking for volcanoes and lava flows that it thinks were once present on Vesta’s surface (not to belabor the point, but the presence of volcanoes would be super Earth-like).

All that points to the possibility that Vesta could be reclassified as a dwarf planet, joining the likes of Pluto and Ceres–another asteroid belt dweller already classified as a dwarf planet and the a research target for Dawn starting in 2015.





It is on this asteroid that the solar system’s tallest mount was discovered. There are two massive impact craters on Vesta, themselves some of the largest to be found in our solar system. They encompass much of the surface area of Vesta, plunging down into the asteroids interior, differentiated like Earth’s own interior.

And it in in one of these impact craters that the mountain is to be found.

The crater is called Rheasilvia, named for the mother of the mythical twins Romulus and Reemus who founded the city of Rome. It is 505 kilometers in diameter, almost as wide as the diameter of the asteroid itself at 525 kilometers. In the center of the crater rises a mound to a height of 22 kilometers/14 miles.

Olympus Mons still has credit for being the tallest volcano, as astronomers are still uncertain exactly how this central mound was formed. It could have been push back from the interior after it formed, volcanic activity following the impact, or a number of other theories. We won’t know for sure until later exploration. Which will be long in coming, with NASA’s ever shrinking planetary budget.