Rick Robinson

Jan 4th 2021

Ceres: From Asteroid to Ocean World


When Italian astronomer Giuseppe Piazzi sighted an unknown celestial object on the night of January 1, 1801, he first thought that he’d started out the new year by discovering a comet.

That “comet” would later become known as the asteroid Ceres.

As Piazzi continued his observations, he grew doubtful about his comet theory. According to NASA, Piazzi wrote that, “owing to its lack of nebulosity, and to its motion being so slow and rather uniform, I feel in the heart that it could be something better than a comet, perhaps.” Moreover, the orbit fit neatly into the wide gap between Mars and Jupiter — a gap where astronomers already suspected another planet might lurk.

Piazzi named Ceres after the ancient Roman goddess of agriculture, NASA also explains, and it was briefly hailed as the eighth planet of the solar system, even though it was much smaller and fainter than its celestial neighbors Mars and Jupiter.

From Planet Ceres to Asteroid Ceres

Over the next few years, though, astronomers discovered other bodies, smaller even than Ceres, with similar orbits between Mars and Jupiter. Along with Ceres, they came to be regarded as less than full-fledged planets, and as reports, all of them collectively were dubbed the asteroid belt. Pigeonholed as simply the largest asteroid, Ceres was somewhat ignored for the next 200 years.

Meanwhile, history was repeating itself in the far outer solar system. Pluto, discovered beyond Neptune in 1930, was regarded as the ninth planet for decades, even though it was much smaller than any other planet or several large moons (including Earth’s).

But starting in the late 20th century astronomers discovered several other Pluto-sized objects in the far outer system. Finally, in 2006, astronomers defined a new class of astronomical body, dwarf planets, smaller than the major planets but larger than any mere asteroid.

Pluto, demoted to a “dwarf planet,” became a cause celebre for schoolkids (who had one less planet to memorize). Hardly anyone but astronomers noted that the object formerly known as the asteroid Ceres was then promoted to a dwarf planet at the same time.

Getting Melty

The distinction between dwarf planets and asteroids is all about geology and how they were formed. The formation process of any large astronomical body produces heat, as smaller particles or chunks bump together and coalesce. If the object grows large enough — about 600 miles across — this heat becomes so intense that it melts the interior. Denser materials sink to the core, while less dense materials float above them.

Ceres is the one and only object in the asteroid belt that grew large enough to undergo this melting and layering process. Thus, it is the only asteroid to be reclassed as a dwarf planet.

As Astronomy magazine notes, not long after this reclassification, NASA launched the Dawn space mission to explore the two largest members of the asteroid belt, Vesta and Ceres. Results from the mission confirmed that Ceres — but not smaller Vesta — underwent the melting and layering process that marks a dwarf planet.

But the Dawn mission findings have done much more than that. The melting and layering of Ceres was expected. What no one expected, says Sky & Telescope, was that the reclassed dwarf planet would turn out to be an ocean world.

The Briny Deep

The most conspicuous surface feature on Ceres, imaged by the Dawn probe, is a crater 57 miles across named Occator (for a minor Roman harvest god who assisted Ceres). And one of the most distinct things about Occator, per Astronomy, is bright areas on its surface.

These have been identified as salt beds. And like salt beds on Earth, they indicate the former presence of water, even if the water has long since evaporated. But in a new twist, according to, closer examination has shown that Ceres is still geologically active, with “cryovolcanoes” that erupt with steam and liquid water instead of lava.

The impact that formed the Occator crater took place about 20 million years ago, and by now, the heat from the impact should have dissipated enough for melted subsurface ice to freeze again. However, the presence of active cryovolcanoes suggests that cracks in Ceres’ crust allow liquid brine (extremely salty water) to push up from the mantle layer below.

Putting it another way, below its crust of dusty rubble, much of the interior of Ceres is a very salty sea. The high salt content of the brine pushes its freezing point far below zero degrees Centigrade, allowing it to remain liquid even at Ceres’ large distance from the sun (nearly three times Earth’s) with a frigid surface temperature of about -130 degrees F.

As one researcher, Julie Castillo-Rogez, notes in a National Geographic article, Ceres’ subsurface ocean is “definitely not for scuba diving — it’s like a big swamp” full of very cold, salty mud. But while Ceres may never become a popular diving spot, the building blocks of life could have taken form in the environment of early Ceres.

From comet to asteroid to potential abode of life, the solar system’s innermost dwarf planet has undergone an extraordinary journey of discovery in the two centuries and two decades since Giuseppe Piazzi first spotted it.