Ceres is an ancient, crater-dominated world. It is the largest of the asteroids, also known as minor planets, at 588 miles in diameter. It is also classified as a dwarf planet along with Pluto because it is round. It was the first asteroid discovered, on January 1, 1801, by Guiseppe Piazzi. It orbits the Sun beyond Mars in the main asteroid belt at an average distance of 257 million miles. Ceres may seem dull, but it became an attractive target for the orbiter mission known as Dawn due to its large size; it makes up 30% of the volume of all known asteroids. Stunning revelation from the mission elevated Ceres into an elite class of worlds worthy of in-depth exploration with the possibility of a sample return mission.

NASA’s Dawn spacecraft blasted off on September 27, 2007, on a historic mission to orbit two asteroids within the asteroid belt. Dawn successfully orbited Vesta from July 2011 to September 2012 and then flew off to Ceres, which was nothing more than a blurry dot. Dawn’s approach to Ceres revealed a lone bright spot that was resolved into a cluster of white spots as Dawn grew closer. NASA’s Dawn spacecraft entered orbit on March 6, 2015, where it photographed and mapped it until it ran out of fuel in 2018. The bright spots were originally thought to be exposed patches of water ice because the Herschel Space Observatory faintly detected water vapor around Ceres in 2014, but the bright spots were too large to be water ice, which ruled out geysers and cryovolcanoes. The pattern of the bright spots was complex, located in a 57-mile diameter crater called Occator, and were determined to be some kind of salt. There is evidence that heating from the impact that created the crater caused underground briny water to flow upward to the surface and ooze through numerous cracks. A fine mist was detected close to the surface within the crater. It is possible that puddles of briny water can freeze at -200ºF. Solid ice would sublimate when exposed to the vacuum of space or settle elsewhere on Ceres leaving behind the dissolved salts. Occator is a young crater, hardly 80 million years old, compared to Ceres at 4.5 billion years old. It contains the largest area of highly reflecting material on Ceres making it the brightest feature and an expansive area of salt from deep within Ceres. The crust is around 25 miles thick composed of water-rich material; about 40% of its volume could be water, enough to fill Lake Erie. These discoveries are what classify Ceres as an Ocean World. These worlds include Europa, Ganymede, Enceladus, Titan, and Pluto, which may still have interiors warm enough where water remains as a liquid beneath an icy crust. Ceres has a lot of carbonates and ammonia on its surface along with brine and potentially a lot of organics. All of this concentrated in one area and dredged up from the interior makes it readily accessible to sample/analyze with no drilling into the crust necessary, plus Ceres is much closer to Earth than Europa and Enceladus. The possibility of a sample return mission is too great to pass up and has put Ceres on the list of recommended medium-class (~$1 billion) missions for the future.

The Ceres Sample Return mission concept, which is now recommended as a priority for NASA’s New Frontiers program, would bring organic material from another Ocean World back to Earth for the very first time. The bright regions are evidence for sodium carbonates - compounds of sodium, carbon, and oxygen - also found on Enceladus, which are a marker of a habitable environment. Landing on Ceres would not be difficult as it is a small, airless world with low gravity. No fancy airbags or sky cranes are needed as the spacecraft would use rocket power all the way down to the surface. The spacecraft that would land in Occator would be a combination orbiter, lander, and ascent module, which would return the samples. The orbiter would be equipped with a high-resolution camera, a thermal imager, and radar. The lander would contain a sampling arm, a camera, and an on-board gas chromatograph mass spectrometer. The ascent module would contain vessels for four surface samples, amounting to a maximum of a few precious ounces. Upon return to Earth the samples would be characterized using high-precision analyses to understand the salt and organic composition from Occator and the habitability and evolution of Ceres as a relic Ocean World from the dawn of the Solar System.

Ceres has another unique feature located along the equator which stands out all by itself, a lone mountain, about 2.5 miles high, known as Ahuna Mons. It is the tallest feature on Ceres, an ancient cryovolcano where water mixed with salts pushed up and sluggishly erupted from beneath the surface. The slow and steady process resulted in a towering dome laced with bright streaks. The eruptive salts and towering dome are proof that Ceres was once an active world and may still be to a lesser extent today.

The main reason to return to Ceres is to explore the salt deposits since the sodium, calcium, carbonates, and ammonia salts are consistent with solid residue expected from alkaline brines formed by carbonaceous chondrite interacting with warm fluids. The source of these fluids beneath Occator is likely a shallow subsurface reservoir as part of a larger underground ocean from long ago. The detection of ammonium chloride on the surface indicates that the ocean is rich in ammonia and/or chloride from a water-rich interior. As water is one of the prerequisites for life as it is known on Earth, investigating these salt deposits gives an opportunity to investigate a potential habitable niche and may help in understanding the variety of potential habitats in the Solar System. Another reason to return to Ceres is to determine its original position as it is high in ammonia, which is volatile and does not exist as liquid at Ceres’ present location relatively close to the Sun. It is possible that Ceres formed in the outer Solar System before migrating inwards. The return to Ceres would also involve collecting soil outside of the salt deposits to analyze the organics and composition of the bulk of Ceres itself. The same planetesimals that created Ceres may have been the building blocks for Earth, which is the main reason for studying the organics.

Dawn answered questions about Ceres but did not find why Ceres’ spectral features do not match any known meteorite groups, nor can the location of its formation be pinpointed to a specific region. How did Ceres form and did it play a role in delivering water and organics to proto-Earth? Ceres appears to contain three of the prerequisites for life: water, carbon, and energy. Ice bodies like Ceres could represent a widespread astrobiologically-favorable niche.

Exploring Ocean Worlds is tough with distance being the villain. Europa and Enceladus take nearly a decade to reach, landing on them is even tougher. Powerful radiation around Jupiter makes landing on Europa challenging and such a mission is decades away. Enceladus has a better chance for landing and sampling as it has active plumes, weaker gravity, and less radiation, but it is a billion miles away. The proposed Enceladus Orbilander is decades away if it is approved once the Uranus orbiter mission is under development and launched. Ceres has everything going for it for an enriching mission; it is closer to Earth, low gravity, no atmosphere, and the salt deposits are already accessible on the surface. A sample return mission is easier than Mars with no atmosphere and lower gravity for the return launch of the samples. All that is needed is the approval and funding for a relatively low-cost mission that could have samples back on Earth during the 2040s. Ceres will contribute greatly to our knowledge of the origin and evolution of Earth and how Ocean Worlds like Earth work.