In 1801, astronomers discovered the first body in the Asteroid Belt, a region in the Solar System between the orbits of Jupiter and Mars. Called Ceres, it was initially, and understandably, classified as an asteroid. Today, more than 200 years later, it's status is actually that of a dwarf planet — it is much bigger than its asteroid companions.
Ceres' unique status also underlies why it was the first dwarf planet to get a visit from a spacecraft with NASA's Dawn mission, which arrived at the strange world in 2015.
Now, five years later, the mission has sent back high-resolution images of Ceres that reveal new, exciting details that suggest it was volcanically active a million years ago — and could be active today.
The new discoveries were detailed in seven different studies published Monday in the journals Nature Communications, Nature Geoscience, and Nature Astronomy.
The Dawn spacecraft gave astronomers an unprecedented look at a massive impact site on Ceres, the Occator Crater. The crater stretches across 50 miles; scientists believe that it formed around 22 million years ago.
Dawn flew above the crater at a distance less than 22 miles above the surface. Previous observations of the area had suggested that some sort of geological activity at the crater site that brought saltwater to the surface.
The latest observations showed that the liquid came from a deep reservoir of brine, or salt-enriched water, and that the brine reservoir is about 25 miles deep and hundreds of miles wide — a global, salty ocean that, possibly, could still be escaping to the surface.
"Dawn accomplished far more than we hoped when it embarked on its extraordinary extraterrestrial expedition," Marc Rayman, Dawn's mission director at NASA's Jet Propulsion Laboratory, explains. "These exciting new discoveries from the end of its long and productive mission are a wonderful tribute to this remarkable interplanetary explorer."
Geologically active — The subsurface brine observed is thought to be a result of cryovolcanic eruptions. This activity is linked to a type of volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock, and is considered a phenomenon of the outer solar system.
The observations suggest that cryovolcanism began just 9 million years ago on the dwarf planet and that it was likely still active up until a million years ago. In fact, the researchers believe that Ceres may occasionally be active until today.
This type of geological activity typically takes place on icy moons, driven by their host planets' gravitational pull. However, this is the first observational data of geological activity on other, ice-rich bodies.
“The evidence for very recent geological activity on Ceres contradicts the general belief that small solar system bodies are not geologically active," Guneshwar Thangjam, a researcher at India's National Institute of Science Education and Research, and lead author of one of the new papers said in a statement. "This has paved a new direction in planetary science exploration toward understanding conditions of the very early solar system as well as having implications for habitability.”
Dawn also helped identify a certain type of salt that has previously only been found on Earth, and now on Ceres. The finding suggests that the process by which the salt was deposited on the surface is still active till today, which may explain why the dwarf planet is relatively warm as it maintains small pockets of liquid water.
A water-rich, icy body — Ceres takes 1,682 Earth days, or 4.6 Earth years, to make one trip around the sun. Additionally, the dwarf planet takes nine hours to complete one full rotation around its axis.
Although it is the largest body in the asteroid belt, Ceres' radius is only 296 miles across. The dwarf planet may make up 25 percent of the asteroid belt's total mass, but its tiny planetary counterpart Pluto is still 14 times more massive.
Scientists have found evidence of water on Ceres, and are therefore suspect that the dwarf planet may sustain some form of life.
In 2007, NASA sent the Dawn spacecraft to Ceres to study its composition, surface, and how it evolved over time. The spacecraft got to Ceres in 2015 and retired in November 2018. After finishing its observations of the dwarf planet, Dawn went into an uncontrolled orbit around Ceres and is still flying around it until today.
Given the new results from Dawn, scientists are looking into sending future missions to further explore Ceres.