Ceres is the largest asteroid at 595 miles across and considered a dwarf planet as it is large enough for gravity to compress it into a sphere. It was discovered on January 1, 1801 by Guiseppe Piazzi. Being the first asteroid discovered and the largest, it was only natural for it to become a target for a mission. NASA’s Dawn spacecraft blasted off on September 27, 2007 on a historic mission to orbit two asteroids using ion propulsion. Dawn successfully orbited Vesta from July 2011 to September 2012 revealing a complex world more closely related to Earth than the other planets. Dawn flew off to Ceres after great successes at Vesta.

Dawn’s approach to Ceres in early 2015 revealed a lone bright spot that can actually be seen with the Hubble Space Telescope. As Dawn grew closer, the spot seemed to glow until finally it became a cluster of bright spots. Dawn entered into orbit around Ceres on March 6, 2015 and the mystery of the bright spots would soon be solved. The original thought was that it was a patch of exposed ice. The idea was helped by the fact that the Herschel Space Observatory had faintly detected water vapor around Ceres in 2014. The bright spots covered too much area to be water ice, which ruled out geysers and cryovolcanoes. The patterns of the bright spots are complex, located in a 57-mile diameter crater called Occator, and were determined to be some kind of salt. It may be that the heating from the impact that created the crater caused the underground briny water to flow upward to the surface possibly through numerous cracks. The surface is so cold at -200ºF that the brine would freeze. 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) considering Ceres is more than 4.5 billion years old. It contains the largest area of highly reflective material on Ceres making it the brightest and most conspicuous on Ceres.

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 and is actually an ancient cryovolcano where water ice mixed with salts pushed up and sluggishly erupted from beneath the surface. The slow and steady process resulted in a towering dome built to impressive heights and laced with bright streaks. Ceres slowly cooled and the eruptions stopped. The eruptive salts and the cryovolcano are proof that Ceres was once an active world, and may still be to a lesser extent today. There is a combination of observed features that build a case for a volcanic dome. The summit of Ahuna Mons has cracks like those seen in uplifted lava domes on Earth. Also, the slopes have lines that resemble those formed by landslides, and the steep flanks surrounding the dome could be formed by piles of debris. Surface features on planets with little or no atmosphere like Ceres are eroded by asteroid and meteoroid impacts and take on a soft, rounded appearance. However, Ahuna Mons is sharp, with fine features like the debris from landslides that should fade with time. Also, older surfaces have a heavily pockmarked appearance from the accumulation of many impacts, but Ahuna Mons has few craters. Mountains tend to get broader as they erode and slump under gravity, but Ahuna Mons is narrow with steep slopes. Finally, surfaces tend to darken as they are exposed to radiation and meteoroid impacts, but Ahuna Mons is brighter than its surroundings, and could have formed within the last billion years and possibly within a few hundred million years. Ahuna Mons appears young on a geological timescale considering the Solar System is about 4.5 billion years old.

A young volcano on Ceres is surprising because Ceres is a small world, with a diameter about the width of Texas. Small bodies like this should quickly lose the heat from their formation, but Ceres still had enough heat to produce a relatively recent cryovolcano. Ice volcano activity is found elsewhere in the Solar System such as Saturn’s moon Enceladus, which has fountains of water-ice particles shooting from cracks in the icy crust at its south pole. Enceladus is even smaller than Ceres, but heat is generated inside it from flexing due to the gravitational pull of neighboring moons and Saturn. Ceres is an isolated world with no neighbor nearby to give it a significant gravitational tug, so it is unique. Ahuna Mons is the first cryovolcano that was produced by a brine and clay mix. The solid worlds in the Solar System are rockier and denser closer to the Sun, and less dense farther out. Ceres is interesting because it appears to be a transition object since it is not completely rocky or icy.

Dawn has detected water in the form of ice at Oxo Crater, a small, bright, sloped depression at mid-latitudes on Ceres. Exposed water-ice is rare on Ceres, but the low density of Ceres along with impact-generated flows and the cryovolcano, Ahuna Mons, suggest that the crust does contain a significant amount of water-ice. Impact craters are the dominant geological feature on Ceres and their different shapes explain the history of Ceres. Craters that appear not truly round and bounded by straight lines hint that Ceres crust is heavily fractured. Several craters have patterns of visible fractures on their floors. Some, like Oxo, have terraces, while others, such as the larger Urvara Crater, have central peaks. There are craters with flow-like features, and craters that imprint on other craters, as well as chains of small craters. Bright areas are peppered across Ceres, with the most reflective ones in Occator Crater. Some crater shapes could indicate water-ice in the subsurface. This gives evidence that Ceres is not purely ice or rock, but rather a mixture of both. Ceres could harbor as much water underneath as Lake Erie.

Ceres is a frigid world orbiting about 250 million miles from the Sun. When it is closest to the Sun, the warmest summer day at the equator is hardly 0ºF. In spite of the cold, Ceres may have a very thin atmosphere. Dawn observed evidence that Ceres had electrons accelerated to very high energies from the solar wind over a period of about six days. The interaction between the solar winds energetic particles and atmospheric molecules could explain these observations. A transient atmosphere would be consistent with water vapor that the Herschel Space Observatory detected at Ceres in 2012-2013. The electrons that were detected could have been produced by the solar wind hitting water molecules, but there could be other possibilities. If this sounds familiar, it is because a similar process occurs at Mercury. The atmosphere would be too thin, however, for haze or a blue sky, but it is still fascinating to realize that Ceres is not a totally dead world.

The possibility of Ceres still being slightly active has led to the decision to leave Dawn at Ceres instead of sending it to another asteroid. Dawn will remain in a stable orbit even after it ceases functioning later this year. In the meantime, it is capable of detecting any faint gas that erupts through the vents. Even if it never detects any geological activity, the Dawn mission will remain the greatest asteroid mission for a long time. No other spacecraft has ever orbited two asteroids and yielded so much science and discovery. The sceneries provided by Dawn are so intriguing that there are plans in the distant future for a land and/or rover mission to deepen our understanding of this fascinating, once obscure world.