Bright spots on Ceres

• 1. Introduction

  ›

  Bright spots on Ceres appeared in images the Dawn spacecraft sent back from the dwarf planet in 2015. Tiny points of light stood out across an otherwise dark, cratered surface. Nothing else on Ceres reflected sunlight so strongly.

  Scientists studying the pictures did not yet know what the light was: ice, salt, volcanic activity, or something else entirely. What the cameras had actually captured, and why one crater held the brightest cluster of all, would take years of analysis to explain.

• 2. A Mysterious Glow

  ›

  During the early orbital phase of the Dawn mission, scientists speculated that the spots' high albedo came from some kind of outgassing. Albedo measures how much sunlight a surface reflects. Closer images taken as the spacecraft's orbit tightened showed a material with a high level of reflection. Ice and salt both emerged as leading candidates.

  Once measured directly, the bright features showed an albedo of about 40 percent, four times brighter than the average of Ceres's surface. That reflectivity ruled out plain rock as an explanation, but it still left the question of composition wide open.

• 3. Salt Left Behind

  ›

  On the 9th of December 2015, scientists reported that the bright spots were probably tied to a salty brine. That brine was a form of hydrated magnesium sulfate, written as MgSO4·6H2O. The same readings linked the spots to clays rich in ammonia.

  Researchers at Arizona State University proposed a specific sequence for how the spots formed. Briny water erupted from Ceres's interior, reached the surface, and then sublimated away, leaving only the salt deposits behind.

  Near-infrared spectra of the bright areas matched a large amount of sodium carbonate, with smaller traces of ammonium chloride or ammonium bicarbonate. Scientists suggested these minerals crystallized recently, as brine reached the surface from below.

  In August 2020, NASA confirmed that Ceres was a water-rich body holding a deep reservoir of brine. That reservoir percolated up to the surface at several locations, including inside Occator crater, producing the bright spots. That crater's scale and position on Ceres would turn out to be just as striking as the brine deposits inside it.

• 4. Occator's Center

  ›

  Occator crater stretches about 80 kilometers across and sits at 19.86 degrees north latitude, 238.85 degrees east longitude. The brightest cluster of spots inside it, called Spot 5, has its largest and most luminous point at the crater's center. Dimmer spots trail off toward Occator's eastern rim.

  The central point of Spot 5 was named Cerealia Facula. The dimmer group to the east became known as Vinalia Faculae. The International Astronomical Union approved both names on the 26th of November 2016.

  One planetary scientist would soon offer a detailed account of how Occator produced that glow in the first place.

• 5. Explaining The Glow

  ›

  Alan Duffy, an astronomer at Swinburne University, offered one detailed account of how the bright spots might have formed. He described the process this way: 'a meteorite strike either shook covering material off the salty ice or heated it so that salty water rose to the surface as a geyser. The water escaped into space and now, only the salt remains.'

  A haze appears periodically around Spot 5, filling around half of Occator crater without spilling past its rim. That pattern added credence to the idea that some kind of outgassing or volcanism was still occurring.

  Andrew Rivkin, an asteroid specialist, raised a related point in an article for Sky & Telescope magazine. He noted that at low viewing angles, a haze appears inside the crater but not outside it. Rivkin speculated that this could be vapor sublimating from ice, possibly linked to the bright spots.

  Reflectivity studies conducted in September 2015 pointed toward salts rather than ice as the source of the glow. That finding implied Ceres's interior was somehow still delivering fresh salt up to the surface.

  Dawn's pictures of the spots led to widespread coverage in news outlets, astronomy magazines, and science magazines. NASA even ran an informal poll in May, asking the public to guess the spots' nature: ice, volcanoes, geysers, salt deposits, rock, or something else. Those salt findings settled what covered Occator's floor, but where inside Ceres that salt actually came from still needed tracing.

• 6. What Lies Beneath

  ›

  Further analysis of the Dawn spacecraft's low-altitude data pointed to two separate sources feeding the bright spots. One was the melting of crustal ice caused by the impact that formed Occator crater itself. The other was a deeper brine reservoir, made up of sodium carbonate or ammonium chloride, sitting near or inside Ceres's mantle.

  Hanami Planum, a raised plain near Occator, presented a puzzle of its own. The plain registered as a topographic high but a gravity low, an unusual mismatch. Scientists read that mismatch as isostatic compensation, most likely caused by buried ice underneath the plain.

  That buried ice beneath Hanami Planum suggested Ceres was still holding frozen water far from Occator's bright center. Scientists had barely begun to map that hidden reservoir.