Want Salt on Your Ceres?

Those mysterious white spots on asteroid Ceres could be outcrops of salt. How did vast amounts of salt get out there?

Surprise findings from the DAWN spacecraft team generated excited headlines:

• Mysterious bright spots on Ceres are probably salt (Nature)
• New Clues to Ceres’ Bright Spots and Origins: Ice also transforms to water vapour in the dwarf planet’s craters, creating an enigmatic haze. (Astrobiology Magazine)
• Ceres reveals its salty secrets – and blurs the line between comets and asteroids (Monica Grady at The Conversation)
• Dwarf planet Ceres may harbor clouds of water ice (Science Magazine)
• Mystery Solved? Ceres’ Bright Spots Likely Made of Salt (Space.com)

The findings are announced in two papers:

• Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres (Nature)
• Sublimation in bright spots on (1) Ceres (Nature)

The numeral (1) before the name indicates that Ceres is the #1 largest asteroid. The salts are in the form of magnesium sulfates, similar to Epsom salt. There are also ammonia-rich clays.  At least, that’s consistent with the spectra. Nature points out they are found in at least 130 spots, indicating widespread occurrence. As for the water ice haze, Nature suggests Ceres is not cold and dead: “Some kind of geological process seems to continually feed ice to the surface, replenishing what is lost.” The scientists covered the bases about where the salt compounds came from:

Our measurements indicate widespread ammoniated phyllosilicates across the surface, but no detectable water ice. Ammonia, accreted either as organic matter or as ice, may have reacted with phyllosilicates on Ceres during differentiation. This suggests that material from the outer Solar System was incorporated into Ceres, either during its formation at great heliocentric distance or by incorporation of material transported into the main asteroid belt.

Compound explanations are problematic in the philosophy of science (“either the dog ate my homework, my brother stole it, or I forgot to do it”). They can serve as placeholders for ignorance and guidelines for modelers, but do not constitute understanding at this point. This is clear from another comment in the paper:

Ceres’ surface enrichment in ammoniated phyllosilicates and relatively low bulk density pose significant challenges to understanding its origin and formation.

The other paper says that Ceres is blurring the line between comets and asteroids.

Ceres is the first identified large body in the main asteroid belt showing not only primitive Solar System material, but also comet-like activity. Our results are consistent with widely distributed subsurface water or ice and water activity in the main asteroid belt, supporting the more recent view of a Solar System with a continuum in composition and ice content between asteroids and comets.

Parts of Ceres’ surface are as dark as charcoal, but the spots are very bright. Monica Grady explains the significance of these surprises:

Dawn has found a number of mysterious features on Ceres so far, including bright white spots on its surface. Its latest results suggest that these are salts left behind as ice vaporised from the surface by sublimation – a process often seen in comets. They also suggest Ceres may have formed far away from its current location in orbit between Mars and Jupiter. This would be surprising as many astronomers believe that a key difference between comets and asteroids is that asteroids form closer to the sun.

Undoubtedly these puzzles will require revisions to textbooks, while providing job security for planetary scientists for years to come.

Negative Knowledge

Planetary scientists are gaining increased understanding of one thing: how the solar system did NOT form. That’s the title, in fact, of an article by Kleomenis Tsiganis in Nature: “How the solar system didn’t form.” Interestingly, he first repeats a myth then debunks it.

About 4.5 billion years ago, the Solar System formed in a disk of gas and dust particles that surrounded the newly born Sun. The ‘giant’ planets (Jupiter, Saturn, Uranus and Neptune) formed first, within the few million years of the disk’s lifetime. Closer to the Sun, the small, rocky ‘terrestrial’ planets (Mercury, Venus, Earth and Mars) took tens of million years to form, by collisions of numerous smaller objects generated in the disk. Myriad small bodies formed the asteroid belt between the orbits of Mars and Jupiter. Despite decades of attempts, no computational realization of standard formation theories has reproduced the mass and orbital distribution of both the terrestrial planets and the asteroids. Writing in the Monthly Notices of the Royal Astronomical Society, Izidoro et al. show that this is not possible.

He explains how that paper shows all models failing to get the asteroids right. But that’s not all. “Standard planet-formation models have been unable to reconstruct the distributions of the Solar System’s small, rocky planets and asteroids in the same simulation,” the sub-headline says. “A new analysis suggests that it cannot be done.” So not only do planetary scientists fail to understand how to fit the pieces together today, it’s unlikely they ever will.

There is hardly a body in the entire solar system that has not surprised planetary scientists. Their predictions are almost always wrong. Their models are wrong and will always be wrong. Are the planets and small bodies trying in vain to tell secular astronomers something about reality?