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On Those Ceres Organics

I set off an interesting conversation with a neighbor when organic material was detected on Ceres, as announced last year by scientists using data from the ongoing Dawn mission. To many people, ‘organics’ is a word synonymous with ‘life,’ which isn’t the case, and straightening that matter out involved explaining that organics are carbon-based compounds that life can build on. But organic molecules can also emerge from completely non-biological processes.

So with that caveat in mind about this word, it’s still interesting that organics appear on Ceres, especially since water ice is common there, and we know of water’s key role in living systems. A new paper looks again at data from Dawn, whose detections were made with infrared spectroscopy using its Visible and Infrared (VIR) Spectrometer. The instrument, examining which wavelengths are reflected off Ceres’ surface and which are absorbed, detected organic molecules in the region dominated by Ernutet Crater on Ceres’ northern hemisphere.

Image: Last year, the Dawn spacecraft spied organic material on the dwarf planet Ceres, largest denizen of the asteroid belt. A new analysis suggests those organics could be more plentiful than originally thought. Credit: NASA / Rendering by Hannah Kaplan.

The paper’s lead author is Hannah Kaplan, now a postdoc at the Southwest Research Institute. What Kaplan and team did was to contrast the Dawn data with laboratory spectra from both terrestrial and extraterrestrial organic materials, the latter derived from meteorites. Comparing these materials with known composition, the researchers looked anew at the Ceres spectra to gain a better picture of their composition and abundance. This analysis could help us make the call on the origin of these organics, whether natural to Ceres or delivered by an impactor.

When they contrasted the VIR data from Ceres with the laboratory reflectance spectra of organic materials formed on Earth, the scientists found that between 6 and 10 percent of the spectral signature on Ceres could be explained by organic material. But folding in comparisons with organic material from carbonaceous chondrite meteorites, the team found a spectral reflectance that differed from the terrestrial.

“What we find is that if we model the Ceres data using extraterrestrial organics, which may be a more appropriate analog than those found on Earth, then we need a lot more organic matter on Ceres to explain the strength of the spectral absorption that we see there,” Kaplan said. “We estimate that as much as 40 to 50 percent of the spectral signal we see on Ceres is explained by organics. That’s a huge difference compared to the six to 10 percent previously reported based on terrestrial organic compounds.”

As to the question of origins, the impact theory would seem to favor a cometary solution, comets being known to display higher abundances of organics than asteroids. The accompanying problem here is that a cometary impact would produce enough heat to destroy such organics. On the other hand, formation on Ceres itself is problematic, because other than the small patches in the northern hemisphere region already noted, organics do not appear.

“If the organics are made on Ceres, then you likely still need a mechanism to concentrate it in these specific locations or at least to preserve it in these spots,” said Ralph Milliken, an associate professor in Brown University’s Department of Earth, Environmental and Planetary Sciences and a study co-author. “It’s not clear what that mechanism might be. Ceres is clearly a fascinating object, and understanding the story and origin of organics in these spots and elsewhere on Ceres will likely require future missions that can analyze or return samples.”

Thus a major lesson: The results depend on what kind of organic material you use to make sense of the Ceres data. The comparison with extraterrestrial organics seems sensible, and it’s one we’ll doubtless invoke again as we move toward upcoming asteroid encounters. It’s worth noting in that regard that Kaplan has recently joined the teaming operating OSIRIS-REx. The spacecraft will arrive at asteroid Bennu in August of this year, while the Japanese Hayabusa 2 is expected to reach asteroid Ryugu in a matter of weeks.

The paper is Kaplan et al., “New Constraints on the Abundance and Composition of Organic Matter on Ceres,” Geophysical Research Letters 21 May 2018 (abstract).


{ 7 comments… add one }
  • Alex Tolley June 14, 2018, 14:34

    This research really highlights the need for more than spectroscopic data.

    I find it interesting that the Jean-PhilippeCombe paper “The surface composition of Ceres’ Ezinu quadrangle analyzed by the Dawn mission” which suggested that tholins were present on Ceres was not referenced.

    While the formation of kerogen’s is from living organisms, the sole reason for suggesting that they fit the Ceres spectral model is based on arguments against the more likely source of IOMs from chondrites due to the required IOM abundance, even though such asteroids are abundant in the asteroid belt.

    I would be very skeptical that kerogens are the organic source of that spectral signal. Hopefully, other analyses may shed a different light on the matter until we can explore this dwarf planet with sampling landers.

    Due to the paucity of missions due to costs, we never get inexpensive, off-the-shelf, equipment that can be added to a spacecraft. We are always designing something new, cutting edge, and inevitably very expensive. A tiny mass spectrometer to test samples could be added to any probe if there was sufficient demand. We will have to wait until costs are driven to low levels so that CubeSat descendants can be propelled and navigated quickly across the solar system to targets, with very small teams to manage them.

  • Robin Datta June 14, 2018, 16:35

    Organics continue to be conceptually associated with origins IN life even though Wohler’s conversion of ammonium cyanate to urea by heating
    dispelled that notion.
    The origins OF life, at least in the Earthly form, would indeed seem dependent on the presence of organics, but even a plethora of organics does not equate to the exquisitely tuned pathways invoked for those origins.

  • Geoffrey Hillend June 14, 2018, 16:47

    I don’t think the percentage volatiles is important. The the idea that chemical composition of the carbonaceous chondrites is similar to to Ceres and some asteroids is what is important. It’s no surprise that there is a higher percentage of volatiles on the surface of Ceres compared to carbonaceous chondrites if we consider how meteorites are formed. We get the high impact temperatures from the collision of meteors with asteroids, so there is some loss of volatiles as a result; meteorites are fragments of asteroids which have hit the surface of asteroids with high velocity and some of them reach escape velocity and are ejected into different orbit or collide with Earth. We also have Moon and Mars meteorites which have landed on Earth due to their escape velocity from high velocity impacts.

    A large collision will expose materials from deeper in the crust where there might be less volatiles. There is some differentiation in asteroids with the radio isotopes and heaver elements sinking to the center and the lighter ones rising to the surface.

  • Geoffrey Hillend June 14, 2018, 17:22

    Excuse me. The carbonaceous chondrites are the ones that have not undergone a lot of heating or formed by differentiation. I don’t think the high percentage of water in Ceres had to come from comets or Ceres migrated from the Kuiper belt. The water most likely came from the accretion disk like the carbonaceous chondrites. It’s a dwarf planet so its very small and it’s outside the life belt or behind the snow line so it kept most of its water.

    • Michael June 18, 2018, 3:15

      “It’s a dwarf planet so its very small and it’s outside the life belt or behind the snow line so it kept most of its water.”

      Near the core of Ceres the temperature and pressure is quite tolerable for life, so the life belt need not rely on the Sun.

  • Michael June 18, 2018, 9:45

    This a nice article about terraforming Ceres, the bit I am very interested in is the ammonia, there should be enough there to use as a fertiliser and the nitrogen component in atmospheres for habitats.


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