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A New Look at Ice on Ceres

Ceres, that interesting dwarf planet in the asteroid belt, is confirmed to be just as icy as we had assumed. In fact, a new study of the world, led by Thomas Prettyman (Planetary Science Institute), was the subject of a press conference yesterday at the American Geophysical Union fall meeting in San Francisco. Prettyman and team used data from the Dawn spacecraft’s Gamma Ray and Neutron Detector (GRaND) instrument to measure the concentrations of iron, hydrogen and potassium in the uppermost meter of Ceres’ surface.

Prettyman, who is principal investigator on GRaND, oversees an instrument that works by measuring the number and energy of gamma rays and neutrons coming from Ceres. The neutrons are the result of galactic cosmic rays interacting with the surface, some of them being absorbed while others escape. The number and kind of these interactions allows researchers to investigate surface composition. Hydrogen on Ceres is thought to be in the form of frozen water, allowing the researchers to study the global distribution of ice.

The result of the GRaND study: The elemental data show that the materials were processed by liquid water within the interior. The top layer of Ceres’ surface is hydrogen rich, with the higher concentrations found at mid- to high latitudes, a finding consistent with near surface water ice, with the ice table closest to the surface at the higher latitudes. Says Prettyman:

“On Ceres, ice is not just localized to a few craters. It’s everywhere, and nearer to the surface with higher latitudes. These results confirm predictions made nearly three decades ago that ice can survive for billions of years within a meter of the surface of Ceres. The evidence strengthens the case for the presence of near-surface water ice on other main belt asteroids.”

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Image: This image shows dwarf planet Ceres overlaid with the concentration of hydrogen determined from data acquired by the gamma ray and neutron detector (GRaND) instrument aboard NASA’s Dawn spacecraft. The hydrogen is in the upper yard (or meter) of regolith, the loose surface material on Ceres. The color scale gives hydrogen content in water-equivalent units, which assumes all of the hydrogen is in the form of H2O. Blue indicates where hydrogen content is higher, near the poles, while red indicates lower content at lower latitudes. In reality, some of the hydrogen is in the form of water ice, while a portion of the hydrogen is in the form of hydrated minerals (such as OH, in serpentine group minerals). The color information is superimposed on shaded relief map for context. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.

But we have no solid ice layer here. Instead, Ceres’ surface appears to be a porous mixture of rocky materials, with ice filling the pores, as this Institute for Astronomy (University of Hawaii) news release makes clear. The GRaND findings show about 10 percent ice by weight.

Also interesting is that the elemental composition of Ceres differs from CI and CM carbonaceous chondrite meteorites, which represent some of the most primitive, undifferentiated meteorites we know (Cl and CM are two of several different subgroupings within the carbonaceous chondrite family). These meteorites were also altered by water, but the GRaND data tell us that their parent body would have differed markedly from Ceres.

The researchers offer two explanations, the first being that large scale convection occurring within Ceres may have separated ice and rock components, leaving the surface with a different composition than the bulk of the object. The other possibility is that Ceres formed in a different location in the Solar System than the parent object of this class of meteorite.

A second paper on Ceres has also appeared, this one in Nature. It is the work of Thomas Platz (Max Planck Institute for Solar System Research, Göttingen) and colleagues, who focus on craters that are found in persistently shadowed regions. These ‘cold traps’ are cold enough (about 110 K) that little of their ice turns into vapor. Bright material found in some of these craters is thought to be ice, and Dawn’s infrared mapping spectrometer has indeed confirmed ice in at least one.

As is the case with the Moon and Mercury, ice in such cold traps is thought to be the result of impacting bodies, although solar wind interactions are also a possibility. Each of these bodies has a small tilt compared to its axis of rotation, producing numerous permanently shadowed craters. “We are interested in how this ice got there and how it managed to last so long,” said co-author Norbert Schörghofer (University of Hawaii at Manoa). “It could have come from Ceres’ ice-rich crust, or it could have been delivered from space.”

But the comparison between what we find on Ceres and elsewhere in the Solar System reminds us how much we still have to learn about the process. From the paper::

The direct identification of water-ice deposits in PSRs [permanently shadowed regions] on Ceres builds on mounting evidence from Mercury and the Moon that PSRs are able to trap and preserve water ice. For the Moon, the abundance and distribution of cold-trapped ice is little understood. On Mercury, the cold traps are filled with ice, and the planet traps about the same fraction of exospheric water as Ceres, so either the PSRs on Ceres are not able to retain as much water ice as those on Mercury or the amount of available water is much lower.

The Prettyman paper is “Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy,” published online by Science 15 December 2016 (abstract). The Platz paper is “Surface water-ice deposits in the northern shadowed regions of Ceres,” published online by Nature Astronomy 15 December 2016 (abstract). Video of the press briefing at the AGU meeting can be accessed here.

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Comments on this entry are closed.

  • Thomas Goodey December 16, 2016, 16:24

    “… Each of these bodies has a small tilt compared to its axis of rotation…”

    What does that mean?

    I guess, it means that the the axis of rotation of the body in question is almost orthogonal to the ecliptic plane… in other words, that the body is sitting straight up, not leaning over…

  • Hop David December 16, 2016, 16:56

    The existence of PSRs on Ceres seems to imply it’s axis of rotation is nearly perpendicular Ceres’ orbital plane about the sun. I’m wondering if this is the case. I have been wanting to know the tilt of Ceres’ axis as well as the time of summer and winter solstice and spring and fall equinox.

  • Albert A Jackson December 16, 2016, 21:24

    The Expanse (TV series) has a major part of the drama taking place on Ceres , a base and colony for asteroid mining. Looks as if Ceres has a supply of a needed resource.
    It is weird that the ‘Sharknado’ (SyFy) channel has a sophisticated solar system science fiction show , but it does. I recommend it to SF fans of the 1940 and 1950’s style of ‘solar system’ space opera , surprisingly good TV SF.

    • andy December 19, 2016, 11:56

      It’s based on a book series by “James S. A. Corey” (Daniel Abraham and Ty Franck). Would recommend the books, generally a fun read, not especially hard science fiction but good storytelling. It’ll be interesting to see how the TV series handles developments in later books.

  • Michael December 17, 2016, 5:54

    When I look at Ceres I can’t help but think of a giant fishbowl. All the materials are there for a massive colony in a low gravity environment.

    • Wojciech J December 18, 2016, 17:02

      I always thought Ceres is much better target for space colonization than Mars. Although I would put O’Neil habitats around it(or other type of space based colony) to provide living space and artificial gravity, and just use Ceres as resource/mining base.
      Based on these findings I think we could probably calculate how much water there is in theory, and how many people this could support(according to Wikipedia it is “200 million cubic kilometers of water, which would be more than the amount of fresh water on Earth”. It would also be interesting to know how deep the surface is before you reach the liquid water is.
      Obviously areas like the Occator with its bright Spos would be protected nature areas for tourists to visit and observe from afar ;)

      • Moebius December 19, 2016, 8:22

        Mars is better for many reasons, it has everything that Ceres has, plus:
        – an atmosphere, albeit a thin one but still more than nothing.
        – 0.38 Earth’s gravity (Ceres’ gravity is 0.028 G)
        – a 24ish-hour day (Ceres’ day is 9 hours)
        – an available sunlight per m² about 1/2.25 of the Earth’s (but more UV make it through the thin atmosphere, so the solar panels’ efficiency won’t get halved). Ceres is much farther and get less sunlight, requiring larger solar panels or nuclear power plants.
        – Although there are more launch windows to Ceres, the distance and thus the time to get there are very long, I didn’t do the math but I would guess something like one year at least with conventional rockets, compared to merely 6 months to go to Mars. If innovative propulsion technologies such as VASIMR make the trip to Ceres as short as 3 months, ok but that also means that with this technology, going to Mars will be much easier (and safer). We must also take into account that due to Ceres’ shallow gravity well, insertion (decelerating from interplanetary to orbital velocity) takes much longer than for Mars. Overall, that means a bigger rocket with more fuel and less payload, not exactly optimal to set up a base prior to the arrival of humans.

        • Michael December 19, 2016, 10:36

          The asteroid belt has more materials easily accessible than on Mars and the gravity is not such a big problem as very large interconnecting torus’s can be built on the surface. Yes it has less sunlight, is harder to stop at and has no atmosphere but for the long term it has much going for it.

  • Enzo December 17, 2016, 20:18

    I have heard that sending an orbiter quickly to Ceres it’s not as easy as, say, Mars because of the weak gravity making capture difficult. Of course Dawn did not have that problem because approached with a very small relative velocity. It also took years to get there.
    Otherwise it looks like a great candidate for a rover+sample return directly from the Occator crater. Both things have been done decades ago on the moon. Landing and taking off Ceres should be even easier.
    According to the “Sample return mission” page on Wikipedia, the Chinese were apparently studying something like that. : a pleasant change to the current Mars obsession that is taking funds from all other targets.

    • Wojciech J December 18, 2016, 17:03

      The Chinese study is just that, a study of the concept so far. I don’t believe it is more than than just analysis of how the mission could be done.
      But it would very interesting if they tried to retrieve a sample, and study it.As always the possibility of organic traces and is the elusive holy grail.

  • Dave Moore December 17, 2016, 21:33

    I attended a public lecture at Caltech on Nov. 9 by Carol Raymond, the Deputy Principle investigator of the Dawn mission and this is what she had to say about Ceres:

    The first point she made was that Ceres’s core is undifferentiated, unlike Vesta which has an iron core and rocky mantle. This implies Ceres formed a lot later than Vesta after the Al 26 from the nearby supernova had time to decay. Because of this and its composition, they think Ceres was originally a KBO or was formed in the outer solar system with a lot of KBO material.

    It is thought that its initial compensation was similar to Io’s or Enceladus with an icy crust over an ocean, but exposed ice sublimates away in under 100 years at Ceres’s distance from the sun, so it is thought that over the billennia, constant bombardment exposing fresh ice depleted Ceres’s ice cap and ocean resulting now in a surface of mixed ice, clay, regolith and alkaline brine that has the properties of concrete. (The salts on Ceres’s surface are very similar to the chemistry of Enceladus’s plumes.)

    If this is true, then exploring Ceres’s surface would give us an idea of what the ocean bottom chemistry of Io and Enceladus is.

    • Moebius December 20, 2016, 10:14

      Io has an ocean of magma under its crust, and no water ice on its surface. I guess you meant Europa.

  • Michael Spencer December 18, 2016, 7:49

    ” Ceres’ surface appears to be a porous mixture of rocky materials, with ice filling the pores”

    Paul: I wonder if the data could characterize the size or scale of the “inter’rock” area a bit more. In other words: just how big a ‘gob’ of water would one find, and the distribution of sizes?

    Are we talking about big frozen pieces of water the size of swimming pools or Lake Erie, in other words? Is the water even so a solution of fines, or is it predominantly H2O?

    So many questions.

  • RobFlores December 19, 2016, 12:25

    So do we think Jupiter’s gravity cause some tidal heating, when the
    sun-ceres-Jupiter alignment occurs. I understand the distances are great,
    but Jupiter is quite the heavyweight too. Does it have a role in keeping
    the deep core liquid, or is mostly radioactive materials and pressure?

    If it Ceres had remained a KBO object, would it be a good assumption that if it was part of the primordial solar nebula it would be frozen solid to the core. (maybe a little liquid reservoir would remain because of pressure)

    I suppose a base could be useful on Ceres, But the porous surface must
    be difficult to deal with. Thinking about making Igloos for habitats, but
    the impurities in the Ice would be make it a bit hazardous to cut ice blocks
    for construction. Your ice house would have to be quite thick to protect against GCRs 4 Feet thick would be a reasonable shielding for an extended
    stay. Of course your Base, should be of a centrifuge design inside this Igloo or you will get very similar ill effects as long duration ISS astronauts.

  • DJ Kaplan December 19, 2016, 13:19

    Apologies, but I must have missed where we discovered that Ceres has a liquid water ocean under its ice?

    • Alex Tolley December 19, 2016, 16:47

      I thought that it might once have had a liquid ocean, but that it has now frozen. If there is a liquid ocean beneath the surface, I have lost a bet!

    • Dave Moore December 19, 2016, 20:17

      The white spots on Ceres are thought to be exposed beds of NaCO3 and NH4Cl. These thick beds, to have formed, must have started off in an aqueous environment.

      As the ocean froze from surface down, the salinity of the ocean would have gone up until the ocean became saturated and the salts started precipitating out.

      Also, there is a lot ammoniated clays on the surface of Ceres, and these can only have formed in an aqueous environment.

      Ceres probably started out with a layer of ice at least 50km thick, which it lost over time. There is plenty of evidence that the regolith is hydrated in places. The is even evidence of exposed beds of ice, where crater sides have slumped away. I don’t think finding water (ice) just under Ceres’s surface is going to be a problem.

      What you are looking at is muddy ocean bottom that has been vacuum dried on the surface, but is still wet underneath.

      • DJ Kaplan December 20, 2016, 13:50

        Thank you for the explanation.
        So we are thinking that, Ceres was once located closer to the Sun, allowing liquid surface water? In that case, I’d say it warrants further exploration.

        • Dave Moore December 20, 2016, 16:52

          No. It had an ocean with a thick icecap above it. Think Enceladus or Io, but the water in the ocean gradually froze into ice, and the ice sublimated away, so all you have left is the core from the original ocean bottom down.