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Ceres: Axial Tilt and Surface Ice

Earth’s axial tilt (its obliquity) is 23.5 degrees, a significant fact for those of us who enjoy seasonal change. The ‘tilt’ is the angle between our planet’s rotational axis and its orbital axis. If we look at Earth’s obliquity over time, we find a 41,000 year cycle that oscillates between 22.1 and 24.5 degrees. Here the Moon becomes useful, with recent studies showing that without it, Earth’s obliquity could vary by 25° (some earlier analyses took this number much higher).

Now we have new data from the Dawn spacecraft at Ceres relating the dwarf planet’s axial tilt to the locations where frozen water can be found on its surface. This is interesting stuff, because it depends upon the spacecraft’s ability to measure the world it orbits.

“We cannot directly observe the changes in Ceres’ orientation over time, so we used the Dawn spacecraft’s measurements of shape and gravity to precisely reconstruct what turned out to be a dynamic history,” says Erwan Mazarico, a co-author of a paper on this work based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

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Image: This animation shows how the illumination of Ceres’ northern hemisphere varies with the dwarf planet’s axial tilt, or obliquity. Shadowed regions are highlighted for tilts of 2 degrees, 12 degrees and 20 degrees. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

What we learn from the paper just published in Geophysical Research Letters is that in the last three million years, Ceres’ axial tilt has ranged from 2° to 20°. The last time of maximum obliquity of 19° was about 14,000 years ago, while its current tilt is just 4°, meaning seasonal effects over the course of a current Cerean year (4.6 Earth years) will be slight.

Charting Ceres’ obliquity allows researchers to examine which areas remain most deeply shadowed even during times of maximum tilt, and the current work, led by JPL’s Anton Ermakov, reports that craters that are shadowed during times of maximum obliquity show bright deposits that are most likely water ice. Ceres’ surface temperatures range from 130 to 200 Kelvin (-143° C to -73° C), but regions that rarely see sunlight are more likely to have ice deposits than sunlit areas where ice can sublimate directly into vapor.

Deeply shadowed areas at the poles never receive direct sunlight when Ceres’ axial tilt is as low as it is today — this is an area of about 2,000 square kilometers — but increasing obliquity reduces the shadow region to as little as 1 to 10 square kilometers. The researchers call craters with areas that stay in shadow over long periods of time ‘cold traps’ because volatiles that readily vaporize cannot escape once deposited there. We’ve already learned from Dawn that 10 such craters contain bright material, and one is already known to contain ice.

The northern and southern hemispheres have two persistently shadowed regions each at 20° tilt, and so far we have found bright deposits in three of the four. All of this should call up thoughts of the polar regions of the Moon, a body that has little variability in its tilt because of the influence of the Earth. Mercury, too, stabilized by its proximity to the Sun, shows little axial tilt, and on both objects, we are finding evidence of water ice in shadowed craters at the poles. As with Mercury, the Moon’s ice surely comes from the impact of asteroids and comets, whereas what we find on Ceres may, at least in part, come from the dwarf planet itself.

Remember that the European Space Agency’s Herschel Space Observatory found a tenuous atmosphere on Ceres several years ago, a possible source of water molecules that can accumulate in the cold traps. Meanwhile, note that Ceres’ axial tilt varies on a cycle of about 24,500 years, a figure researchers consider to be a surprisingly short time given the size of the variation. Ceres’ surface ice, then, gives us insight into its geological history as we continue to probe the question of whether the small body continues to give off water vapor.

The paper is Ermakov et al., “Ceres’s obliquity history and its implications for the permanently shadowed regions,” published online by Geophysical Research Letters 22 March 2017 (abstract).

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Comments on this entry are closed.

  • Hop David March 23, 2017, 15:21

    A very informative and interesting article. Thank you, Paul.

    There is a portion of Ceres’ orbit where it passes through the paths of Hilda asteroids. The Hildas spend a lot of their time in at their 5 A.U. aphelions in the neighborhood of Jupiter’s trailing or leading Trojans. I suspect at some of the Hildas are fallen Trojans. They’re cold enough to be volatile rich. So the Hildas might be an exogenic source of water for Ceres.

    Paul Spudis has talked about features on the lunar landscape that seem to be from outgassing. While Spudis seems to believe most of the frozen volatiles at the lunar poles come from exogenic sources, he suggests at least some of it comes from the moon itself.

    I am hoping to find images of Ceres with latitude lines marked. I want to do some images of Ceresian space elevators which of course would be anchored at Ceres’ equator.

  • Michael March 24, 2017, 6:03

    ‘I am hoping to find images of Ceres with latitude lines marked. I want to do some images of Ceresian space elevators which of course would be anchored at Ceres’ equator.’

    They can be anchored to any latitude and will naturally hang towards the equator.

    • James Stilwell March 25, 2017, 18:01

      What an incredible thought…
      Please think about it on our nearby moon…
      This is more than an article of curiosity…
      I’m so out of date…

      • Michael March 26, 2017, 4:30

        Perhaps if we put the ‘anchor’ on a rail system on the surface we could increase the speed of the ejection of payloads from the surface, they could then reach everywhere in the solar system. And yes it could be used at the water richer poles of the Moon, but the stresses get higher the further you move away from the equator.

    • Hop David March 25, 2017, 23:21

      Attaching to a higher latitude makes for a longer tether and more stress.

      Plus how would you lower a tether to a latitude other than the equator? Clarke and other elevator advocates usually recommend starting in synchronous orbit and unwinding tether both to greater heights and below to keep balanced from a synchronous orbit. Lowering a tether from synchronous orbit would extend the tether to the equator.

      • Michael March 26, 2017, 14:05

        Start at the equator as normal then move it to a new latitude or mining outpost by using a mobile anchor platform, as for the length it would not be much greater.

  • Dani Eder March 25, 2017, 11:14

    > while its current tilt is just 4°, meaning seasonal effects over the course of a current Cerean year (4.6 Earth years) will be slight.

    Ceres has an aphelion of 2.977 AU, and a perihelion of 2.558 AU. Therefore solar flux at perihelion is 35.4% higher than at aphelion. So it will still sustain significant seasonal effects, even at low axial tilt.

    We Earthlings don’t usually consider the orbit eccentricity effect, because solar flux only varies 6.9% on our planet, and axial tilt normally produces a larger effect. Aphelion this year will be on July 3rd, so the low flux on that date tends to compensate for northern axial summers. Our aphelion dates drift in a 21,000 year cycle, so sometimes solar flux will amplify northern summers rather than moderate them.

    For Ceres, we would want to consider both the cycle of tilts and orbit changes to understand the combined effect on surface conditions.

  • Scott Guerin March 27, 2017, 11:11

    I just discovered Ian Webster’s amazing visualization site. One of the models includes all known asteroids with searchable variables. http://www.asterank.com/ Works of science art!

  • ljk April 17, 2017, 19:02

    Dawn reveals Ceres formerly hidden ice:

    https://www.sciencedaily.com/releases/2017/04/170417181605.htm

  • ljk September 19, 2017, 13:29

    New research suggests Mercury’s poles are icier than scientists thought…

    https://news.brown.edu/articles/2017/09/mercury