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.
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).