Garrett P. Serviss was a writer whose name has been obscured by time, but in his day, which would be the late 19th and early 20th Century, he was esteemed as a popularizer of astronomy. He began with the New York Sun but went on to write fifteen books, eight of which focused on the field. He was also a science fiction author whose Edison’s Conquest of Mars (1898) used Wellsian ideas right out of War of the Worlds. It was a sequel to an earlier story, involving master inventor Thomas Edison in combat against the Martians both on the Martian surface and off it.
I think this is the first appearance of spacesuits in fiction. In any case, Serviss anticipates the ‘space opera’ to come, and will always have a place in the history of science fiction. I can only wonder what he would have made of the Dawn mission at Ceres. In Edison’s Conquest of Mars, he refers to a race of ‘Cerenites’ who are, because of the low gravity of their world, about forty feet tall. They are at war with the Martians, and thus allies of Earth.
Image: Author and science popularizer Garrett P. Serviss.
Forgive the digression, but I’m always interested in the work of people who bring science to the public, and it’s clear that Serviss’ interest in Ceres would have been profound, as works like Other Worlds: Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries (1901) demonstrate. From Other Worlds, this look at Ceres and the asteroid belt as understood at the turn of the 20th Century:
The entire surface of the largest asteroid, Ceres, does not equal the republic of Mexico in area. But Ceres itself is gigantic in comparison with the vast majority of the asteroids, many of which, it is believed, do not exceed twenty miles in diameter, while there may be hundreds or thousands of others still smaller—ten miles, five miles, or perhaps only a few rods, in diameter!
Hundreds or thousands indeed. Another Serviss book, Astronomy with an Opera-glass (1888), holds up pretty well even today. Another thing I like about him is that in his science fiction, he writes scientists into the plot. Not just Edison, but Edward Emerson Barnard and Wilhelm Röntgen, to name but a few.
A New Look at Occator
But enough of early science fiction and on to Ceres today. Even before Dawn, we had made recent strides, nailing Ceres’ rotation rate, for example, by 2007, at 9.074170 hours. Those odd bright spots on the surface were evidently picked up in 2003 Hubble measurements, which showed a spot that moved with Ceres’ rotation, and water vapor plumes erupting from the surface have been reported using Herschel data, suggesting possible volcano ice geysers.
Now, of course, we’ve homed in on the bright spots in Ceres’ Occator crater, and the speculation about their origin continues. Are we looking at a relatively recent impact crater that has revealed bright water ice? Cryo-volcanism or icy geysers have also been suggested. As of today, we still don’t know whether the bright spots are made of ice, or evaporated salts, or some other material. A new study using the HARPS spectrograph at La Silla (Chile) has just appeared, showing us that whatever they are, the bright spots show changes over time.
Lead author Paolo Molaro (INAF–Trieste Astronomical Observatory) explains the method:
“As soon as the Dawn spacecraft revealed the mysterious bright spots on the surface of Ceres, I immediately thought of the possible measurable effects from Earth. As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth.”
Image: The bright spots on Ceres imaged by the Dawn spacecraft.
The changes Molaro’s team has found are subtle — remember that nine-hour rotation rate, which means the motion of the spots toward and away from Earth is on the order of 20 kilometers per hour. But the Doppler shift caused by the motion can be revealed by an instrument as sensitive as HARPS. Over two nights of observation in July and August of 2015, the researchers found the spectrum changes they expected to see from this rotation, but they also recorded random variations from night to night that came as a surprise.
The paper lays out a possible cause: The bright spots could be providing atmosphere in this region of Ceres, which would confirm the earlier Herschel water vapor detection. Pointing toward this conclusion is the fact that the bright spots appear to fade by dusk, an indication that sunlight may play a significant role, possibly heating up ice just beneath the surface and causing the emergence of plumes:
It is possible to speculate that a volatile substance could evaporate from the inside and freezes when it reaches the surface in shade. When it arrives on the illuminated hemisphere, the patches may change quickly under the action of the solar radiation losing most of its reflectivity power when it is in the receding hemisphere. This could explain why we do not see an increase in positive radial velocities, but all the changes in the radial velocity curves are characterized by negative values. After being melted by the solar heat, the patches can form again during the four-hours-and-a-half duration of the night, but not exactly in the previous fashion, thus the RV curve varies from one rotation to the other. It is possible that the cycle of evaporation and freezing could last more than one rotational period and so the changes in the albedo which are responsible of the variations in the radial velocity.
As this ESO news release points out, the volatile substances evaporating under solar radiation could be freshly exposed water ice, or perhaps hydrated magnesium sulfates. The spots are brightest in the morning when their plume activity reflects sunlight effectively, but as they evaporate, the loss of reflectivity produces the changes analyzed in the HARPS data. We can add to the mix a recent paper in Nature (see below) that reports localized bright areas consistent with hydrated magnesium sulfates and probable sublimation of water ice.
So the mysterious bright spots of Ceres continue to tantalize us. As the paper notes, the fact that we find several bright spots in the same basin (Occator crater) seems to point to a volcano-like origin, but that would imply that an isolated dwarf planet can be thermodynamically active. Such activity is no surprise in large satellites around the gas giants, where tidal effects are assumed to play a major role, but its sources on Ceres will need further investigation.
The paper is Molaro et al., “Daily variability of Ceres’ Albedo detected by means of radial velocities changes of the reflected sunlight,” Monthly Notices of the Royal Astronomical Society Vol. 458, No. 1 (2016), L-54-L58 (abstract). The Nature paper is Natues et al., “Sublimation in bright spots on (1) Ceres,” Nature Vol. 528 (10 December 2015), 237-240 (abstract).