Surface, geology of Pluto studied via opposition observations
Nearly six years after NASA’s New Horizons spacecraft revealed close up images of Pluto to the world, researchers are teasing out more information about its geology and surface through ground-based observations and laboratory simulations.
The best time of year for ground-based observations is when a planet or moon is at opposition, meaning it is positioned directly opposite the Sun as viewed from Earth. In Pluto’s case, this occurs annually every summer in the northern hemisphere.
In July 2018, scientists took advantage of a rare opportunity, an opposition alignment of Pluto with Earth’s orbital plane. Because Pluto’s orbit is inclined to the plane of Earth’s orbit, known as the ecliptic, this exact alignment occurs only once every 161 years. During other oppositions, Pluto is located either above or below the ecliptic.
Objects at opposition, like the full Moon, receive maximum illumination from the Sun. Through a phenomenon known as “opposition surge,” these objects experience a rise in brightness that provides scientists an opportunity to learn about its surface compositions and textures.
Changes in brightness in various regions on these objects during opposition are caused by the different densities of surface materials.
“By looking at how much an object brightens when it gets full, you can tell something about the surface texture and what the surface is like — is it fluffy? Is it snowy? Is it compact?” questioned Bonnie Buratti, a member of the New Horizons science team who works at NASA’s Jet Propulsion Laboratory.
When Buratti took advantage of the 2018 alignment to view Pluto at opposition using the 200-inch Hale Telescope at Palomar Observatory in California, scientists were able to see Pluto and its large moon Charon resolved into two separate objects for the first time since the 2015 flyby.
In July 2019, Buratti repeated the opposition observations. While plans to observe during the July 2020 opposition were canceled by closure of the observatory due to the COVID-19 pandemic, observation of this year’s opposition will take place several times in June, July, and October. All will be done remotely.
This year, Buratti intends to conduct exact measurements of the opposition surge in an effort to learn how surface activity could be impacting Pluto’s brightening.
Images captured by New Horizons‘ cameras revealed floating glaciers; ices that sublimate into gases, then fall back onto the surface as snow, and even possible exchanges of these ices between Pluto and Charon.
“Pluto is much more active than we thought. We saw stuff we never saw before there,” Buratti said.
Together, images and data from the flyby and those obtained through ground-based opposition viewing give scientists a more complete picture of Pluto’s geological activity, she said.
A paper on the opposition surge study has been published in the journal Geophysical Research Letters.
In a second study aimed at a more detailed understanding of Pluto’s surface activity, a group of researchers at the Delft University of Technology in the Netherlands created tholins, organic compounds seen in the outer solar system, formed via interaction between solar radiation and surface methane and carbon dioxide, in a laboratory.
Tholins tend to have a reddish color. While Pluto is covered by an icy shell, a large area surrounding its equator, especially the whale-shaped region known as Cthulhu Macula, has a red color that could be produced via tholins.
New Horizons‘ detection of methane, nitrogen, and carbon dioxide in Pluto’s hazy atmosphere seemed to confirm that tholins are responsible for coloring the red areas.
Research team leader Marie Fayole produced tholins in a laboratory to compare the way they reflect light with the way Pluto reflects light, as revealed by the spacecraft.
Surprisingly, the researchers found the spectra of the two reflections to be slightly different.
The laboratory tholins, created by combining nitrogen, methane, and carbon dioxide in the same proportions found on Pluto, were irradiated with plasma to match solar radiation at Pluto. Yet following the experiment, the scientists found their tholins had different percentages of the three above gases than does Pluto’s surface. The laboratory tholins absorbed slightly more light than does the actual Cthulhu Macula region.
“From reconstructed reflectance spectra and direct comparison with New Horizons data, some of these tholins are shown to reproduce the photometric level reasonably well in the near-infrared. Nevertheless, a misfit of the red visible slope still remains, and tholins absorption bands present in the modelled spectra are absent in those collected by the New Horizons instruments,” the researchers wrote in a paper on their findings published in the journal Icarus.
These findings could mean tholins are not the only possible explanation for Pluto’s reddish terrains.
The researchers posed several theories to explain the discrepancies they found. One possibility is that galactic cosmic rays alter the way tholins on Pluto reflect and absorb light. Another is that the level of ice sublimation in these equatorial regions changes with Pluto’s seasons. While only small amounts of methane and nitrogen ice were found in these warmer regions, that could change in colder seasons, when both fall back to the surface as snow.
A third theory is that tholins on Pluto’s surface have a light, fluffy consistency resulting from Pluto’s relatively low gravity.
To better their understanding of the interactions between Pluto’s atmosphere and surface, the researchers plan to continue experimenting with artificially produced tholins.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.