Tantalizing Pluto views suggest active surface but won’t be seen again for 161 years
For a brief moment in July 2018, the solar system aligned to show Earth Pluto’s fully sunlit disk, an arrangement that will not happen again for 161 years.
Bonnie Buratti, a planetary scientist, was ready: She had been waiting for a decade for the chance to see the rare sight in the hopes of filling a knowledge gap that even the carefully planned New Horizons mission couldn’t complete. The result is an enigmatic plot of light from Pluto and its moon, Charon.
“We grasped this once-in-a-lifetime — well, it’s once in more than a lifetime, once in two centuries — opportunity to see Pluto fully illuminated,” Buratti, who works at NASA’s Jet Propulsion Laboratory in California and is the lead author on a new paper presenting the observations, told Space.com.
Observations of a solar system objects at and around its maximum illumination allow scientists to assess the “opposition surge,” a sudden increase in brightness of an object when it is completely illuminated that is disproportionate to the extra sliver of surface area illuminated.
And opposing surge isn’t just a strange optical effect: Scientists believe the pattern of the surge is influenced by the density of material on a world’s surface. “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?” Buratti said.
According to her, the opposition surge of the full moon is created by the loose, dusty regolith on its surface. “These particles cast shadows and those shadows rapidly disappear as the face becomes illuminated to the observer.”
The inclination of Pluto’s orbit compared to our own makes it impossible to capture a fully illuminated Pluto from Earth. Skywatchers use the word “opposition” to refer to the time of year when a certain solar system object appears in our skies opposite the sun. However, a celestial body does not always appear fully illuminated at that moment, particularly Pluto, which is frequently above or below the plane of Earth’s orbit around the sun.
Hence the 161 year wait for the next such opportunity and Buratti’s care to make a note of the 2018 opportunity so far in advance. She and her colleagues agreed to use the Palomar Observatory’s 200-inch Hale telescope in San Diego. The Hale telescope is equipped with an optics system that could pick apart Pluto and Charon, rather than seeing one blob of the two worlds.
In addition, the team captured super-illuminated views of Pluto on July 12, 2018, as well as additional days that month and in July 2019. More observations were scheduled for last summer, but the COVID-19 epidemic forced the observatory to shut.
The researchers are still not allowed to use the telescope in person, but Buratti and her colleagues have scheduled observations. She expects that this data will shed light on how Pluto’s opposition surge occurs, providing scientists with the information they need to comprehend what is happening on the ground to generate the visual effect.
Buratti believes the huge increase in opposition is connected to how active a planet the New Horizons spacecraft discovered during its 2015 flyby. “Pluto is much more active than we thought,” she said. “We saw stuff we never saw before there.”
It’s the first solar system world known to have glaciers, for example. Ices vaporize and refreeze, occasionally migrating between Pluto and Charon. “There might be snow on it [Pluto], there’s a lot of frost moving around, it might have a really fluffy, textured surface.”
Although the very near alignment of spacecraft flyby and full illumination from Earth is just an accident, Buratti believes the combination of data demonstrates how planetary research thrives when ground-based and space-based technologies are used in tandem.
During its movement, New Horizons saw night-side and halfway illuminated views of Pluto, but it couldn’t see the fully illuminated disk owing to the trajectory of its brief flyby. “You combine it with these ground-based observations and you have the full thing, you have the full package,” she said. “These are totally complementary.”
And for Buratti, the new discoveries are a poignant reminder of her own work during the New Horizons flyby, because scientists rarely get to use equipment powerful enough to differentiate between Pluto and Charon while observing the pair from Earth.
“We saw Pluto and Charon separately for the first time since the encounter,” Buratti said. “It’s kind of emotional for me … Here’s a thing that was just a point of light, and then in a day or two it becomes this geologic world and you feel like you’re there. It’s an intimate thing. But then it goes back to being a point again.”
The findings were published in the journal Geophysical Research Letters.