Nineteen Years Later, the Pluto Resistance Continues

 

Nineteen years. That is how much time has passed since the controversial IAU vote in 2006. Planetary science has been in a long-term stalemate over the issue of planet definition. The IAU refuses to re-open the debate based on new information, including that returned by New Horizons. At the same time, a large number of planetary scientists are using the geophysical planet definition and ignoring the IAU altogether.

The conference “Progress in Understanding the Pluto System: Ten Years Since Flyby” focused heavily on the planetary characteristics of not just Pluto, but also Eris, Ceres, Makemake, Neptune’s moon Triton, Saturn’s moons Enceladus and Titan, and Jupiter’s moon Europa.

It also included presentations on the persistence of scientists in terms of Pluto’s discovery and in terms of getting the New Horizons mission off the ground.

The term “planetology,” now another name for planetary science, was first coined by the late Percival Lowell, who began the search for a planet beyond Neptune early in the last century.

In 1915, Lowell published “Memoir on A Trans-Neptunian Planet” outlining the reasoning behind his planet search—supposed anomalies in the orbits of Uranus and Neptune (later proven to be in error). Having founded the Lowell Observatory in 1894 to search for evidence of life on Mars, Lowell purchased an astrograph to capture wide-angle images of the sky and a blink comparator to switch back and forth between two images of the same part of the sky in an effort to find something that moved against the background stars.

Ironically, while Lowell died in 1916 thinking his search failed, Pluto appeared in photographic plates of the sky he took in 1915 but was not recognized and thought to be one of many background stars.

Twenty-four-year-old Clyde Tombaugh, hired by the Lowell Observatory in 1929 to continue the search, used the astrograph to image photographic plates and the blink comparator to compare images of the same parts of the sky taken several days apart. Within a year, he discovered Pluto.

It took until 1988, when Pluto was imaged using CCD cameras, for scientists to discover it has an atmosphere.

Similarly, getting New Horizons to launch was another protracted, long-term effort. Principal Investigator Alan Stern noted that the idea of a Pluto mission was first raised in 1989, after Voyager 2 flew by Neptune. Five separate mission proposals were made and subsequently canceled. But the scientists who advocated a Pluto mission, nicknamed the “Pluto Underground,” refused to quit. When the Johns Hopkins University Applied Physics Laboratory (JHUAPL) was finally awarded the mission, they were given only a few years to build the spacecraft and its instruments to fit the necessary launch window.

“Pluto is the story of perseverance, of not taking no for an answer, of saying, ‘yes I can,” emphasized Lowell Observatory historian Kevin Schindler at the conference. “This is one of the reasons that inspires so many people about Pluto.”

Today, we are in yet another situation that requires perseverance, both in terms of getting a better planet definition that restores Pluto and all dwarf planets to full planethood, and in the effort to return to the Pluto system with an orbiter.

Two days ago, the publication Morning Overview reignited hope by publishing an article titled, “Why Pluto Could Be Reclassified as A Planet Again.” The article cited recent scientific studies that reveal Pluto to share many characteristics with the solar system’s larger planets, including a complex atmosphere, geological activity, and varied surface features, such as mountains, valleys, and plains.

“Pluto is unique in many ways. Its size and composition are comparable to other planets in the solar system, albeit smaller. It has a rocky core surrounded by a mantle of water ice, and its surface is covered in nitrogen ice, with traces of methane and carbon monoxide. This composition is not unlike that of terrestrial planets, further blurring the lines between Pluto and its larger counterparts,” the article noted.

The article goes on to mention the possibility of Pluto having a subsurface ocean and the uniqueness of Pluto-Charon as a binary system, the only one in our solar system.

Acknowledging the efforts by so many, not just scientists, the article credits popular culture for keeping the notion of Pluto as a planet alive for close to two decades.

“Popular culture, through media and educational systems, has maintained Pluto’s image as a planet…Grassroots movements and public interest can significantly influence scientific decision-making, as seen in past scientific debates and reclassifications.”

And there we have it. Our efforts have kept this debate and Pluto’s status as a planet alive in both culture and science. Though it may not always seem that way, our efforts are making a difference.

The article goes on to list the benefits of Pluto being recognized as a planet, which include more scientific research and funding to study Pluto and similar objects as well as renewed searching for hard-to-find planets both in our solar system and others.

“As discussions continue, the potential reclassification of Pluto as a planet remains an intriguing possibility that could reshape our understanding of the Solar System,” the article concludes.

Pluto’s story is one of perseverance, and that continues today. Regardless of how much time has passed since the IAU vote, we today need to call upon that perseverance and stick with the effort to undo the travesty of 2006 and gain a better planet definition. Never give up. Never, never, never give up.

Pluto's Hazes and Charon's Geology: Notes from the July Conference

 

The conference “Progress inUnderstanding the Pluto System: Ten Years Since Flyby” covered a wide range of topics regarding Pluto, Charon, other KBOs, ocean worlds, and more. With apologies for the delay in this entry, I will focus now on two of these topics: Pluto’s atmospheric hazes and Charon’s geology and history.

Launched in late 2021, the James Webb Space Telescope (JWST) observed Pluto’s atmosphere in infrared wavelengths. Other than New Horizons, no instrument has been able to study Pluto’s atmosphere because it is so cold. These hazes, which likely have a high ice content, likely control Pluto’s climate and keep its upper atmosphere cool.

While the spectrum of Pluto’s hazes somewhat resembles the spectrum of the atmosphere of Titan, Saturn’s large moon, Titan’s atmosphere has much less ice than Pluto’s. The sublimation and condensation of ices plays a significant role in Pluto’s atmosphere but not in Titan’s.

New Horizons observed layered hazes covering all of Pluto. While both Pluto and Titan have nitrogen in their atmospheres, Pluto’s also contains carbon monoxide. Given that Pluto is farther from the Sun than Titan, it is understandable that the former’s atmosphere is colder than the latter’s.

Several of New Horizons’ instruments observed Pluto’s atmospheric hazes, which are believed to form through photochemistry of methane and nitrogen. These hazes vary with Pluto’s seasons and during its elliptical solar orbit, as well as with the Sun’s 11-year cycle. When Pluto gets closer to the Sun, Pluto’s volatile ices sublimate and are lifted into its atmosphere.

For Charon, New Horizons’ LORRI, MVIC, and LEISA instruments were used to image the planet and create a mosaic making up 80 percent of its surface. Topographic features were best seen when the Sun was low in Charon’s sky while reflectivity features were best seen when the Sun was high.

Though Charon’s non-encounter hemisphere was imaged in low resolution, topographic features such as massive canyon systems, moated mountains, mottled terrain, and alternating areas of smooth and rocky terrain could still be seen. There are many impact craters but not enough to saturate the surface. Patterns of bright and dark ejecta not seen anywhere else in the solar system are visible on Charon’s surface.

These features suggest Charon has a volatile crust as well as strong layering beneath its surface. All of them indicate a long ago freezing of the planet’s ice shell. This freezing of what was once a subsurface ocean happened very early in the system’s history, before Pluto and Charon became tidally locked to one another.

Charon has about 200 scarps or steep cliffs that are taller than the Grand Canyon on Earth. These might be the largest canyons relative to planet size in the entire solar system.

While Pluto has both young and old surface terrain, Charon’s surface is approximately two billion years old. Large craters on both objects were likely created by ancient impacts of KBOs approximately the size of Arrokoth. Charon’s craters contain a record of impactor populations, which created its large craters. Few small craters have been seen on Charon’s surface. Impactors that hit Charon excavated material from its subsurface.

New Horizons’ instruments showed that water ice is ubiquitous on Charon’s surface. Observations by JWST revealed the presence of carbon dioxide, hydrogen peroxide, and ammonia diluted in water on Charon’s northern hemisphere. Significantly, Jupiter’s moon Europa also has carbon dioxide in its spectrum. Impactors that hit ancient Charon appear to have exposed what had been subsurface carbon dioxide.

What is especially significant is that Charon’s surface appears to have accurately preserved its formation and impact history.

The red spot visible on Charon’s north pole is comprised of tholins, complex organic molecules found in space created by the interaction of ultraviolet sunlight with methane, nitrogen, and water on an object’s surface or in its atmosphere. These tholins originate from methane that escapes Pluto’s atmosphere that is then gravitationally captured by Charon. Only Charon’s nighttime temperatures are cold enough for methane to condense on its surface. Most of this methane is converted to ethane; only ten percent is processed into sticky surface materials.

Like Pluto as well as Titan, Europa, and other solar system bodies, Charon may have once been an ocean world. The freezing of that ocean likely caused expansion of surface features once the planet’s internal heating shut down. Features such as canyons, rifts, and fractures, all seen on Charon, have also been observed on other icy moons that may have subsurface oceans.

There is some evidence Charon may once have experienced cryovolcanism, such as shallow fractures and landslides, which are seen on elsewhere in the solar system on Ceres, Europa, and Uranus’s moon Ariel. Charon’s north appears to be heavily cratered while other areas are composed of smoother cryovolcanic planes.

These factors reveal Charon to have once been one of a growing number of ocean worlds in the solar system. While Charon’s ocean has since frozen, Pluto’s, like those of Europa, Titan, Ceres, Saturn’s moon Enceladus, Jupiter’s moon Ganymede, and Neptune’s moon Triton could still exist in liquid form. While no signs of microbial life have yet been found on any of these worlds, the presence of liquid water and organic compounds raises the possibility that these oceans could harbor such life. This is one of many reasons we need further exploration, especially by instruments that can drill through the ice layers and study the liquid below.