Towering Haze, Flowing Volatile Ices On Pluto
Now 20 days after New Horizons' closest approach to Pluto, the science team has been treating us to one tantalizing revelation after another, and Friday's press conference was no different. With only 5% of the fly-by's data down from the spacecraft, Alan Stern and his co-investigators reported that the activity on Pluto's surface and in its atmosphere is once again greater and stranger than we could have expected.
The visual jewel of these new morsels of data is a LORRI image taken by the spacecraft after it had passed Pluto, when it turned itself around to look at the night side of Pluto eclipsing our Sun. The result is incredible:
Pluto's night side, imaged as it eclipses the Sun from New Horizons' perspective. The amount of visible haze stunned the atmospheric team. Credit: NASA/JHUAPL/SwRI
Michael Summers, Co-Investigator, said that this image "stunned the encounter team," and brought tears to the eyes of some of the atmospheric scientists. It's not just gorgeous, it's also a crucial scientific find: the image obviously shows a thick haze layer in Pluto's atmosphere, created by dry particles that are absorbing passing sunlight. On Pluto, these particles are suspected to be aerosolized hydrocarbons like ethylene, acetylene, and hydrogen cyanide, created from atmospheric nitrogen and methane by interactions with ultraviolet energy from the Sun (more on that here).
It was speculated before the fly-by that Pluto may have haze layers, but it wasn't known for sure, or whether they would be substantial. It turns out that Pluto's haze layers extend at least five times higher than they had predicted -- at least 100 miles high. Summers called this "a huge surprise," and conceded that the reason the layers are so much larger is still a mystery.
Yet another atmospheric surprise was the pressure of the atmosphere through which these haze-generating particles travel. Pluto's atmosphere is thought to be in constant flux. Pluto's orbit around the Sun is very eccentric, meaning there is a large difference between its perihelion (point closest to the Sun) and aphelion (point furthest from the Sun). Over the 248 years that it takes Pluto to make one orbit around the Sun, it's expected that this great difference in distances from the Sun, and hence in the amount of solar energy received, would cause dramatic changes to the atmosphere. Pluto reached perihelion in 1989, and since then has been getting further from the Sun in its orbit, causing surface temperatures to drop.
When New Horizons was being developed, it was feared that, by the time it arrived, Pluto's atmosphere would've gotten so cold that it would condense, and "freeze out," falling to the surface as nitrogen snow. There were real concerns that the spacecraft would arrive too late to observe any atmosphere at all. Strangely, and for reasons that aren't yet understood, surface-based observations in the mid and late 2000s were showing the opposite of what was expected -- Pluto's atmospheric pressure at the surface was rising:
Graph presented by the team showing estimates from Earth-based measurements of Pluto's atmospheric pressure at the surface over the time since perihelion (1989). The pressure was counter-intuitively rising, when it was expected to be falling as the atmosphere "froze out." The units on the left are in nanobars. Credit: NASA/JHUAPL/SwRI
It's clear now that New Horizons didn't miss out on Pluto's atmosphere. But a new data point from the spacecraft's REX instrument seems to indicate it was cutting things a bit closely. The REX instrument uses a radio uplink signal, sent from Earth, scheduled to arrive at the spacecraft when the edge of the planet is just about obscuring Earth, from the spacecraft's perspective. The radio signal passes through the thicker atmosphere just above Pluto's surface before being received by the spacecraft. Analyzing how the signal was altered by that passage allows scientists to measure the thickness of the atmosphere at the surface. This is what is referred to as the radio occulation experiment.
The first result from the radio occultation experiment indicates a surface atmospheric pressure much, much lower than expected, and lower than any of the Earth-based measurements above. It suggests that Pluto's surface pressure has decreased by a factor of two in just the last two years. This was another big surprise. Further investigation and contextualization of that measurement is required, and will be conducted when more data comes down from the spacecraft. But it could suggest that the theorized "collapse" or "freeze out" of Pluto's atmosphere as it distances itself from the Sun may have just begun. New Horizons may have gotten to Pluto in the nick of time.
Down on the surface, the focus continued to be on the western side of Tombaugh Regio, including Sputnik Planum and the border region with Cthulhu Regio, which I discussed in detail in this post on Wednesday. In close analysis of the edges of Sputnik Planum, the very large, very smooth region of polygonal ice in Tombaugh Regio, the geology team has identified clear signs of flowing ice, which is invading neighboring territory and surrounding other surface features.
LORRI image of the edge of Sputnik Planum, annotated by the geology team to highlight signs of flowing nitrogen ices. Credit: NASA/JHUAPL/SwRI
At the temperatures on Pluto's surface, as low as -387 degrees Fahrenheit, water ice is as hard and strong as granite rocks are on Earth. Nitrogen, methane, and carbon monoxide, which have a much lower freezing point than water, create much more malleable and volatile ices at these temperatures. Unlike water ice, these ices can flow across the surface, in the same manner that massive ice water glaciers can flow on Earth. At the borders of Sputnik Planum, there are obvious spots where nitrogen ice has filled in old craters, flowed around hills and mountains (like Hillary Montes, shown in the below image), and otherwise intruded upon nearby terrain.
Image of the region where Sputnik Planum meets Cthulhu Regio, buffered by Hillary Montes. Nitrogen ice flow has trickled around Hillary Montes and filled in at least one crater in its path. Credit: NASA/JHUAPL/SwRI
This is another stunner of a result. Like other early finds, it points to sustained, active geology on Pluto, and a very recently evolved surface. How recent?
"There's no reason why this stuff can't be going on today," based on what the team now knows, said mission scientist Bill McKinnon.
Pluto continues to grow more fascinating by the day. Potential massive tectonic features, an atmosphere in flux, and the vast volatile reservoir of Tombaugh Regio have combined to make what Principal Investigator Alan Stern calls "a scientific wonderland." With the first phase of data downlink finished, but much more still to come, Stern reiterated his confidence that the scientific payload selected for New Horizons was the proper one to unravel all of these complicated tales of Pluto, past and presesnt.
"We're going to be able to tell this story very well over the next year," Stern said. With data continuing to rain down from New Horizons, we may be in for a new plot twist each week.