Since the historic New Horizons flyby in 2015, scientists have learned a lot about Pluto. But everyone's favorite dwarf planet still has a lot of secrets waiting to be uncovered.
NASA researchers announced Tuesday that they may have discovered one of these secrets in the form of seven features that appear to be clouds in various images of Pluto taken by the probe, which is currently headed deeper into the Kuiper Belt.
If the information provided by New Horizons is accurate, scientists will have to reevaluate their knowledge about of Pluto's weather system. While it has been known for a while that Pluto has a thin, somewhat hazy atmosphere, confirmation of clouds would suggest that a lot more is going on above the dwarf planet's surface than previously thought.
Each of the seven cloud-like features are all "similar in that they are very low-altitude, and all are very similar in length scales, measured in kilometers or tens of kilometers," New Horizons principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colo., told reporters during a news conference at the 2016 American Astronomical Society's Division of Planetary Science meeting, Space.com reports. "We're not seeing decks or banks of clouds – we're seeing individual, discrete cloud features."
Dr. Stern compared them to the isolated, smaller clouds common in the western United States, floating in otherwise empty skies.
But any clouds at all are impressive on Pluto, which has atmospheric pressure of 3 to 100 microbars at the surface, which is only 3 to 100 millionths of the surface pressure of the Earth's atmosphere. These clouds would not be made of water, like clouds on Earth. Stern told reporters that Pluto's clouds would likely come from trace amounts of acetylene, ethane, hydrogen cyanide, or possibly even methane, floating in Pluto's mostly-nitrogen atmosphere.
This summer, NASA published a photo of Pluto with a cloud-like feature lurking at the edge of the planet between daylight and darkness, known as the "terminator." Researchers were hesitant to definitively label that feature a cloud, but the discovery of these other potential clouds – which also appeared at that dividing line between day and night – lends a bit of validity to that possibility.
The terminator zone may be particularly prone to cloud formation "because there are cooler conditions there where condensation processes are likely to occur," Stern suggested.
Complex weather would only solidify Pluto's already-unique position in the solar system. The dwarf planet is the only known trans-Neptunian object with an atmosphere at all, and its icy "heart" surface feature is one of the most reflective surfaces in the solar system, indicating some sort of activity just below the surface. Despite that activity, however, Pluto evidently has no landslides, a phenomenon made even more unusual by the fact that landslides have been spotted on Charon, Pluto's largest moon, according to a NASA statement. Clearly, the New Horizons mission to the dwarf planet has raised even more questions about Pluto than it ever could have answered.
"We're excited about the exploration ahead for New Horizons, and also about what we are still discovering from Pluto flyby data," said Stern in the statement. "Now, with our spacecraft transmitting the last of its data from last summer's flight through the Pluto system, we know that the next great exploration of Pluto will require another mission to be sent there."