Stunning Backlit Photo of Pluto Shows Atmosphere

NASA’s New Horizons spacecraft not only took fascinating photos of Pluto as it approached and flew past the dwarf planet 10 days ago, but continued snapping pictures as it raced away on the other side.  Today NASA released a photo showing Pluto’s silhouette as the spacecraft looked back towards the Sun.

The artistically stunning image is full of scientific data.  As Michael Summers, a co-investigator on the mission from George Mason University in Fairfax, VA pointed out at a briefing this afternoon, it is the first image of Pluto’s atmosphere and shows a “haze” that reaches out 80 miles (130 kilometers) from the surface — much higher than anticipated.  The photo was taken when New Horizons was 1.25 million miles (2 million kilometers) from Pluto around midnight July 15 Eastern Daylight Time and delivered to Earth yesterday.   North is at the top of the frame.

New Horizons image of Pluto in silhouette, backlit by the Sun, showing atmosphere.   Photo credit:  NASA/JHUAPL/SwRI

That Pluto has an atmosphere is no surprise — scientists have studied it for years, measuring its density using stellar occultation.  Early results from the New Horizons data, however, indicate that in the past two years, the atmosphere has lost half its mass, Summers said.  The rapidity of the loss is unexpected, but not that the atmosphere would dissipate.  As Principal Investigator Alan Stern from the Southwest Research Institute (SwRI) reminded the audience today, concern that the atmosphere would collapse — “freeze out” — as Pluto’s 248-year orbit takes it further from Sun was one factor in NASA’s approval of the mission in the early 2000s.  (The New York Times published an account of the hurdles the New Horizons mission had to overcome to win support after an earlier version was cancelled in 2000 because the cost rose too high.)

Jim Green, NASA’s planetary science division director, explained that only 5 percent of the data captured by New Horizons during its close approach to Pluto and its five moons has been returned to Earth so far.   At a maximum data rate of 4 kilobits/second, it will take 16 months for all of it to be collected.   A number of new images were released today, but no more will come until mid-September.   For the next several weeks, the focus will be on sending back data taken by other instruments on the spacecraft.

Among the other images made public today is a higher resolution photo of Pluto’s surface, shown in false color to highlight differences in surface characteristics. The image was taken when New Horizons was 280,000 miles (450,00 kilometers) from the planet.  The resolution is 1.4 miles (2.2 kilometers).

Pluto surface in hi-resolution false color image.  Photo credit: NASA/JHUAPL/SwRI

The heart-shaped region revealed in earlier black and white imagery is still visible, but the surface contrasts are evident.  Scientists are still trying to understand what all the data mean and stress that these are “early days” and much more data, and lively debate, lie ahead.

At today’s briefing, though, Cathy Olkin from SwRI and William McKinnon of Washington University in St. Louis offered theories to explain the data returned so far.  Some of the surface is geologically young, due perhaps to flows of nitrogen-, methane-, and carbon monoxide-ice moving around on the surface as water glaciers do on Earth, from ice upwelling from beneath the surface, or ice falling from the atmosphere as it freezes. Some scientists are postulating that Pluto has an internal heat source and there may even be a liquid ocean under the icy crust as has been observed on other solar system bodies.  McKinnon advanced that theory today, but stressed that there is no evidence of such an ocean so far.

Pluto’s five moons also are being avidly studied using the New Horizons data, especially Charon, the largest.  Stern, in fact, refers to Pluto and Charon as a “double planet” system because the center of mass around which they orbit each other (the “barycenter”) is in space, between the two objects, not within Pluto.  (The barycenter of the Earth-Moon system is about 1,000 miles or 1,600 kilometers below the Earth’s surface.)
 

Pluto and Charon from New Horizons spacecraft.  Composite image from data acquired July 13 and 14, 2015 in approximate true color with approximated
relative reflectivity, size, separation, and orientation.  Image credit:  NASA/JHUAPL/SWRI

Stern enthused that “we’ve never been to a double planet system before” and it is a “scientific wonderland.”   Data from that wonderland will be making its way to scientists over the next year and a half, and analysis will continue long after that.  As Stern himself cautioned in the days before the spacecraft reached Pluto, “science on the fly is often wrong.”

New Horizons was built and is operated by the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, MD under contract to NASA. 

The spacecraft is currently 7.6 million miles (12.2 million kilometers) beyond Pluto heading deeper into the Kuiper Belt that surrounds our solar system.  Stern estimated earlier that the spacecraft will operate for another 20 years until its radioactive power source runs out.   It is possible that it can be targeted to visit another object in the Kuiper Belt during that time.  Pluto is in the Kuiper Belt, along with another dwarf planet, Eris. 

The International Astronomical Union (IAU) is in charge of designating solar system bodies as planets, dwarf planets, asteroids, or other types of objects based on characteristics defined by consensus by the astronomical community.   In addition to Pluto and Eris, Makemake, Haumea and Ceres are categorized as dwarf planets today.  Another NASA mission, Dawn, is currently orbiting Ceres, which is in the asteroid belt between Mars and Jupiter.