Scientists Surprised, Bewildered by Pluto Data

Scientists studying data still streaming back from the New Horizons spacecraft months after it flew past Pluto presented an overview today of what they know and what they still are trying to understand about the dwarf planet.  Ice volcanoes, an atmosphere much smaller than expected, and rapidly spinning moons are just three of the tantalizing findings discussed at a press conference held in conjunction with the Division for Planetary Sciences (DPS) of the American Astronomical Society conference outside Washington, D.C.

New Horizons flew past Pluto on July 14, but because of the large amount of data, slow data rates, and 4.5 hour one-way signal travel time, it will take a total of 16 months for all of the data to reach Earth. Pluto is about 3 billion miles (4.9 billion kilometers) from Earth right now and New Horizons has passed it and is heading further away.  The maximum data rate is 4 kilobits per second.

Today’s results reflect only initial analysis of data received so far and the overall message is that the data are fascinating, with many unexpected characteristics, and the scientists are still trying to figure it all out.

Pluto and its largest moon, Charon.  Composite image from New Horizons.  Credit:  NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The possibility of ice volcanoes — or “cryovolcanoes” — on Pluto’s surface is without doubt the headline grabber.  Oliver White of NASA’s Ames Research Center described high resolution images of an area south of what is informally called Sputnik Planum with two mountains 100 miles across and seven miles high that have depressions at the top similar to the shapes of volcanoes.  Theoretically they could have been formed by the eruption of ice from the subsurface, a “phenomenal” discovery if verified, White exclaimed.

Apart from the possible cryovolcanoes, perhaps the most
intriguing finding from images of Pluto’s surface is the
discovery that the dwarf planet is geologically active.  Scientists
expected to find a heavily cratered surface somewhat similar to Earth’s
moon, but instead there are large regions where no craters are evident,
meaning that the surface is “new” — perhaps only 10 million years old. 
Other portions do show signs of many crater impacts, indicating an
ancient surface dating back 4 billion years.   Understanding what
geological processes are taking place on Pluto is likely to take many
years to deduce.

Pluto’s atmosphere also is not what scientists expected.  Leslie Young of the Southwest Research Institute (SwRI) explained that before New Horizons arrived, models suggested that the atmosphere was 7-8 times larger than Pluto’s diameter, but instead it is only 2.5 times larger.  It is much more compact with an escape rate thousands of times smaller than anticipated, she said. One effect of this finding is to reopen debate about the red “stain” at the north pole of Pluto’s moon Charon.  Charon is half the size of Pluto and that feature (visible in the image above) was immediately noticed when New Horizons images first reached Earth.  Initial speculation was that material from Pluto’s atmosphere was bleeding over onto Charon creating red tholins.  Now that the escape rate is known to be so low, that seems much less likely.  New Horizons Principal Investigator Alan Stern of SwRI said today that “we are scratching our heads” over it.

Another surprise is
that Pluto’s four smaller moons — Hydra, Nix, Kerberos and Styx — are spinning.  Hydra is the fastest, spinning 89 times during
each orbit of Pluto.  Mark Showalter of the SETI Institute said the moons
“behave like spinning tops” for reasons yet unknown.  Showalter also
said that at least two and possibly all four of them are
the result of mergers of smaller bodies — in essence that Pluto once
had more moons that collided with each other, combining into fewer,
larger moons.

Stern and the rest of the New Horizons team hope the spacecraft has a continuing mission ahead of it.   A long term goal has been to send the spacecraft, launched in January 2006, to investigate another target in the Kuiper Belt at the outer edge of the solar system.  Stern estimates that the spacecraft’s radioisotope power source has another 20 years of operation left to investigate Kuiper Belt Objects (KBOs) that are thought to be pristine bodies left over from the origin of the solar system 4.5 billion years ago. 

The plan is to send the spacecraft to 2014 MU69, a 30 mile (45 kilometer) diameter KBO.  Pluto is 1,400 miles (2,300 kilometers) in diameter by comparison.  MU69 was discovered using the Hubble Space Telescope and is one billion miles (1.6 billion kilometers) beyond Pluto.  The spacecraft would arrive there New Year’s Eve 2018/2019.   Four course correction maneuvers were recently performed to set New Horizons on that trajectory, though officially the extended mission has not yet been approved.

Pluto once was classified as the ninth planet in the solar system, but in 2006 was reclassified by the International Astronautical Union (IAU) as a dwarf planet that is itself a KBO along with its moons.  The “demotion” of Pluto from planet to dwarf planet remains controversial.  Stern, in fact, refers to Pluto and Charon as a double-planet system.