Pluto | What Have We Learned?

Top findings by scientists with the New Horizons Pluto Mission were published in March 2016 in the journal Science. A list of the highlights is available in this news release from NASA. I will attempt to summarize each finding and their significance.

Images used in this post are from Johns Hopkins Applied Physics Laboratory. Click on any for a much larger and impressively detailed view. Their entire collection of released images is available here for your browsing pleasure. I encourage you to look through the images. Our views of Pluto went from a tiny mottled orb a year ago to ones with great detail and mystery. Each of the 120+ images includes an explanation of the important details.

Now on to the top findings…

1. Geologically active

The ages of solar system bodies can be estimated by counting the number of craters. This method tells us Pluto is about 4 billion years old, a little younger than Earth. The heart-shaped light colored, icy region called Sputnik Planum is devoid of cratering. That indicates it is young no more than 10 million years. Sputnik Planum is smooth and about the size of Texas.

2. Charon

The largest of 5 known moons, Charon is half the diameter of Pluto. As with our Moon, it keeps the same face toward the parent body as it orbits synchronously. Charon is old at about 4 billion years. Cryo-volcanic flows erupted to form some smooth plains billions of years ago as an ocean froze beneath the surface.

This computer generated fly-over animation gives a simulated close view of Charon. New Horizons did not actually pass near the surface of Charon.


3. Surface Variety

Pluto has a complex and puzzling variety of surface types. The surface features are unlike any others in the solar system. There are nitrogen-rich, methane-rich, and water-rich regions which challenge scientists to piece together their history and geology. Click this image to see the variety in detail. Smooth icy regions are flowing into rugged mountain valleys reminiscent of glacier flow here on Earth.


4. Cold Atmosphere

Escape of gases from the surface atmosphere into space is a cooling process. The upper atmosphere of Pluto was found to be 70 Fahrenheit degrees colder than expected. Scientists have no explanation at this time. The surface of Pluto averages -229˚C (-380˚F).

5. Atmosphere Composition

The surface atmosphere of Pluto is 99% Nitrogen. It also contains molecular methane, acetylene, ethylene and ethane. The composition of gases compared to altitude has been measured for the first time. The gas escaping most into space is methane. That is a surprising finding which scientists cannot explain.

6. Haze Layers

The atmosphere of Pluto is composed of at least 20 layers. This highly processed image shows the layers backlit by the Sun with Pluto in silhouette. Click to embiggen for detail.


The layers are not always parallel to the surface. The layering can extend hundreds of miles and extend from a few miles altitude as in the arrows at lower left down to the surface shown by the arrow at upper right. Winds blowing over Pluto’s mountainous surface appear to be causing this variation.


7. Moons and Debris

Are the four small moons of Pluto, Styx, Nix, Kerberos, and Hydra, the result of an earlier impact which broke apart a larger moon? If that were the case, much debris would typically be scattered about the moon orbit system. A dust counter detected only one particle during the five days of flyby it measured. There is apparently no debris. This was a surprise and relief to the scientists. Debris could have fatally damaged the New Horizons spacecraft and ended the mission.

The moons were found to exhibit some unusual rotations in their respective orbits. Charon, the largest, is in a synchronous orbit always facing Pluto the same way. Not the others as illustrated in this animation.


8. Moons Have High Albedo

Albedo measures the reflectivity of surfaces. The small moons have high reflectivity, more than the typical albedo of small bodies in orbit beyond Pluto. This suggests their origin is not from that farther region. Instead, their similar high albedo and quite varied rotations (see video above) suggest they were formed from an impact long ago. This poses a mystery since there was no debris detected as New Horizons passed by. Perhaps there will be more to this story later.


9. Atmosphere and Solar Wind

As the solar wind moves past Pluto, it interacts with the escaping gases from the atmosphere and drags them downstream. It forms a sort of bow shock pattern. The interaction with the gases started at six Pluto radii upwind. This was a much smaller distance than expected.

As New Horizons coasted into the downwind tail of gases, seen in blue, it detected a higher concentration of nitrogen ions forming a plasma tail. Such plasma tails have been observed near Venus and Mars.


More to Come

More data and images are stored on New Horizons and are due to be transmitted to Earth in the months to come. Transmission rates are very slow due to the small size of its antenna, the age of the technology, and the distance from Earth. Every week new results arrive. Stay tuned.


7 thoughts on “Pluto | What Have We Learned?

  1. One thought-provoking line came near the beginning of your summary: “The ages of solar system bodies can be estimated by counting the number of craters.” How do they know that? Who came up with the system? I can understand counting the rings in a cross-section of a tree, but this seems — well, not quite so understandable.


    • Good questions. Some broad assumptions are needed. Apply with caution.
      • The more craters…the older that surface.
      • Planets accreted from smaller bodies impacting and adding to their mass.
      • The number of impacting smaller bodies has decreased over time to near zero.
      • More numerous small bodies continued to impact surfaces well after the fewer large bodies were finished.
      • The larger craters are likely the oldest.

      Because we have multiple samples from the cratered parts of the moon, we can carbon and radioactive date these and compare to the crater counting method to verify.

      Several scientists were involved in developing the techniques in the early to mid 60s.


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