Buried Treasure: Digging into Pluto’s Past and Future from Three Billion Miles Away

A simulation of the view of Pluto seen from the New Horizon spacecraft over the 4 hours around the closest flyby. The largest moon, Charon, appears in yellow and the Sun’s inner planets are marked as seen post-flyby. Lines of longitude and latitude are marked on Pluto and the nightside is to the left of the yellow line at close approach. New Horizons will only have a detailed view of one hemisphere of Pluto. Credit: NASA / New Horizons / Geoviz

A simulation of the view of Pluto seen from the New Horizon spacecraft over the 4 hours around the closest flyby. The largest moon, Charon, appears in yellow and the Sun’s inner planets are marked as seen post-flyby. Lines of longitude and latitude are marked on Pluto and the nightside is to the left of the yellow line at close approach. New Horizons will only have a detailed view of one hemisphere of Pluto. Credit: NASA / New Horizons / Geoviz

Using a telescope as a shovel and a forgotten map as a guide, astronomers have begun digging into Pluto’s anatomy from the outside in.

“This was a bit like using a telescope as a digger to mine into Pluto,” said Dr. Jane Greaves from St. Andrews University, who used forgotten data to map the sub-surface of Pluto, “but with less effort!”

The rediscovered map used by Greaves and her assistant Ailsa Whitelaw dates from the late 1990s. Around that time, a telescope called the James Clerk Maxwell (JCMT) on Mauna Kea, Hawaii, trained its eye on Pluto for the first time.

The JCMT is the largest telescope in the world for observing in a very specific range: the submillimeter. Radiation in the submillimeter wavelengths longer is far-infrared, but shorter than microwave radiation. Radiation in the submillimeter range is a thousand times smaller than the human eye can appreciate. Through the eye of the JCMT, this frequency is invaluable for studying the coldest material in the Universe–such as near-zero-degree clouds of interstellar dust–and Pluto, which revolves around the Sun about 39 times further away than the Earth and has an average surface temperature of roughly -230 ༠C(-387 ༠F).

In spite to living far from the Sun, Pluto rotates fast: every 6.4 days, to be precise. The JCMT watched the rapidly spinning dwarf planet for a very specific signal in the submillimeter: 0.85 millimeters. Radiation 0.85 mm in wavelength is emitted not from the icy surface of Pluto, but from beneath the icy surface. Using this wavelength and a light curve, Greaves and Whitelaw were able to look under the surface of Pluto without leaving the comfort of Earth.

The light curve of Pluto in the new submillimeter data, which corresponds to signals from beneath its surface. The small points are from the SPIRE camera on ESA’s Herschel telescope, and the large points are from the SCUBA camera on the JCMT. The darker points represent longitudes on the nightside during the New Horizons closest approach. The red curve is infrared data published from NASA’s Spitzer spacecraft (corresponding to the surface); the new submillimeter (below the surface) results particularly differ from this curve in the nightside longitudes. Credit: Jane Greaves & George Bendo

The light curve of Pluto in the new submillimeter data, which corresponds to signals from beneath its surface. The small points are from the SPIRE camera on ESA’s Herschel telescope, and the large points are from the SCUBA camera on the JCMT. The darker points represent longitudes on the nightside during the New Horizons closest approach. The red curve is infrared data published from NASA’s Spitzer spacecraft (corresponding to the surface); the new submillimeter (below the surface) results particularly differ from this curve in the nightside longitudes. Credit: Jane Greaves & George Bendo

Pluto’s light curve worked something like this: Imagine a black-and-white soccer ball (or a football in Europe) half as wide as the continental US. Coat it in ice. Place it 3 billion miles away. Now spin it on its axis once a week and watch it through a telescope. Even though the image may be faint and/or blurry, there will still be features that you can appreciate. When more black pentagons on the soccer ball’s surface move into your image, the amount of light you are looking at will transiently drop. When more white hexagons move into your image, the image gets temporarily brighter overall.  Similarly, as Pluto revolved around its axis, dark and light patches under the ice showed up in the 0.85 mm wavelength, again and again.

Even though the data has been available for decades, until now no one had plotted Pluto’s light curve in this range. By plotting the change in brightness over time, Greaves and Whitelaw proved that in terms of color, and likely composition, Pluto’s subsoil is patchy.

What these these color changes represent is something we may understand better after next July’s New Horizons fly-by. Different chemicals mixed up in the subsoil is one possibility. Another explanation is that dark areas of water ice and frozen polymers lie over layers of nitrogen and/or methane. It’s important to remember that Pluto’s orbit is far more eccentric, or elliptical, than any of the eight planets. It may be that as Pluto approaches closer to the Sun, in a manner reminiscent of a comet, parts of its surface actually boil away in sunlight. If that were the case, the result would be that Pluto’s dark patches would slowly evaporate, leaving a brighter surface behind.

he track of Pluto across the night sky, as seen from 8 to 20 May 1997 when it was moving southwards in the direction of the constellation of Scorpius. North is at the top in this image. The superimposed red points are the moving point that the JCMT measured for the whole system of Pluto and its moons. Credit: Jane Greaves & George Bendo

he track of Pluto across the night sky, as seen from 8 to 20 May 1997 when it was moving southwards in the direction of the constellation of Scorpius. North is at the top in this image. The superimposed red points are the moving point that the JCMT measured for the whole system of Pluto and its moons. Credit: Jane Greaves & George Bendo

Learning more about Pluto and other Kuiper Belt Objects allows us a look back at the Solar System during its formation, and gives us a better idea of what dwarf planets and other small bodies throughout the solar system might look like.

“Pluto and Charon were part of the original population of scattered planetesimals, and they probably remained near the region in which they formed,” said Dr. Alyssa Rhoden, a NASA Goddard specialist on Pluto. “Pluto and Charon can also give us insight into the materials that were delivered to the region around Jupiter, and perhaps even further inward. Pluto and Charon have a unique story to tell about how our solar system formed and evolved, the types of volatiles that were distributed during giant planet migration, and the extent of habitable bodies in our solar system.”

Without making a single additional observation, Greaves and Whitelaw looked straight through the surface of a tiny, fast-moving, far-away body in the submillimeter, giving the world a glimpse not only into the Solar System’s past and Pluto’s present, but perhaps–if we can solve the mystery of the dark patches–a sneak peek into in its future.

“I’m really excited to see what New Horizons will find a year from now,” said Greaves, “Some researchers think that even deeper down, Pluto has liquid water, kept fluid by remnant heat from a big crash that formed its moons — if so the surface will probably look wrinkled. But the flyby is so quick that we’ll need to follow up — maybe with future radar we can dig down even further.”