This is Pluto’s largest moon, Charon, in the most beautiful, detailed, highest-resolution single frame image we’ll be downlinking from the flyby this month. And it is amazing.

This image was taken just 466,000 kilometers (289,000 miles) from Charon within hours of New Horizon’s closest approach early on July 14, 2015. The image was taken with the spacecraft’s high-resolution high-resolution, monochromatic LORRI camera at 02:41:49 UTC, just over nine hours before the critical mission flyby. The approximately 2.3 kilometers per pixel resolution makes this the most detailed single-frame image of the entire moon in one shot we’ll be getting during these initial failsafe downlinks. We’ll eventually see mosaics of Charon in more detail, but not until after the primary science-gathering phase is past and the probe switches over into downloading all of its data to Earth.


This image was included in the first data downlink early this morning after the New Horizons probe reestablished contact late last night. That data was compressed, so treat it with some caution. The NASA press release explains:

The image has been compressed to reduce its file size for transmission to Earth. In high-contrast areas of the image, features as small as 3 miles (5 kilometers) across can be seen. Some lower-contrast detail is obscured by the compression of the image, which may make some areas appear smoother than they really are. The uncompressed version still resides in New Horizons’ computer memory and is scheduled to be transmitted at a later date.

The Massive Moon of Pluto

Charon is by far the largest of Pluto’s five known moons. At 1,200 kilometers diameter, it’s nearly half the diameter of Pluto, making it the largest moon in the solar system compared to its parent. The entire moon has a surface area of just 4.6 million square kilometers. That’s only a whisper larger than India (3.2 million km2), and half the size of Australia (7.7 million km2).


At just over 10% the mass of Pluto, tidally-locked Charon has enough heft to pull the dwarf planet into a distinctive orbital wobble about their mutual center of mass (barycenter). This makes it not only the largest moon in the solar system compared to its parent, but makes a solid argument for declaring that Pluto-Charon is the first binary dwarf planetary system.

Charon is small enough that its gravity isn’t even 3% that on Earth: the escape velocity to launch from the moon is just 0.58 kilometers per second, or roughly twice the current land speed record at 2,088 kilometers per hour (1297 mph).

Charon, Pluto’s Partner in the Cosmic Underworld

As many astronomical bodies are, Charon was discovered by accident. In 1978, US Naval observatory astronomer James Christy was studying photographic plates of Pluto taken at the 1.55 meter Flagstaff telescope when he noticed a slight, periodic bulge. Later observations would show that the bulge was due to a smaller accompanying body, whose periodicity corresponded to Pluto’s rotational period. This finding—strong evidence that the bulge was due to another close by body in a synchronous orbit—led astronomers to reassess Pluto’s size, mass, and other physical characteristics.


Charon’s discovery at the Naval Observatory Flagstaff Station as a time-varying bulge on the image of Pluto (seen near the top at left, but absent on the right)

But Charon’s existence wouldn’t be confirmed until 1985, when the two planetary bodies entered a five-year period of mutual transits, crossing each others’ path in Earth’s line of sight. This is an event that only occurs twice in Pluto’s 248 year rotation around the sun, when the Pluto-Charon plane is edge on as seen from the Earth. And we’re damn lucky it happened when it did, or we might have spent many additional years doubting Charon’s existence.


Christy first suggested the moon be called ‘Charon’ after his wife Charlene, nicknamed ‘Char.’ Only later did he realize what a coincidence the name actually was: Those versed in Greek mythology will know that Charon is the mythological ferryman of the dead, closely associated with Hades, the god of the underworld. And it just so happens the Roman underworld god, Pluto is a direct derivation of Hades. It’s incredible that these two planetary bodies, dancing around each other in an endless gravitational embrace in the cold, dark hinterlands of the solar system, get their names from two mythological figures who shared very much the same relationship.

Official adoption of the name Charon announced by the International Astronomical Union on January 3, 1986. Notable features on Charon are going to follow an exploration theme. It’s still to be announced if that theme is exploration destinations, exploration vessels (“New Horizons” would make a lovely name for a plateau), or the explorers themselves.

Coming Into Focus

Until the New Horizons flyby, we had very, very few images that resolve Charon as anything more than a bulge off Pluto’s backside. Here’s the best image of the 1990s, taken by the Hubble Space Telescope. Pluto and Charon appear as small, enigmatic worlds, barely visible through one of the most powerful telescopes of the time.


Pluto and Charon seen by the Hubble Space Telescope on February 21, 1994. Image credit: ESA/NASA


Hubble swung around for another look in 2006, getting another shot at the moon that was just as squint-inducing but captured a few pixels of the smaller moons Nix and Hydra. In a bit of historical symmetry, part of the imaging team this time included Alan Stern, Principal Investigator for the New Horizons Mission.

The Pluto-Charon seen by the Hubble Space Telescope on June 22, 2006. Image credit: ESA/NASA

Grainy and low-res though they may be, this fascinating image helped convince NASA that the Pluto system was worth an exploratory trip.


After New Horizons woke up for its Pluto approach, Charon started zooming into focus. In February, the spacecraft beamed back this time-lapse video showing an entire ‘Pluto day’ (roughly 6.5 Earth days) in the Pluto-Charon system, captured from a distance of 203 million kilometers (126 million miles). The composite of shots, taken from January 25-31, shows the gravitational wobble around the system’s mutual center of mass.

Fuzzy, pixelated Charon has continued to grow, sharpen, and develop geologic surface features before our eyes. A batch of images downlinked in mid-June revealed a prominent dark splotch on one of the moon’s poles, taken by New Horizons’ high-definition, monochromatic LORRI camera at a distance of 50.7 million kilometers (31.5 million miles). The dark spot was totally unexpected, sending the science team into a flurry of still-ongoing speculation.


We got our first-ever detailed look at Charon’s surface one week ago on July 8, 2015. From 6 million kilometers (3.7 million miles) away from the Pluto-Charon system, the still-unexplained dark cap on its northern pole started coming into focus. We also got our first hints of the moon’s cratering, an idea that filled Geology, Geophysics and Imaging team leader Jeff Moore with glee as he revelled in the possibility of impact-generated windows below the surface.


Close-up of Charon imaged using the Long Range Reconnaissance Imager (LORRI) on July 8, 2015. Image credit: NASA/JHUALP/SWRI

Days later, we saw the moon in enough detail to start our first geological interpretation. The possibility of craters was upgraded to specific probable locations, and new linear features joined the list as potential chasms. The speculation about that odd northern pole increased — could it be the same dark material we were seeing on Pluto, evidence of a shared atmosphere or at least the capture of Pluto’s sublimating ices clinging to the moon?


The view of Charon by New Horizons on July 11, 2015 reveals potential craters and chasms. Image credit: NASA/JHUAPL/SWRI

As part of the final failsafe data downlink before the New Horizon probe’s closest approach to the Pluto-Charon system, we got our first colour look at the moon yesterday. The colours are real as in the red filter is displayed as red, but enhanced and saturated to over-exaggerate changes so isn’t what we’d see with our own eyes.

Mission scientists offered the interpretation of the red polar cap as hydrocarbons, tholins formed when methane and nitrogen are exposed to ultraviolet radiation. Those elements have already been confirmed in Pluto’s polar ice cap and tholins are molecules small enough to aerosolize, so it’s theoretically possible that Charon’s cap is formed by capturing sublimated ice escaping form the dwarf planet. If New Horizons finds an atmosphere on the moon, and if that atmosphere is uncannily similar to Pluto’s, it gets downright probable the dark cap is a consequence of the two objects are swapping gas.


Alternately, instead of material transported from the dwarf planet, the patch could be coming from Charon’s interior. To confirm that, we’ll need to take a look at that mottled surfaces covering the rest of the moon, checking if any crater interiors reveal matching material.

Charon in enhanced real colour on July 13, 2015. Note the distinct red cap, and the diverse mottling in the southern hemisphere. Image credit: NASA/APL/SwRI


One of the other mysteries that we might solve in the next few days is why Charon’s surface is so much more cratered than Pluto’s. Both objects should be roughly the same age and subject to the same history of bombardment, so something is happening on Pluto to smooth out its surface that isn’t also happening on Charon. If it’s an internal process on Pluto, we’re left confused by how something so small could still be warm enough to be active. If it’s ice-related, we’ve got a new mystery into how the cryogenic processes are different than on Charon.

In that final pre-flyby package of data, New Horizons also sent home one last image of Charon taken with its high-resolution camera. Taken on July 13, 2015 from just under 1.5 million kilometers away from the moon, the 7.2 kilometer per pixel resolution image was impressive but no where near as glorious as the photograph we’d see the very next day.


Unprocessed view of Charon on July 13, 2015 from 1.5 million kilometers away. Image credit: NASA/JHUAPL/SwRI

And here’s the latest image again, as revealed by NASA during a press conference this afternoon:


Charon on July 13, 2015 with the New Horizons probe just 466,000 kilometers away. Image credit: NASA/JHUAPL/SwRI

As we’re learning right now based on the the latest image, Charon has far fewer craters than we expected. This raises the fascinating possibility that the moon is geologically active! But, to be fair, we don’t know exactly how many craters are on this image, and we’ll get additional data tomorrow. The fascinating gash across Charon’s surface is also revealed in the highest resolution to date—this feature is way, way bigger than the Grand Canyon, with a band of cliffs and troughs stretching over 1,000 kilometers (600 miles). Another canyon along the right edge of the image digs in to the moon an estimated 7 to 9 kilometers (4 to 6 miles).

Even weirder, there are fewer craters than we thought. While we’re busy having our minds blown over the sudden transformation of Pluto into a geologically active world, Charon also has something going on to leave it less scarred than cratering rates demand it should be. Yes, Charon has craters, but some process is driving activity.


What type of process is keeping heat and geologic activity going on Charon? It can’t be tidal energy—Pluto and Charon are in tidal equilibrium, meaning there’s no significant exchange of tidal energy between the two bodies. One possibility is that the decay of radioactive material inside Charon is powering a lot of the geologic activity on the surface. It’s also possible that Charon has managed to store heat from its formation for a long, long time. But the New Horizons team doesn’t want to speculate too much—yet.

Meanwhile, that mysterious dark polar region to the north has a diffuse, fuzzy boundary. That suggests it’s a thin veneer deposited on top of the underlaying terrain. Even better, we’re catching a glimpse of some distinct, crisp angular feature distorting part of it. As for what that is? To be determined in the not too distant future by even better, higher-resolution images.

The New Horizons probe has safely completed its closest approach to the Pluto-Charon system, but the mission is far from over. The probe is continuing to collect additional observations on its way out of the system, including more photographs of Charon as a crescent moon and lit by reflected light of Pluto-shine. Because the probe can only make observations or send data home, not both, we won’t be getting any higher-resolution full-disk images of the moon this month. When the probe does switch to data-transmission mode, it’ll take over a year to send all the data back to Earth. After that, it will continue to explore deep space, hopefully with extended mission funding to fly past a second Kuiper Belt Object.


As an extra bonus, we have our first informal place name on Charon: the dark polar region is now know as Mordor.

Top image: Charon on July 13, 2015 from 466,000 kilometers away. Credit: NASA/JHUAPL/SwRI