New Horizons was nearing Pluto in 2015 and the mission team would need names for the surface features they discovered. So they appealed to the public. The names would be informal ones, but they hoped the International Astronomical Union (IAU) would make them official.

Pluto - Names and Places

The Tartarus Dorsa (a ridge) is named for Tartarus, the deepest abyss of the Greek underworld - it's way way below Hades, and a place of torment for the very wicked. The ridge on Pluto, interestingly, seems to contain a feature known as penitentes. This is the first place (other than the Earth) where these erosional snow and ice features have been found.

Why are they called penitentes? In Andalucía and some other parts of Spain, during Holy Week some religious brotherhoods perform penance processions. These people are the penitentes.

A picture of Clyde Tombaugh, discoverer of Pluto shows him not with his telescope, but with the "blink comparator". Tombaugh was hired to look for the Planet X that Perceval Lowell thought was out there somewhere beyond Neptune.

He used the telescope to photograph the night sky, getting images of the same areas of sky at different times. What do planets do that stars don't? They move. the blink comparator "blinked" two images back and forth quickly so that an object that wasn't in the same place in both them would stand out from the fixed stars. That makes it sound easy. You can try to find Pluto yourself - and you'll have the advantage of knowing that it is there to be seen. Tombaugh had quite a job carefully searching pairs of plates just in the hope of finding something.

High mountains on Pluto were a BIG surprise. These mountains probably formed no more than 100 million years ago. (That's young in a Solar System over 4.5 billion years old.) And they may still be building, in other words Pluto could well be geologically active.

The young age is based on the lack of cratering in the area. Smoother surfaces are younger surface because the craters have been covered over by by more recent activity. Old surfaces, like the ones on our Moon and many of those of the outer planets, are heavily cratered.

The high mountains are almost certainly made of water-ice. There's lots of methane and nitrogen ice covering Pluto's surface, but they aren't strong enough to make great mountains. At extremely low temperatures, water-ice is like rock.

Image Credit: NASA-JHUAPL-SwRI
It was taken about 1.5 hours before New Horizons closest approach to Pluto. It resolves structures smaller than a mile across.

Jeanne Baret (1740-1807) was a Frenchwoman who was the first woman to circumnavigate the globe. Baret was a botanist and explorer who traveled most of the way around the world disguised as a man. Here's a short biography of the intrepid Jeanne Baret.

Venetia Burney was the 11-year-old English schoolgirl who suggested the name Pluto for Clyde Tombaugh's discovery. In 2006 the New Horizons team named the Student Dust Counter instrument in her honor. NASA invited her to the New Horizons launch, but she was unable to attend.

Here is a delightful interview with Venetia Burney Phair by Edward Goldstein with NASA Public Affairs. It was republished on her death in 2009. Alan Stern, principal investigator for New Horizons said, "She was a thoroughly intelligent, likable and endearing woman. The entire New Horizons team is saddened by her passing."

A serene image of the Hekla volcano gives no hint of being "the gateway to Hell" as it was called in the Middle Ages. It's a very popular tourist attraction these days.

However it's worth remembering that Hekla is one of Iceland's most active volcanos, and were it to erupt, it could affect air traffic in the same way the Eyjafjallajökull volcano did in 2010. That happened during the Easter break, and European air traffic was extensively grounded, with knock-on effects stranding people all around the globe.

(At least, we will all be able to pronounce Hekla. I think in 2010 a lot of newscasters just referred to "the Icelandic volcano" with only the brave trying Eyjafjallajökull.)

Today - July 14th - is Bastille Day in France. But for astronomy buffs, it's also the 4th anniversary of the New Horizons flyby of Pluto on July 14, 2015.