Let me just admit something up front: before New Horizons, I never thought Pluto would be a geologically interesting place. I mean, it's just this tiny ball of dirt and ice orbiting the Sun way the heck out there on the edge of the solar system. There surely are worlds a lot more exciting for planetary geology, right?
Then we saw this:
And I realized I'd been completely wrong. That is a very geologically interesting world!
I've been collecting links for y'all on Pluto and the New Horizons mission ever since. Today, I've got six links that show why New Horizons rocks.\
For example, one of the most interesting features on Pluto (and there are so many to choose from!) is the nitrogen plains making up the western half of Pluto’s heart, called Sputnik Planum (the entire heart feature is called Tombaugh Regio, after Clyde Tombaugh, the astronomer who discovered Pluto). Much of Sputnik Planum is segmented, which is something you see on Earth in frozen lakes, when warm water rises and colder water sinks (called convection) and the frozen surface forms abutting geometric plates.
On Pluto those plates are a few kilometers thick, but underneath them may be more fluid material with the consistency of toothpaste. It convects, possibly at speeds of several centimeters per year. But this raises the question: Why is Pluto’s interior warm? It’s small enough that it should have frozen solid billions of years ago! This is one of the biggest questions the New Horizons observations have generated. Maybe Pluto suffered a huge collision in the past billion years, creating the moons, and warming its interior. Maybe it’s something else entirely. But the nitrogen plains are pointing toward some huge energy input that made Pluto’s interior warm.
The surface of the heart is a mere –235 C, cold enough to freeze nitrogen. According to the Washington University research, there's a reservoir of nitrogen-filled ice that's likely several kilometres deep in some places. The solid nitrogen is warmed by Pluto's internal heat, and then it rises up in blobs — which has been described as a "lava lamp" sort of effect. So through this convection process, the nitrogen bubbles up to the surface and refreezes.In other words, the heart is pumping nitrogen up to the surface where it hardens again into the heart-shaped pattern that we see.
It’s hard to tell in the image because the resolution makes it difficult to know just how steep the walls are, and if there are ledges along the face.
But, if some of the walls are steep and sheer enough, they could qualify Charon for having the tallest cliffs in the solar system! That record right now is held by Miranda, a moon of Uranus. Verona Rupes on that icy worldlet has cliffs at least ten kilometers high (the NASA article about Charon says five, but many sources list it as 10 and even some as 20). So it’s hard to say whether Charon can edge out Miranda or not.
If Pluto ever had a liquid sea that froze solid, the pressure from the heavy outer ice shell would squish the subsurface ocean into a denser phase called ice-II, which has a smaller volume than liquid water.
“If the oceans were to freeze completely, soon after that you form this ice-II, which would cause all of Pluto to undergo a huge volume contraction,” Hammond says. This would make Pluto’s surface buckle, like the skin of an overripe peach wrinkling as it dries.
But that’s not what New Horizons saw. Instead, it saw deep cracks. That suggests the dwarf planet is slowly growing bigger, with normal ice – which has a larger volume than liquid water – still forming slowly. If this is the case, then no ice-II will have formed, and something must be keeping the ocean wet – probably heat from the decay of radioactive elements in Pluto’s rocky core.
In fact, sending a probe to a planet so far away that it takes light five-and-a-half hours to reach is kind of on the cheap side! It cost $720 million to reach Pluto — by contrast, the construction of the new Minnesota Vikings stadium will cost $1 billion, CBS reports. Metlife Stadium, in New York, cost $1.6 billion to build, and the world's most expensive stadium, Tokyo's forthcoming 2020 Olympic arena, could cost $2 billion (the Pluto trip was a mere 36 percent of the latter's cost).
NASA has now picked its next stop: a small, cold Kuiper Belt object called 2014 MU69 that is nearly a billion miles beyond Pluto.
Although the trip to Pluto has been carefully planned, the trip beyond has been…less so. Kuiper Belt objects at the edge of the solar system are enticing destinations because they’re made up of primitive material largely unchanged since the solar system’s birth 4.6 billion years ago.
Pluto is definitely not the end of fascinating places in the solar system to explore. There will be missions beyond New Horizons, and new discoveries to be made. There's lots of geology in them thar asteroid belts - and between the data New Horizons is still returning and future missions, we'll be rocking worlds for many years to come.