The first stage of planetary exploration took exactly 50 years: On July 14, 1965, a NASA spacecraft named Mariner 4 flew by Mars and took the first-ever close-up photos of another planet. On July 14, 2015, a NASA spacecraft named New Horizons took the first close-up photos of Pluto, the last of our nine planets to be visited. Like a grand-scale Magellan, mankind has completed its first trip around the solar system.
To be technical, while New Horizons was on its way to Pluto, a vote at the International Astronomical Union demoted Pluto from “planet” to “dwarf planet.” Of course, a lot happened while New Horizons was on its way to Pluto; the trip covered 3 billion miles over nine and half years. New Horizons had its closest encounter with Pluto last Tuesday; the first few stunning images it sent home have already become ubiquitous. By now, everyone has seen pictures of Pluto’s “heart,” its gentle mottling, and the vast light and dark fields on its rusty-red surface. Many of you will have seen the first extreme-close-up photo New Horizons sent home, showing gigantic ice mountains near Pluto’s equator. Not many of you will have heard that, beyond the ice mountains, that photo shows something that utterly changes our understanding of planets.
The photo shows about 150,000 square miles of Pluto’s surface. The New Horizons scientific team reported, a day after the photo arrived, that they had studied it carefully, and had been unable to spot a single crater. That’s very big news.
During the autumn, you can estimate how recently a lawn has been raked by how many leaves are on it. In the same way, you can estimate how recently a planet’s surface formed by how many objects have had a chance to crash into it. When things crash into a planet, they leave craters. Not one single crater on 150,000 square miles of Pluto means its surface is brand new.
That doesn’t mean Pluto itself is brand new — it means that Pluto was recently, or still is, geologically active; that its surface is being churned up and changed and covered by new material. That’s an enormous breakthrough.
SLIDESHOW: NASA’s New Horizons Mission
Geological activity requires energy. Earth’s geologic activity — earthquakes, volcanic eruptions, and so forth — is powered by its internal heat, which comes from the friction of Earth’s innards moving around, and from the decay of radioactive elements. The heat in the Earth’s interior is proportional to its immense size; if our planet were smaller — Pluto-sized — it would be colder, and thus, it had been thought, inactive.
Pluto is generating heat. And that has huge implications for the solar system at large.
There are geologically active bodies that are much smaller than Earth, but they don’t generate their own heat. The most active body in the solar system is Io, the innermost of Jupiter’s four big moons; its energy comes from constantly being pulled back and forth by Jupiter and the other three moons. But nothing is pulling on Pluto. It has a big moon of its own — Charon, whose mass is about 10 percent that of Pluto — but Pluto and Charon orbit each other in perfect synchronization. Something else is powering Pluto’s surface changes; somehow, Pluto is generating heat. And that has huge implications for the solar system at large.
Pluto is the eponymous member of the Plutoid family of dwarf planets, which is thought to have about 200 members. Planetary scientists had assumed that Pluto was an icy, dead world. If Pluto is self-heating, some of its ice — ice deep in Pluto’s interior — might be liquid. The same may well be true of all those other Plutoids. Lots of places we had assumed were cold and dead might be warm and wet. Lots of subterranean oceans would mean lots and lots of water. As New Horizons’ principal investigator, Alan Stern, puts it: “Wherever you have water, you have the potential — and I want to stress that word, ‘potential’ — for biology.” These not-so-cold dwarf planets might turn out to be, says Stern, “abodes for life.”
These are damned exciting times.
— Josh Gelernter writes weekly for NRO and is a regular contributor to The Weekly Standard.