We did it! After nine and a half years of travel, the New Horizons spacecraft has finally reached Pluto. In an instant, this indistinct dot of dim light has now become yet another brilliant world known by humans in glorious detail!
Pluto before 2003, (left=telescope) and after 2015 (right=New Horizons)
It is hard for me as an astronomer to express how thrilling it is to see something clearly for the first time. For all my life since grade school in the 1960s, the only thing I have ever known about Pluto's appearance has been a photograph of its star-like shape. Again and again it has been photographed by powerful telescopes on Earth, but they still only reveal it as a dot of light among the star fields. This changed only slightly when in 2003, some 73 years after discovery, the Hubble Space Telescope took its series of photographs of Pluto.
After intense computer modeling an image of sorts was created that showed persistent surface blotches hundreds of kilometers across. Its vast distance of nearly six billion kilometers, together with its diminutive diameter of only 2,360 km means that from Earth, no matter what you try to do, its angular size is only 0.08 seconds of arc or 0.00002 degrees. Our moon in the sky is a full 0.5 degrees and 1800 seconds of arc in diameter! Thinking about it another way, Pluto appears as large as the diameter of a U.S. penny seen at a distance of 30 miles! New Horizons can easily discern features smaller than Lincoln's eyeball!
In a matter of a few months earlier this year as New Horizons approached Pluto, its best images only matched Hubble's at a resolution of about 500 km per pixel, but by May they were ten times better. At its closest approach on July 14, we could at last see details on a limited area of its surface at a resolution of 700 meters per pixel, though these images may not be relayed to Earth until August or September. This will be like viewing Earth from afar and at first seeing only indistinctly the entire blurry landmass of North America, but then in a matter of months being able to discern the Grand Canyon and the Mississippi River!
High-resolution image of surface released on July 15 showing features as small as ten kilometers (Credit: NASA/ JHUAPL)
When you get right down to it, astronomy is a relatively primitive science. Enormous advances in our understanding of celestial objects can spring from nothing more than getting a clear image of what you are studying. This happened on a daily basis when the Hubble Space Telescope began operation. For the first time we could finally see, for example, what a quasar was, while for decades they were only just an indistinct 'quasi-stellar' smudge of light. Huge advances in planetary science have also come from the systematic replacement of fuzzy telescopic images by up-close, high-definition photographs.
Pluto Revealed.
The first impressions you glean from the released New Horizons high-resolution images is that Pluto is vastly different from its dwarf planet cousin Ceres. Ceres lives in the asteroid belt between Mars and Jupiter, and its surface has been pummeled by asteroids, leaving behind thousands of craters from meters to tens of kilometers across. Most of this activity happened during the Late Heavy Bombardment Era, which ended around 3.8 billion years ago. Pluto is very different. It seems all traces of craters smaller than about 50 km have vanished, at least from some regions of its surface. There are only a few ways this can happen that come to mind:
1) The violent formation of the large moon Charon (like the formation of our own moon) erased these craters; 2) The surface has been steadily covered by some material either derived from its atmosphere (nitrogen snow!) or by volcanism (geysers of liquid nitrogen?) from its interior; 3) There aren't as many asteroids in the Kuiper Belt that can make these smaller craters; and 4) Tidal action of Charon upon the surface of Pluto has, over the eons caused the surface to crack and rearrange itself. We don't know which of these, if any, are a good match to what we are now seeing, but any one of these, or ones that astronomers haven't yet worked out, will definitely re-write the textbook about Pluto, as the Media are fond of saying.
By the way, I do hate that hackneyed cliché "re-writing textbooks," and I wish journalists and press release writers would stop using it after nearly 30 years of questionable literary usefulness. You will be hearing it used a lot of times in the coming weeks. For myself as a scientist I find it trite, banal, bombastic and intellectually presumptuous, and it says nothing at all about how big a change is implied by "re-writing textbooks" or their actual historical significance!
Anyway, one thing that startles me about the new imagery is how dramatically different two dwarf planets can look depending on where they were formed. Ceres (950 km) and Pluto (2300 km) both have interiors that must have become warm from radioactive heating, causing the materials to segregate into a dense rocky core overlain by less dense mantle and crust material. But the surfaces are vastly different in appearance, which means they have experienced dramatically different histories. Ceres never was able to erase the scarring of the millions of impacts it received, but distant Pluto either never experienced this kind of pummeling, or its cold -230 Celsius surface was, nevertheless, able to hide the scars through some unknown process of resurfacing that filled in all but the largest impact basins. We see these larger filled-in basins peeking out above the new surface as dark rings. From the available data, it will be a challenge to discern which of several mechanisms are responsible for what we are seeing, and over what timescale they acted.
The next thing that is curious about Pluto's new imagery is that in some ways we have seen these surface features before among the moons of the outer planets. The stark color contrasts are reminiscent of Saturn's moon Iapetus where lighter and darker materials combine in dramatic ways. The complex geometric edges and polygons now coming into view resemble the tortured surface of the Uranian moon Miranda. And of course, many of the larger "geological" elements we are seeing look suspiciously like the surface of Neptune's moon Triton.
Iapetus, Miranda and Triton (Credit: NASA/JPL/Voyager 2 and ESA/Cassini)
Had we not sent the Voyager (launched 1977) and Cassini (launched 1997) spacecraft to fly by these objects, we would never have had them in our pallet to compare with what we now see in the Pluto imagery. Our enormous and growing catalog of surface images from other solar system localities can now be used as a Rosetta Stone to decipher the origin and history of Pluto. In the months and years to follow, the analysis of Pluto's surface will accelerate, and then reach a general consensus explanation that will work for this generation of scientists. We can only hope that in the distant 2030s and beyond, new spacecraft will take even closer looks at this world to continue human exploration of its remaining mysteries.
In the next blog on Pluto, I will discuss more of the preliminary results as the images arrive, and scientists have the time to really think about what they are seeing!