Why the Dark Spots on Pluto?

What’s at Work on this Distant Planet?

A sharpened view of one hemisphere of Pluto as seen by New Horizons. Courtesy NASA/New Horizons/JHU-APL
A sharpened view of one hemisphere of Pluto as seen by New Horizons. Courtesy NASA/New Horizons/JHU-APL

As Pluto comes into sharper focus through the eyes of New Horizons, we’re seeing more definition of the dark and light areas on this distant world. The lighter areas are very likely ice patches, but the dark ones are more intriguing. The most interesting image THIS week, shows four distinct markings that look like craters, or mountains, or… something on one side of the planet. What could be causing these dark markings?

Think about this: Pluto is covered with ices — water, nitrogen, and methane. That means it’s a cold place, indeed. At a surface temperature range of -287 to -369, that’s cold enough for nitrogen and methane to freeze out and coat the landscape. Of course, water is frozen as well.

Nitrogen and methane are susceptible to what’s called “irradiation” by sunlight, and in particular, the ultraviolet radiation emitted by the Sun. That process darkens the two ices and can create molecules called “tholins”. If you could be there and pick up some of this stuff, it might be as Carl Sagan once described: “A complex, organic goo.” Astronomers have found a great deal of evidence of these tholins on Pluto, which could account for the reddish color and dark markings we’re seeing.

Patterns of Tholins?

These tholins aren’t new discoveries. We’ve seen them on other our solar system moons such as Iapetus at Saturn and Umbriel at Uranus. They’re also present in the atmosphere of Titan. So, it makes sense that we’d see them at Pluto, too. And, they’re very likely what is forming the dark areas we’re seeing and turning the overall surface a nice red color. Now, why they’re in those exact configurations? Maybe there are vents or some other sources in those areas that periodically spew water ice over the other types of ice. Maybe those dark areas are where the water ice has gone away, leaving leaving behind the sun-darkened ice.

That’s my educated guess, and I can’t wait to see if it’s right or wrong. If it’s wrong, then, wow — maybe there’s something even MORE interesting going on there than everybody expects. In any case, I’m just part of the “Public Imaging Team”, taking my best educated guess at what’s going on there.

The image I’ve posted here was released on July 1 and was part of a pair taken on June 25 and June 27th. I’ve overprocessed it a wee bit just to bring out the light and dark areas, so don’t read too much into the image based on what I’ve done. In point of fact, the day they were released,  Principal Investigator Alan Stern made it clear that the team was really puzzled by all the dark and light markings on the planet. I suspect they won’t be for long!

It might be tempting to say, “Oh, those look like craters!” Or “Mountains on Pluto!”, the truth is, nobody quite knows yet what they are. The good news is, there are more images on the way, and in 11 days (or less) we’ll know exactly what those things are. Go New Horizons!

To learn more about Pluto’s coloration and markings, check out NASA’s Pluto: The Other Red Planet page. It goes into more scientific detail about the darkening of Pluto’s surface. They’ve also posed a GIF animation/movie of the rotation of Pluto and Charon during the time the last released images were taken.

7 thoughts on “Why the Dark Spots on Pluto?”

  1. Dark spots that merge into a broad continuous dark band along Pluto’s equator that apparently ends sharply along an eastern terminus, where it gives way to a very bright region…

    One intriguing possibility is that the dark regions are probably associated with low-lying basins, where volatile cryo-lavas rich in methane can pool. As you point out, methane and nitrogen exposed to radiation can turn dark with tholins. But the configuration along Pluto’s equator may be roughly analogous to what was discovered on Saturn’s moon Iapetus, with its startling equatorial ridge. Many theories have attempted to account for it, including the deposit of material from a long-vanished ring around that moon raining onto the surface. However, another scenario augments that explanation: a sizeable moon may once have shared Iapetus orbit around Saturn, or even have been formed in close orbit around Iapetus (that’s right: a temporary moon of a moon) following a large impact that produced a temporary ring out of which it briefly reassembled before eventually partially breaking apart into still-sizeable fragments that crashed back into Iapetus. The largest of these can have thrown up a ‘rooster-tail’ of material as it plowed its way over a long relatively low-energy impact footprint, leaving a long linear raised ridge analogous to a ‘central peak’ rebound seen in many large impact craters. Iapetus has long been known to be peculiar in the extreme variation of its surface albedo between its two hemispheres. Besides the incredible high equatorial ridge, its surface is also peculiar for sporting a large number of disproportionately large impact basins…possibly signatures of a large population of sizeable moons that had once existed in the region with Iapetus just beyond the orbit of Titan…

    In Pluto’s case the geometry of small spots that merge into a long continuous dark avenue arranged linearly along its equator is consistent with a sizeable body undergoing a relatively low-energy collision, breaking apart as it came within the Roche limit with smaller fragments impacting first to the western end of the train and the larger and main surviving body impacting shortly thereafter at the eastern terminus.

    Of course, this remains pure speculation for now, but hopefully imagery from New Horizons in the coming days and weeks will reveal whether those features are consistent with such a scenario or refute them in favor of something potentially even more startling.

  2. Adolf, I was thinking about Iapetus, too, and that odd ridge along the equator and wondered about the formation mechanism. The first thing I thought of when I saw that image was a “chain of craters” effect, but the scale bothers me. Something that would make craters that large would have disrupted Pluto significantly, I would think. I like your low-lying basins idea, however. If they were fed by some sort of recurring cryovolcanism, then the darkening would take over.

  3. Yes, that’s the idea: what we are seeing now at low-res at this distance is just what is discernible in terms of the gross local albedo differences. Whatever mechanism or event(s) arranged the spots, it is too early to say whether they may be associated with low-lying or elevated terrain. However, if the spots are dark because of a rich organic composition (i.e., potentially, tholins) than it is reasonable to suppose those volatile materials (methane-nitrogen-rich) may have pooled into low-lying basins; that they are arranged along an apparently straight and long longitudinal swath along the equator is consistent with the hypothesis (thin as it otherwise is!) of cryogenic processing of such fluids in preexisting basins that may have been excavated by some low-energy but complex impact scenario like that I described.

    On the other hand, we have to be careful about assigning excessive importance to what the human perception perceives as significant – in this case, a pattern of dark spots apparently arrayed along a fairly straight (or, that is, a great-circle) line. Its too early to be sure that the dark spots are really that decoupled from the other brighter features adjacent to them , which in the fullness of time and better res may reveal an equally arresting story or mystery, perhaps as part of a global terrain dominated by cryo-tectonic processes. In any case, it sure is an exciting thing to watch it being revealed right before our eyes!

    In all the annals of exploration and discovery to which human perception has been exposed, nothing has ever quite so powerfully stroked the strings of wonder as the looming up of entire worlds out of invisible points of light set at unfathomable distances once deemed hopelessly untraversable…

    I can’t help but think how extraordinarily lucky and privileged we are to be alive during the one brief blink of our beloved planet’s 4.5 billion-year history to witness these wonders that have never (probably) had any eyes and the necessary minds behind them – human or otherwise – cast their attentions upon them…and yet, there they are, whole worlds awesome and full of beauty that have existed for billions of years quite indifferent to whether they are gazed upon by appreciative beings such as we.

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