Raadselachtig Pluto

Nieuws | de redactie
9 februari 2010 | Wat gebeurt er toch met Pluto? Binnen zes jaar zijn de oppervlaktepatronen van de dwergplaneet sterk veranderd. Het noorden is lichter geworden en het zuiden donkerder. Bovendien laat de Hubble telescoop zien dat Pluto een roder kleurtje heeft gekregen. Dit maakt de astronoom Marc Buie nogal zenuwachtig.

Buie ontdekte de verschillen na het vergelijken van foto’s vande Hubble Space Telescope uit 1994 met afbeeldingen uit 2002.Hierbij is de astronoom niet over één nacht ijs gegaan. “It tookabout four years running on 20 computers simultaneously andcontinuously to get these results. The results, when they finallycame, were astonishing: the patterns on Pluto’s surface had changedmarkedly in the six years separating the two sets of observations,a period that covered only a tiny fraction of Pluto’s year.”

Ook de oppervlakte van de Aarde en Mars verandert van tijd tottijd, maar bij lange na niet zo spectaculair als die van Pluto. Opde website van The Planetary Society legt  Buie uit wat de radicalegedaanteverwisseling van de dwergplaneet veroorzaakt. Maar waar dierode gloed toch vandaan komt? Buie tast in het duister.  

The origin of the changes is thought to be seasonal. Spring isadvancing in Pluto’s northern hemisphere, and as it arrives,volatile ices (in particular, of nitrogen frost) are likelyevaporating in the northern hemisphere, to be deposited again inthe dark (hence, invisible) winter pole. However, why thisevaporation should cause the northern hemisphere to brighten is abit uncertain. Buie speculated that it may be a transient effect,that the nitrogen may be sublimating in such a way as to create anintricate “fairy castle” texture to Pluto’s northern hemispheresurface, which would appear brighter not because it wasintrinsically a brighter material but instead because of the wayits change structure made it reflect light toward Earthdifferently.

Unieke vlek
One spot on the map – a particularly bright spot located near theequator at about 180 degrees longitude – was observed to remainfairly constant between the two sets of observations. “This one isunique,” Buie said. “We know from spectroscopic studies that atthat particular longitude, we see the unmistakable strong signatureof carbon monoxide frost. At other longitudes, we don’t seethat.  There’s this nice synergy between imaging data andground based data. There’s got to be a correlation between thetwo. It’s the brightest spot on the surface and it’ssingularly rich in carbon monoxide frost, and that’s the spot onPluto we’re going to get a really good look at with NewHorizons.”

Another peculiarity revealed in the new map is that “Pluto is nowsignificantly redder” than it has been in the past. In fact, Buiesaid, Pluto has had a fairly constant color for 50 years ofastronomers’ observations, up through the year 2000; the new redcolor (by which astronomers mean a change in the relative amountsof longer-wavelength rather than shorter-wavelength light reflectedby a surface) appeared between 2000 and 2002, an unbelievably shortperiod for such a distant, dark body. Buie still can’t quite wraphis mind around this change: “This business about the color change,that has had me scared for a while. I got that result years andyears ago.  But it’s just so hard to understand and believethat I’ve been checking everything I can think of. I am stillnervous about it, I have to admit.  It could be that I’vecompletely screwed this up and got it wrong. But I can’t findit.  Charon is on the same images, and Charon’s got the samecolor throughout, but Pluto changed.”

Nix en Hydra
If Buie got the result so long ago, why is it only beingreleased now? Part of the delay was Buie’s caution about the colorchange. But it was also because of the discovery, in 2005, ofPluto’s second and third moons, Nix and Hydra. “Our [2002 and 2003]images also had data on those two satellites,” Buie said, and hedropped his work on the Pluto color maps to focus on derivingprecise orbits for Nix and Hydra. “Once I got that taken care ofand out of the way, it was time to come back.”

Met dank aan The Planetary Society

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