Am I the Only One who Sees a Crater here?

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Am I the Only One who Sees a Crater here?

from Michael Khan, 24. August 2009, 08:47

This July I read a NASA press release that sent me strolling down memory lane. I will never forget that frantic autumn of 2004. Whoever thinks science is boring should have witnessed that major scientific drama ... and not only scientists were in for a white-knuckle ride.

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Back then I was involved in mission analysis for the Huygens project. This European space probe entered the atmosphere of Saturn's largest moon Titan on January 14, 2005, examining the atmosphere during its almost three-hour parachute descent and then, as an added bonus, continuing to function for hours on the surface after touchdown.

Since July 1, 2004, NASA's Cassini spacecraft, the "mother ship" that carried Huygens to Saturn, had been in orbit around the ringed planet. As December 25 drew closer, a palpable suspense was building up in the team. On that date, Cassini was to separate from Huygens, and the entry probe would then irrevocably be on its way to its destination. But before that, Cassini was to execute two close Titan flybys and use these opportunities to collect scientific data.

We pounced on every new image and every new shred of data. Almost all of it was dramatic and new, as Titan had managed to hide its lower atmosphere and surface from probing eyes very effectively via its dense shroud of hydrocarbon smog. But now the veil was being lifted bit by bit. Everyone hoped to glean some salient information of relevance for the upcoming Huygens landing.

The new press release that made me remember those days referred to a large basin that had been discovered in radar imagery of the south polar region. It could be an impact crater or a collapsed, ancient cryovolcanic caldera.

This is what made me think specifically of the end of October, 2004, when the Cassini radar had been used for the very first time. This radar can be employed in different modes. One of it is an imaging mode, Titan surface is scanned by a probing radar beam during a close flyby. Although this allows imagining of a narrow strip only, the surface features in the strip can be discerned in much greater detail than with infrared imagery.

The first such radar image showed a 250 x 150 km swath around 50 degrees northern latitude and 73 degrees Eastern latitude in amazing detail. This image made me go bananas. I looked at it horizontally, vertically, I zoomed in, I panned out, I enhanced sharpness, then I introduced a blur, I even held a printout of it against the light and looked at it from the back. I stared at it with half-closed eyes; that will remove all fine detail, making major patterns stand out. I viewed this image alone and with friends and colleagues. I don't even know why I was so crazy about this radar image. After all, the region depicted wasn't even close to where Huygens would land. But that didn't matter to me, I had sunk my teeth into this image and was worrying it like a puppy worries its rubber ball.

Perhaps one should not look too long and too closely at astronomical images. The human brain is good at discerning patterns where there are none. Remember Percival Lowell? On the other hand: Who says the pattern I thought I was seeing was not really there?

What I thought I could make out at the time (and still think I can see) are the sunken remains of a large complex crater. In the picture to the left I have marked up the crater rim and the central mountain region in yellow. Careful here: Synthetic Aperture Radar imagery is sometimes counter-intuitive. Brighter regions are not necessarily higher, lighter or better illuminated; they are merely reflecting more radar beams back to the antenna, because the surface is rough or because it happens to be favourably inclined.

If this is a crater, then it must be ancient and largely eroded. But erosion would be entirely consistent with the ambient conditions on Titan, a meteorologically and perhaps also cryovolcanically active world. The crater's diameter of around 150 km would also be consistent with its complex shape (With craters, "complex" means that a central mountain formed before the molten material solidifies, which requires a certain minimum size). Even the dense Titan atmosphere, with over five times the density of the Earth's near the surface, would not pose an obstacle to hurtling chunk of ice or rock capable of producing a crater of this size.

If this is a crater, it would be similar in appearance to the ancient "ghost craters" on the moon, the outlines of which have been blurred during the aeons. On the Moon, this is connected to the crater filling with lava, on Titan, different processes would be involved.

So, what would this discovery mean (if in fact there is a discovery at all)? Not much, perhaps. In February 2005, just 4 months after Cassini's first Titan flyby, a crater was detected, also by radar imagery. This had a diameter of 440 km, so it was much larger, and also much clearer (and younger) than the one I refer to above. This February 2005 discovery is referred to as the first known crater on Titan.

But is it? If the object in the October 26, 2004 image is a crater, then that would be the first known one. Will I ever find out? I have searched for information everywhere, and image PIA06988 has been discussed extensively, without anyone mentioning the term "crater".

So I guess I must be wrong.

Or did any of you see a sunken, complex crater there? Please let me know your opinion, either by e-mail or using the comment function below:

[ ] Sure, anyone can see the shape of a crater there. It's elementary, my dear Watson

[ ] Huh? Crater? No way ... Michael should have his eyes examined

Annex: The Solution

On this web site of the LPL Arizona all SAR swaths on Titan are shown, inclding the one of October 26, 2004 in high resolution. The region in question is right in the middle. Here is that section extracted for your convenience. It should now be pretty obvious that we are in fact talking about a round structure, not just an optical illusion.

I kept on digging and came up with the solution. My initial impression that there is around structure here was right. But it was not an eroded crater, as it turns out. Two and a half years later, NASA-JPL announced that this is Ganesa Macula, a cryovolcano! This explains why in my initial search for answers, I came up empty-handed. Wow! Had we known back then that we we looking at a cryovolcano, we would have been even more excited than we already were. Folks, science is great. I love it.

Further Information

NASA-JPL Cassini-Huygens web site

NASA-JPL photo journal section on Saturn and its moons

LPL Arizona web site showing the SAR swaths on Titan