No, the Chang'e image isn't fake! -- but there's no new feature in it, either
Posted By Emily Lakdawalla
2007/12/03 03:18 CST
An update to this story is posted here.
pparently there is a rumor circulating on the Internet that the Chang'e 1 image released last week was faked, that it looks too similar to a "2005 NASA photo;" the rumor was written up in a Reuters story (thanks to Phil Plait for that link) and in the Telegraph (thanks to Doug Ellison for that one) today. My first reaction to this story was: of course two pictures of the same area on the Moon look similar. First of all, the Moon is pretty much geologically dead, so you don't expect to see any differences over time. Furthermore, the Moon is incredibly gray, meaning that it reflects light across all wavelengths pretty much the same way; what that means that small differences in the color sensitivities of different instruments won't make it look any different. More colorful places, like Mars for instance, can look very different depending on what wavlengths your camera is sensitive to (not to mention the fact that Mars has weather, and changes in the atmosphere and in dust deposition on the surface can make the same place look different at different times).
It's a pretty serious accusation, so I tried to track down the images that the Internet rumor is referring to. A complication is that the NASA image is unlikely to be "from 2005" as the articles say; there wasn't a NASA spacecraft in orbit at the Moon in 2005. (Check our lunar missions page for an accounting of the history of lunar exploration.) ESA's SMART-1 was in orbit then, but I had a hunch that the "2005 photo" might refer to a 2005 release of a global mosaic of Clementine image data. The Clementine orbiter was active at the Moon in 1994, acquiring a global map; newly reprocessed and reprojected versions of these data have been coming out in public release lately. (One easy way for you to access these is the handy-dandy PDS Map-a-Planet tool.)
Anyway, the Telegraph article included two images side-by-side, one from Chang'e and one from "NASA." I dug in to the Clementine data and indeed the "NASA" image shown in the Telegraph article was evidently from Clementine's 1994 mission. Here is a single Clementine image -- taken from the original data, not from any reprojected or mosaicked version -- showing the area in question. For the detail-oriented, this is from the UVVIS instrument, the 750-nm channel. I found it using the Clementine data browser at the Naval Research Laboratory's website.The Telegraph article's Chang'e image was evidently cropped from a low-resolution version of the lovely original image. Here is the Chang'e image from the same area, at its full resolution of 120 meters per pixel, slightly better than the Clementine photo, which was 145 meters per pixel. The two images cover the same areas, but they are evidently not the same image. The biggest difference arises from a different lighting angle between the two: the Clementine image is lit from the top (north), while the Chang'e image is lit from the northwest. There is also much more detail visible in the Chang'e 1 image. The extra detail arises in part because of the slightly higher resolution of the Chang'e 1 image, but it owes more to the much better camera on Chang'e 1; a decade of technological development will do that for you, and also, Clementine's camera was not really a Rolls Royce among spacecraft imagers.
So the notion that China faked their lunar photo can be put to rest. (What is it about the Moon and conspiracy theories, anyway?) At least it certainly isn't a copy of the Clementine image; and it's certainly not a Lunar Orbiter image, either. I am curious -- but don't have the time to hunt -- to see what SMART-1 saw in the same area. But the Clementine/Chang'e 1 comparison contains something exciting: a spot that may have changed between the two images! Both images contain a right-pointing equilateral triangle formed by three smallish craters. The rightmost crater in the Clementine image stands alone, but in the Chang'e 1 image it has an apparently new feature to its lower left. The chief scientist of the Chinese lunar exploration program, Ouyang Ziyuan, was quoted in the Reuters and Telegraph rebuttals of the fake-image accusation as pointing to this spot as proof that the two pictures weren't the same. Amazing! Is this a new crater on the Moon, the place where nothing ever changes? And how ironic that there should be a new feature in an image that was supposed to be "identical" to one taken previously by NASA. Here's the two areas side-by-side:I fired off a couple of emails to lunar scientists asking what they thought of this new feature, then began to study it myself. I immediately realized that this was not a new feature at all. Look at the Clementine image of that crater. You'll see that the crater has another tiny crater on its rim at roughly the 9 o'clock position. Looking at the Chang'e 1 image, you can see that the "new" feature is, in fact, that tiny crater -- the Chang'e 1 image has a seam running diagonally through that crater, and the left side of the image has been offset downward along the seam. In the version below, I fixed the seam, sliding the left side of the image up along the diagonal until it matched the right side. The apparent "new feature" is gone, and it now looks pretty much like the Clementine image of the same area. (The crater looks bigger in the Chang'e 1 image because that camera has a higher resolution, something I didn't correct for.) How did that happen? the Chang'e 1 image is actually a mosaic of 19 image strips taken on different orbits. It is incredibly difficult to get every tiny feature in a large mosaic to line up properly; you have to have very, very precise knowledge of the shape of the body being photographed, and we actually don't have very good topographic models of the Moon (Kaguya will change that). So it's no surprise that there are seams in this image. I commented last week when I first posted this image on the beauty of the mosaic, and the fact that there were no obvious seams. It's evident from this little snafu that the seams are there, just blended well to make it difficult to see them.
I don't have time to do it now but I bet if you go back to the original image you can see other places along the same seam where features are offset. It's a bit embarrassing that Ouyang Ziyuan cited the crater as being a new feature when in fact it's just an artifact of their mosaicking process. This is one reason you often see lots of image seams in mosaics produced by space missions, why scientists generally don't take the time to make their images prettier by blending seams away; such blending is, in a sense, a "fakery," making the data looking prettier than it is. It's perfectly okay to clean up images for use as illustrations, and this first Chang'e image release was clearly designed to wow the public. However, because of the blending of the seams, this is not a product that should be used for scientific research -- including looking for new craters. Some of this problem should go away once the Chang'e team improves their knowledge of their spacecraft's orbit and of the shape of the Moon; with more precise positional information, their mosaics will automatically improve, having less obvious seams with smaller offsets.
So, in a way, the story has come full circle. The Chang'e image isn't a fake as far as I can tell; my personal opinion, based upon the evidence I was able to dig up, is that the Chinese do have an orbiter at the Moon, and that it is producing really beautiful images that are a great improvement over Clementine. But the one released image is a processed product, and was altered slightly (the seams were blended away) to make it pretty. This alteration made it difficult for a scientist to realize that what appeared to be a new feature was in fact an artifact.
Being a scientist type myself, I would love, eventually, to get my hands on the original data. And there's good news on that score. According to a Xinhua news story from Sunday, the Chinese Academy of Sciences is making the data available, in a limited way anyway, to the public. Chinese lunar exploration program chief scientist Ouyang Ziyuan is quoted as saying: "The money used for the Chang'e project comes from the taxpayers and, therefore, the data should also be made public. Any scientist or astronomy lover can apply to the state in accordance with certain procedures to obtain data he needs." I invite any Chinese reader of this blog to attempt to get access to the data to help refute the fakery story, and tell me what happens!