No one claims there are canals on Mars now, but for some time it has been customary to denounce Lowell's observations as "optical illusions," and to declare that there is "nothing really there" where he said he saw canals. Carl Sagan was among those famous for doing it.
And I never felt it was entirely fair. Lowell was said to have been one of the finest planetary observers of his era, though he lacked academic training in the field. His observatory was located in one of the finest observing locations in the country, and Lowell Observatory is still one of the nation's premiere observatories and research centers.
In an earlier post, I juxtaposed an illustration of Lowell's freehand drawing of what he saw in the Solis Lacus region of Mars in 1896 with three telescopic views taken with a CCD camera in 2006, 110 years later in the UK. The similarities are striking.
There are differences to be sure. Especially the size of the "pupil" of the Solis "eye." Lowell's is small and discreet, the "pupil" in the CCD image is large and extended. But many of the other features correspond almost exactly. Those features we now know include dark-floored craters, "fossae," dark and light terrains, volcanoes, and clouds among other things. I was able to toggle easily between the images, so it was startling to see the correspondences and to spot more of them each time I toggled.
Among the most striking correspondences is the appearance of lines on the CCD images, several of them in remarkably similar positions to those delineated by Lowell.
Side by side comparison; features seen on the CCD image are marked with arrows on the Lowell drawing [Note: South is at the top of the images.]
|Side by side comparison of Mars Solis Lacus quadrant|
left drawing by Percival Lowell, c. 1896
CCD telescopic image by D. Peach, 2005
Correspondences noted on Lowell drawing
The faint linear markings on the CCD image are NOT "optical illusions." They are on all the CCD images from that sequence so they are real in the sense that the CCD camera was able to record them. But there are many other CCD images taken by others around the same time that do not show linear markings as this one does.
The "optical illusion" argument tends to fail when CCD images show linear markings similar to those Lowell recorded 100 years before. So other arguments have been put forth proposing that the linear markings are actually a telescopic artifact. Something in the optics of the telescope makes them "appear." This argument was offered in part because other astronomers also reported seeing linear marking corresponding to Lowell's canals, and Lowell for his part reported linear markings on the surface of Venus and Mercury when he trained the telescope on them.
I'm more inclined to accept the optics argument than I am the illusion argument, but even so, it seems to me there's something about the surface of Mars itself that allows and requires the appearance of linear markings in some telescopes but not in others. Given the fact that Lowell's telescope was a huge 24" refractor, and most of the modern amateur CCD images are made through a much smaller reflector or Schmidt/Maksutov Cassegrain telescopes, the "optics" argument becomes difficult to fathom.
[Note (as of 9:30pm Friday, August 10, 2012): a huge limb just broke off a eucalyptus tree down the street, blocking the road, exploding street lamps and power transformers, and causing much to do in the neighborhood. Power went out briefly, but there are wires crossed all over, and the exploded transformer is a big problem. Will probably lose power again as repairs are made. That tree has been there since God was a boy, and it periodically drops fair sized limbs, but this time the branch that snapped was enormous. Good thing nobody (that we know of) was under it at the time. Another big tree on the same property fell a few years ago in a storm, destroying the garage and badly damaging a neighbor's house. A neighbor said he witnessed the whole thing tonight, and he thought it was the end of the world...]
The point I'm making is that Lowell was a very accurate observer who honestly recorded what he saw when he observed Mars. Many of his contemporaries saw similar features. And similar features can still be seen and recorded on the surface of Mars with much different telescopes. I have seen some of them myself. It has never looked to me like the surface is crisscrossed with canals, but it is quite possible to resolve straight lines -- both dark and light colored -- that appear to connect various dark and light spots.
It's not an illusion.
There is something which produces the appearance of these lines, and given the absolute abundance of linear features of all kinds on the surface of Mars -- some of which have never been adequately explained, especially the "searchlight" and other linear features that Mariner 9 revealed, many of which are no longer seen in orbiter images -- it seems worth investigating how the surface features, the Martian atmosphere, the terrestrial atmosphere and telescopic optics all interact to produce the appearance of "canali" as described by Giovanni Schiaparelli in 1877.
|Cerberus Fossae, set of deep troughs with dark interiors, about 1600 km long|
The conventional wisdom is that relief on the surface of Mars cannot be detected with Earth-based telescopes even today, and that the various and numerous fossae on the surface are so narrow -- at most, a few kilometers wide, in the case of the Cerberus Fossae above, only about half a kilometer -- they cannot be resolved by Earth-based telescopes. While volcanoes and craters have long been seen on the surface of Mars, including the vast Valles Marineris canyons, they weren't recognized for what they were because it was impossible to detect surface relief. In fact, prior to spacecraft explorations, it was assumed that Mars had very little surface relief because none could be detected.
This is an unlabeled US Air Force map of Mars from 1962, which will give you an idea of how widely the linear features of Mars were accepted prior to the arrival of spacecraft:
|USAF map of Mars, c. 1962. Clickage will embiggen.|
Lowell's map of Mars, c. 1894 for comparison:
|Map of Mars drawn by Percival Lowell, c. 1894|
Some observers have seen surface features very much like those recorded by Lowell throughout the history of telescopic observations, and some of them can be detected by modern CCD cameras. The appearance of linear markings on the surface of Mars is quite real. They are well and truly there, and they are there at many different scales.
In another post, I may get into the scale issue, for in some ways it may be a key to understanding what is going on.
[Today and tomorrow are travel days....]