CN FORUMS ARCHIVE Telescope Reviews: Resolution limit on planets

Reading over some of the notes with great images of Mars I am
beginning to see a little misunderstanding or at least errors in
interpretation of what observers believe they are seeing on these
images. First, a review of HST images of Mars over the past decade
will give you some idea of what has been revealed by these
magnificent up-close images, but how the surface details become
clearer and more distinct as Earth approaches Mars closer and
closer. In 2001 HST image of Mars we saw what appeared to be
features that could only be interpreted at craters and volcanoes. In
2003 the two planets were even closer and images from HST revealed
even more features that appeared as craters and volcanoes. However,
is this evidence that we are actually seeing topography or even
relief? I should caution those who may jump to these conclusions
that many times an observer interprets visual sensations that may be
illusions and even trained cartographers will at times interpret
features on Mars with a different opinion.

Another problem I have seen is that bright desert regions on Mars may
appear like a dust cloud to some and a dull haze area to others.
These features are difficult to detect at best, but when alerted by
someone that a dust cloud has been identified in a certain region --
this only heightens the excitement for some observers and the game of
illusion begins. It spreads until it becomes quite obvious no dust
exists. Dust cloud are bright in red light, they move and obscure
stuff on Mars. You will certainly see how bright red they are the
first time you see one, and when they cover up a dark feature they
really leave it looking weird. In other words, you will know a dust
cloud when you see one.

While I will not proclaim that everyone has accepted their visions of
what appears as topography in these images never the less I read much
excitement on Internet that many have believed they are actually
seeing true relief on Mars in their images. For what it is worth I
would like to leave yawl with the quote below, by repeating a letter
to S&T by Cornell's Jim Bell wrote to Roger Sinnott in 2003:

"While I acknowledge that there can be moments of incredible clarity
(some would even say clairvoyance) at the telescope, I have never
seen any convincing evidence for the observation of craters on Mars
based on their topography. It's not hard to be skeptical: the
elevation differences are quite small and the phase angles are not
ever really that large. However, I find it easy to believe that
observers have seen craters on Mars in the past, not based on their
topography but instead on their albedo variations.

A good example can be seen in the June 26, 2001 Hubble Space Telescope
image, which is among the highest resolution images of Mars ever
obtained from Earth. One can easily see a number of large and even
smallish craters in the Meridiani and Arabia regions, but they are
resolved because they are just shallow holes in the ground filled
with dark sand scattered amidst a "sea" of bright dust. The craters
in this area (and many others) are natural sinks for coarser-grained
and thus darker particles, leading to these large albedo contrasts.
Even the keenest observer could mistake the dark inter-crater
deposits for shadows, especially for cases where the deposits
fortuitously appear biased towards the direction that one might
expect for shadows."

Now that everyone is mad, please stop and think about what Jim and,
now, I have said. Interpretation is in the eye of the beholder.