If it looks like a ridge-its a valley…1st rule of Mars.
I stole the title of this post from our discussion forum. I hope the author – Paul Johnson – forgives me. It expresses well the feeling, that many people had while arguing if the lines that they see are depressions or elevations. But this 1st rule is not 100% correct. Let me explain!
Like this example:
Do you see those squiggle lines as channels or as ridges? Chances are, you are seeing those as ridges. What about the next image?
Of course, you understand that this is the same image, only in the second case we’ve rotated it 180 deg.
So why do you see same features once as positive topography and once as negative?And oh, by the way, how to know which one is the real way?
Very often I get confused by this myself. But i know, that the trick lies inside my own brain. The brain is an amazing machine for the fast feature recognition. It is very useful in everyday life: we need to fast react to the objects around us, so to become efficient, brain developed “fast tracks” that are very useful in most everyday situations and help making microsecond-fast decisions.
One of them is finding faces in everything around us: leaves, clouds, wallpaper, Martian landscape… It is helpful for a human to recognize another human! That’s why people keep finding faces everywhere, also on Mars. Interesting stretch to this is our amazing ability to see a face “correct way” in the negative-face optical illusion. Here you can see a video about it.
Another hard-wired fact for our brain is that light comes from above. You must agree, most of time it does. So, to decide if the surface bends away from us or towards us, our brain assumes that light is from top (and somehow top-left for most people, but not 100% of people).
Unfortunately, what happened in our project is that most images have light coming from lower right of the image. Opposite to what our brain prefers! This is why it is so hard to see spider troughs as channels. As in the first image above.
The fact that most our images got that “unlucky” illumination is not a coincidence. Here is why:
You can imagine Mars as a globe and a spacecraft that flies around it following almost polar orbit, i.e. it goes from North to South, flies above almost south pole and flies from South to North on the other side of Mars. And then repeats all again.We do not rotate our camera relative to the spacecraft, we only can rotate/tilt the spacecraft as a whole. The image of the southern polar areas, that the spacecraft will take on the descending branch of the orbit (flying from North to South) will have sun light coming from below, because sun comes from the equatorial area, same as on Earth. If we take image on ascending branch (moving from South to North), sun will come from the top of the image.
Now, one side of Mars has day, another one is in the darkness of the night, so we can not image there. This means, we mostly get one of those branches for imaging. It depends on the exact orbit parameters, which one.
As a result, most of the images you have got to see have sun from lower side. Not all of them, because rarely, but we do image on the other side of Mars, mostly in summer, when polar areas get polar day.
It’s unlucky, that we got the most problematic sun position for our project. Preparing the images we first had an intention to map them to the Martian surface, but it turned to be more time consuming than useful. We anyway wanted you to mark dark fans and blotches (not spider channels!) and for that not-mapped images are good enough.
So, the 1st corrected rule of Mars: when you see an image of Martian landscape for the first time, do not believe your brain straight away. First check the direction of the sun. You can see which part is in shadow and which is illuminated and then figure out the topography.