Hi this is Emily from MinuteEarth.
A little while back, Henry made this great MinutePhysics video about how the weird symmetry
of light and shadows on a landscape can make it really hard for your brain to tell which
features are popping out and which ones are dented in, especially if you’re not sure
where the light is coming from.
This re-ignited an old debate we had here at MinuteEarth, about THIS photo of the surface
of Mars.
That sinuous pattern on the landscape is an ancient stream channel – which is cool because
it means that liquid water used to flow on Mars – but that crater above it has a shadow
in the top left, which means the light in the image must be coming from the top left,
which seems to suggest that the stream channel is actually sticking up above the surrounding
landscape.
But...that can’t be right, right?
I mean, river channels should obviously be valleys, since they’ve been carved into
the ground.
This seemed like such a basic fact to me that, even when everyone else on my team said they
saw this as a ridge, my brain kept telling me it was a valley.
It turns out that, not only was my brain lying, but this stream channel is an example of a
weird geological process that essentially turns landscapes inside out, creating inverted
relief.
If you live in a cold climate, you may have actually seen something like this happen with
compacted footprints in the snow – as the sun melts the fluffier snow around them, they
can end up sticking out, like this.
The same thing can happen to a river valley.
For instance, say you’ve got a stream running through a desert.
Sometimes there is so little rain that the stream dries up.
As it does, some groundwater is actually drawn upward by capillary action, and as it rises
and evaporates, dissolved minerals in it are left behind and precipitate out, coating the
sediments in the riverbed and cementing them together.
As the stream flows and dries up over and over again, more and more cement accumulates,
making the riverbed harder and more resistant to erosion than the surrounding rock.
Over the eons, wind erodes that softer rock, turning the channel into an inverted version
of its former self.
We can’t know exactly how this Martian riverbed got turned inside out without getting down
on the ground and digging around a little, but we can make some good guesses by studying
inverted relief on our home planet.
You can even find your own examples on Google Earth; just be warned that trying to figure
out what’s rightside-up and upside-down could turn your brain inside-out.
Hi, this is Ever and I illustrated this video.
Lots of you have asked how I create animated effects like this one.
It’s not magic; in fact, it’s pretty easy to do if you’ve got the right tools - and
the right teacher.
I was never one for formal school, but the online classes from experts at Skillshare
are perfect if you’re a self-directed learner like me.
You can try out skillshare’s impressive library of 17,000 how-to classes on everything
from video editing to T-shirt design.
The first 200 people to click the link in the description will get a 2 month free trial.