Imagine a river or lake on Earth. What would it look like if you took all the water out? Something like this:
|Dry Riverbed in the Himalayas|
On Earth, when a riverbed dries out it leaves a depression, or indentation in the Earth.
So then why on Mars are riverbeds and lakes inverted, or raised features, like in the below image of Gorgonum Basin?
|Gorgonum Basin. Credit: HiRISE (Click through for HiRISE captioned image)|
or Iani Chaos?
|Iani Chaos. Credit: THEMIS (Click through to post on this image)|
It all comes down to erosion. The weight of the water condenses and compacts the sediment beneath it, making it much harder and more resistant to wear from erosion. Thus, over the course of millions (and sometimes billions) of years, the ground surrounding the river bed gets eroded much more rapidly than the riverbed itself. This results in the inverted features that you see on Mars today.
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