On the Moon's surface lie a set of bizarre looking features called Irregular Mare Patches, or IMPs. Here's one, called Ina.
The enigmatic Irregular Mare Patch (IMP) called Ina. Source: NASA LROC on Wikipedia
This IMP is a 2 km depression in the surrounding lava plains, and consists of multiple smooth as well as rough areas. The smooth areas seen in the picture are not actually low craters, but elevated hills with flat/rounded tops! The rough areas on the other hand are depressed in elevation.
You see the bulging areas as depressions and vice versa because your brain is expecting the light to be coming from one direction whereas its actually coming from the other. Here’s an oblique view of Ina to make things clear.
The contrast between the smooth and rough areas stands out in this oblique view of Ina. Source: LROC blog
So how was this weird and fascinating terrain created? Ina lies atop a volcanic dome and is thought to have formed as a result of basaltic volcanic eruptions in the lunar past. As to the nature of these eruptions that formed such weird terrain, nobody knows yet. Knowing the age of Ina is one way to constrain the origin of its unique morphology. The trouble is, however, lunar scientists are not sure about its age either.
Some scientists propose that Ina is less than 100 million years old primarily because its surface has only a few larger impact craters (> 20 meters). Another proposal is that Ina formed as a solidification of magmatic foam, which must give it a highly porous structure. Such a structure would skew the crater counts and how we measure the age of a feature. As such, Ina could also be as much as 3.5 billion years old!
Landing near Ina and possibly using a rover to assess both the smooth mounds and uneven terrain could solve Ina’s age mystery. It would be a crucial step in understanding how such IMPs form.