What are those long, smooth, longitudinally-opposing areas from pole to pole on Mercury? Look at this model and spin it - Mercury 3d model


Are those artifacts of stitching together images, or does Mercury really have these opposing, huge pole to pole lines of smoothness?
During the Messenger mission, there were (among many others) two global images of Mercury taken.
One is the so called &quot;Low Incidence Angle Mosaic&quot; which is created from combining images taken at local noon when the Sun is high in the sky and the amount of shadows is reduced. This image is useful to see different types of surfaces.
The other one is the &quot;Morphology Mosaic&quot;, taken at a higher incidence angle at local morning / afternoon. This shows enhanced shadows and therefore provides a better view of craters, but there are some areas were not photographed at the right time.
The Mercury 3D model is filled with images of the Morphology Mosaic.
You can see the same smooth areas present in the mosaic if you view it in higher resolution at the link below.
https://messenger.jhuapl.edu/Explore/Images.html#global-mosaics 
And, comparing the two mosaics, the smooth looking area is a copy of the low incidence angle image that has been used to replace missing images. So, the only difference between the areas is the incidence angle of the light, making many of the craters almost disappear. The resolution of the image of both areas is the same, but shadows are not.
This is also described in the image caption provided by NASA:

Any remaining gaps in the global mosaic were filled by using images
obtained in support of the high-incidence imaging campaign.

https://photojournal.jpl.nasa.gov/catalog/PIA17230