Sponge - Why do astronomers believe the lunar highlands are older than the maria?

Surface Rocks - There are basically two kinds of surface rock on the Moon. Highland rock - 2.9 g/cm3, rich in aluminum, lighter in color, crust material Maria rock - 3.3 g/cm3, richer in iron, darker in color, mantle material

Many rock samples are impact breccia, a rock that consists of many smaller rock fragments stuck together due to meteoric impacts.

The maria are believed to have been formed by great volcanic upwellings of mantle material about 4 billion years ago. These may have been caused by lunar impacts with very large space debris.

Meteoric collisions caused the layer of pulverized ejecta that covers the Moon to an average depth of 20 m.

This “soil” is also called lunar dust or regolith, which means “fine rocky layer”. It is thinnest on the maria (10 m), thicker on the highlands (over 100 m in some places). (The term regolith is also used for the soil on other moons and planets.)

The bootprints left in the regolith by the Apollo astronauts will remain for millions of years due to the slow erosion.

Lunar regolith contains no organic matter or water, unlike Earth soil.

Most all lunar craters are meteoric, but some are volcanic.

Volcanic rilles are ditches where molten lava once flowed.

No rocks on the Moon have been found to be younger than 3 billion years, so this was the time of the last volcanic activity.

Lava flows