What is the oldest named geologic period in Martian history?

The oldest named geologic period in the standard Martian timescale is the Noachian. It began roughly 4.1 billion years ago and lasted until about 3.7 billion years ago, although the boundaries remain approximate because Mars lacks a complete radiometric rock record.

The Noachian is named after Noachis Terra, a heavily cratered highland region. Mars was more frequently struck by large asteroids during this era, and geological evidence indicates that liquid water may have existed at the surface in lakes, rivers, and possibly a northern ocean. Extensive volcanism also helped reshape the planet.

Noachian terrains are especially important in the search for ancient habitability because they contain clay minerals formed through water-related chemical alteration. The period predates the Hesperian and Amazonian, which are younger named Martian periods. “Pre-Noachian” is sometimes used for even older material, but it is generally an informal interval rather than the standard oldest named period in this three-era timescale.