What is the primary component of the northern polar ice cap on Mars?

The primary component of Mars’s northern polar ice cap is water ice. The permanent, or residual, north polar cap is made mainly of frozen water, although seasonal carbon-dioxide frost and snow cover it during part of the Martian year.

Mars has two polar regions with important differences. In the north, carbon dioxide ice largely disappears during northern summer, leaving a residual cap dominated by water ice. In the south, a more persistent layer of carbon dioxide ice remains over much of the residual cap, which is why the two poles are often described differently.

The northern cap is not merely a frozen reservoir; it is also a record of Martian climate history. Its layered deposits preserve dust and ice accumulated over repeated changes in Mars’s orbit and axial tilt. Radar observations have revealed internal layers that help scientists reconstruct ancient ice ages and the movement of water between the poles and lower latitudes.

The main confusion is treating all visible polar frost as the permanent cap. Seasonal carbon dioxide ice is important, but it is not the principal material in the residual northern cap.