Harpalyke’s grey colour and what it might mean

A small colour difference that complicates Jupiter’s moon family tree.

Published 2026-10-23

Colour is one of the few things astronomers can measure for a moon a few kilometres across.^[2]

Measuring colour

Astronomers compare brightness through different filters. B−V compares blue and visual light; V−R compares visual and red. Higher values mean redder surfaces.

Harpalyke and two others

Harpalyke, Praxidike and Iocaste display similar grey colours, averaging B−V 0.77 and V−R 0.42.^[4] NASA likewise describes Harpalyke as a similar grey to those two.^[2]

And Ananke?

Ananke itself is on the boundary between grey and light red.^[4] If the family came from one body, why the difference?

A possible answer

Some studies suggest secular resonance has mixed members of the Ananke and Pasiphae groups.^[4] Harpalyke’s colour is a small clue in that bigger puzzle.

Numbered citations link to the sources list on the Harpalyke overview page.