A Lunar Island Surrounded by Lava

This image shows the Western Crisium Kipuka centered in the middle surrounded by the Mare Crisium. The kipuka appears as a gently-sloping raised landform in the center of the image. Shades of yellows, oranges, and reds represent different elevations of the uplifted WCK, with fractures covering the surface shaded blue. There is a crater on the eastern edge of the kipuka. North is towards the top of the page. — Color-shaded relief map showing the elevations of the Western Crisium Kipuka (WCK) located on the western edge of Mare Crisium. The elevation ranges from -4,218 m (blue) to -3,195 m (red) [NASA/GSFC/Arizona State University].

We typically think of islands as pieces of land surrounded by water, but by replacing that water with lava, you can get what is known as a kipuka. Kipukas, a Hawaiian term for an older pocket of land surrounded by younger lava flows, are present on the Earth as well as on the Moon. The topographic model below shows a feature thought to be a kipuka, the Western Crisium Kipuka (or WCK), located in Mare Crisium. The yellows, oranges, and reds show that it is higher in elevation than the surrounding area, with lower-lying fractures crisscrossing its surface.

In this image, which is the same as the featured image at the beginning of the post, the WCK is centered in the middle surrounded by the Mare Crisium. There is a key in the bottom left corner that shows the elevations of the WCK ranging from -4,218 m (shaded a dark blue) to -3,195 m (shaded red). The fractures cutting across the kipuka are yellow or light blue, indicating that they are lower in elevation. North is towards the top of the page. — Color-shaded relief map showing the elevations of the WCK from a digital terrain model (DTM) mosaic created by the LROC team (NAC_DTM_CRISMMELT). This mosaic is centered at about 15.2°N, 50.3°E [NASA/GSFC/Arizona State University].

In recent studies of this and other kipukas in the Mare Crisium region, scientists proposed two different origins for these islands of older material. One study suggests that the impact that formed the Mare Crisium basin was enormous and had enough energy to melt the material inside the crater, creating what is known as impact melt. After this material cooled, magma from the interior was emplaced underneath the surface, resulting in an area of high elevation. Lava then flowed into the basin, burying all but the highest points, which resulted in these Mare Crisium kipukas.

Another study suggests that rather than originating from impact melt like some of the other kipukas in the Mare Crisium region, the WCK specifically resulted from post-Crisium volcanism. While the kipukas in the northern and eastern areas of Mare Crisium seem to be composed of melted highland rock, the ejecta from impact craters superposed on the WCK has a composition closer to that of basalt, a volcanic deposit that makes up the lunar mare.

Controlled low-Sun NAC mosaic showing the WCK centered in the middle surrounded by the Mare Crisium. The boundary of the kipuka (where the fractures end) is traced in white. There is a small crater on the right-hand side of the kipuka (labeled Yerkes V), as well as a partial crater in the top right corner of the image (labeled Yerkes E). North is towards the top of the page. — Controlled NAC mosaic showing the WCK region. The margin at which the fractures covering the WCK terminate mark the boundary (traced in white) between it and the surrounding Mare Crisium [NASA/GSFC/Arizona State University].

While the origin of the WCK is still being debated, both studies agree that it was uplifted by magma emplaced from below. They even relate this process to that of floor-fractured craters; in fact, we see similar large fractures covering the WCK. These fractures form because when the WCK was uplifted, the tension in the surface caused it to crack, similar to cracks that form when bread rises in the oven.

This feature must be older than the surrounding mare to truly be a kipuka. Perhaps if astronauts were to visit this area during a future mission, they could collect samples to determine the age of the features and use the presence of these fractures and the raised surface to identify where the lunar mare ends and the kipuka — if it even is a kipuka — begins.

Explore the WCK and its fractures in the Zoomify window below!

Lunar Kipuka

Faulted Kipuka

Fractured Impact Melt

Picard Crater Impact Melt

Karpinskiy Floor Fractures

Published by Melissa Thomas on 1 April 2024