Basic Feature Guide

The following guide is designed to accompany Planet Mappers: Mercury Edition Simply Craters.

There are many, many different types of features on planetary surfaces such as the moon, though many of them are variations on themes. They generally fall into a few kinds of feature classes: Craters, volcanic, linear (can be tectonic or volcanic), and a broad “other.” All of these are interesting in their own way and we want you to help us find them.

Light Albedo Feature

Bright Albedo Feature

When craters are formed, the impact scatters about the crater a blanket of bright material. Called an ejecta deposit, it is made of pulverized material. This ejecta appears very bright when first formed, but slowly fades as it ages due to interactions with micrometeorites and the radiation of space. Studying young craters is an important part of planetary science, but to do that, we need to know where they are!

Dark Albedo Feature

Dark Albedo Feature

Dark features can come from either volcanic eruptions, or through special impact craters. Just like the Earth’s surface is made of many layers of soil (top soil, clays, bedrock, etc) Mercury’s surface is also made of multiple layers, which happen to have different colors.

Sometimes an impact event that forms a crater punches through the top, light-colored layer and excavates a darker layer from underneath.
In other places, dark material could be evidence of volcanic activity on the surface of Mercury.

Crater Chain

Crater chain

These streaks of craters can be formed through a broken object hitting Mercury or material scattered during a “primary” impact event.

Sometimes, when a large crater is formed, the impact can hurl large blocks of material (ejecta) that land in a row. This forms chains of craters that we call “secondary craters” (since they formed as a result of the main, primary crater). If you see a bunch of craters in a row, the most common explanation is that they formed from these secondary impacts.

In some, much rarer cases, crater chains can also form when a string of broken up asteroid or comet pieces hits Mercury. Gravitational disruptions and impacts between objects can sometimes break up comets and asteroids. These broken up pieces may continue along the same orbital path, but spread out as they travel. When these strings of rock or ice hit an object like the moon the can form crater chains.

Secondary Craters

Secondary craters

Clusters of secondary craters form when a large impact somewhere on the surface throws up debris which rains down on another part of the surface. These smaller craters are thus secondary to the main crater event.

The image at right shows two secondary crater clusters highlighted with yellow arrows.

Hollows

Hollows are a unique feature to Mercury that were first discovered in MESSENGER images in 2011. These fresh-looking pits are often seen in the floors of craters. They may even still be actively forming on the surface.

It is hypothesized that hollows form when the intense solar wind blasts away volatiles, or substances that boil away easily, as they are unearthed by impacts.

Boulders

Boulder fields are strewn across planetary surfaces, but they are highly non-uniform. Sometimes we find them at the bottom of craters, sometimes on crater walls. Other times they are strewn around the rims of large craters.

And yet, there are many craters that don’t seem to have any boulders in or near them whatsoever.

Concentric Craters

Concentric craters

Sometimes craters look like there is one crater nested just inside a slightly larger crater. This is not a freak accident, where one crater impacted just inside another one (though that does occur, but very very rarely).

Scientists think these craters form when the region impacted consists of two layers of very different material; a weak rubble (or regolith) layer on top of a harder rock layer. The weak layer is more easily ejected by the impact than the hard layer, forming a slightly larger crater in the regolith than in the rock below.

These craters allow us to understand the depth of different layers of material in different regions.

Linear Feature

Linear feature (bottom right)

The surface of Mercury is criss-crossed by linear features which we’ve built a special tool for you to mark. Examples include the long lobate scarps, or cliffs, that seem to cut across craters and plains. Linear features also include graben, which are like cracks or troughs, which are thought to form from tectonic processes that pull the surface apart.

Some of these linear features are indeed curved, not straight lines. Use the linear tool as shown in the tutorial video to make multiple line segments which curve to follow the features. Some example linear features are provided in the images here.