Chains of Craters
Most of the time, craters occur randomly across planetary surfaces. They may seem to occur in clumps or clusters, groupings or voids, but because craters usually form from an extraplanetary impactor striking the surface, there is no preferred location on a planet.
This is one reason that actual chains of craters are interesting features that some people like to study. Chains of craters will usually form in one of two ways. The first is through a process that is endogenic – meaning that it is produced by forces on or within the planet itself. These are known as “pit craters” and form overtop an evacuated lava tube when the surface collapses into it. We see these on Earth in regions of volcanism like Hawai’i where we even have “Chain of Craters Road” on the Big Island. We also observe these all over the major volcanic regions of Mars. And, they are present on the moon.
An aerial (left) and ground-based (right) view of the “Devil’s Throat” pit crater on the Big Island of Hawai’i. Image courtesy USGS.
Very large pit craters on the flanks of Ascraeus Mons on Mars. Image from THEMIS data (ASU).
Since Moon Zoo users are asked to identify impact features, it’s the second type that we’re more interested in: Secondary crater chains. Secondary craters form when an extraplanetary impactor strikes the surface, creates large, cohesive blocks of ejecta, and these blocks strike elsewhere on the surface and create their own craters. Hence, they are secondary to the primary impact (astronomers are not that creative in naming things). Because of the way material is ejected, these can occur randomly spaced around the primary impact, in clumps, or in long chains. We ask you to identify crater chains in Moon Zoo in part to identify regions of secondary craters.
Chains of secondary craters. Small white dots indicate secondary craters that are larger than about 1 km in diameter. Image from CTX data (Malin Space Science Systems).
Chain of secondary craters. Image from LROC NAC data (ASU) and found by our forum member Tom128.
Understanding secondary craters is one of the “new old frontiers” in crater populations research. Secondary craters were first identified by a fairly famous planetary scientist named Eugene Shoemaker (remember Comet Shoemaker-Levy 9 that hit Jupiter in 1994?) in the early 1960s based on telescopic and early space-based photographs of the moon. The opinion of secondary craters – whether they are statistically important at a given size, how many are produced, how they are produced, the pre-requisites for their production, and their populations – has varied widely throughout the last half century in the planetary community. In fact, secondary craters were generally ignored. Until recently.
Over the last several years, interest in secondary craters has picked up after a pair of papers in 2005 that identified 10s of thousands of secondary craters produced across Jupiter’s moon Europa from a single large impact (Pwyll), and identification of 100s of millions of secondary craters from a single, young impact crater on Mars (Zunil). (The papers are Bierhaus et al. (2005) and McEwen et al. (2005).) These began to reignite debate in the planetary geology community about how important secondary craters are in crater populations.
This is still an open debate. People generally fall into three or four camps. The first would be those who say that secondary craters don’t really exist at all except in localized areas. Very few people actually believe this. The second is that secondary craters exist, affect crater statistics, but that on a surface with a large number of craters the “background” secondary crater population is generally uniform and we don’t have to make any special consideration for them. The third is that secondary craters are important, are non-random, and need to be taken into account when using craters for most applications. A fourth potential camp is that they are important but are hopelessly tangled with the primary crater population so there’s “no use crying over spilled milk,” as the saying goes. I personally fall mostly in the third camp and a little in the second.
But, a better understanding of secondary craters is one of the main science goals of Moon Zoo. Helping to identify chains of craters – one of the few ways secondary craters appear on a planetary surface – is an important step in this goal. After all, we can’t really study them if we don’t know where they are! Many large secondary craters are known over the lunar surface, such as those produced by the relatively “youthful” Mare Orientale (roughly 3.8 billion years old) (see image below). With Moon Zoo, we’re asking you to look at very small-scale, meter-sized features as opposed to 10s of kilometers, and there is simply too much of the lunar surface for planetary scientists to do it all.
Large crater chains emanating radially from Mare Orientale on the Moon. The white lines are drawn above the chains so you can still see them. Image from LROC WAC mosaic (ASU).
So next time you’re identifying features on the moon, take a moment and look for any kinds of crater chains. Post them in our forum. If you’re adventurous, take some time to explore the lunar surface on your own with the wonderful ACT-REACT tool made available by the Lunar Reconnaissance Orbiter Camera (LROC) team. For more information on how to use it, see this helpful thread on our forum. Posting permalinks from the ACT-REACT tool is a fast and easy way to draw attention to a particular region and allows us to extract the relevant images and use them in our next round of Moon Zoo images.