The Case of the Lobate Scarps

Planetary Shrinkage in Action

Scarps cutting across craters on the surface of Mercury. These cliffs formed as the planet cooled and shrank, early its history. Courtesy NASA.

Welcome to and the page for the November 25 episode of 365 Days of Astronomy.  If you haven’t listened to the podcast, go check it out!  If you just listened — here’s some more information about those lobate scarps she talked about in the podcast!

For many years, the jagged cliffs on Mercury — offically called “scarps” and “lobate scarps” were a kind of puzzling mystery to planetary scientists. There have been a number of suggestions as to how these cliffs formed, including deposits by volcanic flows and cracks induced by large-scale impacts on Mercury from incoming projectiles.

With the advent of the Mariner 10 mission in the early 70s, plus flybys by other spacecraft, we got a lot of images of scarps over the years.  The MESSENGER mission currently circling the planet is giving us the highest-resolution imagery yet, showing scarps set amid craters and other surface features.

The story now seems to be that the scarps mostly formed when the planet was still cooling off, early in its history. As this rocky world cooled, it shrank and whenever you shrink a solid surface, it cracks. Some of the surface areas were thrust up a bit by the cooling action — called thrust faulting — and those cliffs are the scarps we see today. Some scarps seem to cut across craters — which  means that the surface was cratered long before the surface cooled and cracked.  Of course, that makes perfect sense, since Mercury (and the other inner planets) were subject to intense bombardment by solar system debris throughout the early history of the system.

Our inner planets swept up a lot of that debris and got intensely cratered as a result. On Venus, the craters have long been mostly surfaced over by ongoing volcanic deposits. Here on Earth, the record of the early and late bombardments is mostly erased by erosion, volcanic eruptions, and other processes. Mars shows some ancient craters, but the record of the earliest bombardment is also gone, probably erased by erosion and the action of the wetter, warmer Mars of eons past.

So, we’re left with Mercury as an open book showing us those days of yore, when newborn planets were hot, bombarded, and starting to cool. It’s a something of a treasure trove of things to learn, and the MESSENGER mission is uncovering more treasure all the time!

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