Over the ramparts

Amid all the distractions of life it can be easy to forget that we live on a rock flying around a big fiery ball at over 100,000 kilometres per hour. Beyond our little world there are an essentially infinite number of planets out there and we’re lucky enough to be alive at a time when we are just beginning to understand their diversity. Ranging from our neighbouring rocky worlds to the gas giants and dwarf planets of our solar system, and beyond to the rapidly growing menagerie of exoplanets. Arguably the alien world to most capture our imagination is Mars, close enough that shifting details on its surface have been tantalising observers for centuries but still at a distance where any mission requires years of preparation and a speed dial to a cardiologist.

I started my PhD in late 2011 and I’ve been investigating how certain minerals may play a role in our search for organic molecules on Mars. However, I want this blog to be focussed on other research and to give a broad overview of the planet. I hope to bring you weekly to fortnightly posts about how Mars has both surprising similarities to Earth and many extreme differences. I’ll also try and occasionally write summaries of new or classic papers and I’m always open to suggestions for posts in the comments or through twitter/reddit. I will probably post about my own research in the future (almost certainly excessively when I eventually manage to publish something, comments telling me to shut up are encouraged at this point). However, I’m also liable to make (hopefully rare) mistakes as a combination of a third year PhD workload and not knowing everything. If you identify any errors or take issue with something please let me know and I will update the offending article and credit you.

In this first post I wanted to give a quick overview of one of my favourite discoveries on the red planet. If you visualise a crater the first thing you probably think of are the classic sharply defined bowls that cover bodies such as the Moon and Mercury. These craters are often surrounded by a layer of bright powdery material blasted out by the impact that formed them, a sheet known as an ejecta blanket. Such features are present on Mars but many appear radically different to the ejecta blankets seen on the Moon and Mercury. Martian craters are commonly surrounded by sheets that seem to have flowed around objects rather than landing on top of them and they terminate in an abrupt escarpment called a rampart. They are therefore called rampart craters. The ejecta blankets look a bit like a splat into a muddy puddle and the way they form is not too dissimilar. The wonderfully named Tooting Crater is an excellent example and is shown in the figure below alongside a more familiar looking lunar crater.

The lunar crater Proclus is partially surrounded by rays of fine scattered ejecta. Tooting Crater to the left appears dramatically different, ejecta is thick and layered and terminates in high relief ramparts. The ejecta looks fresh indicating this is a relatively young rampart crater.

The lunar crater Proclus is partially surrounded by rays of fine scattered ejecta. Tooting Crater, in the right hand image, appears dramatically different, ejecta is thick and layered and terminates in high relief ramparts. The ejecta looks fresh with few craters indicating that this is a relatively young rampart crater. (Credit to http://astrobob.areavoices.com/tag/tycho/ and http://www.higp.hawaii.edu/~pmm/planetary.html)

Interpretation of rampart craters has led scientists to believe that they form when an impacting asteroid or comet hits a substantial reservoir of underground water ice. The ice melts and fluidises the ejecta, which then flows along the surface like a nightmarish surprise giant mud flow. This is an awesome process to imagine but it’s also of academic interest as well. Smaller craters tend not to form rampart craters because they don’t dig deep enough into the crust to encounter the water frozen below. However, as you move away from the Martian equator the minimum size needed for a rampart crater to form decreases. This suggests that the hidden ice layer under the Martian surface exists at shallower depths at higher latitudes.

Rampart craters have also been discovered on Earth (such as the Ries Impact Crater) and the icy moons Europa and Ganymede. In these planets ground ice or water is obviously present but Mars is often portrayed as a dry barren world. While this may be true at the surface it seems that substantial ground ice exists at depth and rampart craters are one line of evidence revealing this hidden subsurface layer. Mars is full of secrets and this is a good example of how one piece of surface evidence can start to tell us more about the planet’s hidden environments. I will cover rampart craters in greater detail and the current debate over them in a future post but in this introductory article I just wanted to give you an overview of how they most likely form and their significance to Mars science.

Thanks for reading, more coming soon!

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One Response to Over the ramparts

  1. Pingback: A beautiful new impact crater on Mars | Fourth rock from the Sun

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