Hello, friends!  Welcome back to this year’s A to Z Challenge.  My theme for this year’s challenge is the planet Mercury, an often under-appreciated but still fascinating little world.  In today’s post, L is for:

LOBATE SCARPS

The planet Mercury is shrinking!!!  How do scientists know this?  Well, the story begins with photos taken by NASA’s Mariner 10 space probe, back in the 1970’s.  Mariner 10 found these long, serpentine features on Mercury, winding their way across the planet’s surface.  Scientists decided to call these strange features “lobate scarps” (a sciency way of saying “curvy cliffs”).

Mariner 10 only photographed part of Mercury, but in the 2010’s, NASA’s MESSENGER space probe was able to map almost all of the planet’s surface, confirming that these lobate scarps are everywhere.  Rough, heavily cratered terrain?  Covered in scarps.  Smoother, flatter terrain?  Interrupted by scarps.  Young terrain, old terrain, middle-aged terrain?  Doesn’t matter.  The lobate scarps are all over the place (though one source I looked at suggested that some parts of Mercury are more scarp-y than others—especially parts of the southern hemisphere).

Images from both Mariner 10 and MESSENGER show scarps that are hundreds of kilometers long, and by looking at the scarps’ shadows, scientists are able to determine how tall they are—up to three kilometers in height, in some cases!  Just imagine that: a cliff three kilometers tall!  The tallest cliff on Earth isn’t even half that high.

The most likely explanation for all this is that Mercury is shrinking.  The planet’s core is cooling off, plus gasses trapped beneath Mercury’s crust are slowly leaking to the surface and escaping into space.  This ongoing loss of internal heat and mass puts stress on the planet’s crust, causing thrust faults and earthquakes Mercury-quakes.  I’ve read several sources that said Mercury is shriveling up like a raisin, but it sounds to me more like Mercury is very slowly crumpling like a tin can.

I’ve seen many different estimates for how much Mercury has shrunk, from as little as 2 kilometers in radius to as much as 20.  Since lobate scarps are found on young and old terrain alike, this process of global shrinkage must have been happening for billions of years, and it’s likely continuing to happen to this day.

WANT TO LEARN MORE?

Here’s an article from The Atlantic, quoting one of those larger estimates for how much Mercury has shrunk.

And here’s an article from Space.com, quoting a much lower estimate.

And here’s another article from Space.com, which talks about how some parts of Mercury are more scarp-y than others.

Lastly, if you want to get a better sense of what a lobate scarp looks like, click here to see a picture of one cutting across one of Mercury’s craters.