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• 1 media/Carnegie Rupes.jpg 2016-05-26T12:00:22+00:00 Mercury 60 image_header 2022-02-04T17:02:21+00:00
  
  So far, Mercury has been studied only by orbital sensing missions. Since the temperature on Mercury alternates between diurnal extremes of up to 740 K when facing the Sun (hot enough to melt some metals) and 90 K (183 below freezing) when facing away from the sun, orbital missions are the most reasonable type to study the planet. Fortunately, the recent Messenger mission orbited for four years and has provided views of the surface features of Mercury. Even though the Messenger craft crashed into the surface of Mercury on April 30, 2015, it often takes time before the data has been processed.  And so we'll start with the Topography of Mercury, which was released in May 2016. This is what Mercury looks like:
  
  If you prefer a static version of that image, this is it, with all of Mercury's features labelled: 
  
  
  Examining the surface of Mercury makes it obvious that cratering has been a dominant surface process.  Let's start there.
   

  Cratering

  We'll only examine one example of cratering on Mercury, the crater Abedin because it's, well, awesome. If you would like to see more craters on Mercury, we suggest you visit the Messenger site and check out the galleries where there are many examples of craters small to large. 
  
  

  Awesome Abedin

  Release Date: April 16, 2015
  Scale: Abedin crater is 116 km (72 miles) in diameter

  
  Of Interest: This mosaic of oblique images highlights the spectacular interior of Abedin crater. The crater floor is covered with once-molten rock melted by the impact event that formed Abedin. Cracks that formed as this melt cooled are visible. Particularly intriguing is the shallow depression that lies amidst the central peaks of the crater and may be volcanic in origin. Color imaging shows that this depression is surrounded by reddish material, as seen at other sites of explosive volcanism across Mercury.
  Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

  
  Fortunately Mercury has more to offer than simply craters.  As one might expect from any terrestrial planet, there is ample evidence of volcanic activity. 

  Volcanoes

  Messenger discovered extensive evidence of volcanism; here is an example from early in the mission:
  

  MESSENGER Discovers Volcanoes on Mercury

  Release Date: July 11, 2008
  Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
  Resolution: 270 meters/pixel (0.17 miles/pixel)
  Scale: This image is about 270 kilometers across (170 miles)
  Spacecraft Altitude: 10,500 kilometers (6,500 miles)
  
  Of Interest: As reported in the July 4, 2008, issue of Science magazine, volcanoes have been discovered on Mercury's surface from images acquired during MESSENGER's first Mercury flyby. This image shows the largest feature identified as a volcano in the upper center of the scene. The volcano has a central kidney-shaped depression, which is the vent, and a broad smooth dome surrounding the vent. The volcano is located just inside the rim of the Caloris impact basin. The rim of the basin is marked with hills and mountains, as visible in this image. The role of volcanism in Mercury's history had been previously debated, but MESSENGER's discovery of the first identified volcanoes on Mercury's surface shows that volcanism was active in the distant past on the innermost planet.

  Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

  In addition to volcanoes, there is evidence of volcanic vents and flows.

  Volcanic vents and flows

  Spectacular Volcanic Features on Mercury

  Release Date: September 29, 2011
  Of Interest: Candidate volcanic vents, lava flow-related channels, and teardrop-shaped features associated with smooth plains deposits in Mercury's northern hemisphere. (Upper left and right) Image and sketch map of the assemblage of volcanic flow-related features. Blunt arrows, flow-front-like embayment; long arrows, teardrop-shaped hills (TH). (Lower left) Pits interpreted as source vents. (Lower right) Teardrop-shaped hills and channel interpreted to be formed by lava erosion.

  When planets expel large volumes of lava, the lava will flow downhill and fill in low areas, which, in many cases are craters.  This results in large areas being covered by volcanic material that forms large plains.  These are referred to as volcanic plains.

  
  Volcanic plains

  
   

  Expansive Northern Volcanic Plains

  Release Date: April 16, 2015
  Scale: The large ghost crater at the center of the image is approximately 103 kilometers (64 miles) in diameter
  
  Of Interest: Mercury's northern region is dominated by expansive smooth plains, created by huge amounts of volcanic material flooding across Mercury's surface in the past. The volcanic lava flows buried craters, leaving only traces of their rims visible. Such craters are called ghost craters, and there are many visible in this image, including a large one near the center. Wrinkle ridges cross this scene and small troughs are visible regionally within ghost craters, formed as a result of the lava cooling. The northern plains are often described as smooth since their surface has fewer impact craters and thus has been less battered by such events. This indicates that these volcanic plains are younger than Mercury's rougher surfaces.
  Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
   

   

  The Small (and Lonely) Planet Guide to Tectonism

  The topography above should make it clear that Mercury does not have a system of plate tectonics, all the large scale features (trenches, spreading ridges etc.) are missing.  But small planets without plate tectonics do all have a special type of tectonism in common.  During geological time, all planets will cool, and eventually run out of internal heat sources. As most substances cool, they shrink (we'll look at an important exception in a later module).  As a result of that shrinkage these worlds get smaller.  If you examine that desiccated apple that may be lurking in the back of your fridge or on the kitchen counter, you'll notice that it has become very wrinkled.  The apple will have gotten smaller by loss of water and the skin remained pliable and wrinkled. By the time planets shrink, they tend to have a very thick crust which is no longer pliable.  Instead of folding the entire crust into wrinkles, faults disrupt the crust and may produce scarps that give evidence to a reduction in diameter.
  Results of the early Mariner 10 mission led to estimates of a reduction in Mercury's radius on the order of 1 km.  The Messenger mission refined that estimate to a more significant 7 km of reduction in radius!  Planetary scientists use two different terms to describe the features. The first, "wrinkle ridges" is pretty self explanatory. It describes a large wrinkle on the surface, no scarp is needed. The outermost layers of the crust have not broken but we believe that there is a big fault just below the surface.  The second term is "rupes", which sounds like it was named after a root vegetable. But the term rupes is simply derived from the latin term for cliff.  So, it does require the cliff to be visible, which means that the fault broke all the way through the crust.
  
  Here are some of the spectacular examples from Mercury:

  Wrinkle Ridges

  Wrinkles on Mercury

  Release Date: April 16, 2015
  Scale: The ridge is approximately 140 kilometers (87 miles) long
  
  Of Interest: This image mosaic shows an unnamed ridge in the northern volcanic plains of Mercury. Wrinkle ridges like this are interpreted to be tectonic in origin and are usually only found in volcanic plains. Six individual MDIS images are part of this mosaic.
   

  Rupes

  
  
  

  Enterprise Rupes

  Release Date: March 16, 2015
  Scale: Scale bar of 100 km provided on the image
  Center Latitude: -31.56°
  Center Longitude: 74.97° E
  
  Of Interest: Enterprise Rupes, (indicated here with white arrows) about 1000 km long and with over 3 km of relief, is the largest lobate scarp on Mercury! MESSENGER has confirmed that the contraction of Mercury has resulted in a global array of lobate scarps, tectonic landforms that are the surface expression of thrust faults. Many lobate thrust fault scarps, such as Enterprise Rupes, are large, often hundreds of kilometers long, and display hundreds to thousands of meters of relief, in contrast to the small scarps more recently being identified on Mercury.
   

  This is what Mercury has to offer:

  Hollows

  And finally, every self-respecting planet has to retain some features that we don't fully understand. Hollows on Mercury are mysterious features that we don't fully understand.  For a full discussion of them read this.
  
  

  Sublime Sublimation, Not Subliminal

  Release Date: September 30, 2011
  Of Interest: View of a section of the floor and peak-ring mountains of the Raditladi impact basin [...]. The individual frames in the mosaic are about 20 km wide. The rounded, depressions, called "hollows" are a fascinating discovery of MESSENGER's orbital mission and may have been formed by sublimation of a component of the material when exposed by the Raditladi impact. This image was created by merging high-resolution monochrome images from MESSENGER's Narrow Angle Camera with a lower-resolution enhanced-color image obtained by the Wide Angle Camera.

   

  Summary

  Mercury is not a welcoming world.  As you saw in our solar system overview, its surface alternates between very hot and very cold depending on the direction the surface is facing.  Its surface is dominated by cratering, the oldest part of that surface is approximately 4 - 4.1 billion years old.  That cratering would have been accompanied by widespread volcanism.  The large volcanic plains are thought to be ~3.8 - 3.55 billion years old.  Eventually that volcanism stopped, Mercury contracted, and then was subjected to a relatively light bombardment. Most likely, the last 3 billion years on Mercury have been very quiet.
  
  Mercury is not a planet you would want to visit. And in fact the best Hollywood treatment for a Mercury-like planet is the prison planet Crematoria in the Chronicles of Riddick.  Even though it is not accurate (breathable atmosphere, fairly warm during the dark), the escape scene does give an idea of what it might be like to see the approaching sunshine on a world like Mercury.  Here you go: