Cratering

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