• Conglomerate vs. breccia: Both conglomerate and breccia are coarse-grained and poorly sorted clastic sedimentary rocks. However, the grains in conglomerate are rounded whereas the grains in breccia are angular.
• Sandstone vs. shale: These are both clastic rocks. Sandstone is medium-grained so you may be able to see individual sand grains. Shale is fine grained, so individual grains will be too small to see and it will feel smoother than the sandstone. Also, shale is distinctive in that it breaks apart in layers.
• Travertine vs. limestone formed in a reef: Travertine and limestone are both made of the mineral calcite. One way to distinguish between these rocks is to look for fossils of reef-building organisms in the biochemical limestone. Travertine is a chemical sedimentary rock, so it won’t be built from the shells of marine organisms.
• Chert vs. gypsum, an evaporite rock: Both are chemical sedimentary rocks. Chert is made of silica (the same composition as quartz) and gypsum is a mineral as well as a rock. You could test the mineral properties of the samples (chert will scratch glass, but you can probably scratch gypsum with your fingernail), but you could also add water to the samples. Gypsum will dissolve but chert will not. (But don’t do that if you want to keep your gypsum sample safe.)

Mysterious Rocks on Mars

The image below was taken by NASA’s Curiosity Rover (field of view is approximately 1.2 m across.) The image shows a slab of red stone surrounded by sand. The slab is covered in a complex network of interconnected lines.

Which sedimentary structure might the lines represent?

The pattern of lines on the slab of rock is most likely from mud cracks.

What would that sedimentary structure imply about past environments on Mars?

The presence of mud cracks is evidence that Mars had abundant liquid water in the past.

Read more about this image.

View the original article on the NASA JPL website.

Jan. 17, 2017

The network of cracks in this Martian rock slab called “Old Soaker” may have formed from the drying of a mud layer more than 3 billion years ago. The view spans about 4 feet (1.2 meters) left-to-right and combines three images taken by the Mars Hand Lens Imager (MAHLI) camera on the arm of NASA’s Curiosity Mars rover.

Mud cracks would be evidence of a time when dry intervals interrupted wetter periods that supported lakes in the area. Curiosity has found evidence of ancient lakes in older, lower-lying rock layers and also in younger mudstone that is above Old Soaker.

MAHLI was positioned about 3 feet (90 centimeters) above the surface when it took the component images on Dec. 31, 2016, during the 1,566th Martian day, or sol, of Curiosity’s work on Mars. This observation was planned as part of assessing a hypothesis that the target preserves evidence of drying mud. The location is within an exposure of Murray formation mudstone on lower Mount Sharp inside Gale Crater.

The slab bears a network of four- and five-sided polygons about half an inch to 1 inch (1 to 2 centimeters) across, which matches the pattern commonly formed when a thin layer of mud dries. Some edges of the polygons are ridges of material the same color as the surrounding rock. This could result from a three-step process after cracks form due to drying: Wind-blown sediments accumulate in the open cracks. Later, these sediments and the dried mud become rock under the pressure of multiple younger layers that accumulate on top of them. Most recently, after the overlying layers were eroded away by wind, the vein-filling material resists erosion better than the once-muddy material, so the pattern that began as cracks appears as ridges.

Note that some of the cracks contain material much brighter than the surrounding rock. These are mineral veins. Curiosity has found such bright veins of calcium sulfate in many rock layers the rover has investigated. These veins form from circulation of mineral-laden groundwater through underground cracks. Rover-team scientists suggest that a likely scenario for the history of Old Soaker is more than one generation of fracturing: mud cracks first, with sediment accumulating in them, then a later episode of underground fracturing and vein forming.

The target rock’s name comes from the name of an island off the coast of Maine. The names informally assigned by the rover team to features in the area of lower Mount Sharp that includes this slab are from a list of islands, hills and other sites in or near Maine’s Bar Harbor.

Malin Space Science Systems, San Diego, built and operates MAHLI. NASA’s Jet Propulsion Laboratory, a division of the Caltech in Pasadena, California, manages the Mars Science Laboratory Project for the NASA Science Mission Directorate, Washington, and built the project’s Curiosity rover. More information about Curiosity is online at http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl/.