Mars Science Lesson

Mars has caves just like we do here on Earth, however, they are a little harder to discover snce we have yet to set foot on the red planet. While we can't just stroll across the planet, we have already been able to catalogue many caves using imaging. High resolution surface imaging data taken over the past couple of decades by instruments like the Mars Reconnaissance Orbiter Context Camera system (CTX), together with Mars Odyssey's thermal emission imaging system (THEMIS), have enabled scientists to search for signs of deep pits, caves, and lava tube structures on the planetary surface.

In general an opening to a deep enough cave will exhibit a combination of extreme shadowing and thermal characteristics that suggest a large interior volume. In practice the data, often with 6-meter per-pixel resolution, is challenging to interpret. Nonetheless, work had been underway over the years on a Mars Global Cave Candidate Catalog, or MGC³. Fortunately for us, it is quite a bit easier to locate and identify caves on our own planet. 

Earth Science Lesson

What is a cave?

A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word cave can also refer to much smaller openings such as sea caves, rock shelters, and grottos, though strictly speaking a cave is exogene, meaning it is deeper than its opening is wide, and a rock shelter is endogene.

Speleology is the science of exploration and study of all aspect of caves and the cave environment. Visiting or exploring caves for recreation may be called caving, potholing, or spelunking.

Formation types

The formation and development of caves is known as speleogenesis; it can occur over the course of millions of years. Caves can range widely in size and are formed by various geological processes. These may involve a combination of chemical processes, erosion by water, tectonic forces, microorganisms, pressure, and atmospheric influences. Isotopic dating techniques can be applied to cave sediments, to determine the timescale of the geological events which formed and shaped present-day caves.

It is estimated that a cave cannot be more than 3,000 metres (9,800 ft) vertically beneath the surface due to the pressure of overlying rocks. This does not, however, impose a maximum depth for a cave which is measured from its highest entrance to its lowest point, as the amount of rock above the lowest point is dependent on the topography of the landscape above it. For karst caves, the maximum depth is determined on the basis of the lower limit of karst forming processes, coinciding with the base of the soluble carbonate rocks. Most caves are formed in limestone by dissolution.

Caves can be classified in various other ways as well, including a contrast between active and relict: active caves have water flowing through them; relict caves do not, though water may be retained in them. Types of active caves include inflow caves ("into which a stream sinks"), outflow caves ("from which a stream emerges"), and through caves ("traversed by a stream").

Solutional

Solutional caves or karst caves are the most frequently occurring caves. Such caves form in rock that is soluble; most occur in limestone, but they can also form in other rocks including chalk, dolomite, marble, salt, and gypsum. Rock is dissolved by natural acid in groundwater that seeps through bedding planes, faults, joints, and comparable features. Over time, cracks enlarge to become caves and cave systems.

The largest and most abundant solutional caves are located in limestone. Limestone dissolves under the action of rainwater and groundwater charged with H₂CO₃ (carbonic acid) and naturally occurring organic acids. The dissolution process produces a distinctive landform known as karst, characterized by sinkholes and underground drainage. Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation. These include flowstones, stalactites, stalagmites, helictites, soda straws and columns. These secondary mineral deposits in caves are called speleothems.

The portions of a solutional cave that are below the water table or the local level of the groundwater will be flooded.

Lechuguilla Cave in New Mexico and nearby Carlsbad Cavern are now believed to be examples of another type of solutional cave. They were formed by H₂S (hydrogen sulfide) gas rising from below, where reservoirs of oil give off sulfurous fumes. This gas mixes with groundwater and forms H₂SO₄ (sulfuric acid). The acid then dissolves the limestone from below, rather than from above, by acidic water percolating from the surface.

Primary

Caves formed at the same time as the surrounding rock are called primary caves.

Lava tubes are formed through volcanic activity and are the most common primary caves. As lava flows downhill, its surface cools and solidifies. Hot liquid lava continues to flow under that crust, and if most of it flows out, a hollow tube remains. Such caves can be found in the Canary Islands, Jeju-do, the basaltic plains of Eastern Idaho, and in other places. Kazumura Cave near Hilo, Hawaii is a remarkably long and deep lava tube; it is 65.6 km long (40.8 mi).

Lava caves include but are not limited to lava tubes. Other caves formed through volcanic activity include rifts, lava molds, open vertical conduits, inflationary, blisters, among others.

Sea or Littoral

Sea caves are found along coasts around the world. A special case is littoral caves which are formed by wave action in zones of weakness in sea cliffs. Often these weaknesses are faults, but they may also be dykes or bedding-plane contacts. Some wave-cut caves are now above sea level because of later uplift. Elsewhere, in places such as Thailand's Phang Nga Bay, solutional caves have been flooded by the sea and are now subject to littoral erosion. Sea caves are generally around 5 to 50 metres (16 to 164 ft) in length, but may exceed 300 metres (980 ft).

Corrasional or Erosional

Corrasional or erosional caves are those that form entirely by erosion by flowing streams carrying rocks and other sediments. These can form in any type of rock, including hard rocks such as granite. Generally there must be some zone of weakness to guide the water, such as a fault or joint. A subtype of the erosional cave is the wind or aeolian cave, carved by wind-born sediments. Many caves formed initially by solutional processes often undergo a subsequent phase of erosional or vadose enlargement where active streams or rivers pass through them.

Glacier

Glacier caves are formed by melting ice and flowing water within and under glaciers. The cavities are influenced by the very slow flow of the ice, which tends to collapse the caves again. Glacier caves are sometimes misidentified as "ice caves", though this latter term is properly reserved for bedrock caves that contain year-round ice formations.

Fracture

Fracture caves are formed when layers of more soluble minerals, such as gypsum, dissolve out from between layers of less soluble rock. These rocks fracture and collapse in blocks of stone.

Talus

Talus caves are formed by the openings among large boulders that have fallen down into a random heap, often at the bases of cliffs. These unstable deposits are called talus or scree, and may be subject to frequent rockfalls and landslides.

Anchialine

Anchialine caves are caves, usually coastal, containing a mixture of freshwater and saline water (usually sea water). They occur in many parts of the world, and often contain highly specialized and endemic fauna.

Logging Tasks

To satisfy the logging requirements of this EarthCache, please answer the following questions by email:

Questions:

1. Does this structure meet the definition of a cave on Earth as listed in the above description?
2. Is this a primary cave?
3. Based on your observations, what type of cave is this?
4. Post a photo of yourself, or a proxy, near GZ (No Spoilers Please).

References

1.  "Cave" Oxford English Dictionary Second Edition on CD-ROM (v. 4.0) © Oxford University Press 2009
2. Moratto, Michael J. (2014). California Archaeology. Academic Press. p. 304. ISBN 9781483277356.
3.  John Burcham. "Learning about caves; how caves are formed". Journey into amazing caves. Project Underground. Archived from the original on May 3, 2009. Retrieved September 8, 2009.
4. "Cave" https://en.wikipedia.org/wiki/Cave
5. Scharf, C. (2020, March 09). The 1,000 Caves of Mars. Retrieved January 11, 2021, from https://blogs.scientificamerican.com/life-unbounded/the-1000-caves-of-mars/

6. Cushing, G. (2019, July 26). The Caves of Mars. Retrieved January 11, 2021, from https://www.usgs.gov/center-news/caves-mars