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Similar-Looking Ridges on Mars Have Diverse Origins
Jet Propulsion Laboratory
January 25, 2017

Fast Facts:

* Many places on Mars have networks of ridges that intersect at angles
to form polygons.

* Martian polygonal-ridge features vary in size and origin.

* A new project seeks volunteers to examine Mars images and identify
sites with polygonal ridges

Thin, blade-like walls, some as tall as a 16-story building, dominate
a previously undocumented network of intersecting ridges on Mars, found
in images from NASA's Mars Reconnaissance Orbiter.

The simplest explanation for these impressive ridges is that lava flowed
into pre-existing fractures in the ground and later resisted erosion better than material around them.

A new survey of polygon-forming ridges on Mars examines this network in
the Medusae Fossae region straddling the planet's equator and similar-looking networks in other regions of the Red Planet.

"Finding these ridges in the Medusae Fossae region set me on a quest to
find all the types of polygonal ridges on Mars," said Laura Kerber of
NASA's Jet Propulsion Laboratory, Pasadena, California, lead author of
the survey report published this month in the journal Icarus.

The pattern is sometimes called boxwork ridges. Raised lines intersect
as the outlines of multiple adjoining rectangles, pentagons, triangles
or other polygons. Despite the similarity in shape, these networks differ
in origin and vary in scale from inches to miles.

Small and Large Examples

Mars rover missions have found small versions they have been able to inspect up close. Some of these polygonal ridges, such as at "Garden City" seen
by Curiosity, are veins deposited by mineral-laden groundwater moving
through underground fissures, long before erosion exposed the veins. Curiosity recently also imaged small boxwork ridges that likely originated as mud cracks.

At the other end of the size scale, ridges outline several rectangles
each more than a mile (more than 2 kilometers) wide at a location called
"Inca City" near Mars' south pole. These may have resulted from impact-related faults underground, with fractures filled by rising lava that hardened
and was later exposed by erosion.

"Polygonal ridges can be formed in several different ways, and some of
them are really key to understanding the history of early Mars," Kerber
said. "Many of these ridges are mineral veins, and mineral veins tell
us that water was circulating underground."

Polygonal ridges in the Nilosyrtis Mensae region of northern Mars may
hold clues about ancient wet, possibly warm environments. Examples of
them found so far tend to be in the same areas as water-related clues
such as minerals that form in hot springs, clay-mineral layers and channels carved by ancient streams. A larger sample is needed to test this hypothesis.

Volunteers Sought

Kerber is seeking help from the public through a citizen-science project
using images of Mars from the Context Camera (CTX) on Mars Reconnaissance Orbiter.

"We're asking for volunteers to search for more polygonal ridges," she
said. Finding as-yet-unidentified polygonal ridges in CTX images could
improve understanding about their relationship to other features and also
will help guide future observations with the High Resolution Imaging Science Experiment (HiRISE) camera to reveal details of the ridge networks.

This citizen-science program, called Planet Four: Ridges, began Jan. 17
on a platform released by the Zooniverse, which hosts dozens of projects
that enlist people worldwide to contribute to discoveries in fields ranging from astronomy to zoology. More information is at:

http://ridges.planetfour.org

Other Zooniverse Mars projects using data from CTX and HiRISE have drawn participation from more than 150,000 volunteers.

On Earth, too, polygonal ridges have diverse origins. Examples include
grand walls of lava that hardened underground then were exposed by erosion, and small ridge networks inside limestone caves, where erosion can be
chemical as well as physical.

With CTX, HiRISE and four other instruments, the Mars Reconnaissance Orbiter has been investigating Mars since 2006.

Malin Space Science Systems, San Diego, built and operates CTX. The University of Arizona, Tucson, operates HiRISE, which was built by Ball Aerospace
& Technologies Corp. of Boulder, Colorado. NASA's Jet Propulsion Laboratory,
a division of Caltech in Pasadena, California, manages the Mars Reconnaissance Orbiter Project for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, built the orbiter and collaborates with
JPL to operate it. For additional information about the project, visit:

http://mars.nasa.gov/mro

News Media Contact
Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster@jpl.nasa.gov

Laurie Cantillo / Dwayne Brown
NASA Headquarters, Washington
202-358-1077 / 202-358-1726
laura.l.cantillo@nasa.gov / dwayne.c.brown@nasa.gov

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