[meteorite-list] Similar-Looking Ridges on Mars Have Diverse Origins

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=6725

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 at jpl.nasa.gov

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

2017-015