The Navajo Sandstone is an iconic symbol of the American Southwest, renowned for striking rich red to creamy white colors and for sweeping expanses of cross-beds, the remains of ancient sand dunes. Recently I hiked across a vast expanse of these rocks in Snow Canyon State Park, near Zion National Park in the southwestern corner of Utah. During my wanderings, I came across millions of iron-rich nodules, or concretions, that are weathering out of this formation. They reminded me of similar concretions found on Mars, a sign of the presence of water.
The Navajo Sandstone concretions are referred to as Moqui (or Moki) marbles. Scholars are uncertain about the name’s origin, but since “moqui” means “the dead” in the Hopi language, the name possibly relates to ancestor worship traditions. The iron-oxide cement tightly binding the marbles in the Navajo Sandstone is composed primarily of hematite (Fe2O3 ) but may also include other iron-oxides such as goethite (FeOOH).
Most of the Moqui marbles I saw in Snow Canyon were roughly spherical, and many were similar in size to some of the glass marbles I collected and traded during games at recess in elementary school. Others were the size of small potatoes. Sometimes I found two or more concretions joined like soap bubbles. In other areas of Navajo Sandstone, a variety of shapes are common, including iron-oxide sheets and pipes.
The Action of Water in Ancient Dunes
Approximately 180 million years ago during the Early Jurassic, a series of major “sand seas” swept across what is now the Colorado Plateau, depositing the enormous accumulation that forms the ancient dunes in the Navajo Sandstone. Over millions of years, the array of colors in the formation developed from water moving through porous sediments and accumulating iron-rich elements, which provide the vivid reds and pink colors of the rock. At other times, reducing fluids “bleached” the rock to pale, creamy whites.
In southern Utah, Navajo Sandstone rocks contain small amounts of metallic, carbonate, and quartz cement. Locally, the rock is porous and permeable and can easily transmit fluids. Over millions of years, weathering processes, combined with near-surface and surface waters from rainfall, broke down the iron-bearing minerals and iron oxide coatings formed on quartz grains. Deep below the surface, reducing fluids eventually dissolved the coatings. Then, some fluids mixed with oxidizing groundwater, causing the iron to precipitate out as hematite and goethite and forming the abundant concretions in the Navajo Sandstone.
Since the composition of the sandstone is uniform, the concretions tend to grow equally in all directions into spheres. The concretions have a different chemical composition from the surrounding rock, and the high concentration of iron makes them relatively resistant to chemical and mechanical weathering. Wind and rain have gradually eroded the surrounding exposed sandstone, preserving the Moqui marbles in loose accumulations.
Iron-rich concretions, such as those found in the Navajo Sandstone, occur in a variety of geological settings, including other sedimentary rocks of diverse compositions and in volcanic rocks. Although the chemical details can vary, we know that on our planet Earth, iron-oxide is an indicator of fluid flow, forming only in the presence of liquid water.
Water and Possible Life on Mars
Back in the mid-1990s, research priorities at NASA, the U.S. National Aeronautics and Space Administration, included gathering evidence on Mars for water and signs of life, plus identifying landing locations for robotic rovers. From 1997 to 2002, the NASA Mars Global Surveyor spacecraft orbited the planet to obtain a variety of data. A thermal emission spectrometer (TES) collected and mapped surface hematite levels, used as a proxy for water.
Map of surface hematite levels at the Meridiani Planum; the ellipse labelled TLA indicates the target landing area for Opportunity rover and the blue line labelled OT indicates the traverse route of the rover (2004, Wikipedia)
Within a large plain that straddles the equator of Mars, the TES mapped high hematite levels in an area now called the Meridiani Planum. That region became the target site for landing the NASA Opportunity rover. When the rover landed successfully on January 24, 2004, images immediately revealed thousands of small (0.16 to 0.24 in/4 to 6 mm diameter) roughly spherical pebbles, found on the surface as well as embedded in bedrock. Although they are gray, researchers dubbed them “blueberries” because they look bluish compared to the dusty reds that dominate on Mars.
Subsequent analysis shows that the abundance of blueberries is responsible for the strong hematite signal detected by the TES during orbit. Since these blueberries have spherical shapes and hematite compositions like the Moqui marbles found in southern Utah, this suggests similar origins — including subsurface fluid flow and a porous host rock (Chan et al., 2004). The smaller size of the Mars blueberries may reflect the limited area that Opportunity examined, or possibly a difference in how the concretions formed, given the basaltic (volcanic) composition of the surrounding rock.
Mars blueberries; most diameters in this image are 3 – 6 mm (2004, Wikipedia)
During past decades, researchers have found clear evidence for solid, liquid, and gaseous forms of water on Mars. I clearly remember the early reports about these findings, as the existence of water on a distant planet seemed extraordinary to me! Although the presence of liquid water today is controversial, there is clear data showing that the planet had abundant water early in its history. Much of that water is currently ice on the planet’s poles. (The Wikipedia entry “Water on Mars” has an excellent summary of this topic.)
And there is more intriguing data about the Red Planet. Scientists are learning that some hardy microbes can protect themselves from damaging ultraviolet and survive in the stratosphere. Researchers have found that one particular microbe, named Methanosarcina barkeri, obtains energy by converting carbon dioxide and hydrogen into methane (no oxygen required!). Tests at low, Mars-like gas pressures showed that M. barkeri continues to produce methane (Fox, 2026). Rovers and orbiters collecting data on Mars have repeatedly detected methane gas, so possibly, this is appearing as a byproduct of microbial life beneath the surface. Amazing!
Modern Times—Changing Priorities
In our current times, government funding priorities have changed. Sadly, many scientific research projects, including those at NASA, have hit the chopping block. As one example, the Perseverance rover has been collecting and caching Mars rock samples for return to Earth, where analyzing these rocks would be easier than at our current extreme distance from Mars (depends on the elliptical orbit; average distance is 140 million mi/ 225 million km). NASA engineers and researchers developed plans to retrieve the rock samples, but the current situation has stalled these plans.
One rock in particular could provide important information. Perseverance drilled into rock that contains iron phosphate and iron sulfate molecules around the rims of distinctive spots. On Earth, similar patterns in rocks result from chemical reactions associated with ancient microbial life. So, this “potential biosignature” is very interesting. However, scientists need to perform analyses on Earth to understand what is actually represented.
At the same time, some entrepreneurs are looking towards the future. To my surprise, I found an article titled “Iron Oxide Harvesting on Mars” (Olson, 2021). This paper focuses on harvesting blueberries to turn hematite into steel that robots on Mars could then use to construct infrastructure for instruments such as radio telescopes and for scientific research facilities.
Our world—and the universe—hold infinite surprises!
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I remember seeing these during my first hiking trip to southern Utah, and couldn’t imagine how they got there. And so many! Toys left behind by the children of alien visitors? Unfortunately, no geologist along to explain concretions. Thanks for the information. How long before Martian blueberries show up in my local market? They sound wonderful.
Thanks for the comment, Steven! I’m glad that I could explain concretions– they are certainly unusual!