Blueberry Stem Mystery Explained
The genesis of the mysterious "stems" is clear now
The troublesome "stems" that many of the fossil spherules appear on have been difficult to explain until now. Some appeared to have formed due to minerals dissolved in water, and others might have been formed by wind and dust eroding away softer material from behind the fossil spherules. But now, some have been photographed in the midst of the formation process, showing clearly the mechanism responsible for their formation.
Just inside Endurance Crater, Opportunity has been exploring the various sedimentary formations with its instruments and the RAT. Panoramic images of some of the slabs of broken stone showed clear evidence of the recent flow of liquid water. Then, it became clear how many of these stems formed.
| If you look at this image,
you will see some stone slabs of the now familiar sedimentary formations
that form the foundation of much of the Meridiani Planum area.
Look at the spherules that are emerging from the surface of the stone slab. You can see lines behind the spherules that indicate that something has moved over the surface, eroding the stone in a direction different from the layering of the sediments. Now see how the ground is darker, and the smaller bits have been sorted into the crevices. The movement of water from a geyser or spring is responsible for this. As the water washes over the surface, carrying sand and debris, it washes the material- but any scouring or erosion is shielded by the more durable spherules. As a result, the spherules create a "lee" behind them that protects the material from wearing away. This is the birth of a stem. |
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| In this magnified view,
you can see how the wash lines are all aligned exactly with the stems that
are forming.
Because of how fluid dynamics works, the water washing over the spherules will cling to them and run around the back, making the spherules' stems narrower than the spherules themselves. By the time the water has flowed around the spherules, it has lost some of its force and the erosion continues to form the stem as a long, thin structure. These stems are forming close to the rock surface- eventually, they will emerge as long unsupported stems when the rock beneath them wears away due to the action of the water. See how the ground appears dark and muddy? And also take note of the fact that the stone face is absolutely clean of any loose sand or material. The water has washed it off. |
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| On a recent trip through
the Ocala National Forest, I found this simple terrestrial analog of the
stem formation process. This is the result of small bits of
limestone mixed with sandy soil.
While this material does not have the same hardness or durability that the Martian sediments and fossils have, it does show the same basic process and results. This is a close-up shot of an area that rain has eroded. See the stems capped by harder lime fragments? |
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| Now we can see a broader
field and how some of the stems, while made of nothing more than sand,
show impressive elongation and altitude in comparison to the small rock
fragments that crown them.
Now imagine soft sediments and harder "blueberries" in them being blasted by tiny, high-speed water droplets that carry sand and silt as the eroding agent. The results would clearly have this shape and structure, but could be formed at angles because, being solid rock instead of sand, they would have the durability needed. |
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This simple mechanism can explain nearly all of the stems that we see. Water spraying onto the rocks and shielded by harder spherules will wash away at the soft sediments and leave long, narrow stems behind. In other words, the spherules were fossilized in the rock, and then the rock wore away due to sand and water, leaving the spherules exactly where they started out.
And, it has been suggested that the water may be carbonated due to the carbon dioxide content of the environment. This would result in a slightly acidic solution containing carbonic acid. In contact with minerals, this often dissolves them more easily than pure water.
So the action of liquid water is the culprit. It is responsible for the mystery of the stems. Case closed.