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On the cause of the Namibian fairy circles
Posted by DeepSand on 07 Jul 2012 at 13:03 GMT
When reading elsewhere about this phenomenon, at first I also considered biological causes for the fairy circles. But upon reading that these had been dismissed by the author of the article, I immediately thought of a similar phenomenon.
One or two years ago, when walking to the public library, I noticed what may be called fairy snow circles. It was cold. It was thawing. There was no wind and the area was on the lee side of a wall. The underground was dark. There was sunlight on the ground. The snow circles, where the snow had thawed away, leaving the bare pavement, varied in size from tiny to coin-sized, if I recall correctly. Incongruously, in the exact center of each fairy snow circle was an ice crystal the size of which was proportional to the diameter of the circle. While passing by this phenomenon, inspecting some of the circles, I wondered about the cause. Clearly the circles and the fairy ice crystal at their center were growing in size. It occurred to me that tiny dark spots in the pavement would be heated by the sunlight through the thin layer of snow, causing a tiny spot in the snow to melt. The air above this tiny open spot of heated pavement would heat in turn, causing it to expand sideways and to rise. The sideways expanding warm air would melt the snow walls of the tiny fairy snow circle, enlarging its diameter. The rising warm air at the periphery of the circle would cool and sink down at the center of the circle, causing that center spot to become very cold and the water vapor in the descending air flow would freeze at this very cold spot and grow into the observed center fairy ice crystal. I do not know whether this phenomenon has ever been described in a literature, nor whether anyone solved the cause of the phenomenon before me, but countless people must have observed such fairy snow circles when the conditions were right for them to form by way of such micro-circulation of air.
I propose that the same mechanism is at work in the Namibian fairy circles. When a tiny patch of ground becomes bare, due to the death of a plant or by some kind of damage, it heats up, causing the air above it to become hot and to rise. As the air rises it expands sideways and cools down and sinks again, cooling the periphery of the circle. As this air cools down the water vapor in it condenses and wets the ground at the periphery, causing the vegetation there to become lush. Having deposited its water at the periphery, the now dry air flows back to the center of the circle, heating up as it goes and extracting moisture from the ground. Thus at the center of the circle the ground will have the least moisture, causing moisture in the ground to move down this gradient from the periphery towards the center. Plants at the very edge of the circle will therefore be short on water and no new plants will take seed there when they wither away. It is by this mechanism that the circle enlarges itself.
At some phase an equilibrium may be attained.
When the wind hollows out the fairy circles - I wonder whether they then resemble concave mirrors with an optical focal point somewhere in the air above them - the air may start to rise at the periphery of the circles and descend at the center, as it does in the fairy snow circles, and the process is reversed with water vapor condensing and wetting the ground at the center of the circle, enabling plants to recolonize the circle.
RE: On the cause of the Namibian fairy circles
wtschinkel replied to DeepSand on 07 Jul 2012 at 20:27 GMT
Actually, contrary to the respondents claims, I have not dismissed biological causes of fairy circles. I did not delve deeply into causes because my purpose was to describe the variation of circles, to sequence the life cycle and to estimate the life span.
I have received many emails suggesting causes, some quite ingenious like this one, and many suggesting fungus as a cause. A fungal disease can account for many attributes of fairy circles, but not for their regular dispersion.
As for the causal mechanism proposed by this reader, there are a number of problems. First, the Namib Desert is a very windy place, so the suggested convection cells would not have a chance to form. Second, the relative humidity is extremely low, and for most of the year, condensation (dew) simply does not occur. Third, grass growth follows summer rains and is proportional to their amount. The rest of the year, the soil is bone dry down to over a meter (that's where I stopped digging). Fourth, the circles seem to form in a rather short time, faster than the repeated cycles of dessication implied by the reader's mechanism.
Readers have come up with a number of possible causes, some quite complex. The thing is, one can make up explanations that are logical and seem reasonable, but to follow up on any of these, one would want some empirical evidence. Moreover, the mechanism should not be contradicted by known facts and conditions.
I am impressed by how much fun readers have coming up with suggestions, and how interested they are in this phenomenon. I think that is wonderful.
RE: RE: On the cause of the Namibian fairy circles
DeepSand replied to wtschinkel on 10 Jul 2012 at 21:17 GMT
Hm, interesting objections. I appreciate the challenge.
Yet, googling a bit, recalling water gathering darkling beetles and fog, I read: "fog that can reach as much as 100 km inland from the coast . Fog brings water in the form of minute droplets that can deposit up to a litre of water per square metre on the mesh of an artificial fog screen during a day in the Namib Desert . These fog events occur approximately 30 days per year in the inland desert "
The above quote from the article "Fog-basking behaviour and water collection efficiency in Namib Desert Darkling beetles" by Thomas Nørgaard and Marie Dacke.
Thus, provided that the fairy circles are within reach of this fog, which occurs approximately 8 percent of the days, the condensation problem is not an issue. It stands to reason that bone dry ground will absorb the water in the fog like a sponge.
Secondly, I gather that the prevailing winds are from the east and that the sea fog moves inland from the west. At some moments these opposing winds must balance: a time of no wind. Moreover, I gather that the wind in the area appears to pick up as the desert heats and the day progresses. And as well, it is my experience that when there is fog, there usually is no or but little wind. Thus it is to be expected that in the mornings when the fog lifts, there will be some small amount of time for the proposed micro-circulation of air to form and to exist.
Also I read that the air below is rather cold with an inversion layer of hot air above it. The micro-circulating air will therefore easily cool within the low layer of cold air.
And in another artcle I read: "Namib dune bushman grass (Stipagrostris sabulicola, Poaceae) collect water directly from the fog".
The above quote from the article "Animal or Plant: Which Is the Better Fog Water Collector?" by the same authors as the one above, plus Martin Ebner.
So now I wonder about the water collecting properties of the "halo of taller grass (Stipagrostis giessii Kers or S. hochstetteriana (Beck ex Hack.) at the rim of the fairy circles. I venture to predict that those properties will be found to be very good.
The circles form in a short time? Well, it is an exponential process - and therefore to be expected. Vegetation does take time to grow and whither, though, so I expect that the concept 'short' is a relative concept.
Thus, the conditions for the mechanism of micro-circulation of air as the cause of the fairy circle all appear to have been met.
RE: On the cause of the Namibian fairy circles
It has occurred to me that the same fairy circle principle explains the recurring slope lineae (RSL) that have been observed on the planet Mars. These RSL are narrow streaks of low reflectance up to five metres wide that appear to flow down steep, equator facing slopes during warm seasons when temperatures reach about 250–300 K, i.e. above minus 23 degrees Celsius, and then fade in cooler seasons. They have a wide distribution, extending from the tops of ridges and peaks [of craters only?], on equatorial dunes composed of permeable sand, they are abundantly present in the Vallis Marineris troughs (e.g. up to 1.2 kilometers long RSLs have been observed at a site near Eos and Capri Chasma), on a heavily-dissected (eroded) hill in southern Juventae Chasm, Mars (latitude 4.7 degrees S, longitude 298.6 degrees E). They are the most active on slopes that receive more direct sunlight.
Lujendra Ojha et al. found “Spectral evidence for hydrated salts in recurring slope lineae on Mars” (Nature Geoscience, Published online 28 September 2015) in four observed locations.
The salts lower the melting point of water, forming a liquid brine – if water is present.
It was suggested by Chevrier, V. F., and E. G. Rivera-Valentin, in "Formation of recurring slope lineae by liquid brines on present-day Mars", (2012, Geophys. Res. Lett., 39, L21202), that a recharge mechanism is active in order to maintain a source of brine over even a short geological timescale.
The best hypothesis for the origin of the water, in my opinion, is suggested by McEwen, A.; Chojnacki, M.; Dundas, C.; L. Ojha, L. “Recurring Slope Lineae on Mars: Atmospheric Origin?” (28 September 2015. European Planetary Science Congress 2015.) They suspect near-surface hygroscopic salts to seasonally adsorb water provided by the atmosphere, analogous to water tracks in Antarctica.
The fairy circle atmospheric convection cell mechanism neatly explains the origin and circulation of the water in the RSLs. The convection cells are protected from the larger atmospheric movements by the slopes of craters and dunes and by the Vallis Marineris walls. The water is evaporated into the dry Martian atmosphere from the hygroscopic soil and rock of the slopes. As the air rises, it cools down and moisture condensates higher up the slope, to flow down as brine. This downflow powers the direction of the air circulation in the convection cell: its downflow too is only down the slope. The convection cell therefore has an asymmetrical air circulation. As the brine flows down, getting into the lower, warmer region it heats up and evaporates again. As the sun rises higher during the warm season, it warms more of the surface area of the slopes, causing more water to be evaporated, which increases the size of the RSLs.