Tuesday, April 19, 2016

The Secrets of Jupiter’s Great Red Spot

A recent article on space.com (http://www.space.com/30236-jupiter-great-red-spot-color-secrets.html?cmpid=NL_SP_weekly_2015-08-14) tells of the efforts of a team of NASA scientists at Goddard Space Flight Center to replicate the striking brick-red color of Jupiter’s famous and long-lived Great Red Spot (GRS).   Most of Jupiter is covered by alternating bands of bright clouds (zones) and dark clouds (belts), in which the GRS is embedded: however, the red color of the GRS appears to be distinctly different from the brown belts, suggesting two or more different coloring agents. 

Carl Sagan and his colleagues long argued for organic matter as the coloring agent; this suggestion, however, depends on the achingly slow destruction of methane by ultraviolet sunlight, which makes largely uncolored products at such a slow rate that the atmosphere would have to remain stable and unmixed for millions of years to accumulate a detectable tinge of brown.  Carl gave these largely imaginary organic coloring agents the name “tholins”, a name that has stuck with us while the organic coloring agents that supposedly justified the name have largely disappeared from the Jovian literature as being quantitatively indefensible: another clear example of the victory of the charmingly qualitative over the less-romantic quantitative.

The Goddard team wisely concentrates on the predicted ammonium hydrosulfide (NH4SH) cloud layer (misidentified in the article as ammonium sulfide, (NH4)2S), the level that we see when we peer into Jupiter’s belts, the next cloud layer below the white ammonia-crystal clouds that cover most of the planet, especially the bright zones.  They presumably chose that layer because fresh ammonium hydrosulfide, a colorless crystalline substance, is very sensitive to ultraviolet light and rapidly turns brown when exposed to sunlight.  Space.com explains, “Studies predict that Jupiter's upper atmosphere is composed of clouds of ammonia, ammonium hydrosulfide and water”.  I’m rather partial to these cloud layers because I am the author of the generally accepted cloud models of Jupiter and its fellow giant planets [J.S. Lewis, The Clouds of Jupiter and the NH3-H2O and NH3-H2S Systems. Icarus 10, 365 (1969)].  Yes, that’s 1969. 

The space.com article explains that the Goddard team is “baking some of the components of Jupiter's atmosphere with radiation, mimicking cosmic rays”.   They also report that their simulation “heats up hydrogen sulfide and ammonia” to make ammonium hydrosulfide, a remarkable assertion that makes no sense.  Actually, the way to make ammonium hydrosulfide, both on Jupiter and in the lab, is to cool down a mixture containing ammonia and hydrogen sulfide gases to precipitate a “snow” of the solid.  As for the “baking”, the temperature of that cloud layer is both predicted and measured to be about 225 K (-48 oC; -54 oF), a pretty bracing temperature for baking! 

OK, now they have solid NH4SH.  What next?  They blast the solids with high-energy particles, “much as cosmic rays blast Jupiter's clouds”.  Now, they have good reason to expect color changes because the much less violent and simple exposure of this cloud-stuff to sunlight has the same effect.

But wait a minute!  Doesn’t the Sun also shine on Jupiter?  How important are cosmic rays compared to the ultraviolet part of sunlight?  Good question!  The cosmic rays hitting Jupiter carry about 0.001 ergs of energy per square centimeter per second (of which only a tiny proportion actually goes to make colored products).  The energy supplied by the part of ultraviolet sunlight energetic enough to make colored sulfur compounds out of H2S (all the sunlight with wavelength less than 270 nanometers) is nearly 1000 ergs per square centimeter per second.  In other words, whatever the importance of cosmic rays, sunlight is about a million times more important! 

So hydrogen sulfide makes Jupiter-colored products.  How could we have missed this, back in that earlier millennium?  Well, we didn’t.  Ron Prinn and I pointed this out long ago: J.S. Lewis and R.G. Prinn, Jupiter's Clouds: Structure and Composition. Science 169, 472 (1970).  In that article, we showed that the rate of solar UV destruction of hydrogen sulfide (and production of yellow-, orange-, and brown-colored sulfur compounds) should surpass the total rate of methane photolysis claimed by Sagan and coworkers by a factor of 100,000.  This would occur only in those regions of Jupiter where the topmost (crystalline ammonia) cloud layer was thin or absent; i. e., in the belts but not in the zones.  The belts remain white because the ammonia-snow clouds block sunlight from reaching the deeper levels where the sulfur compounds reside.

But this explanation did not apply to the Great Red Spot, which is, after all, red.  Prinn and I addressed this issue a few years later, after phosphine gas (PH3) was detected on Jupiter (R.G. Prinn and J.S. Lewis, Phosphine on Jupiter and Implications for the Great Red Spot. Science 190, 274 (1975)).  Our argument was straightforward: that the dynamically active GRS was the best place on Jupiter for accumulation of red phosphorus made by solar UV destruction of phosphine: strong vertical winds blow phosphine gas up to altitudes above the protective ammonia clouds, where it encounters UV light and makes red phosphorus; the vertical winds then help levitate the particles of red phosphorus up where we can see them.  This process would occur at a rate governed by the relatively large proportion of UV radiation that is energetically capable of destroying PH3 and NH3 compared to that capable of destroying methane: red phosphorus would be produced at a rate hundreds of times faster than the total rate of destruction of methane (the ultimate source of all organic matter, both colored and uncolored), and thousands of times faster than the rate of formation of colored organic products.

Where do cosmic rays figure in this argument?  They don’t.  The total energy flow from cosmic rays is about a million times smaller than the rate of production of colored sulfur compounds.  Even if the cosmic rays produced colored products with 100% efficiency, which they don’t, their effects would remain negligible.

Then there is that spectacular image of “cosmic rays blast(ing) Jupiter's clouds”.  “Blasting” at one millionth of the intensity of ultraviolet sunlight”?  Really?  Sounds more like an advertising slogan to me.   

These color “secrets” haven’t been secrets for over 40 years.

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