Thursday, March 31, 2016

A Large Airburst over Iran (?)

Last summer, on 30 July 2015, a spectacular airburst occurred over northern Iran, in the mountains west of Tehran.  The site of the reported fall is near the town of Avaj, in Qazvin County.  Press reports mention shattered windows and light structural damage to buildings.

There are reports of recovered meteorites, complete with plausible, if not wholly convincing, pictures.  I even received an email from someone in Iran offering to sell me a stone from the fall, accompanied by a putative analysis that claims 20% carbon and doesn’t mention silicon.  At least, that what the message appears to mean: it bears all the earmarks of a machine translation from low Martian.  So what hard facts do we have to work with?  Virtually none.

Perhaps the most notable fallout from this event has been the fuss kicked up on the internet.  There has been the most astonishing display of ignorance, prejudice, millennialist vitriol, and bigotry, liberally salted with insane conspiracy theories.  I have seen the following charges: 1) it’s a lie by the Iranian government, 2) a cover-up by NASA, 3) a harbinger of the end of the world, 4) a divine portent of unknown significance sent by Allah, 5) evidence of the God of Israel’s intent to destroy Iran, 6) a stray Russian missile, 7) an Israeli missile, 8) a baseless rumor denied by the Iranian government, etc.  A number of comments appear to have no content, simply serving as vehicles for incoherent rantings, misspellings, tortured grammar, and severe mental confusion from which no meaning can be extracted.  It’s a paranoid madhouse.  I have read close to 50 such comments, of which two show evidence of both knowledge and sanity. 

So, dear reader, here is my summary: we don’t know diddly-squat about this particular event.  Statistically, however, airbursts are not rare and reports of minor damage have many historical precedents.  As for using this natural event as an omen, well, any idiot can make up some such nonsense.  There is overwhelming evidence that they can—and do—exactly that.  Many such predictions of the end of the world have been issued, none of which have come true.  For your amusement, read

The question of why something happens is enormously interesting, but these examples of “man’s search for meaning” show our pathetic incompetence in this task.  It is wonderful to contemplate why something happened, but any explanation beyond physical causality is often simply baseless speculation, rarely testable by observation, and, when tested, almost invariably found to be wrong.  We would be far better engaged in studying the how, what, when and where of events, where physical evidence can be brought to bear. But speculation about underlying causes is fun!

Test yourself: The (true) given fact is, “The first German artillery shell fired on Leningrad in World War II landed in the zoo and killed the only elephant in Russia.” 

Now, propose an answer to the question, “Why?”

An Earthlike Planet

The press is full of the news that a new Earthlike planet, Kepler 452b, has been discovered by the revived Kepler planet-hunting spacecraft.  The discovery of a planet “much like Earth” garners more attention because the planet orbits in the so-called “Goldilocks zone” around its star, the range of distances within which water can exist as a liquid on its surface rather than only as ice or hot vapor.

Kepler 452b is 60% larger in diameter than Earth and is presumed to have Earthlike composition, although it is important to note that there is as yet no way of measuring its mass.  Nonetheless, the phrase “Earthlike world” has the press reeling, including suggestions that worlds like this are the places radio astronomers should look to find radio signals from intelligent aliens.

OK, let’s play this game and see what Earthlike composition would imply.  A diameter of 1.6 times Earth’s means a surface area of 1.6x1.6, or 2.56 times Earth’s, and a volume of 1.6x2.56, or 4.1 times Earth’s.  Assuming Earthlike material, this planet would have a core/mantle/crust structure very similar to Earth’s, but the internal pressures would of course be significantly higher, and the core and mantle material must be compressed to higher density than Earth’s average of 5.5 grams per cubic centimeter, probably close to 7.5 for the whole planet.  Now, that would generate a planet with 5.6 times Earth’s mass.  This mass and diameter would correspond to a surface gravity that is 5.6/2.56 times Earth, 2.18 times as large, or 21.4 meters/second2. 

“Similar composition” means similar abundances of radioactive elements, which decay inside the planet and eventually lose their heat by radiating heat from the planetary surface into space.  This planet, generating 5.6 times as much heat as Earth, which it radiates into space through a surface with area 2.56 times Earth’s, so the heat flux (Watts per square meter per second) is about 2.2 times Earth’s.  At steady state, with heat loss rate equal to heat production rate, this requires that the temperature gradient in the crust (the rate at which temperature increases with depth) must be 2.2 times as large as on Earth. 

Mountains can build up only to a finite height because the temperature gradient under them leads to softening and melting of the continental rock deep under them.  On Earth the Himalayas rise about 13 kilometers above the abyssal plains of the oceans.  Mars has half the radius of Earth, so its heat flow and temperature gradient should be about half that of Earth, meaning that softening of the rock should occur about twice as deep, and Martian mountains should build to about twice the height as on Earth.  In fact, the highest peak on Mars, Olympus Mons, rises about 26 km above the plains.  Of course, to calculate this precisely we need to account for the slightly lower density of Mars and its slightly lower surface temperature, but we still get a similar answer. 

Now let’s apply that to Kepler 452b.  The topography, Earth’s standard scaled down by a factor of 2.2, would have the highest mountain regions about 6 kilometers above the abyssal plains.  Now let’s think about the oceans on this world.  On Earth, there is enough water to make a layer about 4 km deep covering the entire planet.  Kepler 452b, if it has the same composition as Earth, would contain 5.6 times as much water spread out over a surface area that is 2.56 times as large.  Thus it would contain enough water to make a layer 2.56 times as deep as Earth’s 4 km, or about 10.2 km deep.  Since the highest land would rise about 6 km from the abyssal plains, this means that the tops of the highest mountains would lie roughly 4 km below mean sea level.  Thus Kepler 452b, if truly Earthlike in composition, would be a true water world.

Now consider the conditions on the sea floor.  An ocean 10.2 km deep in a gravity field of 2.2 Earth gravities would exert tremendous pressure at the ocean floor: the weight of the ocean exerts an average pressure of 2240 atmospheres, which is essentially the highest pressure at which pure liquid water can exist, irrespective of temperature.  At approximately this pressure, water freezes to make dense (sinking) ice III rather than familiar (floating) ice I.  Such an ocean could start to freeze from the bottom up.

We’re also told the planet has had liquid water for 6 billion years or more, without mentioning how the luminosity of its parent star (and the surface temperature of the planet) have changed over time.

Of course, with data on the mass of the planet we could see whether it really is a “terrestrial” planet or a sort of warmed-up ice ball: if the latter is the case, then the planet could be much less dense and the ocean much deeper.

So this is “Earth 2.0”? 

If you are interested in the wonderful game of designing planets that accord with the laws of physics, astronomy, and chemistry, you can find a number of examples in my 1998 book Worlds without End, which explores the possibilities for many types of planets allowed by nature, but not present in our Solar System.

Pluto in the Rear-View Mirror

The New Horizons flyby of the Pluto system reveals a new world in stunning detail.  The progress made over the last few decades in understanding Pluto has suddenly undergone another round of explosive growth.  Perhaps this is a good time to review just how our knowledge of Pluto has evolved since I first became interested in this outpost of the Solar System.

My first source of information about space was Stars, a Golden Nature Guide, written by Herbert S. Zim in 1951.  This little pocket guide contained, on pages 104 and 105, a table of data on “The Planets”.  The table was populated with a lot of quaintly obsolete data and a liberal sprinkling of question marks.  The column devoted to Pluto, unabashedly listed as a planet, gave good information on its distance from the Sun, but all the rest seemed designed to pique the curiosity of a child.  The diameter, which could not be measured directly, is given conjecturally as “3600  (?)” miles, comparable to Mars—but New Horizons, viewing from up close, measured only 1473 miles.  The volume of Pluto is therefore only 6.85% of what was then accepted.  The mass of Pluto was based on the wildly wrong diameter and an equally wild guess about its density; its mass relative to Earth was given as “0.8 (?)” and its volume as “0.07 (?)” that of Earth.  One need not have mastered calculus to figure that the density of Pluto was 11.4 times that of Earth, or 63 grams per cubic centimeter.  Considering that the densest chemical elements (osmium and iridium) have densities of 22 g/cm3, this presented an obvious problem for a budding young chemist. 

Further paradoxes abounded.  The number of moons of Pluto is given as “0.1 (?)”.  I can perhaps be forgiven for wondering how a planet could have a tenth of a moon.  The length of the Pluto day was given simply as “(?)”, and its axial inclination is reported the same way.

Is it any wonder that I became intrigued with this mysterious planet?

In 1972, years before we had reliable data on the size and mass of Pluto, I published two papers on the structure of ice-plus-rock bodies, so-called “dirty snowballs”, with the large satellites of the Jovian planets in mind.  The densities of these bodies constrained their proportions of ice and rock; the rocky component, with its endowment of uranium, thorium, and potassium, contributed substantial heat to their deep interiors.  Considering the size of the heat source, the surface temperatures of the large icy satellites, and the melting behavior of ices, I predicted that bodies like Europa and Ganymede could have deep oceans covered by thin crusts of water ice.  Their surfaces would then be quite susceptible to resurfacing, and would be very poor at preserving evidence of impact cratering.  Later, in 1979, Stan Peale, Pat Cassen and Ray Reynolds (Science 203, 892) proposed another, stronger heating effect for the Galilean satellites, through flexing driven by the tidal interactions of the moons. This model became entrenched in the literature, even to the point that most scientists ignored the radioactive heating component. 

We had no way to measure the mass of Pluto until its big satellite Charon was discovered in 1978, finally letting us track the orbital motions of Charon and Pluto around their common center of mass.  Mutual eclipses of Pluto and Charon provided much-improved data on their sizes.  But the best guesses on their densities still relied on condensation theory, which could not be tested with the best Pluto data in hand.

All that changed when New Horizons flew by Pluto.  Since Pluto and Charon are locked into a 1:1:1 spin-spin-orbit resonance, heating by tidal flexing is ruled out.  The other satellites of Pluto are tiny and have almost no effect.  Yet impact craters are absent and the whole planet has been recently resurfaced.  Clearly the driving force must be radioactive decay.  But how does it work?  What could the fluid be that resurfaces so efficiently?  Pluto’s surface is far, far below the melting temperature of water ice.  Clearly this is not a place for silicate volcanism: the resurfacing must be connected with the ices that make up a third of the mass of Pluto.  But which ones?

It turns out that there is a significant difference between ices formed in the Solar Nebula and ices formed in the satellite systems around the giant planets.  The environment in a protoplanetary disk girdling Jupiter or Saturn generally has much higher gas pressure than in the nearby Solar Nebula.  The effect of pressure strongly influences the chemistry of both nitrogen and carbon because their reactions with hydrogen (the dominant gas in the Universe) are driven to the right by higher pressures:

            3H2 + N2 à 2NH3         and       3H2 + CO à CH4 + H2O.

Thus ammonia and methane are minor constituents of ices formed in the Solar Nebula, but can be major components of ices formed in sufficiently dense and cool protoplanetary disks, such as those surrounding Saturn, Uranus, and Neptune.  These are available as the raw materials out of which their satellite systems formed.   Each disk was warmer near its center and cooler near its outer edge; in the case of Jupiter, the region inside Europa’s orbit (including Io) was too warm for even water ice to condense, thus making rocky moons.  Europa, forming close to the “snow line” in Jupiter’s nebula, retained only a small proportion of water ice and essentially none of the other, more volatile ices.  Ganymede and Callisto, formed farther out, are much more ice-rich.

Ammonia and methane can enter solid ices at temperatures too high for direct condensation of solid ammonia or solid methane because both gases can react with water ice to make solid hydrates.  This is how Saturn’s largest moon, Titan, retained vast stores of ammonia and methane.  Heating of Titan’s interior, whether by radioactive decay or tidal flexing, caused early melting of ammonia hydrates: in fact, ammonia-water ices begin to melt at only 100 K, or -173 oC.  Once melting begins, separation of the ice component from the “dirt” proceeds to generate a muddy core and a deep water-rich ocean with an ice crust.  Interestingly, the average surface temperature of Titan is 94 K, just 6 K colder than the onset of ammonia/water melting.  One can easily imagine cold, viscous ammonia/water melt being extruded onto the surface as cryovolcanic eruptions.  At these temperatures, little ammonia is released as a gas, but methane is given off in large quantities.  Ammonia is also very vulnerable to destruction by solar ultraviolet light, producing nitrogen and hydrogen (which is so light it readily escapes from Titan).  Not surprisingly, Titan today has an atmosphere dominated by nitrogen and methane.  Neptune’s large satellite Triton should be regarded as a colder version of the same scheme.

But Pluto and other Kuiper-belt bodies, formed in the much less dense Solar Nebula, would have experienced much more limited conversion of CO and N2 into methane and ammonia.  Both CO and N2 gases readily form solid hydrates, permitting them to be important constituents of the ice.  Any heating of the interior (in Pluto’s case, by radioactive decay) will release CO and N2 into the atmosphere.  Thus low-temperature resurfacing is not only possible, but very important—and the key to the process is contained in a theory that dates back to 1972.

Oh yeah, is Pluto really a planet?  I DON’T CARE!!