Hot stuff in the Big Cold

A HOT DAY ON ENCELADUS

 As much as we love natural history , we hold  environments that would kill us in short order in limited esteem. Living on the Earth’s well-shielded surface ,at the bottom of a virtual ocean of air,  we rarely encounter materials that have suffered the ravages of irradiation in the vaccum of deep space We're used to a fraction of one rad a year of background radiation, , but the state of nature elsewhere in the solar system is far more hostile ,including the moons of its giant planets.

 Jupiter and Saturn have immense magnetospheres , and radiation belts that are to  Earth's Van Allen belt as a blast furnace is to a toaster oven. Close to Jupiter, space  is filled with particles enough to deliver up to 10 rads an hour , and the thousand rad flux on Europa would  deliver enough Grays (Joules of absorbed ionizing energy) to bring you to death's door in an hour.

On Saturn’s moon  Enceladus, the environment is almost as hostile, because while Saturn traps solar wind ions and electrons at a somewhat lower density,  its  moons sweep up  disproportionate amounts of radiation damage because  its magnetosphere can trap energetic solar wind particles for years, and focus cosmic rays, amplifying the particle flux. If cells perish in hours out in the ionizing heat of the Big Cold, what happens to inorganic dust exposed over geological time? to find out,  Read on, or take a look at  Seitz et al: 'Xenomict energy in cold solids in space' Naturwissenschaften, March 2006

So energetic are the particles trapped and accelerated by the field lines of the Jovian and Saturnian magnetospheres  that both solar wind protons and alpha particles, and cosmic ray nuclei  generate an appreciable flux of x-rays and neurons as they collide with atoms and nuclei.. Europa receives a total surface dose reckoned in hundreds of megarads a year- roughly a hundred watts  per kilogram of radiation energy is being deposited constantly in  the ice and mineral dust grains exposed in the  regolith’s surface .This is not enough thermal energy to greatly shift its radiative equilibrium because most of the energy  deposited in the first few microns of ice and soil, and that wattage  may be spread over an acre . But other things besides thermalization happen as ions and protons chew on the regolith materials inside the radiation belts. There are surface phenomena of course- wholesale sputtering of surface atom and molecules into space, contributing mass to the ring system in Saturn’s case or creating the thing fluorescent plasma atmospheres of Io and Titan.

  But what about the particles that drill into surface particles and are braked to a halt within them? Solar wind ions in the radiation belts have an energy spectrum on the high side of that encountered in ion implantation in semiconductor processing on Earth - hundreds of KeV to a few tens of MeV. Each ion can accordingly knock hundreds or thousands of atoms out of their places in the crystal lattice of whatever they strike. . When this happens to a silicon crystal in a semiconductor wafer processing plant, the radiation damage can generally be repaired by annealing- warming the silicon several hundred degrees above room temperature to allow thermal motion of atoms and vacancies in the damaged crystal to let the displaced atoms snap back into their equilibrium positions.

 But room temperature is an Earthly concept. In fact, rooms do not have temperatures- the things in them do, and each materially different thing in a room has a different melting point- the air, for instance is so far above it that it has boiled into a gas, while the mineral grains in the cement floor might remain solid even if a fire raised the room’s temperature high enough to melt the glass in its windows. Air does not need annealing , and water is so close to its melting point in an unheated New England room that before long the weather will repair any radiation damage you can inflict on an ice crystal. But what about the weather on the moons of Saturn?

 On the surface of Enceladus, earthly air would liquefy- it is only 72 Kelvin out there and nitrogen boils at 77K. Ice  200 degrees K below its melting point is , from the perspective of solid state physics as far from being ‘hot’ in the sense of ready to melt as a brick pavement in Siberia in the middle of January.. What matters in determining relative atomic mobility in solids is the ratio of their temperature to their melting point, and on a cold day on Enceladus, ice is only a quarter of the way to its 273 K melting point. This puts it as far from liquidity- and unlimited atomic mobility, as copper or glass at room temperature on Earth. But Enceladus is a lot warmer - by a factor of 2- than the temperature really far out in the solar system- on a cold day on Pluto, earthy air would freeze solid- only hydrogen, helium and the rare gas neon remain liquid at one tenth the melting point of water in degrees above absolute zero. Yet exotic and indeed alien as such temperatures may seem , they prevail throughout most of the solar system- the farther out you go , the bigger and colder it gets, and in the almost unlimited fastness of the Oort cloud on the outside of the solar system, temperatures fall close to the frigid norm of interstellar space- the single digits Kelvin.

 The chilling truth is that very little of the matter between us and the stars ever gets as cold as liquid nitrogen, let alone ice water. This includes the billions of Oort cloud planetesimals that provide rare glimpses of matter from the Big Cold as gravitational perturbations launch them as comets into the inner solar system. These objects are thought to form at first rather like dust bunnies by he gradual accretion of small bits and grains of flotsam, with bits of fluff eventually forming clouds that gradually collapse into more solid form under the growing force of their own gravity as they enlarge and attract more mass. This can take many millions of years, and throughout those eons, all of the finely divided particles in question are constantly being zapped by cosmic ray nuclei and solar wind particles, each of which can knock atoms out of place in the truly cold grains. On the surface of a moon like Europa, every water molecule exposed to space in surface ice gets knocked apart every thousand hours or so, but the protons and hydroxyl ions in water molecules instantly recombine even in extreme cold.

 What they are made of is less important than the temperature they have in common- at just tens of degrees Kelvin, all minerals are so far from their melting points that atomic mobility is close to nil- theses solids are truly frozen solid. Below 100 Kelvin not even vacancies in crystals move, and absent diffusion, new radiation induced atomic defects can last almost forever ,adding up over megayears until most materials turn into an odd sort of glass , their crystalline order erased by atomic displacement. This is a very strange  solid state of affairs, for although these materials stay frigid , they have in a sense been melted- all their atoms have been moved around. So what becomes of the energy  of the phase change we associate with the molten state?

 The  radiation that does the damage is deposited too slowly to move the thermometer much- the lethal radiation flux on the surface of Europa or Enceladus has no more warming power than a light bulbin a hockey rink, But that feeble hundredth of a watt per square meter of penetrating radiation never ceases- it does as much damage in an eon as standing next to an H-bomb for a microsecond .  The warmth arising from  thermalization of the rays that displace atoms by inelastic collisions leaks away, but the damage stays  behind, biding its time until  something- tectonics perhaps, or a solar flare , generates enough palpable warmth to release it . So there may be sandy grains in the surface of such moons as rich in stored energy as a stick of dynamite.

 Those objecting  that metastable materials are not long for this world , should recall one of the more remarkable insights of percolation theory- small additions of inert matter to mixtures can change reaction and diffusion rates dramatically ( There would be no Nobel Prize  fund if Alfred hadn’t hit on adding a cupful of kieselguhr to a quart of nitro ) , and field geologists have been known to burn  surplus dynamite in campfires to avoid return trip paperwork- the hard part is getting the fire started when the wood is wet , and a comet that’s half frozen gas may prove hard to light..

 Saturn yields some other odd returns on trifling investments in dimensional analysis. Consider its ring system. On the one hand that random assortment of gritty ice is a million times less dense than a buzz saw blade, but on the other, the orbital velocity of its particles is a thousand times higher than the rim speed of an Earthly diamond saw.

So the factor of a million is a wash relative to the square of the velocity. That makes the ring system, which is only 100 meters thick, look very like a cheese slicing machine or a silicon wafering saw from the perspective of a planetoid spiraling in edge on.

 As ice moons are more or less in the same regime of abrasion resistance as a frozen tuna , there may have been painful episodes in the history of ring formation when large captured bodies have entered orbits in the plane of Saturn's rings. This may even have happened.  Since ring particles carving a tangential slot in a massive body will loose a lot of their velocity, they will immediately fall into lower orbits- and the rings are indeed less icy and grittier on the inside.

It would be a grand to see  Enceladus  sawn open like a geode by the buzz saw of the Gods, - what geophysicist could resist taking a core drill to a core -mantle boundary freshly exposed? But if it does happen, it can’t last . This buzz saw's  flat side  would reduce an ill captured new  moon to ring debris  in under an eon .  Cutting edge experimental geophysics will just have to make do the tools it finds to hand .

Comments

[wkwillis]

We did work with frozen free radical propellants in the fifties, but we used not only isotopes and xrays to make the radicals, but a high strength magnetic field to keep the radicals from recombining.
Once enough radicals were stored to melt the ice, a single cosmic ray could release the energy and it would anneal the entire piece of ice at once.
Which is why we can't use it for Orion.
Pity.