Moon Atmosphere and SW Interactions Kyle Willett Bruce Ferguson Timothy Monk Alexa Halford Outline Observations of the lunar atmosphere physical chemical Evolution of the lunar atmosphere over time The Moon as a comet Influence of solar wind on the moon The lunar atmosphere observations and structure Exospheres vs atmospheres An exosphere has purely ballistic trajectories and distinct speciation Earth s atmosphere distinct stratification winds higher pressure and temperature gradients phase changes our exosphere begins 5001000 km up Moon is an example of a surface boundary exosphere SBE Stern 1999 Observations Telescopic searches only gave upper limits on a lunar atmosphere Ion detectors SIDE CPLEE aboard the Apollo missions 12 14 15 were the first to detect a lunar ionosphere Spectrometers on same Apollo missions detected neutral components Difficulties Low number density Absorption in Earth s atmosphere Artificial contamination Lunar Prospector detected daughter products of Rn and Po also searched for water in polar caps Stern 1999 Observations continued Suspended dust in LA picked up by Surveyor and Apollo 17 missions Detections of lunar ions in the solar wind Orbital motion of the moon restricts periods capable of observing the lunar atmosphere Future missions SELENE LUNAR A Stern 1999 Physical characteristics Number density 105 107 particles cm3 Scale height varies w species zNa 120 km Dynamics particles travel on ballistic trajectories vesc 2 4 km s Alkali metals have two distinct energy distributions barometric and coronal Mendillo 2001 Stern 1999 Chemical composition Few chemical interactions take place in LA due to separation by species Most chemical reactions take place on the surface Problem of the missing elements measured number densities are far less than totals 105 107 cm 3 measured by Apollo instruments Stern 1999 Exospheres on other bodies Moon He Ar Rn CH4 no B Mercury He Na K high B solar proximity Io Europa SO2 SO S O Na K Cl high induced B O Na volatile surface SBE Ganymede Callisto Deimos Phobos Schleicher et al 2004 Overall Dynamics in the Lunar Atmosphere Sources and Sinks The Sinks Total lunar atmospheric loss rate 10 g s sinks control density High compared with total mass 2 x 107 g and abundances Gravitational escape if velocity greater than escape velocity get direct loss depends on initial speed surface atmosphere photodissociation enhanced escape in the tail due to radiation pressure Ionization loss Ions are generally lost through acceleration by solar wind E at low altitudes lose about half increasing with altitude caused by photoionization and solar wind impact photoionization is dominant and always acting Sources and Sinks The Sinks cont d Chemical loss 2 types collisions at the surface temporary chemical bonding before ballistic ejection probably important for p He and O from solar wind gas gas collisions extremely unimportant Condensation occurs on lunar nightside adsorption due to contact with cold surface Ar Na and K still undiscovered no doubt present H2O temporary usually and regained on dayside Sources and Sinks The Sinks cont d Comparison of loss processes photoionization is dominant loss timescales are short all species must be continuously replenished vertical profile for Na and K imply species are largely bound ballistic hop time vs escape time many hops recycling Sources and Sinks The Sources Different source mechanisms are species dependent No one mechanism supplies all of one species Thermal desorption sublimation from the uppermost regolith diurnal cycle 29 5 days peaks at morning terminator due to finite reservoir and thermal timescale locally affected by albedo emissivity conductivity and terrain slope produces bound ballistic atoms H 10 km Sources and Sinks The Sources cont d Sputtering ejection from a lattice site due to impulse of energy evidence from rims of grains and glasses 10 100 g s affected strongly by local characteristics photon sputtering diurnal and seasonally dependent characteristic temperatures 800 2000K charged particle sputtering pressure falls off as cosine of zenith angle diurnal and concentrated at subsolar latitudes contributes to high altitude corona and direct escape shielding when Moon is within the magnetotail Sources and Sinks The Sources cont d Chemical sources chemical reactions due to solar wind protons provide excess energy to desorb atomic molecular fragment high yields with thermal distributions 500 1000K Meteoritic sources impact generated vapor magma release of volatiles highly dependent on composition of impactor and target site temporal variation set by crossing of cometary asteroid debris fields meteor showers vapor temperature 2000 5000K outgassing volatiles 3000K bound and directly escaping species Sources and Sinks The Sources cont d Internal release degassing through vulcanism diffusion and seismically induced seepage vulcanism only important early on diffusion only important for radiogenic species 40Ar seismic seepage correlations between release of Ar and these events by Apollo 17 Rn production at mare highland boundaries random Evolution of the Lunar Atmosphere The exospheric nature of the lunar atmosphere is easily perturbed Epoch 1 with a magma temp of 1500K you could get 100bars Epoch 2 a significant single volcano would create a collisional albeit thin atmosphere Epoch 3 impacts could lift the atmosphere out of exospheric on a timescale 100 yr The Moon as a Comet z Common characteristics z Atmospheric measurements z z z z Number density Pressure Visible tail Significant Differences z z z z General features General orbital characteristics Atmospheric generation and composition Magnetosphere Commonality Atmosphere z Moon atmospheric characteristics z Daytime 107 particles cm3 z Nighttime 105 particles cm3 z Cometary atmospheric characteristics z Varies greatly depending on orbital position z From observations at 0 8 AU of Comet Halley z Gas production rate varies from 6 9 13 x1030 s 1 Commonality Visible Tail 1998 Leonids meteor shower Na tail Visibility from 0 to 128 Rayleighs Subsequent observations report that this is a permanent feature Cometary Tails Hale Bopp 1997 Differentiation General features Moon radius equatorial 1733 1 km z Comet radii on the order of several km s z Examples Hale Bopp 50km Halley 15km Moon composition oxygen silicon magnesium iron calcium titanium and aluminum among others z Comet composition dust and frozen volatiles ice in the form of H20 CO2 CO etc z