Atmospheric Sputtering+ Isotope FractionationApplication to MarsAlso: comments on Titan Nitrogen
Mars Atmosphere and Ionosphere exobase altitude dashed line Hot O in ‘corona’ (exosphere) exobase Mars exobase altitude is depends on the solar activity: 180km is an average. Note: there are molecules and molecular ions at the exobase, unlike at earth. Corona, region above exobase, is eventually dominated by O and O+.
Hot Atom Production Suprathermal particles with kinetic energies E >~10 Tx are produced in various nonthermal processes: Because there are molecules and molecular ions at the exobase: Dissociative recombination of molecular ions Photon and electron impact dissociation Produce kinetic energies up to a few eV: at Mars O2+ and CO2.
‘Sputtering of Atmosphere’Johnson,Space Science Revs. 69, 215, 1994 On non-magnetized bodies energetic ions, A+, can penetrate the exobase and collide with atmospheric molecules, B Charge exchange and momentum transfer collisions by solar wind (H+,He+2) ions or by pick-up ions formed in the corona (O+ at Mars) Momentum transfer:the suprathermal atoms with kinetic energies up to 100’s of eV Charge exchange:high-energy ion is converted to an neutral with approx. the same energy.
Ballistic Trajectories lcol >> H (Kn >> 1) Particle Tracking Atmospheric Corona lcol ~ H (Kn ~ 1) Exobase Thermosphere Fluid-likelcol << H (Kn << 1) Exobase = Transition Region: Goes from collisional to collisionless Can describe using Boltzmann Transport Equations or Monte Carlo Simulations
Boltzmann Transport Equations Evolution of suprathermal atoms in the atmosphere is often calculated via Boltzmann-type kinetic equations with sources, sinks and collisions One such equation for the density and speed distribution for each species. This is the distribution we used for Jeans escape Nonequilibrium: Typically too difficult so simulations are used
Direct Simulation Monte Carlo Model: Approximate the atmospheric gas by a finite number of modeling particles each with a very large weight Calculate the motion of the model particles subject to gravity in a time step, dt B) Calculate the probability of a collision for every particle in dt C) If collisions occur in dt: calculate new speeds of colliding particles and return to A for a new dt
Deflection of Solar Wind Non-Magnetized Planet Solar Wind Planet H+, He+, etc plus Magnetic + Electric Fields Photons produce ionosphere Solar fields try to move through ionized region Induces currents Currents deflect the fields (Lenz’s law) Ionopause : Solar Wind Pressure = Pressure from Ionosphere Fluid Picture ButIons can have large gyroradii + penetrate ionopause and exobase
Interaction of solar wind plasma with the Martian atmosphere Hot O in the corona can become ionized, O+. The O+ are picked up and accelerated by the solar fields (~keV) and can be swept away or impact the atmosphere
Impacting ions produce atmospheric sputtering A+ Exobase Molecules B B might escape A A B B A+ can be an ion with large gyro radius that reaches exobase or a neutralized ion that then ignores the fields and penetrates the exobase and collides with atmospheric molecules B Sputtering Yield, Y Number of molecules with energy greater than Ees that cross the exobase Escape Flux = Y x Incident Ion Flux Therefore, need to calculate: Number of recoils molecules, B, set in motion with energy, E, greater than Ees
A Simplified Transport Equation collisions between identical particles: B energy distribution only(ignore spatial dis.) G(T,E)dE is number of recoils set in motion with energy between E and E+dE by a particle of energy T produced by an incident ion, A+. G(T,E)dE = [Probability that a particle of energy T creates a recoil of energy E] plus [Probability that an earlier recoil produced attains energy E or produces another recoil with energy E
Atmospheric Sputtering (cont.) Use the Recoil Distribution Isotropic Cascade (like Jeans escape)
Hot oxygen at Mars • Dissociative recombination • ionosphere is O2+ • Escape energy for O: 2.1eV • processes 1 and 2 might lead to escape • 3 and 4 only produce hot O in corona • Dissociative recombination primarily populates corona • Sputtering by O+ picked-up in corona • ( ~keV) primarily causes escape • Simulated an O corona: low and high solar activity, and then a multispecies corona • energy spectra from Luhmann and Koyzra 1991
Hot Oxygen at Mars LOW SOLAR ACTIVITY Energy Distribution Function Calculated – solid Thermal – dashe Left vertical line –suprathermal region Right vertical line –escape energy. Suprathermal tail includes escaping flux; Atoms between vertical lines populate corona.
Hot oxygen corona at Mars LOW SOLAR ACTIVITY • Thermal fraction: • exospheric temperature • T=180 K • (solar min.) • Nonthermal fraction: O2+ dissociative • recombination + atmospheric sputtering. • Compared to: • Nagy & Cravens 1988; • Lammer & Bauer 1991. Different height scales!
Hot oxygen at Mars atmospheric loss Extrapolated back in time using changes in EUV flux Net Loss rate ~4.5х1025 O/s Equivalent to 1 earth’s atmosphere
The Martian Corona Use of a 3D Monte Carlo model for the non-thermal component + a thermal component. Examples of exospheric density in the equatorial plane Solar minimum activity n~10cm-3 n~100cm-3 Soleil n~1000cm-3 rotation Soleil Solar maximum activity
Results of a 1D Multi-Species 7 4 0 -2 7 4 0 -2 f(r,v) (cm-6 s3) Net escape to space ~10 m of water ~0.15 bar of CO2 large uncertainties Enough to lose greenhouse effect? -5 0 5
Atmospheric Loss at Mars When magnetic fields collapsed did atmospheric sputtering remove enough gas to cause loss of Greenhouse effect And, therefore, cause the freezing out of the remaining atmosphere? From Chassefiere and Leblanc (2004)
Loss to space after dynamo extinction Loss of Magnetic fields PHOBOS (solar max) MEX (solar min)) Calibrated using the fact that epoch 2EUV before present has, roughly, the same average solar wind and EUV flux as we have at present at solar maximum. Therefore, model can be tested by Mars Express measurements.
Atmospheric Sputteringat Mars Extended hot oxygen corona at Mars is populated mainly by the suprathermal oxygen atoms formed in the O2+ dissociative recombination in both low and high levels of solar activity. Atmospheric sputtering results in the additional population of the extended hot corona and in a large increase of the oxygen loss rate, especially at high solar activity However, it is a complicated feedback process (Johnson and Luhmann, J.Geophys. Res. 103, 3649, 1998) But by testing simulations against accurate space craft measurements (Mars Express) of the corona and the solar wind and solar fields we can hope to simulate earlier epochs and learn whether there were sufficient green house gases for and early wet Mars: will need to include sulfur species
Nitrogen Isotopes Titan Atmosphere: 96% N2 Parts per 1000 Requires 40 Earth’s atmospheres to be lost!! Outgassing is likely NH3 Eventually converted by photodissociation to N2 Whereas the N2 does not escape efficiently large amounts of NH3 (Ees =0.41eV) can escape by non-thermal processes (primarily atmospheric sputtering)
Nitrogen at Titan Remember For non-thermal, nearly mass independent, escape processes, diffusive separation determines population at exobase and, therefore, the isotopic fractionation Present day enhancement of light species at exobase --> large loss required??
Titan's atmosphere is believed to be similar to Earth's early atmosphere. Toby Owens said: "What we've got is a very primitive atmosphere that has been preserved for 4.6 billion years. Titan gives us the chance for cosmic time travel . . . going back to the very earliest days of Earth when it had a similar atmosphere.” The proportion of heavy nitrogen-15 in the atmosphere of Titan is much greater than that around other planets. Scientists believe that the lighter nitrogen-14 was lost over geologic times scales for reasons that remain unknown. Requires that most of the atmosphere evaporated into space, a process in which the nitrogen-14 would have escaped more easily than nitrogen-15. But it would mean that Titan once had an atmosphere 40 times as thick as Earth's - making it a dwarf version of a gas planet. 'This bizarre world may be far more complex that we have begun to imagine,' says Soderblom. The nitrogen isotopes are telling us something about the way planetary atmospheres are formed rather than how they evolve. Why do we insist that a star's "children" all be born at the same time? Hannes Alfv始 wrote in Evolution of the Solar System, "..the Laplacian concept of a homogeneous gas disc provides the general background for most current speculations. The advent of magneto-hydrodynamics about 25 years ago and experimental and theoretical progress in solar and magnetospheric physics have made this concept obsolete but this seems not yet to be fully understood.” The electrical model of planet birth proposes that planets are born by electrical expulsion of some of the matter of a star or gas giant in a tremendous "flare.” http://www.thunderbolts.info/
Summary • Titan should be simple: large 15N/14N ratio. Now mostly N2, likely derived from NH3which is lighter; atmospheric sputtering is likely the principal loss process (e.g., this can remove all atmosphere from large Jovian moons) (Johnson ApJ572, 1077, 2002) But it is not simple--loss appears too large and C is not fractionated Affected by atmospheric structure in earlier epochs and recycling in the surface: CH4 --> hydrocarbons • Mars is complex But--if you start the clock when the intrinsic field froze, there is hope Still all species are not similarly fractionated: reservoirs?
Mars Isotopes • Some elements in the martian atmosphere display large isotopic fractionations suggesting loss of a major portion of the atmosphere • D/H ratio (measured by earth-based spectra and in martian meteorite water) five times Earth’s • 15N/14N ratio: 60% enriched over Earth's (measured by Viking and in martian meteorites) • 38Ar/36Ar ratio: 30% enriched over Earth's • 136Xe/130Xe ratio that is 16-25% enriched over solar Xe and in carbonaceous meteorites (Ar and Xe measurements are from shock-implanted gases in EETA79001) • Kr isotopes in EETA79001 closely resemble solar as do Xe isotopes in Chassigny. Minor component of N in EETA79001 + Zagami has 15N/14N similar to Earth's. • Findings indicate that at least two reservoirs of these gases exist on Mars, one a mass-fractionated atmospheric component and one likely an unfractionated mantle-derived component. • How can such different reservoirs be used to define in detail the volatile evolution of Mars?
Summary Things to Know: Hot Atom Processes Atmospheric Sputtering Modeling Non-equilibrium Regions Recoil Energy Distribution Atmospheric Sputtering Yields Model of Mars Corona Atmospheric Loss Estimates Isotope Fractionation at Mars Nitrogen Fractionation at Titan