J.D. Mathews Penn State

Transcription

1 Metal Ions in Sporadic-E Layers Along with Some Speculation About Analogous Phenomena on Other Planets Iron Meteorite - Meridiani Planum J.D. Mathews Penn State NASA/JPL/Cornell image PIA07269 This is the first meteorite found on another planet. Its maximum dimension is 31 centimeters.

2 Non-Issues: Altitude-narrow E-region layers (ANELs; aka Sporadic-E) are continuously present at most latitudes. ANELs are largely composed of atomic metal ions of meteoric origin. The wind-shear mechanism depicted below is the ANEL generation mechanism. B-field into diagram 1.) 0 = e(v i! B + E) " M# in (v i " V n ) Steady state ion momentum equation. 2.) v i = (u,v,w); V n = (U,V,0) Ion and neutral wind velocity vectors in the magnetic south, east, and upward directions. 3.) B =!B 0 (cos I,0,sin I) Magnetic field vector (dip angle I; magnitude B 0 ). U cos I sin I + rv cos I 4.) w = 1 + r 2 Ion vertical speed for electric field E = 0. 5.) r =! in " ci Ratio of ion-neutral collision frequency (! in ) to ioncyclotron frequency (! ci = eb 0 / M ) for an ion of mass M and electron-charge e.

3 Issues: How to extend consideration of ANEL layering processes in Earth s MLT to the other planets? 1. Meteoroid flux and in-atmosphere speed at atmospheric densities corresponding to Earth s meteor zone (~ km) must ultimately be determined. Thus, what are the salient features of the other planets that we must consider? 1. Planetary gravity, atmosphere density/distribution, and magnetic field presence/ organization. 2. Planetary orbital and escape speed give some idea of where the peak of the meteoroid speed distribution occurs. This is ~55 km/sec for small particles in Earth s meteor zone with a maximum of 72.8 km/sec. A. Duvall, C. G. Justus, V.W. Keller, Global reference atmospheric models for aeroassist applications, 3rd International Planetary Probe Workshop, Anavyssos, Attiki, Greece, 27 June - 1 July 2005.

4 Summary of ANEL Relevant Properties Use Venus, Mars, and Neptune for maximum contrast on existence of a meteor zone. Planet Escape Speed wrt planet (km/sec) Solar Distance (AU) Orbital Speed (km/sec) Escape Speed wrt Sun (km/sec) Max/Min speed in atmosphere (km/sec) Atmospheric Pressure (Earth atmospheres) Magnetic Field (! Earth s) Meteor Zone (range, km) Venus 10.3 ~ /17.8 ~90 ~ Earth / Mars ~ / ~ Neptune 23.5 ~ /23.6 >>100 ~ Immediate Conclusions: 1. IF there is a significant meteoroid flux at Venus, Mars, and Neptune there will be a meteor zone and thus meteoroid-derived metal ions & dust. 2. Neptune will likely have an interestingly narrow meteoroid speed distribution?! 3. The Mars meteor zone will be low but not that low As Venus has no magnetic field, ANELs as we know them won t exist. 5. As Mars has a weak crustal B-field, ANELs as we know them may exist.

5 What are the prospects for ANELs on Mars? Immediate problem: no organized B-field... However, there is an ionosphere and radar basically an ionosonde! The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) yielded the follow example profiles. Nielsen, E. et al., Vertical sheets of dense plasma in the topside Martian ionosphere, J. Geophys. Res., 112, E02003, 2007

6 Mars dayside Ne profile from Mars Express. Venus Ne profile from Venus Express. Patzold et al., 2009

7 MARSIS has shown that the Martian ionosphere is basically photochemical in origin but that it is organized with maxima at those points in the crustal B-field (~100 nt) where the field lines are nearly vertical and open to the solar wind. A partial conclusion... ANELs are possible away from this region as there is an appropriate neutral wind at these levels and the meteoroid flux would be similar to or even higher than at Earth due to the nearby asteroid belt. Nielsen, E. et al., Vertical sheets of dense plasma in the topside Martian ionosphere, J. Geophys. Res., 112, E02003, 2007

8 A further thought... The Martian upper atmosphere contains CO2 ice particles.

9 The CO2 ice particle form cirrus-like clouds at altitudes of km. These NLC-like cirrus clouds inhabit the meteor zone and so, if viewed by an appropriate surface based radar, would likely exhibit PMSE-like scattering...!! Montmessin et al., High-altitude CO2 clouds on Mars: OMEGA and HRSC observations, Third International Workshop on Mars Polar Energy Budget and the CO2 Cycle,

10 And Neptune? Neptune's magnetic field is tilted 47 degrees from the planet's rotation axis, and is offset at least 0.55 radii (~13,500 km) from the physical center. The field strength at the surface varies, depending on which hemisphere is being measured, from a maximum of more than 1 gauss in the southern hemisphere to a minimum of less than 0.1 gauss in the northern. Because of its unusual orientation and the tilt of the planet's rotation axis, Neptune's magnetic field goes through dramatic changes as the planet rotates in the solar wind. Additionally, Neptune has a strong neutral wind structure at all relevant altitudes. Conclusion: Neptune should have ANELs distributed throughout the meteor zone. Voyager mission page: