Extracting atmospheric density estimates at 400km from Mars Odyssey radio signals

1. Extracting atmospheric density estimates at 400km from Mars Odyssey radio signals Retrieval of atmospheric density variations in the Martian upper atmosphere using Mars Odyssey radio tracking data EGU 2005 Vienna 2005-04-29 15:30 Erwan M. Mazarico Maria T. Zuber Frank G. Lemoine Dave E. Smith

2. Outline • The Mars Odyssey spacecraft • Goal, motivation and obstacles • Method • Results • Short-period variations • Conclusion

3. Mars Odyssey Spacecraft X-band telecom system Uplink 7.18 GH Downlink 8.44 GHz ref: NASA Mars Odyssey Press Kit

4. Mars Odyssey Orbit • a ~ 3800 km • Period just under 2 hours • Small eccentricity • altitude ~ 400km • Polar orbit (i~97°) • Close to sun-synchronous • but not frozen like MGS • Perifocus near the South Pole MOLA perifocus

5. Mars Odyssey Attitude • Accelerations due to non-conservative forces depend on cross-sectional areas • Very different cross-sections for radiation and drag ARAD ~ 2 ADRAG ref: JPL D-19010

6. Sampling of the atmosphere • Eccentric orbit (e~0.01) • Altitude varies between 390 and 450km • z > H → more sensitive near periapse • Orbital geometry → sampling occurs mostly on latitudes south of 40°S, slightly smeared by planetary flattening

7. Goal and motivation • Measure the atmospheric density at the spacecraft altitude in order to: • Assist future spacecraft navigation • Obtain data for lander entry design • Understand the structure and dynamics of the upper atmosphere, and constrain GCMs

8. Lemoine et al., 2001 Obstacles • At 400km, atmospheric drag order of magnitudes smaller than many forces … hard to measure

9. Method • Radio Science / Precision Orbit Determination with GEODYN II from GSFC • ~2 years of radio tracking data • Normal equations created after the convergence of ~5-day trajectory arcs • Different parameters are estimated • Spacecraft position and velocity • Spacecraft attitude thruster firings • Ground station biases • Drag and Radiation coefficients (CD and CR)

10. Density Recovery • CD and CR are defined as: • GEODYN outputs aD, AD, V,  • CD estimated per arc or per day

11. LS dust Results from MGS TES data

12. albedo? emissivity? k2? Results -β Secular variation of CR has something to do with the beta angle Observed secular changes on CD is questionable but very stable CR recovery/estimation

13. LS Temporal Variations in Atmospheric Density Smaller scale height in the upper atmosphere ? Retrieved values much smaller (ρODY~10% ρmodel) High variability compared to model Seasonal cycle not visible at 400 km

14. LS Short-Period Variability • Several periods with good data coverage and varied conditions were selected to study the variability of the density

15. Short-Period Variability LS ~ 255 – 275° dust ~ 0.3  ~ -60 – -57 °  ~ -68 – -64 ° NOAA ACE

16. Conclusions • No official Radio Science mission on Mars Odyssey but the tracking data has scientific value • it can be used to estimate the non-conservative forces • these estimates are stable and consistent from arc to arc • Estimated densities smaller than model predictions • at 400km, atmosphere thinner than what models predict • The Martian atmosphere seems to respond to solar forcing on short timescales • higher coupling due to the absence of a protective magnetosphere ? • Decoupling with lower atmosphere? • No obvious atmospheric dust opacity influence

17. Future work • Challenges remaining • Understand long-term variation in CR (and CD) • More data ! and more spacecraft ! • more Mars Odyssey data • Technique can be applied to more spacecraft • Mars Global Surveyor (MGS) • Mars Express (MEX) • Mars Reconnaissance Orbiter (MRO) • MEX and MRO lower altitude → much higher drag • at least one order of magnitude greater (comparable to solar radiation)

18. Thank you