Deuterated Methane and Ethane in the Atmosphere of Jupiter

1. Deuterated Methane and Ethane in the Atmosphere of Jupiter Christopher D. Parkinson1,2, Anthony Y.-T. Lee1, Yuk L. Yung1, and David Crisp2 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA 2Jet Propulsion Laboratory, Pasadena, CA, USA Conclusions Why we are solving this problem • All D present today synthesised during the first few minutes of the Big Bang and is a sensitive tracer of the standard Big Bang model and galactic evolution • Jupiter is considered to be an undisturbed deuterium reservoir since the formation of the solar system 4.5 billion years ago, • Therefore, any measurement of Jovian D abundance may link estimates of primordial values to present time ones Jupiter's D abundance appears to be primarily governed via production by reaction of H with vibrationally hot HD and loss by reaction of D with H20,1 and CH3. Below 540 km, CH3reacting with D acts to transfer D to deuterated hydrocarbons. The D Lyman-a emission due to D abundances can be seen quite clearly on the wings of the H line and we note that subsolar viewing will provide much better observations since the D Lyman-a is limb darkened and the best contrast between D and H Lyman-a is most noticeable at subsolar locations. We have found that a warmer neutral temperature profile in the lower thermosphere increases the deuterium abundance in the scattering region and subsequently results in a brighter Jovian D emission by about 15% when compared to the standard reference case. Increasing the vibrational temperature above Tv=2.5T causes dramatic increases in the deuterium abundance above tCH4=1 for all cases. The CH3D, and C2H5D columns increase with increasing vibrational temperature. The CH3D and C2H5D profiles are enhanced in the lower thermosphere due to the source of deuterated non-methane hydrocarbons in the mesosphere. Higher vibrational temperature profiles, viz. Tv = 4T or greater, are expected in auroral regions which should result in brighter D Lyman-a airglow at these latitudes. However, since Kh should be stronger at higher latitudes (Sommeria et al., 1995), which would affect the D Lyman-a emissions in the opposite way, brighter D Lyman-a airglow may not obtain. This work concerns studies of the thermosphere of Jupiter with the view to better understand some aspects of the chemistry and airglow of deuterated species. Thermospheric estimates of D/H ratio are difficult due large uncertainties in Tv, but very useful in determining abundances and transport properties of deuterated species. What we have seen in this work is that a synergistic relationship exists between the modelling and the measurements which may reveal surprises, viz., HD vibrational chemistry impacts D in the thermosphere, CH3D and C2H5D are vibrationally enhanced in the thermosphere, and variations in abundance of CH3D and C2H5D in the thermosphere may reflect dynamical activity (i.e. Kh) in the Jovian upper atmosphere. These are examples of testable phenomena and an observing program dedicated providing such measurements would provide further insight to the aeronomy of the Jovian atmosphere. • Parkinson et al. (2002) previously consider D, HD, CH3D abundances and D & H Ly-a emissions assuming mixing ratio of D to H2 is given by HD/H2 and well determined by the GPMS instrument (Mahaffy et al., 1998) • thermospheric HD will be vibrationally excited • Solve continuity equation treating He as a minor constituent in a background gas of varying mean molecular mass (allowing for H2, He, and CH4) • utilise C2H5D reactions from Lee et al. (2000). Solving the problem Figure 1 Figure 2 Vibrationally Hot H2 in the Jovian Thermosphere Sources of H2(v’): • H2(v=0) + hn H2* • H2*  H2(v’) + hn (flourescence) • Low densities in thermosphere implies slow quenching of excited H2, H2* • H2(v=0) + e  H2(v’) + e • H3+ H2(v=0) + H Sinks • H2(v’) + H2  H2(v-1) + H2 + KE • H2(v’) + H H2(v-1) + H2 KE • H2(v’) + H2(v’’)  H2(v’-1) + H2(v’-1) Figure 4 Figure 3 Relevant Thermochemistry Figure Captions Figure 1: The model atmosphere of some of the more relevant species considered, viz., H2, CH4, CH3, CH2D, CH3D, C2H5D, HD, H and D. Here, the standard reference temperature profile with Tv = 3T was used. Figures 2, 3, and 4: Various D profiles resulting from calculations utilising vibrational temperature profiles corresponding to Tv = nT, where n = 1, 2, 2.5, 3 and 4. Figure 5: H and D Lyman-a intensity profiles for several solar zenith angles with the same viewing angle (i.e. SZA = viewing angle) for the standard reference atmosphere, Figure 6: D Lyman-a subsolar intensities as a function of vibrational temperature. • H + HD(n=1) --> HD (n=0) + H • H + HD(n=0) --> D + H2 • H + HD(n=1) --> D + H2(n=0,1) • H + CH2D --> D + CH3 • D + H2 (n=0) --> HD + H • D + H2 (n=1) --> H2(n=0,1) + H • D + CH3 --> H + CH2D • D + H + M --> HD + M • H + CH2D --> CH3D • CH2D + CH3 --> C2H5D • C2H5 + C2H4D --> C2H5 + C2H5D Figure 5 Figure 6