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

• 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

Why we are solving this problem
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 Tv2.5T causes dramatic increases in the
deuterium abundance above tCH41 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(v0) hn ? H2
• H2 ? H2(v) hn (flourescence)
• Low densities in thermosphere implies slow
  quenching of excited H2, H2
• H2(v0) e ? H2(v) e
• H3 ? H2(v0) 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(n1) -- HD (n0) H
• H HD(n0) -- D H2
• H HD(n1) -- D H2(n0,1)
• H CH2D -- D CH3
• D H2 (n0) -- HD H
• D H2 (n1) -- H2(n0,1) H
• D CH3 -- H CH2D
• D H M -- HD M
• H CH2D -- CH3D
• CH2D CH3 -- C2H5D
• C2H5 C2H4D -- C2H5 C2H5D

Figure 5
Figure 6