Uranus Composition profiles

In Chapter 2 we saw that the observable atmosphere of Uranus (Figure 4.19, see color section) has much higher levels of methane than Jupiter and Saturn, and a much greater D/H ratio. The methane v.m.r. was estimated to be 2.3% from Voyager 2 radio occultation measurements (Lindal et al., 1987). Later ground-based visible hydrogen quadrupole measurements suggested a figure closer to 1.6% (Baines et al., 1995b), and more recent estimates from ground-based near-IR observations put the abundance as most likely between 1% and 1.6% (Sromovsky and Fry, 2008). Hence the C/H ratio appears to be between 40 x and 50 x the solar value. This, and other indicators, suggest that the hydrogen-helium atmosphere observed is merely a thin shell, accounting for only 20% of the radius and 20% of the mass, and that the bulk of Uranus is composed of "icy" materials such as water and methane, albeit hot and fluid.

The troposphere of Uranus (and Neptune) is much colder than that of Jupiter and Saturn, and together with the higher abundances of condensable species such as water, ECCM calculations predict that clouds such as water and NH4SH condense at correspondingly deeper levels, as can be seen in Figure 4.20. The deep abundances are here assumed to be: O/H = 100x the solar value, N/H = the solar value, S/H = 11 x the solar value, and C/H = 40 x the solar value. The reasons for these assumptions are outlined in the following paragraphs. Water is expected to condense at very deep levels, with methane condensing near 1.5 bar. Considering the measured (or assumed) high abundances of these gases, the SALR can be seen to be very different from the DALR and thus the dry and wet temperature profiles differ significantly. The sharp variation in the DALR at 1.5 bar is due to the reduction in atmospheric heat capacity caused by the condensation of methane. It should also be noted that the sudden change in the mean molecular weight at this cloud base (and at the base of the deeper water cloud) causes a substantial change in the pressure scale height. The ortho-hydrogen/para-hydrogen was here assumed to be in the "intermediate" state outlined in Section 4.1.4.

Figure 4.20. Equilibrium cloud condensation model of Uranus' atmosphere (as Figure 4.7). Calculated cloud layers: H20 cloud (water, then ice) atp > 1,000 bar, NH4SH at ~40 bar, H2S ice at bar, and CH4 ice at ~1.2 bar. Assumed composition: O/H = 100x the solar value, N/H = the solar value, S/H = 11x the solar value, C/H = 40x the solar value.

Figure 4.20. Equilibrium cloud condensation model of Uranus' atmosphere (as Figure 4.7). Calculated cloud layers: H20 cloud (water, then ice) atp > 1,000 bar, NH4SH at ~40 bar, H2S ice at bar, and CH4 ice at ~1.2 bar. Assumed composition: O/H = 100x the solar value, N/H = the solar value, S/H = 11x the solar value, C/H = 40x the solar value.

Because the observable atmospheres of Uranus and Neptune are extremely cold, it is difficult to determine composition profiles using thermal-infrared spectroscopy since the emitted spectrum has such low power. Hence, we know a lot less about the composition of the atmospheres of Uranus and Neptune than we do about the warmer atmospheres of Jupiter and Saturn. What has been determined about Uranus' composition is outlined in Table 4.8 and Figure 4.21. Methane is indeed found to condense near the 1.5 bar level and the v.m.r. drops very rapidly with height above this level reaching a minimum of approximately (0.3-1) x 10~4 at the tropopause. In the stratosphere, photodissociation of methane occurs between 0.1 mbar and 1 mbar, giving rise to hydrocarbon products. Acetylene (C2H2) was detected by ISO (Encrenaz et al., 1998) with a maximum v.m.r. of 4 x 10~7 peaking at the 0.1 mbar pressure level and ethane (C2H6) has been detected by Spitzer (Burgdorf et al., 2006) with a v.m.r. at the 0.1 mbar level of (1.0 ± 0.1) x 10~8, along with methyl acetylene (CH3C2H or C3H4) and diacetylene (C4H2).

Ground-based microwave observations of Uranus between 1 mm and 20 cm indicate that both ammonia and water vapor are substantially subsolar (by a factor of several hundred) down to pressures of approximately 50 bar, but that H2S is much more abundant than in the atmospheres of Jupiter and Saturn at levels of (10-35)x the solar value. The low abundance of water vapor is not surprising since it is expected to have mostly condensed by 100 bar, but the low abundance of ammonia is very surprising, especially when the abundance of methane is so high, and considering that both Jupiter and Saturn have significant quantities of ammonia. It would appear that either almost all the available ammonia reacts with H2S to form NH4SH at levels of approximately 40 bar (which would require the S/N ratio to be greater than 4x the solar value), or that substantial quantities are incorporated into a massive aqueous ammonia cloud at deep levels. It has even been suggested that ammonia might dissolve into water at even greater depths (Atreya et al. 2006), forming a water-ammonia ionic ocean, which may also help explain features of the observed magnetic field.

Ground-based microwave observations with the VLA have also revealed that the deep abundance of ammonia (5-50 bar) appears to vary with latitude by almost an order of magnitude with higher levels detected at equatorial latitudes (de Pater et al., 1991; Hofstadter and Butler, 2003). Such a variation may be indicative of a large-scale Hadley cell with air rising at the equator and descending at the poles or alternatively that the atmosphere is convectively overturning at low latitudes, but convectively suppressed closer to the poles, with the transition latitude at ±45°. VLA observations in 2005 also show some banding and structure at midlatitudes and equatorial latitudes (Orton et al., 2007c). Higher in the atmosphere, the Voyager IRIS observations suggest that the meridional circulation seems to be somewhat different with upwelling at midlatitudes and subsidence at the poles and equator (Flasar et al., 1987; Orton et al., 2007c), a topic we will return to in Chapter 5. If cloud absorption is neglected, the observed microwave spectra suggest that the atmospheric temperature profile becomes isothermal at depth (de Pater et al., 1989). Such a profile would be consistent with Uranus having a very low internal heat flux since atmospheric dynamics would then be driven primarily by absorbed sunlight. How-

Table 4.8. Composition of Uranus.

Gas

Mole fraction

Measurement technique

Reference

He

0.15 at p < 1 bar

Voyager far-IR

Conrath et al. (1987)

f a JeH2

0.85 < /eH2 < 1.0

Visible hydrogen quadrupole

Baines et al. (1995b)

nh3

Solar/(100-200) at p < 10-20 bar

Ground-based microwave

de Pater and Massie (1985)

h2s

(10-30) x solar

Ground-based microwave

de Pater et al. (1991)

s/n

>5x solar

Ground-based microwave

de Pater et al. (1991)

h2o

(6-14) x 10~9 at p < 0.03 mbar

ISO/SWS

Feuchtgruber et al. (1997)

ch4

(0.3-1) x 10~4 at tropopause, <3 x 10~6 at 0.1 mbar

Radio occultation Visible hydrogen quadrupole ISO/SWS

Encrenaz et al. (1998)

ch3d/ ch4

3.6±|;6 x 10-4

Ground-based 6,100-6,700 cm-1

de Bergh et al. (1986)

ph3

<6x solar (2.2 x 10~6). No evidence of strong supersaturation <8.3 x 10-7

Ground-based 1-1.5 mm

HST 5 ^m

Encrenaz et al. (1996) Encrenaz et al. (2004)

AsH3

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