The Atmospheres of Uranus and Neptune

Transcription

1 The Atmospheres of Uranus and Neptune Jim Norwood currently funded by NASA grant NAG

2 Outline 1. Stratosphere 2. Troposphere 3. Dynamics

3 Stratosphere

4 Stratospheric temperature profiles for Neptune and Uranus. Original temperature information from Voyager UVS occultations (high altitude) and radio occultations (lower altitude). Some gaps filled in by ground-based observations. Stratospheric profiles courtesy of J. Moses.

5 Stratospheric temperature profiles for Neptune and Uranus. Neptune: fch4 = 0.05% Uranus: fch4 = ppm Stratospheric profiles courtesy of J. Moses.

6 Neptune Moses et al. (2005) peak mixing ratios: methane: 0.05% ethane: 3 ppm ethylene: 0.2 ppm acetylene: 0.2 ppm Stratospheric profiles courtesy of J. Moses.

7 Uranus Moses et al. (2005) peak mixing ratios: methane: ppm ethane: ppb acetylene: ppb Moses et al. (2008 LPSC): Uranus acetylene abundance is increasing Burgdorf et al. (2006) Spitzer findings: Stratospheric profiles courtesy of J. Moses. ethane: 10 ppb methylacetylene: 0.25 ppb diacetylene: 0.16 ppb

8 changing ethane (and methane) emission on Neptune (Hammel et al. 2006)

9 changing ethane emission on Neptune (Hammel et al. 2006)

10 Uranus CO: Discovered by Encrenaz et al. (2004) from near-ir emission. Teanby and Irwin (2013): sub-mm observations, upper limits only. Observed in the sub-mm by Cavalié et al. (2014). Best fit to observations has CO abundance greater in stratosphere than in troposphere external source. Uranus CO2: Burgdorf et al. (2006): mixing ratio of at 0.1 mbar also of external origin

11 Neptune CO: Discovered by Marten et al. (1993) in sub-mm emission. Analysis by Lellouch et al. (2005) and Hesman et al. (2007): CO exists in the troposphere and stratosphere, but is more abundant in the upper stratosphere dual origin (both internal and external). Neptune HCN: Also discovered by Marten et al. (1993) in sub-mm emission. Trickier: expected to form out of nitrogen photochemistry but are the N supplied from above or below? Marten et al. (2005) think internal source, based on the results from eddy mixing profiles.

12 Neptune s south polar region appears bright in the mid-ir. (Orton et al. 2012)

13 Troposphere

14 Based on thermochemical modeling: methane: 1.4 bars ammonia: several bars NH4SH: ~40 bars water: very very deep

15 Based on thermochemical modeling: methane: 1.4 bars ammonia: several bars H2S: several bars NH4SH: ~40 bars water: very very deep

16 Why is the NH3 abundance so low? Where is the missing nitrogen? CH4 H2S Possibilities: NH3 dissolved in the liquid water cloud. or in the speculatedabout ionic ocean. NH4SH H2O Or the missing N could instead be in the form of N2, which is difficult to observe.

17 Keck images of Uranus by L. Sromovsky.

18 Keck II images of Uranus in the near-ir, presented as a poster at the 2012 DPS by P. Fry, L. Sromovsky, K. Rages, H. Hammel, and I. de Pater. Each is a combination of over 100 images from July 2012, derotated, with a highpass filter applied. Two filters were used: H (cyan, areas with strong and weak methane absorption) and H-continuum (red, weak methane absorption only).

19 Images of Neptune from Keck s NIRC2 (H filter), Gibbard et al. (2003).

20 Keck II images of Neptune, from de Pater et al. (2013). The two filters probe to different depths of the atmosphere: the H- band delves deeper than K.

21 Near-IR spectrum of Uranus from IRTF/SpeX (Tice et al. 2013).

22 from Karkoschka & Tomasko (2011). Uh-oh: Methane abundance varies with latitude it is depleted at the poles.

23 A fuller picture of Uranus varying latitudinal methane abundance, from Sromovsky et al. (2014).

24 Dynamics general circulation, zonal winds, cloud features, seasonal/temporal evolution

25 Sromovsky et al. (2014) tackled Uranus circulation by starting with the poles, which appeared to be depleted in methane as well as other absorbers like H2S and NH3. But how can a model like this produce numerous isolated cloud features, probably from the H2S cloud level?

26 Maybe a more complicated scenario.

27 In trying to figure out Neptune s atmospheric circulation, de Pater et al. (2014) started with the midsouthern and northern latitudes.

28 Uranus wind speeds from cloud tracking in data from The overall pattern is slightly asymmetric. From Sromovsky et al. (2012).

29 Neptune wind speeds based on cloud tracking data from HST and IRTF in 1996 (Sromovsky et al. 2001)

30 Uranus in 2003, showing south polar collar, the Berg, and northern-hemisphere activity. The long-lived Berg feature oscillated between 32 S and 36 S until 2005, when it started drifting toward the equator. Hammel et al. (2005)

31 Observed temporal changes: overall brightness changes in Uranus and Neptune (Hammel & Lockwood 2007) disappearance of Uranus southern collar and formation of a northern collar disappearance of Uranus bright polar (70 S) ring before equinox (Rages et al. 2004) and possibly other observational disagreements (e.g., Neptune s ethane emission)

32 Current priorities for ice-giant atmosphere studies: Spatial resolution Temporal baseline to distinguish seasonal effects from perspective effects from weather

33 Years that have elapsed since discovery: Uranus: 2.78 Neptune: 1.02