The period covered by this cooperative agreement included the analysis of data from the Voyager encounter with Neptune and Triton and the primary Galileo mission to Jupiter (including the Galileo Probe entry into Jupiter's atmosphere), as well as continued work on Uranus' seasonal variability using the Voyager encounter data as a baseline

Rages, Kathy A.

English

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Summary of Research
NASA-Ames Cooperative Agreement Number NCC 2-662
MODELLING ATMOSPHERIC SCATTERERS USING SPACECRAFT
OBSERVATIONS
From April 1, 1990 through April 30,1999
Submitted to
Dr. Christopher P. McKay, Technical Officer
National Aeronautics and Space Administration
Ames Research Center, MS 245-3
Moffett Field, California 94035-1000
Prepared by
Dr. Kathy A. Rages, Principal Investigator
Space Physics Research Institute
572 Hyannis Drive
Sunnyvale, California 94087-1315
(408) 736-9705
https://ntrs.nasa.gov/search.jsp?R=19990062656 2020-06-15T21:35:07+00:00Z

Modelling Atmospheric Scatterers Using Spacecraft Observations
The periodcovered by thiscooperativeagreement includedtheanalysisof datafrom theVoyager
encounterwith Neptune and Tritonand the primaryGalileomissiontoJupiter(includingthe
GalileoProbe entryintoJupiter'satmosphere),aswellascontinuedwork on Uranus' seasonal
variabilityusingthe Voyager encounterdataasa baseline.
Analysis of Voyager Triton Limb Images ...
High resolution images of planetary limbs can be used to derive information about the vertical
structure and single scattering properties of scatterers in the upper atmosphere (assuming, of
course, that the planet has an atmosphere). Because of the very long slant path through the
atmosphere of lines of sight grazing the edge of the planet, such observations_generaUy reveal
information about altitudes corresponding to optical depths in the range 10-" to -0.03. In this
respect limb studies complement observations of the disk of the planet, which are most sensitive to
structure and composition at optical depths of 0.1 to -3. Also, extinction as a function of altitude
can be derived in a more-or-less direct fashion from a limb intensity scan. If there are any discrete
scattering layers present they can be observed directly instead of being inferred from (probably
ambiguous) fits of atmospheric scattering models to observations of specific intensity (I/F) from
the planet as a whole (Pollack et al. 1987; Rages and Pollack 1983).
Voyager images of Triton's limb reveal the presence of scattering clouds and hazes above the
surface, with considerable latitudinal variability (Pollack et al. 1990). Standard algorithms for
calculating scattered specific intensities (I/F's) near the limb of a spherical atmosphere were
combined with Mie theory to derive models for the single scattering properties and the vertical
distribution of these clouds and hazes. Near 14°S latitude, 275°W longitude only haze was present,
with optical depth -0.01 in violet and -0.004 in orange. Near 52°S latitude, 124°W longitude
condensation clouds were also present below 6 km, with optical depth > 0.1 and mean particle radii
either near 0.25 pm or near 0.7/.an (Rages and Pollack 1992).
... and Neptune Limb Images ...
Voyager 2 obtained a high-phase-angle clear-filter image of Neptune's limb at -30°S latitude
with 2 km spatial resolution. Inversion of the radial intensity profde across the limb reveals a rapid
increase in extinction with decreasing altitude near the 12-mbar pressure level, about where
photochemically produced ethane would be expected to condense. At about 30_rnbar, the haze
particle radius and number density are found to be 0.13:£-0.02 pm and 5:13 cm -J, respectively. The
total haze extinction optical depth is -0.003 above 15 mbar, and is mostly due to scattering rather
than absorption, which makes heating due to absorption of sunlight by the haze inadequate to
account for Neptune's observed stratospheric temperature inversion (Baines et al. 1993; Moses et
al. 1995).
The most prominent albedo feature on Neptune at the time of the Voyager encounter was the
Great Dark Spot and accompanying Bright Companion at ~27°S. There has been speculation that
the cores of features such as the Bright Companion might be convective columns with tops
punching through the "cold trap" of the tropopause at ~ 150 mbar, and as such might serve as
sources for the observed excess of methane gas in the stratosphere. A comparison of the center-to-
limb brightness curves of the Bright Companion in the Voyager wide-angle filters with those of its
unperturbed surroundings revealed that, despite its high contrast in the methane absorption band at
619 nm, the top of the Bright Companion may lie as deep as the 400 mbar pressure level. This is
well below the tropopause, and would eliminate the Bright Companion and similar features as
sources for the observed excess methane mixing ratio in the stratosphere (Rages et al. 1996).
... and Galileo Images of Venus' Limb ...
The Galileo spacecraft flew past Venus on its roundabout trajectory to Jupiter in February 1990.
Imaging data included six images of the limb between -10°N and -60°N (three pairs consisting of
one violet image and one 1-#m image each). These images were inverted to yield vertical extinction
profiles using the previously developed procedure. A region of enhanced extinction was clearly
visible in both falters at the highest latitude and an altitude of 91 km (pressure =0.3 mbar). At 48°N
a similar feature was visible in the 1/an filter at an altitude of 88 km ( =0.6 mbar) and there was
also some enhancement in the violet extinction at this altitude, although it was far less conspicuous.
Near the equator there was a change in the mean scale height near 87 km (1 bar), but no sign of any
other feature. The fact that the extinction at a given altitude is much the same at 0.41/an (violet)
and 1 _an indicates that the dominant sources of scattered light at all three latitudes are probably
greater than 0.1-0.2/Jm in size. Particles smaller than this would be Rayleigh scattering, and the
extinction coefficient for Rayleigh scatterers would be expected to drop by more than an order of
magnitude between 0.41/an and 1/ma. A comparison of the extinction profiles at 57°N and 48°N
gives the impression that the layer of enhanced extinction may drop to lower altitudes at lower
latitudes. (Belton et aL 1991).
... and (finally) Jupiter's Limb
In December 1996, during orbit F_A,Galileo used its Solid State Imaging subsystem (SSI) to take
four images of Jupiter's equatorial limb and four images of the limb in the transition zone between
north temperate latitudes and the northern polar region. Galileo made images of each region in
violet (mean wavelength 411 rim) and near infrared (756 nm) filters at two high solar phase angles.
The images were taken close enough to Jupiter to give spatial resolutions of -15 km/pixel,
comparable to a scale height near the tropopause. They reveal the presence of stratospheric hazes in
the equatorial region (9°N -) and the north polar transition zone (60°N). A discrete layer detached
from the limb is present at 60°N, 315°W, but not 20°further east at 60°N, 285°W. Bright streaks
running roughly north-south are also present on Jupiter's crescent at 60°N. No such discrete
features are seen at 9°N, where the haze appears to be more uniformly distributed in height and
longitude.
Radial specific intensity (I/F) profiles across the limb were inverted to give vertical extinction
profiles over -200 km in Jupiter's stratosphere at both 9°N and 60°N. The maximum FF in each
image was used to estimate the single scattering phase function for the corresponding scattering
angle, wavelength, and altitude. These phase functions required different particle sizes in the violet
and near-IR filters, which probed two different altitudes. Haze distribution models were found for
both latitudes in which the near-IR images constrain the haze properties near or above the 100 mbar
pressure _vel, where the mean particle radius is about 0.45/an and the haze number density is near
0.15 cm-L In these models the violet images constrain the haze properties near or above the 20
mbar pressure level, where the haze particle size is about 0.32/.an at 9°N and about 0.27/.an at
60°N. At this altitude, the haze number density increases by almost an order of magnitude between
the equator and the polar transition region. To fit Hubble Space Telescope (HST) observations in
the ultraviolet and the methane absorption band at 890 rim, these models must be modified by the
addition of another scattering component, consisting of < 0.02 pm particles which are
conservatively scattering in the near-IR and have single scattering albedos near 0.5 in the UV. This
0.02-/zrn haze accounts for more than 90% of the Rayleigh scattering (originally ascribed to
scattering by the hydrogen-belium gas) in both f'flters, with a corresponding drop of a factor of ~10
in the pressure levels probed (Rages et al. 1999).
Latitude Bands and Temporal Variability on Uranus
Uranus undergoes extreme seasonal variations as a result of its 98 ° axial tilt. Since the Voyager
encounter in January 1986, the subsolar latitude has shifted from its southern solstice position of
82°S to about 30°S. During the past 20 years, Uranus' brighmess has varied by about 4% in blue
2
light (wavelength0.472:_-0.021pro) and about 11% in yellow (0.551:L-0.023/an) (Lockwood et aL
1985; Lockwood, private communication). These changes appear to be seasonal rather than being
related to solar variability, which would show up as an ~ 11 year periodicity. Some part of this
seasonal variability may be due simply to the changing aspect of the planet as seen from Earth,
since Voyager found that southern polar latitudes were somewhat brighter than regions nearer the
equator.
Radiative transfer models were derived for five southern latitudes in addition to the two
(22.5°S and 65°S) previously studied using Voyager images (Rages et al. 1991). Both _, the
optical depth of Uranus' methane cloud layer, andfH, the product of the mean methane cloud
particle size and the fraction of stratospheric haze material mixed into the methane cloud, vary by
factors of ~2 between the latitudes of 10% and 85°S. The variability observed by Lockwood
exceeds that which would be caused by these latitudinal variations in atmospheric structure, by
about a factor of two in the blue and about a factor of five in the yellow. Therefore Uranus is
undergoing actual temporal changes in its scattering properties. Calculations show that even the
most favorable combination of changes in the methane cloud optical depth and the concentration of
stratospheric haze and its residue below the methane cloud produce only about 2/3 of the observed
brightness changes in both filters. Some other factor, such as hemispheric asymmetry in the
atmospheric structure, must be introduced to account for Uranus' observed long-term albedo
changes (Rages and Pollack 1994).
Galileo Probe Nephelometer
The Galileo Probe entered Jupiter's atmosphere on December 7, 1995 and returned data for 57
minutes, down to a pressure level of about 20 bars. One of the instruments carded on the probe
was a nephelometer intended to measure the scattering properties of the regions the Probe passed
through. The Probe entered Jupiter's atmosphere near the edge of a 5-ban hot spot, a region where
the atmosphere is much clearer than on most of the planet. The nephelometer definitely saw one
thin cloud at about 1.3 bars, and very preliminary analysis indicates that this cloud is composed of
particles with mean radii of 1-5 pro. The data can be fit either by spherical scatterers having radii of
0.8/an, a real refractive index n r of 1.7-1.8, and an imaginary refractive index ni~0.05, or by
scatterers with radii around 8/an, a very low value ofn r (< 1.2), and ni=0. The high-n r solution is
consistent with the little that is known of the optical constants of ammonium hydrosulfide, the most
likely constituent of a cloud at 1.3 bars, but would require the presence of some additional
component to account for the absorption. The low-n r solution requires either a very unusual
composition or "fluffy" particles with many internal voids, since none of the likely bulk
constituents have refractive indices below about 1.3. The cloud optical depth is near 2, over a
vertical distance of-20 km (Ragent et al. 1996; Ragent et al. 1998).
Bibliography
BAINES, K.H., H.B. HAMMEL, K.A. RAGES, P.N. ROMAN], AND R.E. SAMUELSON 1993. Clouds
and hazes in the atmosphere of Neptune. In Neptune and Triton (D.P. Cruikshank, Ed.)
University of Arizona Press.
BELTON, M.J., P.J. GIERASCH, M.D. SMITH, P. HELFENSTEIN, P.J. SCHINDER, J.B. POLLACK,
K.A. RAGES, A.P. INGERSOLL, K.P. KLAASEN, J. VEVERKA, C.D. ANGER, M.H. CARR, C.
R.CHAPMAN, M.E. DAVIES, F.P. FANAI_, R. GREELEY, R. GREENBERG, J.W. HEAD, III, D.
MORRISON, G. NEUKUM, AND C.B. PILCHER 1991. Images from Galileo of the Venus cloud
deck. Science 253, 1531-1536.
MOSES, J.I., K. RAGES, AND J.B. POLLACK 1995. An analysis of Neptune's stratospheric haze
using high-phase-angle Voyager images. Icarus 113, 232-266.
RAGENT, B., D.S. COLBURN, P. AVRIN, AND K.A. RAGES 1996. Results of the Galileo Probe
Nephelometer Experiment. Science 272, 854-856.
3
RAGENT,B., D.S. COLBURN, K.A. RAGES, T.C.D. KNIGHT, P. AVRIN, G.S. ORTON, P.A.
YANAMANDRA-FISHER, AND G.W. GRAMS 1998. The clouds of Jupiter: Results of the
Galileo Jupiter Mission Probe Nephelometer Experiment. J. Geophys. Res. 103, 22,891-
22,909.
RAGES, K., AND J.B. POLLACK 1992. Voyager imaging of Tdton's clouds and hazes. Icarus 99,
289-301.
RAGES, K.A., AND J.B. POLLACK 1994. Modeling Uranus' Temporal Variability. Bull. Amer.
Astron. Soc. 26, 1093.
RAGES, K. 1995. WFPC2 photometry of features near the Galileo probe entry latitude. Bull.
Amer. Astron. Soc. 27, 1134.
RAGES, K.A., H.B. HAMMEL, ANDC.P. MCKAY 1996. Vertical structure of the 1989 Great Dark
Spot and Bright Companion. Bull. Amer. Astron. Soc. 28, 1077.
RAGES, K., H.B. HAMMEL, AND G.W. LOCKWOOD 1998. Changes in Uranus' atmosphere since
the Voyager encounter. Bull. Amer. Astron. Soc. 30, 1097.
RAGES, K., R. BEEBF_., AND D. SENSKE 1999. Jovian stratospheric haze_: The high phase angle
view from Galileo. Icarus 139, 211-226.
Other References
L(X_KWOOD, G.W., B.L. LLrFZ, D.T. THOMPSON, AND E.S. BUS 1985. The albedo of Uranus in
1985. Bull. Amer. Astron. Soc. 17, 744.
POLLACK, J.B., K. RAGES, S.K. POPE, M.G. TOMASKO, P.N. ROMAN/, AND S.K. ATREYA 1987.
Nature of the stratospheric haze on Uranus: Evidence for condensed hydrocarbons. J.
Geophys. Res. 92, 15,037-15,065.
POLLACK, J.B., J.M. SCHWARTZ, AND K. RAGES 1990. Scatterers in Triton's atmosphere:
Implications for the seasonal volatile cycle. Science 250, 440-443.
RAGES, K., AND J.B. POLLACK 1983. Vertical distribution of scattering hazes in Titan's upper
atmosphere. Icarus 55, 50-62.
RAGES, K., J.B. POLLACK, M.G. TOMASKO, AND L.R. DOOSE 1991. Properties of scatterers in
the troposphere and lower stratosphere of Uranus based on Voyager imaging data. Icarus 89,
359-376.
4
APPENDIX
Subject Inventions Certification
There were no subject inventions required to be disclosed to NASA which resulted from
this work. There were no subcontracts awarded under this Cooperative Agreement.



Modelling Atmospheric Scattering Using Spacecraft Observations

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