Review and Updates on
Uranus and Neptune as Flux
Calibrators
Glenn Orton
Jet Propulsion Laboratory
California Institute of Technology
Roadmap
• Assessment of the Uranus model that is
based on Spitzer IRS data analysis
• Assessment of the Neptune model that is
based on ISO LWS/SWS data
– Potential modification from Spitzer IRS data
– Assessment of missing information
• Bruce Swinyard’s comparison of SPIRE
FTS cross-calibration of Uranus and
Neptune
Current status – Uranus spectral models (more robust than for Neptune)
Current (Moreno) model: based on Voyager-2 radio subsystem
(RSS) occultation profile along one low-latitude atmospheric
tangent, with NH3 absorption below ~300 GHz
Alternative (Orton) model is based on inversion of 2007 Spitzer
Infrared Spectrometer (IRS) low-resolution observation
~4 Kelvin maximum difference between the two models
(maximum 5% difference in radiance prediction)
Orton et al (2012, in preparation): retrieval of disk-averaged T(p)
from spectral regions controlled by well-mixed H2
collision-induced absorption (CIA)
CH4
H2 CIA
H2 CIA
C2H6
C4H2
C3H4
H2 S(3) quardupole
H2 S(1) quardupole
C2H2
Orton et al.
Lindal et al. RSS T(p)
(current model)
Longer wavelengths need another
opacity: revisiting a conclusion
from an old calibration mini-workshop:
← a model with only H2 CIA absorption is too warm
Griffin and Orton (1993)
Gurwell and Hofstadter, SMA data
- - - Including NH3 gas does better,
with 1 ppm VMR at depth (Moreno’s suggestion)
But with our derived T(p), it’s too bright for the
5-10 cm-1 (150 – 300 GHz) data, particularly
the recent 1.4-mm (7 cm-1) SMA measurement
An additional opacity must be added to the model to
provide a fit to the data: we tried H2S
(PH3 might also work but is a less
likely candidate on the basis of chemical equilibrium)
- - - Lindal et al. RSS T(p)
(continuum only)
Gurwell & Hofstadter data (SMA, c. 2008)
Sandel & Dowell data (c. 2005)
Orton et al., from Spitzer-based model T(p)
Lindal et al. RSS T(p)
(current standard model)
Evaluation of the model
 Uncertainties in the absolute calibration
±3% from measurement
±5% is Spitzer’s quoted absolute calibration
uncertainty
Total estimated as ±6%
This propagates into an absolute temperature
uncertainty that is very small: ±0.2 K
This uncertainty would propagate into a submm
radiance uncertainty of ±0.4%.
Errors are thus more likely to arise from other
systematic uncertainties or assumptions.
Did the Voyager-2 RSS sample a cold latitude?
No, if anything, the near-equatorial temperatures
should be warmer than the global mean.
Other possible systematic problems:
• Spitzer lost flux from the IRS slit more than point sources
– but correcting to add more flux makes the incompatibility worse
• The H2 CIA spectrum is wrong in the 9-11 μm region
– but it must be off by a factor of ~2
• The radio-occultation technique is wrong
– Radio occultation approach provides only a (T/m) parameter vs.
altitude
– There are known problems when applied to solving for m using
simultaneous mid-infrared observations
– Jupiter He/H2 ratio derived from the Voyager RSS-IRIS
comparison differs by substantially more than 1 standard
deviation from Galileo probe results
– Saturn He/H2 ratio derived from the Voyager RSS-IRIS
comparison differs by a factor of two from the results obtained
using IRIS spectra alone
Other possible systematic problems:
• Mean molecular weight is wrong, it must
be increased
– add more He, but the amount required is
23%, which isn’t likely because it’s hard to
add He to the ‘solar mixtures’
– add more CH4 in deep atmosphere; ~4% is
required
– Sromovsky et al. (2011) suggested this for
independent reasons, but they also want less
He than 15% to match their cloud-level
constraints
Sromovsky et al. (2011) raises the mean-molecular weight by increasing
the volume mixing ratio of CH4 in the deep atmosphere up to
4% from the 2.3% in the nominal RSS “best” model ---- .
But they decreased the He mixing ratio in order to preserve
the depth of the condensation to coincide with the depth
they detect for the primary CH4 condensate cloud. This
drops temperatures substantially above the
CH4 condensation level.
Orton et al.
--- Lindal et al. nominal model
The Sromovsky et al. high CH4 abundance model lets us match the
temperatures needed in the deep atmosphere, but the reduced
He makes temperatures above the CH4 condensation level
much too low. We can’t fit the continuum higher up.
If we ignore the Sromovsky et al. cloud-level requirement and just
use the 4% “upper limit” model in Lindal et al.’s original set of
possible solutions, we can match the temperature required at
depth without changing the He:H2 ratio
- - - - original RSS, 2.3% max. CH4
- - - - RSS, 4% max. CH4
- - - - original RSS, 2.3% max. CH4
- - - - RSS, 4% max. CH4
We can explore various assumptions about the He abundance.
Orton et al., from Spitzer-based model T(p) 15% He / 85% H2
- - - - - 12% He / 88% H2
- - - - - 18% He / 82% H2
Lindal et al. RSS T(p)
15% He / 85% H2 (2.3% CH4)
The mid-infrared spectrum is very sensitive to the He/H2 ratio.
Orton et al., from Spitzer-based model T(p) 15% He / 85% H2
- - - - - 12% He / 88% H2
- - - - - 18% He / 82% H2
Lindal et al. RSS T(p)
15% He / 85% H2
(2.3% CH4)
The nominal model does extremely well using the Voyager He/H2 ratio.
Orton et al., from Spitzer-based model T(p) 15% He / 85% H2
- - - - - 12% He / 88% H2
- - - - - 18% He / 82% H2

ISO LWS
 ISO SWS
- - - Lindal et al. RSS T(p)
15% He / 85% H2
H2 S(0)
H2 S(1)
The spectrum is really consistent with (internal, unpublished) ISO observations.
(LWS data were calibrated to Mars, SWS to a suite of calibrators.)
Plans: Uranus spectrum in 17-37 μm
region will be proposed for SOFIA
FORCAST grism spectroscopy.*
These will extend the Spitzer
IRS spectrum.

ISO LWS
 ISO SWS
H2 S(0)
*Cycle-1 proposals are due in one week! :-0
H2 S(1)
Uranus model assessment
• What’s good:
– Fits the Spitzer IRS data
– Characterizes the whole disk
– Matches ISO data with standard He/H2 ratio
– Better match to SPIRE FTS results relative to Neptune (explained
below)
– Close to Voyager-2 IRIS disk-averaged results
(requires more work: translate pole-on to near equinoctial orientation)
• What’s not so good:
– Does not match a ‘nominal’ Voyager RSS profile without a ‘fix’, such
as moving 2.3% CH4 to ~4% CH4
– Requires an additional millimeter opacity (H2S ?) [testable]
– Damn thing ain’t published!
• Time variability
– Longitudinal variability expect to be low
– Seasonal variability can be assessed
ISO Heritage: LWS and SWS studies of Neptune:
application to FIRST Herschel (Burgdorf et al. 2003)
LWS
SWS
ground-based: IRTF
(Orton et al. 1989, 1990)
calibration based on the Griffin & Orton “standard model” for Uranus
T(p) models
for Neptune
Herschel PACS
(Lellouch et al. 2010;
basis of ESA3)
Spitzer IRS
(Line et al. 2008)
ISO + ground-based
(Burgdorf et al. 2003)
Akari
(Fletcher et al. 2010)
Voyager RSS - - (Lindal et al. 1990)
Neptune
Burgdorf et al. (2003)
LWS
SWS
IRTF
Limited constraints from Spitzer IRS
Neptune
Spitzer IRS: Short-Low-1 (SL1)
(photometrically accurate)

 no continuum
LWS
SWS
IRTF
Spitzer IRS SL1 (photometric)
Spitzer IRS SH (scaled to SL1)
Neptune
Burgdorf et al. (2003)
LWS
SWS
IRTF
Spitzer IRS SL1 (photometric)
Spitzer IRS SH (scaled to SL1)
Neptune
Burgdorf et al. (2003)
LWS
SWS
IRTF (scaled up by 28%)
Spitzer IRS SL1 (photometric)
Spitzer IRS SH (scaled to SL1)
Neptune
Burgdorf et al. (2003)
Lellouch et al. (2010) / ESA3
Incomplete fit to combination of
ISO and Spitzer data (2012)*
LWS
SWS
*last Tuesday!
IRTF (scaled up by 28%)
Spitzer IRS SL1 (photometric)
Spitzer IRS SH (scaled to SL1)
Neptune
Burgdorf et al. (2003)
Lellouch et al. (2010)
Incomplete fit to combination of
ISO and Spitzer data (2012)
SOFIA “Basic Science”
FORCAST results
(3 weeks ago):
pipeline calibration
Spitzer IRS SL1 (photometric)
Spitzer IRS SH (scaled to SL1)
Neptune
SOFIA “Basic Science”
FORCAST results:
default calibration
Burgdorf et al. (2003)
Lellouch et al. (2010)
Incomplete fit to combination of
ISO and Spitzer data (2012)
SOFIA “Basic Science”
FORCAST results:
pipeline calibration
Neptune (H2 CIA continuum models only)
Burgdorf et al. (2003)
Lellouch et al. (2010 / ESA3)
Incomplete fit to combination of
ISO and Spitzer data (2012)*
 Griffin & Orton (1993)
*last Tuesday
Neptune models- assessment
• The a priori basis for Fletcher et al., and, indirectly,
Lellouch et al. models is some perturbation of Voyager
RSS results and ISO results (Burgdorf).
• Unlike Uranus, there are no direct constraints on
temperatures in the relevant part of the atmosphere.
Herschel (e.g. SPIRE) may provide those constraints, or
independent measurements at longer wavelengths.
• Seasonal variability is tiny in the Herschel time frame.
The short-term tropospheric variability is likely to be low
on a hemispheric average, but it’s actually unknown.
Bruce Swinyard’s SPIRE FTS Uranus vs
Neptune comparison
•
Use HIPE v8 with a bolometer temperature conversion stage to generate
averaged interferograms. This now includes the proper phase correction
and everything up to the generation of the interferogram is the standard
SPIRE processing.
•
Check the interferograms for proper phase correction (it works very well
now).
•
Process the on source and darks of the day separately and subtract and
calibrate using a telescope model. This is based on a combination of
modified versions of the instrument throughput and telescope emissivity
versus wavenumber – as described below.
•
The generated spectrum is compared to the Uranus model provided in
brightness temperature by Glenn Orton on 14/2/2011 and one in Jy for
OD423 by Raphael Moreno in December 2010. In all that follows here I
have adjusted everything to fit the Orton Model.
•
Note the Uranus observation used is the one taken on OD423 (10/07/2010
Uranus, Neptune SPIRE FTS results &
absolute calibration
• The best calibration approach has been to use
the telescope emissivity itself
We observe the whole of the telescope –
therefore use mean of the thermistors on
each mirror to construct model of telescope
emission
Original telescope emissivity function (Fischer et al.)
Telescope emissivity function model modified to fit
the GO Uranus model:
- The change is small and well within the error bars
of the original function.
Uranus spectrum
GO model
RM model
Spectrum/Model ratio
Derived Uranus spectrum
GO model
RM model
x Sandel & Dowell
Derived Neptune spectrum
GO Neptune model
(Burgdorf et al. 2003)
RM model
(Lellouch et al 2010 / ESA3)
Using the Spitzer (“GO”) Uranus model
mode. produces self-consistent spectra
of Neptune.
Neptune: observed spectrum / model ratios
RM (Lellouch et al.) / ESA 3 model
GO (Burgdorf et al. 2003) model
Conclusions (Swinyard)
• Using the Orton (Spitzer-based) spectral model
for Uranus provides a calibration for the
Neptune model that provides a reasonable
agreement with ESA3 and “GO”.
• It is better than the current Uranus model + 10
Jy used in the SPIRE pipeline and should be
adopted immediately.
• Glenn’s comment: more work should be done on
the Neptune models. They both seem too dim
by 3-5%.
Supplementary Material
• But we must be careful in making such comparisons. There
is a big change in the projected geometry over decades.
In 1986, Voyager IRIS observed the whole disk of the planet
pole-on; in 2010-2012 we’re just past equinox.
1999
HST: enhanced R, G, B
(Karkoschka, press release)
2004
Keck AO: enhanced J, H, K
(de Pater and Hammel, press release)
• There is also genuine change in time whose influence
we have to evaluate on the disk-averaged spectrum –
the north polar region has cooled.
VLT/VISIR 18-μm images of Uranus 1,2 Sept 2006
North pole should be warm, but
it has cooled since 1986. How
deep are these differences?
Uranus obs.
deconvolved
IRIS-based simulation
- Assess using 19-21 μm
imaging, sensing deeper
Spitzer IRS Spectrum of Neptune
CH4
Note minimal
region where
H2 CIA spectrum
is available
in SL spectrum

C2H6
CH3D
C2H2
Not much of spectrum
is useful to constrain
the global-mean T(p),
but an attempt was
made by student, Mike
Line (2008).
Examine He sensitivity
Best fit for He VMR: 7.1-11.5%
*15% He
 10% He
Uncertainties in the bulk composition are based on those of
Voyager for mixing ratios of H2 , He ~ ±0.033
Negligible influence at the
longest wavelengths/
shortest frequencies.
Para-/ortho-H2 uncertainty
propagates an uncertainty
in the spectral radiance
that is very small.
Biggest uncertainty is in far infrared:
±3% at 100 cm-1/100 μm,
±6% at 175 cm-1/ 57 μm
Mitigation of compositional uncertainty:
SOFIA Cycle-1 proposal to constrain He/H2 ratio
with 19-37 μm photometry and spectroscopy using
FORCAST instrument.
Current status – Neptune spectral models
Current model: based on Lellouch et al. (2010) fit to
HD lines via perturbation of Akari mid-infrared spectrum for the
stratosphere (Fletcher et al. 2010) and earlier model (Martin ?)
Alternative model is based on ISO LWS/SWS data (Burgdorf et al.
2003), preliminary fit to Spitzer IRS data (Line et al. 2010, DPS),
and ground-based spectrum (Orton et al. 1090)
~X Kelvin maximum difference between the two models
(maximum Y% difference in radiance prediction)