Titan Science wi e James Webb Space Telescope

Titan Science with the ��James Webb Space Telescope

Titan Science Focus Group: Conor A. Nixon (Lead), NASA GSFC, Richard K. Achterberg, Univ. of Maryland, Mate Adamkovics, UCB, Bruno Bezard, Observatoire de Paris, Gordon L. Bjoraker, NASA GSFC, Thomas Cornet, ESA ESAC, Alexander G. Hayes, Cornell University, Emmanuel Lellouch, Observatoire de Paris, Manuel Lopes-‐Puertas, IAA-‐CSIC, Mark T. Lemmon UofA, SebasLen Rodriguez, Univ. de Paris Diderot, Christophe SoLn, JPL, Nicholas A. Teanby, Univerity of Bristol, Elizabeth P. Turtle, JHU APL, Robert A. West, Caltech/JPL.

Exploring The Universe with JWST October 13th 2015, ESA-‐ESTEC, Noordvijk, NL

JWST Titan Focus Group The Titan SFG is a volunteer membership of 15 individuals drawn from the internaQonal scienQfic community acQve in Titan research.

SFG TEAM GOALS: •  Produce a paper summarizing capabiliQes of JWST for Titan. •  IdenQfy special needs in terms of observatory capabiliQes. •  Verify that needs will be supported, or describe those needs so that

they can (potenQally) be addressed by the JWST project.

WHITE PAPER/JOURNAL ARTICLE SHOULD: a)  Describe specific scienQfic quesQons that could be addressed using

JWST data b)  Summarize observaQon scenarios and data products needed to

address those quesQons. c)  Examine JWST instrument and observatory performance in light of

the above.

Titan Surface Map From Cassini

!"#$ !"#% !"!" !"!# !"!! !"!& !"!' !"!( !"!) !"!* !"!$

"+)(

"+*

"+*(

"+$

"+$(

"+%

"+%(

,-./

012-34./56-57

3

34.7

!"#$ !"#% !"!" !"!# !"!! !"!& !"!' !"!( !"!) !"!* !"!$'

!

"

!

'

)

,-./

893/.:-34;

1<<1./56-5=6-57

4>7

!"#$ !"#% !"!" !"!# !"!! !"!& !"!' !"!( !"!) !"!* !"!$#+(

#

"+(

"

"+(

#

#+(

!

!+(

,-./

?@A

3/.:-34;

1<<1./56-5=6-57

457

!"#$ !"#% !"!" !"!# !"!! !"!& !"!' !"!( !"!) !"!* !"!$"

("

#""

#("

!""

,-./0B<./3@

<BCD.:1BC34E-D7 4E7

F>6-/G.:1BC63CB:3HB661><-F>6-/.G:1BC63IB661><-

Titan Observability

JWST is restricted to observe solar elongaQon angles 85-‐135 degrees (lower right) due to the sun shield, therefore imposing limitaQons on observability. (Figure: N. Teanby.)

0.01$

0.1$

1$

10$

0$ 5$ 10$ 15$ 20$ 25$ 30$

Wavelength*(microns)*

Diffrac5on6limited*spa5al*resolu5on*of*JWST*for*Titan*

Titan$pixels$(10.5$AU)$

Titan$pixels$(8.5$AU)$

resol$(arcsec)$

resol$(microrad)$

JWST Titan Science Themes The Titan SFG idenQfied five themaQc areas to focus on:

1.  Surface -‐ near-‐IR spectroscopy of Titan’s surface to determine features, composiOon, geology, history etc.

2.  Tropospheric clouds -‐ long-‐term monitoring campaign and quick-‐response observaOons of transient clouds and surface darkening.

3.  Lower atmosphere composiQon – measure spaOal/temporal variaOon of the abundance of methane and other gases in the troposphere by near-‐IR spectroscopy.

4.  Stratospheric composiQon -‐ abundances of trace gases in the stratosphere by mid-‐infrared spectroscopy. SpaOal and temporal monitoring, and search for new gases and isotopes.

5.  Haze -‐ mulO-‐spectral monitoring of Titan near-‐IR wavelengths to determine spaOal and temporal variaOons in haze distribuOon, aerosol composiOon and atmospheric dynamics.

1. Surface •  Titan’s surface has been observed

by HST and ground-‐based observatories since mid-‐1990s.

•  JWST spaQal resoluQon is ~2.5x Hubble based on primary aperture.

22

(Schaller et al., 2009), and in 2010 Cassini observed a major outburst of equatorial clouds, with the surface darkening for weeks thereafter interpreted to be soil damp from methane rain (Turtle et al. 2011).

By 2017 we will be left with the following questions:

1. What does the surface look like in higher-resolution (R ~ 3000 vs. 200) near-IR spectroscopy? 2. What time-variable phenomena might occur due to seasonal (decadal) variations or stochastic surface events in

the near-infrared and in that part of the mid-infrared (640 cm-1) where the atmosphere is once again optically thin enough to see the surface?

JWST can make NIRCam images, and NIRSpec IFU spectral imaging of Titan to build on the 2004-2017 Cassini mission survey, creating a potentially long (10 year +) baseline of spaceborne near-infrared observations of Titan's surface and atmosphere during a seasonal configuration hitherto unexplored in the infrared. The pixel size on NIRCam gives about the same spatial resolution on Titan as Hubble (Table 2), but the signal-to-noise is much higher. Spectral resolution a factor of 6 better than on Cassini can be accomplished using the NIRSpec, allowing for spectra far more diagnostic of the types of organic species present on the surface. Thus while Cassini gets better spatial resolution, JWST will achieve higher spectral resolution with useful spatial resolution (as shown in Figure 12) over the mid-latitude regions of Titan. Of interest is whether surface changes or secular atmospheric changes are in evidence over a decadal timescale. With NIRSpec, the ability to probe the atmosphere over several levels down to the surface provides a unique long-term capability that is unavailable from Hubble and will cease to be available from Cassini after 2017. Thus JWST provides long-time baseline continuity throughout the infrared.

Figure 12. Example of the spatial coverage provided by the NIRSpec IFU on Titan.

One approach is to take NIRCam, NIRSpec and MIRI data on Titan over three equally spaced intervals during the 16-day orbit of Titan, which is phase-locked to Saturn. This provides images and spectra centered approximately 120º apart from each other, and hence global coverage. Cloud movement in the stratosphere, based on our

Lec: HST from Smith et al. 1998 Above: from Norwood et al. 2014

Figure: SebasQen Rodriguez

•  JWST NIRSpec IFU mode saturaQon thresholds compared to Cassini VIMS spectrum. •  Shorter wavelengths will likely saturate NIRSpec, especially at lower resoluQons.

Figure: SebasQen Rodriguez

2. Clouds

Above: Gemini adapQve opQcs observaQons of Titan showing a cloud outburst in 2008 (Schaller et al., Nature, 460, 873-‐875, 2009.)

•  Clouds on Titan have been detected by ground-‐based telescopes (2002+), HST and Cassini.

•  JWST can enable: •  Long term monitoring of

cloud acQvity. •  Quick-‐response monitoring

of cloud evoluQon over days/weeks.

•  Higher sensiQvity and addiQonal bands enabled (no sky background.)

•  Bejer esQmates of alQtude (maybe), monitor surface darkening etc.

2011 ‘Chevron’ Cloud •  Huge (1200x1500 km)

cloud spojed by Cassini in 2011.

•  Caused rainfall that darkened the surface over 500,000 km2.

•  Over following months, methane re-‐evaporated causing surface to brighten again.

Figure: S. Rodriguez and E. Turtle: Cloud laQtude distribuQon and predicQons 2000-‐2030 using model of Schneider et al. (2012) and cloud data points from published observaQons.

Seasonal monitoring of clouds

Figure: SebasQen Rodriguez –simulaQons based on degrading VIMS images to JWST diffracQon-‐limited resoluQon.

Wide-‐band filters

Medium-‐band filters

Narrow-‐band filters

3. Tropospheric ComposiQon •  Use regions of weak CH4 absorpQon (center or wings of the

methane windows) or regions of CH3D absorpQon as a proxy for CH4 (cf Penteado & Griffith 2010 for 1.56 µm CH3D band) to probe the troposphere.

•  Any CH4 variaQons (esp. with laQtude) could then be correlated with surface/troposphere temperature measurements at other wavelengths (far-‐IR).

•  If CO can be measured simultaneously it would provide a check of the results as CO is not expected to show any variability.

•  Measurements should be repeated to search for seasonal evoluQon.

0

0.02

0.04

0.06

0.08

0.1

0.12

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

I/F a

t zer

o ph

ase

angl

e

Wavelength (micrometer)

Methane profile from GCMS

0

0.02

0.04

0.06

0.08

0.1

0.12

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

I/F a

t zer

o ph

ase

angl

e


Methane profile from GCMSRelative humidity divided by 2

0

0.02

0.04

0.06

0.08

0.1

0.12

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

I/F a

t zer

o ph

ase

angl

e



0

0.02

0.04

0.06

0.08

0.1

0.12

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

I/F a

t zer

o ph

ase

angl

e



0

0.02

0.04

0.06

0.08

0.1

0.12

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

I/F a

t zer

o ph

ase

angl

e



0

0.02

0.04

0.06

0.08

0.1

0.12

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

I/F a

t zer

o ph

ase

angl

e



Figure: Emmanuel Lellouch and Bruno Bezard

C2H2

C3H4 C4H2 C2H4

CH3D

CH4

C2H6

C2H6

CO2

H2

HCN

JWST MIRI Range 28 μm (360 cm-‐1) to 5 μm (2000 cm-‐1)

Labeled Cassini CIRS spectrum (low laQtude average) by D. Jennings

4. Stratospheric ComposiQon

5 6 7 8 9 10 11 12 13 14 15 16 18 20 22 24 26 2810

−7

10−6

10−5

10−4

10−3

10−2

10−1

100

101

102

0.2" 0.2" 0.24" 0.27"

C2H

6

CH4

CH3D

C2H

4

C2H

6

C2H

2

HCNCO

2

C3H

4

C4H

2

Wavelength (µ m)

Radia

nce

(Ja

nsk

ys/p

ixel2

)

MIRI MRS Disk−resolved

R=2800 CIRS based Titan modelSaturationSensitivity (3 sigma, 60s integration)

Figure: Nick Teanby and Rich Achterberg

•  Science invesQgaQons: •  Measurements/detecQons of trace species. •  Seasonal changes in temperature and composiQon.

•  Challenges: •  Possible saturaQon of MIRI in strong bands of C2H6, C2H2. •  Low spaQal resoluQon(4 pixels at 5 μm, 1 pixel at 20+ μm)

3.0 3.5 4.0 4.5 5.0Wavelength (µm)

0.00

0.02

0.04

0.06

0.08

0.10

Jy a

rsec-2

400 km

500 km

600 km

700 km

800 km

900 km

HCNC2H2

PAHs

CH4

CH4 CH3DCO

VIMS dayQme limb spectra – various alQtudes – showing gases and PAH band.

5. Haze

Earth’s. However, the tholin haze is blue-absorbing, and soimage darkness implies a relative enhancement of dark hazeabove the bright atmosphere. In near-infrared methane bandsthe lower atmosphere is dark, while haze is bright, and thusabundant haze at high altitudes leads to a bright appearance.[9] The observations are consistent with a seasonal cycle

wherein circulation like that of thermally-direct winds (i.e.,a pole-to-pole Hadley cell) chases haze away from thesummer hemisphere to the winter one [e.g., Hutzell et al.,1996]. This conceptual picture, a simple analytical model ofwhich was developed and compared with HST data in thework of Lorenz et al. [1999] has been captured in detail innumerical models that couple general circulation with hazetransport [e.g., Tokano et al., 1999; Rannou et al., 2002].[10] Remarkably, the 619 nm profile is essentially un-

changed (see Figure 2) over the 8 year period and indicates

an enhanced opacity in the S hemisphere. 953 nm images(Figure 1 and Lorenz et al. [2001]), probing down to Titan’ssurface also show a persistent brightness in the deepsouthern hemisphere.[11] The 619 nm data require comment. Either (1) there is

no seasonal cycle at this wavelength, that the altitudesprobed are so stagnant that the total haze amount at eachlatitude, dominated by lower altitudes, does not change withtime. If so, the persistent asymmetry requires an explana-tion, perhaps due to the eccentricity of Saturn’s orbit aroundthe sun such that southern summers are hotter but shorter.More probably (2) the phase of the seasonal cycle down tothese altitudes is delayed with respect to that seen above100 km or so at 439 nm and 889 nm. It is known that thedisk-integrated albedo cycle recorded by Lockwood [seeLorenz et al., 1999] is delayed by !0.5–1 year at 550 nmcompared with 467 nm: at these wavelengths where thehaze reflectivity is low, the corresponding unity opticaldepth levels are !100 km and 85 km [Lorenz et al.,1997]. The corresponding phase lag at 619 nm is evidently>5 years. At this wavelength, where the haze scatters ratherthan absorbs, the altitude probed is somewhere between!70 km where the haze optical depth is unity and !15kmwhere the methane absorption optical depth is unity [Younget al., 2002]. Thus, if haze is responsible for the 619 nmNSA, there is a substantial phase lag between 85-100 kmand 15–70 km. We point out that the opacity appearedstronger in the south before the subsolar point crossed intothe south in 1995: this would be inconsistent with atropospheric cirrus origin (i.e. indirectly related to convec-tive activity, which is now seen to dominate in the South,e.g., Brown et al. [2002]).[12] The changes are most obvious at 889 nm which

probes only the stratosphere. The limb brightness profiles atthis wavelength are shown in Figure 3a, which gives aquantitative representation of the reversal of Titan’s ‘‘smile’’into a ‘‘frown’’ [Lorenz et al., 2001]. The limb brightnesscorresponds to the high-altitude haze number density. Themodel results of Tokano et al. [1999] show a similar trendswith time (see Figure 3b) although the detailed shape differssince 889 nm data probe only the stratosphere while themodel 640 nm opacity is a total column.

3. Comparison With Other Observations

[13] Variable seeing for groundbased observers, and dif-ferent filter widths, make comparison between data difficult,

Figure 2. North-South cuts along the central meridian ofimages in 1994,1997 and 2002. All data are normalized tothe same maximum brightness (1000). The reversal ofthe north-south asymmetry in blue (439 nm) and (in theopposite sense) in the 889 nm methane band is obvious. The619 nm data is remarkably unchanged.

Figure 1. HST Titan images from the UV to the Near-IRsince 1992. All images are north-up, scaled to samemaximum brightness, and show brightness cubed toenhance contrast. Globes at right show the changing aspect:L is the declination or Subsolar Latitude. The apparentchange in diameter with wavelength is due to changinglimb-darkening. 1990 images (not shown) are of poorquality [see Caldwell et al., 1992].

Table 1. Season on Titan

Date Observation/Event Ls (deg) Solar Latitude (deg)

Sep 1979 Pioneer 11 encounter 354 "2.9Feb 1980 vernal equinox 360 0Nov 1980 Voyager 1 encounter 8 +4.1Aug 1981 Voyager 2 encounter 16 +8.0Nov 1987 northern summer solstice 90 +26.7Aug 1990 HST WFPC 122 +23.0Aug 1992 HST WFPC 145 +16.0Oct 1994 HST WFPC2 168 +5.8Aug 1995 HST WFPC2 177 +1.3Nov 1995 autumnal equinox 180 0Nov 1997 HST WFPC2/STIS 202 "10.7Dec 2000 HST WFPC2/STIS 242 "24.0Oct 2002 southern summer solstice 270 "26.7Dec 2002 HST WFPC2/STIS/ACS 271 "26.7Apr 2004 Cassini approach science 292 "25.0Oct 2004 first Cassini flyby (Ta) 300 "23.5May 2008 end of nominal Cassini tour 345 "7.2Aug 2009 vernal equinox 360 0May 2010 possible Cassini extension 8 +4.1

L10702 LORENZ ET AL.: TITAN FROM HST L10702

2 of 4

•  Titan has a very complex, mulQ-‐layered haze as seen by Voyager and Cassini (lec).

•  HST imaging showed seasonal reversal of haze (below, from Lorenz et al. 2004) with an increase in blue-‐absorbing haze (439 nm) in the north from 1992-‐2002.

•  JWST will enable long-‐term monitoring of global haze distribuQons using imaging and enable composiQon inference from NIR/MIR spectroscopy.

Conclusions •  JWST will have significant capability for Titan science in the

post-‐Cassini era, complemenQng ground-‐based observatories in 2018-‐2028 Qme frame, especially: 1.  Imaging and spectroscopy of Titan’s surface to constrain

possible composiQon, monitor surface response to rainfall.

2.  Long monitoring of cloud distribuQons and quick response observaQons of cloud system evoluQon.

3.  Measurements of methane relaQve humidity in troposphere.

4.  Spectroscopy of trace stratospheric gases, stratospheric temperature measurements.

5.  Haze evoluQon over Qme.

Current Status

•  2-‐page Titan JWST informaQonal flyer available, was distributed at 2014 DPS.

•  JWST Titan paper accepted for publicaQon in PASP – special volume on solar system science (10 papers).

•  Results (poster) will be presented again at DPS in November 2015.

Titan Science wi e James Webb Space Telescope

Documents

Transcript of Titan Science wi e James Webb Space Telescope