The Cassini CIRS Instrument and Science during the Titan-A Encounter

Public Outreach Presentation JPL/Telecon - October 26th 2004

Conor Nixon (University of Maryland)on behalf of the CIRS Team

What is CIRS?• CIRS is the Composite Infrared Spectrometer:

Composite: contains two Fourier Transform spectrometers.

Infrared: covers 7-17 microns in mid-infrared (IR), 17-1000 microns (1 mm) in the far-IR.

Spectrometer: rather than imaging (building a spatial picture), CIRS concentrates on achieving high spectral resolution: distinguishing different wavelengths of heat radiation.

• CIRS was assembled at NASA GSFC, with hardware contributions from the UK, France and Germany.

Where is CIRS?Location on the Cassini Spacecraft

CIRS—>

Cassini optical remote sensing instruments:

(or why CIRS is the greatest!)

What does CIRS do?

• CIRS is a versatile instrument, which can be used to investigate many different aspects of the Saturn system:

• Surfaces: CIRS can be used to help find out what the surfaces of the moons are composed of.

• Atmospheres: CIRS will be used to investigate both the temperature structure and the composition of the atmospheres of Titan and Saturn.

• Rings: CIRS spectra yield information on ring particle sizes, shapes and composition.

Atmospheric Remote Sensing with CIRS

• CIRS can observe planetary atmospheres in two distinct ‘geometries’:

• The NADIR geometry looks through the atmosphere to the surface beneath (or deep atmosphere for Saturn).

• The LIMB geometry is a grazing view through the atmosphere and back into space.

• We’ll see how these views give us very different spectra, on the next couple of slides.

CIRS vs IRIS:• CIRS is the successor to the IRIS spectrometer which flew on the Voyager spacecraft.

• CIRS however is improved in almost every way:

more sensitive (better detectors);

higher spectral resolution (compare the ‘IRIS limb viewing’ right, to ‘CIRS limb viewing’)

• CIRS also covers a wider range of wavelengths than IRIS: CIRS ventures into the longer wavelength ‘sub-mm’ regime (up to 1 mm) whereas IRIS stopped at 50 microns (0.050 mm).

• The figure (right) shows predicted CIRS spectra of Titan, in both the ‘nadir viewing’ (top) and ‘limb viewing’ (bottom) geometries.

• Molecular signatures of water, carbon monoxide and hydrogen cyanide are identified.

Titan’s atmosphere:• A major goal for CIRS is to map the atmospheric temperature of Titan.

• Using a mixture of limb and nadir viewing modes, CIRS can sense the temperatures from the top of the troposphere (0.15 bar) right up into the mesosphere (6x10-6 bar)

T0,… T0, Ta, Tb,… Ta, Tb,…

(Tb, T3-T5), T6,…

Tb,...

T4,…

T4,…

T4,… T4,… T6

TA: Mid-IR Nadir Temperature Map

Mid-IR Nadir Temperature Map (-22:00 to -16:45 hrs)

• North-South scans at 10 rad/s with CIRS FP3/4 arrays, to cover the entire visible hemisphere centred on -14.5°S, 151°W.

• Will sound temperatures at 4.0--0.4 mbar in the stratosphere via the 4 band of methane.

• Spectral resolution of 3 cm-1 will yield 2 scale height vertical resolution (or better).

• First look at the northern hemisphere, allowing stratospheric wind fields to be derived.

• Will also map global variation of stronger hydrocarbons and nitriles (C2H2, C2H6, HCN, CH3D etc).

Probe Health and Safety

• The CIRS global temperature map will further serve to characterize the atmospheric temperature and wind field of Titan, which is poorly known at present.

• All such information is of high value to the Huygens probe mission, the health and safety of which is dependent on correct altitude/pressure deployment of parachute(s), and hence on the atmospheric temperature field.

TA: Far-IR Nadir Composition Integration

Far-IR ‘Nadir’ Composition Integration (-15:30 to -11:30 hrs)

• 4-hour integration at 30°S, 200°W on Titan (airmass ~1.5), with some time spent on space for calibration purposes.

• CIRS highest (0.5 cm-1) spectral resolution.• Over the mission, spatial coverage will

eventually provide global map of CH4, HCN, CO in stratosphere.

• Will also build up S/N over time to search for new species in far-IR (17m to 1mm).

CIRS Riders

• CIRS will also ride-along during the VIMS (-11:30 to -5:00), ISS (-5:00 to -1:50) and UVIS (+5:30 to +8 hrs) PRIME observations, in a mixture of spectral resolution modes, to further supplement CIRS PRIME observation requests.

END OF PRESENTATION

(additional materials follow)

Table 1: CIRS Instrument Characteristics

Telescope Diameter (cm): 50.8Interferometers: Far-IR Mid-IRType: Polarizing MichelsonSpectral range (cm-1): 10 - 600 600 -1400Spectral range (m): 17 - 1000 7 - 17Spectral resolution (cm-1): 0.5 to 15.5 0.5 to 15.5Integration time (sec): 2 t o 50 2 t o 50

FOCAL PLANES: FP1 FP3 FP4Spectral range(cm-1) 10 - 600 600 - 1100 1100 - 1400Detectors Thermopile PC HgCdTe P V HgCdTePixels 2* 1 x 10 1 X 10Pixe l FOV (mrad) 3.9 0.273 0.273Peak D*(cm Hz1/2 W-1) 4 x 109 2 x 1010 5 x 1011

Dat aTelemetry Rate (kbs) 2 & 4Instrument Temperature (K) 170Focal Planes 3 & 4 Temperature (K) 75 - 90———————* Singl eFOV, tw opolarizations