1. VIRTIS activities during Global Mapping and Close Observation Phases
F. Capaccioni for the VIRTIS team
WG1 Meeting ESOC 18th May 2010

2. Outline VIRTIS principles of operations
VIRTIS capabilities demonstrated through observations during the cruise phase
VIRTIS operation strategy during the pre-landing phases
VIRTIS compatibility with lander requirements

3. VIRTIS Summary
VIRTIS-H point spectrometer with high spectral resolution. Needs S/C motion to build “pseudo-images”. Uses a HgCdTe detector to store high resolution spectra in the range 1.88µm-5.03µm
VIRTIS-M Hyperspectral imager. Has a scan mirror to build images. Uses a CCD and a HgCdTe detectors which cover the range 0.25µm-5.06µm

4. VIRTIS-H Spectral orders
Grating dispersion (432 pixels)
Prism dispersion (256 pixels)
Order
Order
Order
Order
Order
Order
Order
Order
VIRTIS-H uses a Prism to disperse the light on the vertical axis and a diffraction grating to further spread each order and achieve high spectral resolution.
The 8 orders are laid on a single frame of the 2D HgCdTe detector;
Only the lighted pixels are read and each acquisition represent a full spectrum.

5. Operation of VIRTIS-M Hyperspectral Imager

6. What VIRTIS can provide
VIRTIS-H is particularly useful for the study of the composition of gases, owing to its spectral resolution which is always larger than 1200.
VIRTIS-M, due to its imaging capability, is more effective in the production of compositional maps of a surface.
Both channels will measure the radiance emitted by the target and consequently will provide a measurement of the surface temperature.
Both channels can work completely independently from each others, so from the operability point of view can be considered as two separate instruments.
In the next slides we shall give some examples of VIRTIS capabilities

7. Earth Observation VIRTIS-M
Fig1. VIS channel; spatial resolution of about 50 km. RGB imaging (0.44, 0.55, 0.7μm).
Fig 2 VIS Channel; contrast enhanced image (0.474μm, 0.785μm, 1.0μm).
Fig 3 VIS Channel; contrast enhancement of chlorophyll absorption feature.
Fig 4 IR Channel; and Radiation emitted from the night side clearly shows up in this image. The cyan spots are high altitude clouds, while Oceans appears in red having a thermal emission and inertia larger than the landforms (in pink).
Fig 5. Thermal emission region at 5.0 μm; the Earth looks fairly uniform on the day and night side. The northern American continent (in the top-left quadrant of the image), mainly at night and during the winter season, appears as the coldest area of this image (in blue).

8. Earth Observation VIRTIS-M
In this high resolution images is shown the region of the north-west coast of Africa, including Morocco, Gibraltar, Canary Island and the atlantic ocean.
Top panel VIS true colors,
Bottom panel IR false colours

9. Classification of surface units
VIRTIS-M image capabilities allows to classify different surface units according to certain spectral properties.
LAND MASK: (R707 / R527) ≥ 1.39
OCEAN MASK: : R758 < 0.04
THICK VEGETATION MASK: (R713 / R685) ≥ 1.18
LOW CLOUD MASK: R758 ≤ (after removal of Land, Vegetation and water)
HIGH CLOUD MASK: R758 > (after removal of Land, Vegetation and water)

10. Average radiances in the visual region
Classification of surface units
Average radiances in the visual region
Average radiances in the infrared region

11. Atmosphere Observations VIRTIS-H
CO2 Non-LTE emission observed by VIRTIS-H ROSETTA on Earth (ESB1, ESB2, ESB3), Mars (MSB Feb. 2007) and Venus (Venus Express regular limb observations since Apr. 2006).
We measured abundances as well as limb profile
Earth (ESB2)
66 km
79 km
89 km
100 km
~10-2 to mbar
Whole CO2 band,
Averaged from 4.2 to 4.4 mm
Radiances in mWm-2sr-1/ mm

12. Mars VR03 = S/C scan; start 24/02/2007 at 21:50 ends at 22:10
Longitude coverage 5E to 185E
Latitude coverage: full disk (slit = 1.1 Mars Disk size)
Spatial resolution 30km at Nadir
Whole CO2 band,
Averaged from 4.2 to 4.4 mm
~10-3 to 10-9 mbar
Altitudes
73 km
110 km
149 km
188 km
Radiances in mWm-2sr-1/ mm

13. Venus
Limb observations : regular scans of the limb at most latitudes and solar zenith angles
Example : observations of 2008/10/29 Orbit 922
Whole CO2 band,
Averaged from 4.2 to 4.4 mm
~10-3 to 10-9 mbar
Altitudes
98 km
107 km
116 km
126 km
Radiances in mWm-2sr-1/ mm

14. Quantitative analysis (Modeling by Lopez-Valverde)

15. Steins Observations VIRTIS-M spectral profile of a single pixel.
The spectrum does not show significant absorption features in the range 1 to 4 µm
In this particular spectrum the region between 2.5 and 3 µm is affected by the defective region of the order sorting filter; the effect will be considerably reduced for larger sized targets
The region between 4 and 5 µm shows a thermal emission with superimposed absorption features.

16. M data projected on shape model
Time
VIRTIS-M imaging capabilities in pushbroom mode will strongly depends on the S/C motion.
A post fly-by reconstruction is needed to build a real image

17. Temperature Profiles
Horizontal Profile
Vertical Profile

18. Cassini Phoebe Observation
CASSINI observed Phoebe in July 2004.
Phoebe is the farthest Saturn satellite
at a distance of 13 million kilometers;
is on a retrograde orbit and is not co-rotating.
Its albedo is below 6% and its spectrum is very different from the other Saturn satellites.
Every clue points to a captured origin of a body that represent a sample of a very primitive object, possibly originated in the Kuiper Belt

19. CASSINI-VIMS spectra of Phoebe

20. Comet Observation Planning

21. Extract from RO-LAN-MOM-WG1-2010_0402 Minutes of the WG1 executive meeting 4.2.2010
……….
PIs should reply using the following template list:
a. Observation type
b. Trajectory
c. Attitude
d. Ops Mode
e. Data volume
f. Frequency of observations
………….

22. To describe what will be the VIRTIS contribution we have started from the presently available information contained in
CREMA doc (RO-ESC-RP-5500 issue 5 August 2003)
The presentation given by Accomazzo during the WG1 Mtg, held in ESOC in December 2008.
We have provided the required information for the general phases Global Mapping and Close Observation

23. Phases definition WG1 Mtg, ESOC 12 December, 2008

24. Global Mapping Phase From the previously mentioned docs we got:
Duration 7 days observation plus 28 days analysis
During the 28 days the S/C will keep orbiting the comet and thus additional observations can be performed, within the limitation due to safety considerations
The angle between the viewing direction and the mapped surface normal shall be less than 70º.
The Distance from Sun is 3.5AU, while the distance from the comet is between 10 to 25 cometary radii (between 25km and 62.5km)

25. VIRTIS Performances during GMP Resolution and Data Volume
Spatial resolution (10 to 25 cometary radii):
VIRTIS-M (VIS and IR): m -15.5m
VIRTIS-H: m x 36.75m – 30.6m x 91.9m
FOV V-M 6.25m x 1600m – 15.6m x 3990m
The S/C is capable of dumping 1.2Gbit/day in this phase
VIRTIS-M approximate Data Volume for global mapping, assuming full spectral resolution (432 bands) and lossless compression:
Spatial resolution 6.25m 9.2Gbit (1.3Gbit/day) Max Resolution at 10Rc
Spatial resolution 25m 2.3Gbit (0.33Gbit/day) 4x binning at 10Rc
Spatial resolution 15.5m 1.5 Gbit (0.21Gbit/day) Max Resolution at 25Rc
Spatial resolution 62m 375Mbit (0.05Gbit/day) 4x binning at 25Rc
In the case of VIRTIS-H the goal shall not be the global coverage but to support site selection observations providing spectral information at the highest spectral resolution. A reasonable assumption would be a data volume of about 100Mbit/day.

26. Comet Radiance in the IR
Assumptions:
reflectance flat 4%; no phase reddening, neither compositional variegation
surface T 160K – 200K
Start of this phase : Distance 3.5AU
T=200K
T=160K

27. VIRTIS-M Radiometric Performances IR (1)
T=200K
Incidence angle 60°
T=160K

28. VIRTIS-M Radiometric Performances IR (2)
T=200K
Incidence angle 0°
T=160K

29. VIRTIS-M Radiometric Performances IR (3)
T=200K
At the end of this phase D=3.1AU
T=200K
We might want to use more than one integration time to optimise signal in the 1-2m and 3-5 m ranges

30. VIRTIS-M Radiometric Performances VIS
Assumptions:
reflectance flat 4%;
Distance 3.5AU
Incidence angle 0°

31. Gas abundances determination
For WG2 the test case consisted in an evaluation of the sensitivity to H2O, CO2 and CO at 3AU
Instr.
Pointing
H2O CO
CO2
Alice
Just above limb ND
Q ≥
Virtis-H
Limb height : 1-5 km
Q ≥
Q ≥ 1026
Q > 1025
Miro
Limb height : km
H218O: Q ≥
H2O : Q ≥ 1023
N/A
Rosina
nadir
TBD
Osiris
ND (TBC)
CONCLUSION
detection of the main species should be possible, even for low CO, CO2 outgassing rates ( mol/s)
for remote instruments, best detectability is with limb sounding.
long integration required for remote instruments

32. Detection of CO and CO2 is clearly possible
VIRTIS-M NESR VIS+IR
Expected signal
continuum plus gas emission
In these cases the S/C is orbiting at 10km from the surface of the nucleus.
Detection of CO and CO2 is clearly possible

33. Observation requirements VIS + IR Global Mapping Phase
The orbit needs to be polar to optimise coverage (this is already included in the CREMA analysis)
The phase angle should be preferably below 60°
VIRTIS-M can work in pushbroom mode (using the S/C motion to build images):
For a ground resolution of about 50m, a 10s repetition rate, max relative speed to ground < 5m/s
At maximum resolution of 6.25m the relative speed must be < 0.625m/s
In pushbroom mode we require continuous acquisition
For lower speeds VIRTIS-M can use the internal mirror to build images.
Limb sounding shall be used, when not in nadir mode, to allow VIRTIS gas abundance determination

34. Close Observation Phase RO-ESC-RP-5500_5_0__CReMA_for_C-G_2003Aug
Orbital Constraints
The observed area shall be illuminated during fly-over with a phase angle below 70º.
The surface-viewing angle shall be below 30º .
Duration 8 days observation plus 14 days site selection
During the 14 days of analysis the S/C will keep orbiting the comet and thus additional observations can be performed, within the limitation due to safety considerations

35. VIRTIS Performances during Close Observation Resolution, Data Volume and frequency
Spatial resolution (2 cometary radii):
VIRTIS-M: 1.25m x 1.25m (pixel) ; 1.25m x 320m full slit
VIRTIS-H: 2.45m x 7.35m
The S/C is capable of dumping 0.91 Gbit/day in this phase
Area of each potential landing site? Depends on landing error ellipse?
VIRTIS-M approximate Data Volume for each landing site, assuming full spectral resolution (432 bands) and lossless compression a 300mx300m area is assumed:
Spatial resolution 1.25m 300 Mbit Max Resolution at 2Rc
Spatial resolution 5m 75 Mbit 4x binning at 2Rc
For VIRTIS-H also in this case an hour worth of observation in nominal operative mode (1s integration time) adds to 75Mbit (lossless compression, max operative frequency)

36. VIRTIS Performances during Close Observation Resolution, Data Volume and frequency
With a pixel resolution of 1.25m to cover the 300m we need about 240 single acquisitions, adding dark current and background calibration acquisition we end up with 256 frames.
With a repetition time of 10s, the full area is covered in about 40 minutes
Possibly more than one integration time shall be needed (Composition Vs Temperature)
Temperature measurements shall provide information on active Vs non Active area distribution

37. Comparison with Philae needs RO-LAN-RD-1000 issue 2_6,_Feb_2010
PROPERTY REQUIREMENT VIRTIS
Albedo Global 1000m 6.25m to 60m
Local m 1.25m
Accuracy ±20% ±20%
Composition Global 100m 6.25m to 60m
Local m 1.25m
Temperature Global 100m 6.25m to 60m
Accuracy 10K < 5K for T>160K
Gas Flux Global 10m – 10km V-H H2O > 1025 mol/s
CO2 = CO > 1026 mol/s
V-M CO2 = CO > 1026 mol/s