A 5 Micron Bright Spot on Titan

Transcription

1 A 5 Micron Bright Spot on Titan Barnes, Brown, Turtle, McEwen, Lorenz, Janssen, Schaller, Brown, Buratti, Sotin, Griffith, Clark, Perry, Fussner, Barbara, West, Elachi, Bouchez, Roe, Baines, Bellucci, Bibring, Capaccioni, Cerroni, Combes, Coradini, Cruikshank, Drossart, Formisano, Jaumann, Langevin, Matson, McCord, Nicholson, Sicardy. False color image of Saturn's Moon Titan Science 310, (7 October 2005). Presented by James Ledoux Slide 1

2 Background on Cassini Titan in Optical Cassini Relasing Huygens Mission to Saturn and Titan (largest of 18 moons) Launched in 1997, Arrived at Saturn (9.5 AU) July 2004, main mission until June Huygens descended to Titan Nov 27, 2004 Cassini will orbit Saturn 74 times with 44 Titan flybys Slide 2

3 Cassini's VIMS Titan from Cassini's VIMS Titan from Keck (1999) The Visual and Infrared Mapping Spectrometer (VIMS) images in 352 colors simultaneously. Wavelength range µm incl spectral windows at.92, 1.06, 1.26, 1.57, 2.0, 2.7, 5.0 µm which can view through Titan's haze and atmospheric absorption. Slide 3

4 Cassini's ISS and Radar Radiometer Imaging Science Subsystem (ISS) is a CCD Visible/Near IR Camera With.938 µm filter can observe Titan's Surface The Radar Radiometer scans for spatial variation in an active and passive mode: Active: Sends 2.17cm radar Passive: Receives Microwave Flux from the Surface Slide 4

5 New 5µm Titan Bright Spot Cassini's VIMS found an unusual bright spot at 80ºW, 25ºS. Blue is 1.57µm window, green is 2.0µm window, and 5.0µm window is red. Slide 5

6 Titan Bright IR Areas Dark spots on Titan hypothesized to be from dark organic haze particles that settle from the atmosphere. Bright spots would be regions washed away from methane rain storms and left with dirty ice. Earth based observations found evidence of liquid hydrocarbon lakes and methane clouds on Titan. Active exchange of liquid between atmosphere and surface. Slide 6

7 Images of 5µm Bright Spot Keck Image with AO with K' filter ( µm) VIMS images have 5 µm as red, 2.0 µm as green, 1.57 µm as blue and were taken on 3 July 2004 (T0), 26 October 2004 (TA), 13 December 2004 (TB), 31 March 2005 (T4), and 16 April 2005 (T5). No spectral/spatial change in B.S. between T4 & T5 Slide 7

8 5 µm Bright Spot The 5 µm spot is consistently observed between 7/04 (T0) and 4/05 (T5). T5 T4 T4 and T5 had best similar resolutions of 30km per pixel N S and 150km per pixel E W, with size 450km N S, and 400km E W. Points 1,2,3 were taken at similar emission angles to compare spectral features from the 5µm B.S., Xanadu, and 'Tsegihi'. Also point 4 was included to represent dark regions. Slide 8

9 Spectral Comparison of Regions All Titan bright regions aren't the same: Xanadu brighter than Tsegihi at wavelengths below ~2.5 µm Tsegihi is significantly brighter at 5 µm. This implies an icier composition for Tsegihi. The unusual bright spot is spectrally more similar to Tsegihi than Xanadu. Slide 9

10 Smile Feature in ISS VIMS (T5) A thin semicircular smile feature is apparent in ISS map (.938 µm) along southern edge of bright spot. Feature is 650km long, up to ~90km wide, could be heavily eroded crater but that wouldn't explain bright spot. ISS (TB) Observed in ISS both at TB (12/04) and T3 (02/05) with no change. Shown are identical regions of close up cylindrically projected maps (13º VIMS + ISS 34º S, 59º 96º W).938 and 5 µm albedo's not correlated. Slide 10

11 2.2 cm Radar Imaging of Spot Passive radiometry from T4 at 2.2cm in one polarization with resolution of 250km is shown Correlation of higher radar brightness T with lower near IR albedo. Slide 11

12 What is the bright spot? Not a new crater as ISS Smile doesn't extend to North Cannot be specular reflection as a flat surface is much to the east, and VIMS observations vary from 12º 57º in phase angle with no variation in spectral character (and ground based Keck AO observations show phase matches surrounding terrain from 0º 70º). Possibly cryovolcanism (Bright spot is hot spot for volcanic activity) Possibly the spot is a topographic high, so sunlight passes through less of Titan's atmosphere and appears brighter. Possibly a cloud feature of some sort. Possibly surface albedo marking of some sort. Possibly CO2 ice Slide 12

13 Bright Spot as Volcanos? Low density of craters suggest high resurfacing rate possibly from cryovolcanism Bright Spot 5µm excess is consistent with gray body emission at T ~ 180K Some features of ISS morphology consistent with this hypothesis (that 5µm is hot spot of volcanic activity): bright optical albedo, crenulated margins, possible linear flow features. However, modeling the microwave brightness radar data (with an antennae temp of ~77K) gives Tb(V) ~ 83K and Tb(H) ~ 67K in the two polarizations, typical with other regions of Titan and lower than thermodynamic surface temp, so no evidence for elevated surface temperature. Therefore not cryovolcanos. Slide 13

14 High Spot or Cloud? If the spot is topographic high then the sunlight travels through less of atmosphere. However, Titan's haze absorbs less at higher wavelengths, this effect should increase the intensity at lower wavelengths more than at higher wavelengths (in contrast to the observed increase in 5 µm brightness). Also this would create a strong negative brightness temperature in radiometry that is not present. Slide 14

15 A Cloud? Probably not a cloud either, as the spot's spectrum doesn't match known clouds and has persisted much longer than known clouds. Keck and Cassini measurements have shown spot to be bright and maintain spatial distribution for 4.5 years. Clouds see in past have formed and dissipated on scale of hours or days. High spatial and good time resolution ISS monitoring showed no apparent changes over tens of hours. If it was a cloud it would have to be persistent low lying cloud or ground fog. It would have to be very tightly controlled by the surface like it was correlated with fog over a lake, hot springs, or volcanic field. But higher altitude clouds seen so far are much brighter than Xanadu at ISS wavelength, and this is only seen at the Smile not the whole spot. Slide 15

16 Surface Albedo Markings? Could represent an area of Titan with distinct crustal composition, an area that was unusually weathered, or thin surface coating. Because H2O ice highly absorbs at 5 µm the spot cannot be water rich. The area could be in an unusual state of weathering: very young geologically or recently cleaned by methane rainfall. This could alter the chemistry though it seems it would leave the area more icy. If the radiometric brightness Tb(V) ~ 83K and Tb(H) ~ 67K are from emission from a dielectric surface at 83K, then the ratio of temps suggests dielectric constant ~3 as opposed to ~2 elsewhere on Titan. Consistent with ice rich composition, but radio data probes deeper than near IR, so a thin coating a few tens of microns thick could give the surface a very different near IR reflectance. Slide 16

17 Conclusion One possibility is that the bright spot's reflecting material is CO2 ice, as it reflects highly in short wavelength atmospheric windows, and is more reflective than water ice in 5 µm window. However, most diagnostic wavelengths for CO2 are not within Titan's spectral windows. Based on a recent abundance of CO2 estimate by Kress & McKay based on comet impacts the bright spot and Tsegihi could be eroded layers of CO2 or recent overlying deposits. This could account for the observed bright 5 µm spot, that appears static over period of years. Slide 17

18 Sources Barnes, Brown, Turtle, McEwen, Lorenz, Janssen, Schaller, Brown, Buratti, Sotin, Griffith, Clark, Perry, Fussner, Barbara, West, Elachi, Bouchez, Roe, Baines, Bellucci, Bibring, Capaccioni, Cerroni, Combes, Coradini, Cruikshank, Drossart, Formisano, Jaumann, Langevin, Matson, McCord, Nicholson, Sicardy. A 5 Micron Bright Spot on Titan: Evidence for Surface Diversity, Science 310, (7 October 2005). Astronomy Picture of the Day (various images of Titan) Cassini Huygens Webpage (info on Cassini and its instruments) Slide 18

19 Poems Tsegihi Navajo Nightsong: In Tsegihi, oh you who dwell! In the house made of the dawn, In the house made of the evening twilight, In the house made of the dark cloud, In the house made of the he rain, In the house made of the dark mist, In the house made of the she rain, In the house made of pollen, In the house made of grasshoppers, Where the dark mist curtains the doorway, The path to which is on the rainbow, Where the zigzag lightning stands high on top, Where the he rain stands high on top, Oh, male divinity! [...] With beauty all around me, I walk. In beauty it is finished. Xanadu First verse of Kubla Khan (Coleridge): In Xanadu did Kubla Khan A stately pleasure dome decree : Where Alph, the sacred river, ran Through caverns measureless to man Down to a sunless sea. So twice five miles of fertile ground With walls and towers were girdled round : And there were gardens bright with sinuous rills, Where blossomed many an incense bearing tree ; And here were forests ancient as the hills, Enfolding sunny spots of greenery. Slide 19