The Cassini Visual and Infrared Mapping Spectrometer (VIMS) detected an unusual bright spot on Titan southeast of Xanadu during the T4 flyby (lower-left figure). The 500 kilometer diameter spot is brighter at 5µm than normal bright terrain by 30-50%. It is brighter than normal bright terrain at shorter wavelengths as well, but by a much smaller margin. The fact that the feature is unusually bright at all wavelengths implies a recent origin.During the talk, Jason showed some RADAR radiometry data which suggested that this feature was not thermally unusual.
Comparison to Cassini Imaging Science Subsystem (ISS) data (lower-right figure) shows correspondence with a unique, semicircular bright feature that bounds the 5µm-bright spot to the south. The feature’s symmetry seems to imply structural control. Also, the ISS image shows a crenulated margin, perhaps due to surface flows. After we knew to look, we noted that VIMS saw a bright area consistent with this bright feature on 4 other flybys. Similarly, ISS monitored this hemisphere for tens of hours during the Tb approach and saw no evidence for clouds or variability associated with the feature.
The feature might be a cloud, but its longevity implies that if it is a cloud it must be one that is somehow controlled by the surface. It is also possible that the spot might represent higher topography. The 5µm spectral window through Titan’s atmosphere is not perfectly clear, so a mountainous area could peek up over enough of the absorbing air to appear brighter than the surrounding terrain. The most intriguing possibility is that the spot might be thermal in nature. Simulations show that the spectrum is consistent with an area of 180K average temperature, very near the water/ammonia eutectic temperature of 173K. This hypothesis will be tested on the T15 flyby 2006 July 2 when VIMS images the area during local night.
Thursday, May 26, 2005
How old is Titan's surface? For years, Saturn's moon Titan was thought to have mastered the cosmetic surgery of the cosmos, with barely a mark or wrinkle to betray its true age. Close-up views provided by Cassini instruments show that Titan is nearly as flawless as it seems from a distance, with only two impact craters found so far. A world with a more youthful surface may be more likely to harbor life.
The problem with determining an age based on crater counts is that we don't know the production function for impactors in the Saturnian system (we don't even have a full grasp of it here in the inner solar system). In the inner solar system, we can rely on the moon as a crutch to determine ages, but without a late heavy bombardment in the outer solar system and with different populations of impactors, you can use that crutch in the outer solar system. Certainly the relative lack of craters on Titan compared to its peers in the Saturnian system can give us a relative age, certainly its surface, or at least the parts observed so far, are much younger than Rhea, but without even the slightest grasp of the production function or gravitational scaling (more craters will impact worlds closer to Saturn than away, ignore Iapetus) except for some rough estimates by Zahnle et al. (2004?), I don't think we can put an absolute age on the surface.
Wednesday, May 25, 2005
I have to admit that this released caught me off guard. I wasn't expecting this for a few more weeks. Regardless, the ISS and VIMS teams have announced the discovery of a very bright spot on Titan. This feature was first noticed in ISS images as a concave-north "smile-shaped feature south-east of Xanadu. In the image from December above, combined with VIMS false color, the smile has an odd, crenulated margin. VIMS was able to observe this feature during the T4 and T5 flybys earlier this year. In VIMS images, the feature is a very bright spot at 5 microns (as opposed to the ISS image which used a filter centered at 0.938 microns). The smile feature bounds the bright spot to the south and west, though it extends farther to the west (this may be an emission angle effect since the bright spot is near the limb in VIMS images).
So what is the bright spot? The spectrum obtained by VIMS is very different from other landforms on Titan, save a very bright region in southwestern Xanadu (seen in VIMS and ISS images). It is not a cloud since clouds generally appear brighter than their surroundings at all methane-window, VIMS wavelengths (and are seen slightly away from the center of those windows since they are at higher altitude) while this spot is much brighter than its surroundings at 5 microns and is only slightly above at shorter wavelengths. In addition, VIMS and ISS images since T0 last July show that the spot hasn't changed in 9 months. The press release does mention topographically controlled clouds, however.
Another possibility is that the spot is a hot spot, not unlike the hotspots seen on Io observed by the SSI and NIMS instruments on Galileo, resulting from volcanic activity that is much warmer than its surroundings. In Titan's case, the hot spot would result from water-ammonia cryovolcanism, rather than basalt. The hotspot could also result from a recent large impact on Titan at that site. The best test for seeing the thermal signature of a hotspot comes by observing Titan's night side, where the only signal coming from the surface would be from emission, whereas day side detections have to be seperated from reflected sunlight. So far no thermal anomalies have been discovered in night side observations, but a test for this feature will come on July 2, 2006 when this feature is observed at night.
Finally, this spot could simply be compositionally distinct from the rest of the surface (a hypothesis I have to admit in the interest of disclosure I favor). In the press release, Alfred McEwen states, "This bright patch may be due to an impact event, landslide, cryovolcanism or atmospheric processes. Its distinct color and brightness suggest that it may have formed relatively recently." It would seem to me that this spot should be a patch of relatively clean ice, again this feature would have to be relative young for it not be covered in haze crud.
Anyways, a very interesting discovery by the VIMS team. It is also nice to finally have some collaboration between different instrument teams since it definitely appears that some collaboration is necessary for us to understand what is going on here.
Two images releases are associated with this news item:
Flasar et al. Titan's Atmospheric Temperatures, Winds, and Composition. Science 308 (5724), 975-978.
Temperatures obtained from early Cassini infrared observations of Titan show a stratopause at an altitude of 310 kilometers (and 186 kelvin at 15°S). Stratospheric temperatures are coldest in the winter northern hemisphere, with zonal winds reaching 160 meters per second. The concentrations of several stratospheric organic compounds are enhanced at mid- and high northern latitudes, and the strong zonal winds may inhibit mixing between these latitudes and the rest of Titan. Above the south pole, temperatures in the stratosphere are 4 to 5 kelvin cooler than at the equator. The stratospheric mole fractions of methane and carbon monoxide are (1.6 ± 0.5) x 10-2 and (4.5 ± 1.5) x 10-5, respectively.Shemansky et al. The Cassini UVIS Stellar Probe of the Titan Atmosphere. Science 308 (5724), 978-982.
The Cassini Ultraviolet Imaging Spectrometer (UVIS) observed the extinction of photons from two stars by the atmosphere of Titan during the Titan flyby. Six species were identified and measured: methane, acetylene, ethylene, ethane, diacetylene, and hydrogen cyanide. The observations cover altitudes from 450 to 1600 kilometers above the surface. A mesopause is inferred from extraction of the temperature structure of methane, located at 615 km with a temperature minimum of 114 kelvin. The asymptotic kinetic temperature at the top of the atmosphere determined from this experiment is 151 kelvin. The higher order hydrocarbons and hydrogen cyanide peak sharply in abundance and are undetectable below altitudes ranging from 750 to 600 km, leaving methane as the only identifiable carbonaceous molecule in this experiment below 600 km.Waite et al. Ion Neutral Mass Spectrometer Results from the First Flyby of Titan. Science 308 (5724), 982-986.
The Cassini Ion Neutral Mass Spectrometer (INMS) has obtained the first in situ composition measurements of the neutral densities of molecular nitrogen, methane, molecular hydrogen, argon, and a host of stable carbon-nitrile compounds in Titan's upper atmosphere. INMS in situ mass spectrometry has also provided evidence for atmospheric waves in the upper atmosphere and the first direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atmosphere. The bulk composition and thermal structure of the moon's upper atmosphere do not appear to have changed considerably since the Voyager 1 flyby.Wahlund et al. Cassini Measurements of Cold Plasma in the Ionosphere of Titan. Science 308 (5724), 986-989.
The Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP) sensor observed the cold plasma environment around Titan during the first two flybys. The data show that conditions in Saturn's magnetosphere affect the structure and dynamics deep in the ionosphere of Titan. The maximum measured ionospheric electron number density reached 3800 per cubic centimeter near closest approach, and a complex chemistry was indicated. The electron temperature profiles are consistent with electron heat conduction from the hotter Titan wake. The ionospheric escape flux was estimated to be 1025 ions per second.Mitchell et al. Energetic Neutral Atom Emissions from Titan Interaction with Saturn's Magnetosphere. Science 308 (5724), 989-992.
The Cassini Magnetospheric Imaging Instrument (MIMI) observed the interaction of Saturn's largest moon, Titan, with Saturn's magnetosphere during two close flybys of Titan on 26 October and 13 December 2004. The MIMI Ion and Neutral Camera (INCA) continuously imaged the energetic neutral atoms (ENAs) generated by charge exchange reactions between the energetic, singly ionized trapped magnetospheric ions and the outer atmosphere, or exosphere, of Titan. The images reveal a halo of variable ENA emission about Titan's nearly collisionless outer atmosphere that fades at larger distances as the exospheric density decays exponentially. The altitude of the emissions varies, and they are not symmetrical about the moon, reflecting the complexity of the interactions between Titan's upper atmosphere and Saturn's space environment.Backes et al. Titan's Magnetic Field Signature During the First Cassini Encounter. Science 308 (5724), 992-995.
The magnetic field signature obtained by Cassini during its first close encounter with Titan on 26 October 2004 is presented and explained in terms of an advanced model. Titan was inside the saturnian magnetosphere. A magnetic field minimum before closest approach marked Cassini's entry into the magnetic ionopause layer. Cassini then left the northern and entered the southern magnetic tail lobe. The magnetic field before and after the encounter was approximately constant for ~20 Titan radii, but the field orientation changed exactly at the location of Titan's orbit. No evidence of an internal magnetic field at Titan was detected.
Tuesday, May 24, 2005
New on the raw images page is a sequence of images showing the shapes and geology of Saturn's inner small satellites: Pan, Atlas, Prometheus, Pandora, Epimetheus, and Janus. For many of these (except Epimetheus) these are the highest resolution images yet obtained for these small moons. Not much can be said for many of these moons. Pandora appears to be relatively young with only one crater visible in this view (though the polygonal shape may betray the presence of several, larger craters along the limb). Not much can be said for Pan and Atlas other than they appear to be oblong. Janus is most interesting in this view (shown above). The surface appears to be bright with number dark splotches on its surface. A crater to the upper right appears to have dark ejecta around it. Perhaps Janus is the opposite of Phoebe, with a bright surface layer with dark material beneath that is dug up by impacts.
The title link goes to a comparison showing all 6 moons side by side.
Monday, May 23, 2005
The ISS camera snapped this image of Enceladus over the weekend, now released on the JPL raw images page. Much of this area seen here was seen on prior encounters at much higher resolution so nothing to get too excited about. However, the "tiger stripes" near the south pole are new as far as I am aware of. The northern tips of these features may have been seen on Rev04 where they were characterized as arcuate ridges or scarps surrounded by blue to blue-green material. Here they are characterized as dark, roughly parallel bands with brighter lanes in the middle (at least in the left band). In this image, they appear similar some of the ridges on Europa where the ridge appears bright and is surrounded by dark material on both flanks. Luckily, we get a much better view on July 14 of these features.
The slight albedo patterns seen north of these tiger stripes are associated with deep fractures and shear features seen in earlier Cassini images as well as low resolution Voyager observations. In these cases, the dark material observed in clear filter images was found to be bluish to blue green in UV-Green-Near-IR false color images.
The ISS talk was on the types of land forms seen by the imager during the two flybys (though most of the images used come from the first encounter). Chewed up craters, fractures, kilometer-high domes, and ridges appear to be the highlights of the presentation. Still need to listen to the talk to see what Zibi did actually say...
Speaking of Enceladus, watch out for possible observations of Enceladus taken during the non-targeted encounter over the weekend. The presentation gives a distance of 93,000 km on the encounter. Not very high, but good enough to get an idea of what is going on at the south pole.
CICLOPS has released this view of Mimas and the outer edge of Saturn's main and F rings. Herschel, the great crater of Mimas, can easily be seen in this view. Fine scale structure is just starting to be discernable in the rings, such as the halo surrounding the F ring and the moonlet Atlas. Resolution on Mimas is 13 km/pixel.