Thursday, April 13, 2017
The Mystery Of Titan's Missing Clouds
Imaging of Saturn's Moon Titan's northern latitudes from the Cassini spacecraft. Interestingly one camera (VIMS) shows clouds (bottom, yellowish area) which the other instrument (ISS camera) misses.
Space scientist Elizabeth Turtle, based at the Johns Hopkins Applied Physics Laboratory in Baltimore, Maryland, is currently puzzling over the matter of Titan's clouds - which appear to be "absent without official leave". All serious models of Saturn's moon indicate storm clouds ought to have appeared by now but evidently have not.
First, a bit of review: Titan intrigues space and planetary scientists on Earth because it is the only other body (apart from Earth) in the solar system featuring an Earth-like liquid cycle i.e. between rain and stable liquid surface features. On Earth it's called a "water cycle" but on Titan it's a methane cycle. This is given Titan's average high temperature hovers around 94 Kelvin (for comparison, the freezing point of water is 273 K)
Methane condenses high in Titan's sky as clouds then rains onto its icy surface as a liquid. It then erodes the surface ice as it runs into lakes and rivers then evaporates once more into the atmosphere,. Like Earth, Titan features an axial tilt as it revolves around its parent world and hence displays "seasons" which affect its atmosphere in a similar way to how Earth's axial tilt affects its own seasons. The diagram below can help fix these ideas with reference to our planet, for its seasons in the northern hemisphere:
Note the axial tilt (at 23.5 deg) always remains fixed in the same direction. But at the summer solstice this translates to more direct reception of sunlight while on the opposite side of its orbit there is less.
In like manner, during Titan's summer one pole is always sunlit and one pole is always dark. Warm temperatures on the more sunlit hemisphere give rise to convective storm cells , just as summer storms form on Earth. Just as Earth in the above diagram was in the midst of its northern winter Titan was in the middle of its January in 2004 when Cassini arrived at Saturn. Not surprisingly, the Cassini team observed lots of southern hemisphere summer storms.
Given Titan's year last almost 30 Earth years Titan's seasons have shifted over the last few Earth years and so we are seeing the moon entering its "June", according to Turtle. Since this shift began APL scientists have been waiting for the same summer storms to now form in the northern hemisphere.
Were they looking too early? Who knows? But as of mid-February on Earth no storms or clouds had yet been observed in Titan images. But wait! Turtle's colleague Jason Barnes - who works with Cassini's Visual and Infrared Mapping Spectrometer (VIMS) reported observing the clouds.
Turtle questioned her colleague, 'What clouds?' - Whereupon she checked all her images and found no evidence at all of clouds. She then viewed imaged via the VIMS data and saw a swath of smudgy clouds obscuring the exact features visible in the ISS (Imaging Science Subsystem) instrument. The latter detects electromagnetic wavelengths in the near infrared, e.g. between 980 nm and 2 micrometers.
On the other hand, the VIMS sees further into the infrared, e.g. to wavelengths that extend to 5 micrometers. So, it appears the spectral band accessed - by the particular instrument - determines whether one observes Titan's clouds or not. Turtle l evinced surprise, asking "they're only showing up in some wavelengths and not others?" But why not? In solar physics this observational divergence is common and one sees different features depending on what wavelength is used. So it shouldn't be totally surprising.
Turtle presented her "puzzle" at the fall meeting of the American Geophysical Union and those interested can read more here: