Spectroscopy of Titan

Right at this moment I'm working on studying Titan's atmosphere and surface using near-infrared spectroscopy. A very general description of this topic is below, including some of the most recent results from throughout the community. For more details, please take a look at the publications that I have contributed to. Soon I'll be posting some of the IDL routines I've written in the course of my research, so please check back for that, and feel free to contact me if you have any questions or comments.

from Cassini/ISS map

Image of Saturn on aperture plate while acquiring spectra

The image of Saturn (above) was acquired using the guider camera on the Shane 3-meter telescope at Lick Observatory. The camera is imaging the light that is reflected off of a shiny metal plate with a rectangular opening, or aperture, that leads into the Hamilton Spectrometer. At the time we were using the aperture that's near Tethys in the image. Although we were taking spectra of Titan, which was out of the guider camera's field of view, roughly where the spinning animation is.

The animation of Titan (top right) was created using a surface albedo map of Titan that was assembled together by the Cassini Imaging Team. This type of reprojection is useful for comparing ground-based observations. For an brief description of Titan with beautiful pictures, please check the wikipedia.

Tools: IDL code for the planetary scientist (coming soon).

Recent research on Titan: The Surface, Hazes, & Clouds
(last updated 06 March 2006)

The Surface

Titan's has many diverse geological features, which suggest that (much like Earth) there are a range of surface interactions with the subsurface and the atmosphere. One slight complication in studying the surface is that to see it, we have to take pictures with infra-red cameras. Using IR wavelengths is not too much of a problem as far as analyzing images of structures that we clearly resolve: pebbles and rocks, hills and valleys, places where fluid (probably liquid methane) once flowed, and lakes (no lakes have been confirmed, but this thing looks suspicious) are all relatively easy to interpret. However, there is a bit of a problem in deciphering the images of features that we don't make out clearly (or resolve), like many of the structures seen here in the animation at the top of the page.

Map of Titan's surface from Cassini/ISS
Surface albedo map from Cassini/ISS

To get a feel for the problem, first consider Earth. We know immediately from looking at a satellite image of Earth that the blue regions are water, the green and brown parts are land (plants, trees, dirt) the white parts are snowy or cloudy. Try it: take a look at Google Earth and test if you can stump yourself, start zoomed in (where it's easy) and then zoom out. Better yet, start zoomed out, make a prediction, and zoom in to see if you were correct. Taking another dimension of information besides just color, like brightness (you intuitively do this looking at pictures), one could add a level of sophistication to their interpretation. For example, smart people who study satellite photos of Earth for a living [link] can tell you the difference between light brown and dark brown, or light green and dark green [link]. Now think of a black and white satellite photo (or look at these). Here the only information you have is brightness, and sometimes it's easy to know what you're looking at, other times it's trickier.

On Titan, we haven't yet developed the vocabulary to figure our how to translate 1-micron, 2-micron, and 5-micron into (for example) blue, green, and/or brown. We also don't quite know what it means to be brighter or darker at any of these wavelengths. This is one of things we're working on. We know that Titan is dark. At the infra-red wavelengths that we use to see the surface, only about 5-20% of the light that reaches the surface is reflected back. This is the same at most wavelengths, which means that if we could combine Titan's infra-red wavelengths to make colors like our eyes do with visible wavelengths, we would see that Titan isn't just dark, it's dark grey. Mostly dark grey. Since there are these spots that jump out at you at 2- and 5-microns (Barnes et al., 2005). We've been using a Very Large Telescope (literally) to look at regions of Titan that have been found to be particularly bright at 5-microns. Imagine walking along a dark-grey landscape and seeing a big red thing in front of you -- that's what researchers using Cassini saw [link]. They couldn't quite tell if the bright `red' (we don't really know if 5-micron translates to red) spot was on the ground, or in the air, but they thought it was on the ground. Our measurements with telescopes further support the 'on the ground' idea, but we're still working on understanding what the spot is.
The Enshrouding Haze of Aerosols

Smogs, fogs, caps, hoods, and collars. These are some of names for the aerosol particles suspended in Titan atmosphere. Altogether they are called hazes, and the different names are for regions (regions the size of continents) where there is more haze or less haze, causing images of Titan, at certain wavelengths, to be brighter or darker. That way it works is that all of the infra-red light that we see when observing Titan is sunlight scattered by aerosols or reflected from the surface. The size, structure, and chemical composition of a particular aerosol will each determine how it scatters light. Any remote observations will measure scattering from a large number particles with a distribution of sizes, structures, and compositions --- each of which depend on the altitude and location of the particular ensemble of particles being probed. On Titan, some combination of these properties change with time and result in seasonal changes in Titan's albedo. These changes in albedo are commonly interpreted as changes in the aerosol density, primarily driven by circulation.

Titan's Global-Scale Hazes
The stratospheric haze (left) is concentrated near the north pole while the tropospheric haze (right) us concentrated near the south pole.

Using spectra from all infrared spectroscopy, we've been able to measure just how much of an increase there is in both the stratospheric and tropospheric aerosols near the poles. We have also measured how the aerosol extinction (the total amount of aerosol scattering and absorption) changes with latitude. In the troposphere, there is a hood around the South pole to 60 degrees South latitude where the aerosol extinction is 50% greater than near the equator and nearly constant. There is a decrease in aerosol density from 60 to 40 degrees, marking the edge of the hood. The stratosphere shows the opposite trend. We find that starting from right about the Southern polar hood the stratospheric haze increases linearly towards the north pole where there is 70% more haze than near the south pole.

These analysis of these data provides a new level of constraints of models of Titan's atmospheric circulation, models that have until now only needed to reproduce qualitative trends in aerosol extinction.

The Clouds

Titan is covered with clouds. A "zoo" of them, proclaims a prominent editorial. We don't have names for all the different clouds yet (no `methano-nimbus' or `Titano-stratus'), but mechanisms for cloud formation have been proposed based on how quickly they form and change with altitude. Microphysical models of the clouds? It's true! French researchers describe many of the observable characteristics of Titan's clouds (e.g., where and how often they form, how long they persist) using the same physical models as for cloud formation on Earth. This is particularly interesting because it has been suggested that Titan is a model of was Earth used to be like, and it has been a surprise to everyone to learn just how similar the atmospheric and geological processes on Titan are to those on Earth right now.

Old News: Check out our press release from a while back, or the old page.

  Máté Ádámkovics
Postdoctoral Researcher
Astronomy Department
University of California
Berkeley, CA 94720

(510) 642-6111

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