Remembering Cassini - Four Windows on Saturn
April 3, 2024
Dear Friends,
On this day, 20 years ago and a billion miles from here, our Cassini cameras acquired a series of images that nicely demonstrates what knowledge can be gained by viewing only a narrow slice of the electromagnetic spectrum.
Each of the Cassini cameras can be simply described as a telescope that focuses the light it gathers and passes it through spectral filters and finally onto a CCD detector sitting behind the optics and in the camera’s focal plane. Alongside and behind the CCD — similar to the one in your cell phone — are all the electronics that control the operation of the CCD and the 2 filter wheels in front of it, the pre-processing of the data coming from the CCD, data compression, internal system temperature, synchronization with the main spacecraft clock, etc.
The human eye can typically detect light with wavelengths between 380 and 700 nanometers (nm), where a nanometer is a billionth of a meter. These wavelengths form the ‘visible’ part of the spectrum. The spectral range of the cameras aboard the Voyager spacecraft, 300nm to 640nm, was close to this.
But there were good scientific reasons to push the limits of vision of the Cassini cameras into the ultraviolet at short wavelengths and into the near-infrared at long wavelengths. Determining the composition of Saturn’s icy moons, for example, and how their compositions relate to the geology and modification processes acting on their surfaces would come down to determining their mass densities, by measuring how much each moon’s gravity altered the spacecraft’s trajectory during close flybys, and establishing the composition of their surfaces through global and high resolution imaging at different wavelengths. Strong diagnostic compositional signatures exist at both ultraviolet and infrared regions of the spectrum, and together these aid in discrimination of surface materials of special interest, such as carbonaceous materials, on the moons and within Saturn’s rings.
For the atmospheres of Saturn and Saturn’s largest moon Titan, imaging in the ultraviolet would provide the best visibility of aerosols high in the atmosphere and of aurorae in the polar regions if there were any. And red/near-infrared imaging would allow searches for lightning, access to spectral bands where methane, a constituent of the atmospheres of both bodies, strongly absorbs light, as well as access to the clear windows in between the methane bands on Titan through which we hoped to see down to the surface for the first time. In combination with what we would see in the visible, the new capabilities would allow us to sound the atmosphere: ie, see down to different levels by looking in different wavelengths. By sounding, we could work out the cloud and aerosol vertical structure and energy balance in the atmosphere, discover previously unseen atmospheric phenomena, map the three-dimensional general atmospheric circulation, including indirect inferences about conditions below the visible cloud tops, and determine the spatial, spectral, and time-varying properties of Saturn’s auroral and lightning emissions, the latter being especially important as tracers of atmospheric storms.
Of course, the visible wavelengths in between the extreme end members could be used to image in the different colors that our eyes can see.
Consequently, the Cassini cameras were designed, and filters were chosen, to reach all the way up to the near-infrared wavelength of 1000nm. And by using a special UV-sensitive coating on the CCD detector, we pushed the response of the narrow angle camera (NAC) at short wavelengths down to 200nm. (The wide angle camera (WAC), a refracting telescope built around a lens and not a mirror, was UV-blind and could only go as low as the violet wavelengths at 380nm.)
In the end, the imaging system on Cassini was tremendously more capable than that on Voyager and, with its new ultraviolet and near-IR eyesight, would reveal to us vistas that had never been seen before. We were stoked.
The images we took 20 years ago today on approach to Saturn proved there was more to the ringed planet than meets the eye. These four images were acquired over a period of 20 minutes when the spacecraft was 27.7 million miles from the planet. All four show the same face of Saturn with an image scale of 166 miles per pixel.
Caption: Four Cassini images taken on approach to Saturn, April 3, 2004. They were imaged at … in upper left, ultraviolet wavelengths centered at 298nm; in upper right, blue wavelengths centered at 440nm; in lower left, far red wavelengths centered at 727nm; and in lower right, near-infrared wavelengths centered 930nm.
What is revealed are the effects of absorption and scattering of light at different wavelengths by both atmospheric gas and clouds of differing altitudes and thicknesses. They also show absorption of light by colored particles mixed with white ammonia clouds in the planet's atmosphere. The contrast in the images has been enhanced to aid visibility of the atmospheric features.
In the upper left image, Saturn is seen in ultraviolet wavelengths centered at 298nm; at upper right, in visible blue wavelengths at 440nm; at lower left, in far red wavelengths just beyond the visible at 727nm; and at lower right, in near-infrared wavelengths at 930nm.
All gases scatter sunlight efficiently at short wavelengths. That's why the sky on Earth is blue. This effect is even more pronounced in the ultraviolet than in the visible. On Saturn, molecular hydrogen and helium gases, the main atmospheric constituents, scatter ultraviolet light strongly, making the atmosphere appear bright in the upper left image. In this image, only high altitude cloud particles, which tend to absorb ultraviolet light, appear dark against the bright background. This explains the dark equatorial band in the upper left image.
The contrast is reversed in the lower left image taken in a methane-absorption band at 727nm where light is absorbed by methane gas but scattered by higher clouds. The equatorial zone in this image is bright because the high clouds there reflect this long wavelength light back to space before much of it can be absorbed by methane.
Scattering by atmospheric gases is less pronounced at visible blue wavelengths than it is in the ultraviolet. Hence, in the top right blue image at 440nm, the sunlight can make its way down to deeper cloud layers and back to the observer before it get scattered, so that both deep clouds and the high equatorial cloud particles, which are reflective at visible wavelengths, are apparent. This view is closest to what the human eye would see.
At bottom right, in the near-infrared region at 930nm, some methane absorption is present but to a much lesser degree than at 727 nanometers. At this point in the mission, we were not certain whether the contrasts here are produced mainly by colored particles or by latitude differences in altitude and cloud thickness.
The sliver of light seen in the northern hemisphere appears bright in the ultraviolet and blue wavelengths (upper images) and is nearly invisible at longer wavelengths (bottom images). The clouds in this part of the northern hemisphere are deep, and sunlight is illuminating only the cloud-free upper atmosphere. The shorter wavelengths are consequently scattered by the gas and make the illuminated atmosphere bright at these wavelengths, while the longer wavelengths (lower images) are absorbed by methane, the light never is returned to the observer, and that region of the atmosphere is dark.
[Saturn's rings also appear noticeably different from image to image, whose exposure times range from 2 to 46 seconds. The rings appear dark in the 46-second ultraviolet image because they inherently reflect little light at these wavelengths. The differences at other wavelengths are mostly due to the differences in exposure times.]
Throughout the rest of the mission, this technique of sounding was used in concert with information from Cassini’s other instruments to yield an unprecedented view of the 3-dimensional structure and motion of the Saturn atmosphere. At this point in the mission, 20 years ago, we were just getting started.
— Carolyn Porco
Every time I see an image of Saturn it makes me feel that it is calling to us asking us when we are going to see it with our own eyes. Hopefully I will live to see the first crewed mission and if we’re really lucky we will see for real what filmmaker Erik Wernquist imagined in his new short film One Revolution Per Minute; got me really fired up! 🪐
https://youtu.be/iiPmgW21rwY?si=uIjfl1GCZni6hgYa
Excellent post! As a wildlife biologist, I think all the time about the way other life forms experience the Earth, be it through seeing different wavelengths of light, hearing different wavelengths of sound, or smelling/feeling things that are totally undetectable by humans. Designing observatories and cameras to see what we cannot is certainly not too nerdy and well worth knowing. I look at it as widening the horizons in the way we humans can experience the universe (or multiverse). The decisions about how the JWST was designed to work are a great example.