Images of Saturn’s moon Titan taken by NASA’s Cassini spacecraft less than two months apart have scientists puzzled, as one of the images appear to show a widespread, bright cloud cover in the near-infrared spectrum while said clouds seem to be absent in other wavelengths.
As Space.com reported on Wednesday, Cassini captured the images using its Imaging Science Subsystem (ISS) and its Visual and its Infrared Mapping Spectrometer (VIMS) instruments. The first was taken on June 7 and the second on July 25, and despite the fact that relatively little time passed between the photos, they differ dramatically in the amount of visible cloud cover.
Why are the clouds visible in some images, but not others? Credit: NASA/JPL-Caltech/SSI/Univ. Arizona/Univ. Idaho
In the near-infrared image of Titan, the skies above the cloud appear to be relatively cloud free, the US space agency explained in a statement. However, in photographs taken at longer infrared wavelengths, Cassini showed a large field of luminous clouds that scientists believe should have been at least partially visible in the other picture. So why didn’t they appear in both images?
“The answer to what could be causing the discrepancy,” NASA explained, “appears to lie with Titan’s hazy atmosphere, which is much easier to see through at the longer infrared wavelengths that VIMS is sensitive to (up to 5 microns) than at the shorter, near-infrared wavelength used by ISS to image Titan’s surface and lower atmosphere (0.94 microns).”
NASA scientists continue to search for an explanation
At longer wavelengths, high, thin cirrus clouds are optically thicker than the atmospheric haze, but are optically thinner than said haze at the shorter wavelengths used by the ISS instrument, the agency said. Thus, they could be detected by the VIMS, but not by Cassini’s other imager.
During both the June 7 (T120) and July 25 (T121) flybys of Titan, the probe obtained views of high northern latitudes over periods of more than 24 hours. While surface features can easily be identified in the monochrome ISS observations, the color VIMS observations revealed that there was widespread cloud cover during both of the summer flybys, NASA officials said.
The monochrome ISS image was captured from a distance of about 398,000 miles (640,000 kilometers), according to Space.com, while the VIMS image showing the cloud cover had been taken at a distance of about 28,000 miles (45,000 km). The agency processed the VIMS image in order to enhance the visibility of the clouds, which appear to be nearly while against the pink hue of the atmospheric haze and the green shade of the moon’s surface areas.
“The observations were made over the same time period, so differences in illumination geometry or changes in the clouds themselves are unlikely to be the cause for the apparent discrepancy,” they added. “This phenomenon has not been seen again since July 2016, but Cassini has several more opportunities to observe Titan over the last months of the mission in 2017, and scientists will be watching to see if and how the weather changes.”
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Image credit: NASA
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