Rev 61 Enceladus (March 12 2008) Options

[Holder of the Tw...]

The mission description PDF is now online.

Enceladus 3 Flyby

[edstrick]

"... Is the new highest temperature the average over the sensor footprint,... "

A surface that's an efficient emitter (near blackbody, like ice) that is all one temperature, puts out a blackbody spectrum. When you convert infrared brightnesses to estimated temperature, the estimates are the same at all wavelengths.

When you have an unresolved checkerboard mosaic surface with two temperatures, one cold and one warm, you get an average of two blackbody spectra and the result is NOT a blackbody spectrum of the average temperature. The warm surface emits more at shorter wavelengths and the cold surface emits more at longer wavelengths. You can model a checkerboard pattern to get the temperatures of the two components AND the proportions of the components. But there's slop in the modeling. Youi usually have more than two temperatures and varying fractions at the surface with temperature and there's just not enough data in the spectrum to tell you what's really going on.

At Enceladus, you probably have regional "cold" temperatures, possibly a generally "slightly warmed" surface between tiger stripes, "warm" surfaces matching the blue areas (in color pics) of the stripes, "hot" areas that are very close to the vents and "very hot" areas within the vents themselves.

If you do 2 component modeling with the better resolution of the new data, you can get the inter-stripe temperatures and in those regions model the mix of cold and slightly warmed surface, then over at the tiger stripes, model "slightly warmed" beside the stripes and "warm+hot" temperatures and coverage fractions on the stripes. If "hot" is different enough from "warm" and is abundant enought in the hottest appearing areas, you may be able to get a better overall fit to the data with 3 components and be able to report a "hot" temperature and estimated surface fraction.

Right now, they're probably NOT reporting that. In 2 or multi-component circumstances, just looking at the raw data, short wavelengths give warmer readings than long wavelengths. They're probably simply reporting the pixel-average temperature in the hottest pixel measured at the shortest wavelength where they have useful signal-to-noise. We can assume if that's correct, the 180 K reading is a lower limit to the real temperature of the hottest areas in the field of view.

[ugordan]

We can assume if that's correct, the 180 K reading is a lower limit to the real temperature of the hottest areas in the field of view.

... which is exactly the kind of thing I wanted to know.

On a related note, John Spencer's blog entry says the stripes were detectable even in the 7 micron wavelength range. That's pretty close to the 5 micron max VIMS can observe. I realize that's stretching it a bit too far, but would it be even remotely possible for VIMS to see the stripe thermal glow (as it can in case of Jupiter and Saturn, for example)?

I wonder if the VIMS S/N ratio is too low for this combination of instrument sensitivity and the wavelength a tad too short (likewise, temperature too low), or would it be feasible? I don't suppose VIMS was active during the outbound leg of the flyby, during the eclipse?