The Cassini radar performed fine on Titan, giving nice detailed ground images through Titan atmosphere and ionosphere.
But what if we aim this radar at Saturn itself?
Of course from greater distance the images would not be so detailed. And perhaps the echoes would be too weak, or too delayed, to be picked by the Cassini radar receiver. Perhaps too the Saturn ionosphere is too opaque.
But if it works?
Saturn being essentially gas, it could be transparent to radar waves at a great depth, perhaps hundreds of kilometres, until the gas is hot enough to be ionized.
So, in the extend a radar test would work, we could see cloud layers, or places with turbulence.
With my opinion, the most interesting thing to look at would be all the numerous vortexes in Saturn atmosphere. Which of them are only local storms, due to solar heating of subsurface layers, and which of them have deep roots into the core of the planet.
Explanation: convection models of giant planets predict the existence of cylindrical vortexes, with axis parallel to the rotation axis of the planet, but with an offset (as the vortexes cannot go through the solid core) so that the wortexes axis reach the surface of the planet at mid latitude. On Jupiter, there are two such mid-latitude bands exhibiting dark vortexes in one hemisphere, and white vortexes on the other. But on Saturn, we see nothing such, only one vortex much larger than the others, the main produced of lightning activity.
So if we could see the deep root of a Saturn storm, we should see the inner convection of the planet, due to core heat production. If we see no root, we know it is caused by solar heating.
At least if we could see only the ionosphere it would even be worth the try.
Anyway it would be interesting to aim the radar at icy moons, at time of close encounter. The ice may be somewhat transparent to radar waves, so that we could detect liquid circulation in ice (Enceladus), faults, ocean, core, etc. With several shots of the same features at some minutes interval, we could even have 3D models of inner features of ice crusts.
I do not know what it would be possible to see, but perhaps more than expected.
Unfortuantely, at the very short wavelength of Cassini's radar (2.3 cm, if I remember correctly) there is almost no ice penetration. Originally the plan was to have it also operate at about 13 cm, but that had to be cancelled -- and I don't believe that would penetrate significantly through ice either.
As for radar scatterometry of Saturn's atmosphere, I haven't heard anything to suggest that this is possible at Cassini's distance from Saturn -- nor have I even heard nything about possible scaterometry studies of the rings.
Yes this is not possible as it. The Saturn radar signal would be 80db under that of Titan
But if we go MUCH CLOSER FROM SATURN???
Consider this: when the ship is getting out of fuel, and eventually need to be sacrificed like Galileo, use a last gravitationnal manoeuver wih a moon, and send Cassini on a very elliptic orbit, grazing the ring plane all its width through, with its periapsis at 50kms above Saturn clouds!! This orbits intercepts the ring plane at its periapsis, just above Saturn surface, where the particule density is the lowest. This trajectory could last for several revolutions without stabilisation, but sooner than later Cassini will burn into the atmosphere or slam into the rings.
So the result would be...
-sampling Saturn high atmosphere with the instruments designed for Titan
-imaging and radar probing Saturn clouds at 50kms!
-imaging and radar probing the rings at 100, no 10kms!!!
Even at 1000kms of the ring we would get a paralax, and then a direct measurement of their thickness.
The RADAR instrument, in radiometry mode, preformed scans of Saturn's atmosphere, to look for ammonia in the deep atmosphere, in late September.
Sadly, the Cassini radar instrument (in an active mode) is strictly limited in erp (effective radiated power). When it is aimed at an object, the outbound signal is subject to an inverse square law. Double the distance, and the power falls off by a factor of four. Additionally, the echo of that signal on its way back to Cassini is also subject to the inverse sqaure law. So for a round trip, double the distance, the power falls off by a factor of 16 (assuming no absorption by intervening atmosphere or the surface being scanned).
This how the above referenced figure of -80dB is realized.
There are 'tricks' to booost the range. (I don't know if Cassini uses any of these) The signal can be pulsed instead of continuous. The signal can change frequency during transmission and a filter can follow the expected freq. change in the returning signal. The dish can be made larger. A given area can be scanned more than once, or scanned at different frequencies. The radar can fix its gaze at a given spot while the transmitter moves, and achieve a long baseline for analysis of the signal.
(sorry for the quick descriptions, I have glossed over heaps of stuff due to time constraints)
Scanning the radar instrument at Saturn, or even the rings, fascinating though that would be, just isn't real feasible.
Now, put a radar on Prometheus, wow!
Note: A close approach to the rings seems to imply a ring plane crossing close to the rings at a very shallow angle. That leaves you in the ring plane maybe a little too long for comfort. Save for the end of the mission as suggested above. A close approach to the rings also means short exposures to compensate for spacecraft motion during the observations, and the closer you get, the worse that problem gets. "Spinning down" Cassini's orbital path around Saturn would help. Apogee at Mimas distance instead of Iapetus helps enormously.
Last week, I saw an image of Saturn by RADAR. It looked very interesting. Does anyone have a web address for this?
This discution about the biological hazard posed by Cassini into the Saturn system is now enlightened witht he discovery of liquid water on Enceladus. So it is all the more important to park Cassini on a safe orbit or place, when she wil have no more fuel. From here a greatest interest of the trajectory I propose, which can end only on Saturn or on the rings.
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