Cassini Radar To Probe Saturn?, Is it possible to see the in Saturn? Options

[Guest_Richard Trigaux_*]

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.

[Guest_BruceMoomaw_*]

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.

[Rob Pinnegar]

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.

It might be worth doing just to see what happens. There wouldn't be any danger, and it might turn up something unexpected. (Could it give some idea of the size distribution of the ring particles?)

I guess it would have to be done during a close approach to Saturn or the rings, though, so maybe there'd be data storage and transmission considerations.

[Guest_Richard Trigaux_*]

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 Messenger]

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?

If we don't try it, the answer is unknown. Any other answer would be better.

[volcanopele]

The RADAR instrument, in radiometry mode, preformed scans of Saturn's atmosphere, to look for ammonia in the deep atmosphere, in late September.

[Guest_Richard Trigaux_*]

The RADAR instrument, in radiometry mode, preformed scans of Saturn's atmosphere, to look for ammonia in the deep atmosphere, in late September.

So, despites the distance, THIS radar was able to pick signals from Saturn.

Alas, in radiometry, I understand that it was spontaneously emitted signals, not radar echoes in the strict meaning.

But for this the receiver must be very sensitive, I think.

Imagine if we come closer, on a trajectory crossing the ring plane near the Saturn surface?

What I wonder is AT WHAT DEPTH into Saturn this radar can go, and what it will show, when firing at close range. Layers of clouds? Turbulence? Deep convection?

Certainly not to the rocky core, as the gas here are very hot and certainly ionized. Only sound wave could probe the entire Satrun atmosphere. But Cassini is not equiped for this. Unless the radar detects oscillations of cloud layers? This would allow to have some hints at the inner structure.

[tasp]

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.

[Guest_Richard Trigaux_*]

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.

Sorry, I think there is a mistake: radar power falls with the power four, not 16. This is because the emitted signal falls with a power two over distance, and the received echo also falls with a power two (as any electromagnetic wave). So the overall ratio is power 4. I calculated the 80db figure assuming a distance of 2000kms from Titan, and 200000 kms from Saturn, 100 times more, so at a power 4: 100 000 000 times less power. To put in db I took 10dB by decade, as we speak of power, not voltage. I am sure of the power 4 law, it is well known in radar and antenna techniques.

Scanning the radar instrument at Saturn, or even the rings, fascinating though that would be, just isn't real feasible.

Alas yes.

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.

This is what I meaned. The safest way to cross the ring plane is, I think, just near Saturn surface, say 100kms altitude. No way to cross it in the A, B, C... ring , and no more in gaps, which still contain particules. Near the surface is the safest, as particules here quickly decay down. The best place is where some air begins to brake the particules, but will not impede Cassini. We have more choice about the apoaxis, and we may have to do with gravitationnal assistance mechanics. But this is less relevant.

What I speak is what would be feasible with Cassini, especially at the end of the mission. The most common choice would be to left Cassini in a round orbit, to be used as an observatory. But anyway when it will run out of fuel it will be unable to keep its orientation (unless it is equiped with reaction wheels, but I think it is not). So there is anyway an abrupt limit to the life of Cassini. To send it on a more dangerous but more interesting orbit is, I think, a better choice.

Once on a ring-crossing Saturn-grazing orbit, Cassini will be unable to keep a safe path without fuel. So at a moment she will crash on Saturn or on the rings. Anyway such a crash was deliberatelly provoked with Galileo, to keep it safe from biologically contaminate Europa. Will such a concern apply with Cassini? I think the guess is less, but who knows, icy moons may have inner liquid water like Europa. So the idea to send Cassini ending its life on a dangerous orbit may be welcome, see mandatory. And sending it to the Satrun-garzing orbit is a better thing that just send it slamming on Saturn. And the orbit I propose will, in any way, lead Cassini to crash on a place with no biological hazard. (Saturn or rings).

And, anyway, flying over saturn at 100kms, or even 1000kms, will provide an opportunity to radar-probe it, and even to analyse the upper atmosphere.

[remcook]

Last week, I saw an image of Saturn by RADAR. It looked very interesting. Does anyone have a web address for this?

[Guest_Richard Trigaux_*]

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.