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Not really happy about the plutonium contamination- plutonium is the longest lasting toxic substance we know of. I guess that's better than bringing it back here though
, in order to assertain this you need to get in depth with the orders of magnitude involved.How much plutonium we have thrown into Saturn?The Cassini spacecraft had
3 GPHS-RTGs (General-Purpose Heat Source - Radioisotope Thermoelectric Generators
), each containing 7.8 kg of Pu-238 inside. Cassini also has 82 small RHUs (Radioisotope Heater Units
), each containing no more than 10 grams of Pu-238. This means that overall Cassini had 24.22 kg of plutonium inside.
But this is tricky, because the fact that plutonium is used as a radioactive heat source means that it gets transformed in energy and decay residuals. So considering thatPu-238 has a half-life of 87.74 years,
the exponential decay law
says that of the 24.22 kg present at the start of Cassini's voyage only 20.7 kg of plutonium
were still there at the moment of entry in Saturn's atmosphere. The other 3.5 kg of plutonium where converted primarily in Uranium (a natural ocurring element) and also in the heat that powered the mission for two decades.How long the plutonium will last in the Saturnian atmosphere?
By the law of exponential decay those 20.7 kg of plutonium should have decay in 20.68 kg of uranium in no more than 880 yearsHow the decayed uranium would affect the enviroment?
No effect at all. Consider that there are8 ppb (parts per billion) of uranium in meteorites
. Considering that asteroids have usually densities in the range from 2 g/cm3 to 5 g/cm3
this means that an asteroid between 100 and 135 meters in size would have the same amount of Uranium that Cassini left on Saturn.An asteroid of that size is expected to impact Earth between each 5000 and 11000 years
. This means that Saturnian atmosphere has received this Uranium dosis at least 400.000 times
since it was formed. I don't see saturn really affected by this naturally ocurring phenomena.
The uranium in Saturn is in fact a lot more. 1) asteroids impact more usually Saturn than Earth (from where the impact frequencies where considered), this is because Saturn is bigger and easier to get hitted and also has more gravitational atraction than Earth, making it a easier target (so probably there have been millions of asteroids like those I've described dropping uranium at Saturn), 2) the rate of impacts is dependant on time (I calculated it with current rates but those are the lowest in the entire history of the solar system), if we consider all the uranium left by the late heavy bombardment we would be outstanded, 3) Uranium was in the first place on Saturn when it formed (even tons of plutonium has to be there until this day), consider that the internal heat of Earth is partially due to radioactive elements sitll throwing heat, Saturn has to have greater amounts of those just by sheer mass.
How would plutonium dilute in the Saturnian atmosphere?
Let's care about the plutonium now and not the uranium in the future. Those 20.7 kg of plutonium are been mixed with all the gasses in Saturn and diluting in the atmosphere this week. Let's assume that this plutonium dosen't mix with the liquid hydrogen mantle and the core of Saturn but just with the 1000 km gaseus laye
r (something that hasn't to be remotly true at all but just to be optimistic let's take that). I estimated the volume of the Saturnian atmosphere as been 42 septillion liters (4.2x1022
). If you sparse plutonium in that volume you get that the increase in plutonium in the atmoshpere would be just of 50 septillionth of a gramm per cubic centimeter. This ammount is so absurdly tiny that we exceed the realms of homeophathic dilutions. The molar mass of Pu-238 is 238.05 g/mol
so in each cubic centimeter you would find 0.001 plutonium atoms
If you believe the Cassini mission has contaminated Saturn then you must belive in homeopathy in the end.What is the radiation harm to the enviroment?
Well once again with the exponential decay law and considering that the decay heat of Pu-238 is 560 W/kg
, those 20.7 kg of plutonium released by Cassini are going to release 4.6x1013
Joules of energy at Saturn for over a milenia of time. Is that much? Well consider the fact that the energy released by an hurricane here on Earth in just one second is more than 10 times bigger. Saturn has bigger hurricanes than that, that last for years (not one second), and they don't destroy the enviroment at all. Ten times more energy still is the Chelyabinsk meteor impact
(a rock the size of a house), an impact like that occurs at Saturn probably each year (and don't forget we are releasing this energy into saturn not in a mere second like the meteor strike but over 1000 years!!). No harm at all, we could send all the plutonium on Earth and nothing would happened at Saturn.