[X] My Assistant

[Guest_BruceMoomaw_*]

One additional note: one of the Galileo mission's major discoveries -- thanks to its gravitational-field profiles of the moon's inner density -- is that "dead old Callisto" is actually every bit as geologically interesting as the other Galilean moons, and maybe even more so. The very fact that its internal rock is only partially differentiated into a core separate from its ice mantle has become a major puzzle/clue in understanding how the Galilean moons (and the moons of the other giant planets) formed -- since standard theories had predicted that the heat from the impacts of the inrushing small "satellitesimals" that crashed together to form Callisto should, by themselves, have generated enough heat to allow its rock to completely separate from its temporarily melted ice and settle in its core, as with Ganymede. The distribution of material in the debris disk orbiting early Jupiter must be radically different from what had been thought.

[David]

Well, I certainly don't want to say anything against a mission to Europa, or anywhere else in our still mostly-uncharted Solar System. I might question whether the discovery of life really is or ought to be the only reason for exploring space, but this is probably not the time or place. But I suppose I can say that on a scale of planetographical interest (my reason for being here), where 10=Earth and 1=any random heavily cratered inactive icy moon, Titan has just leapfrogged up to a 9.5 (with Io a 9 and Mars maybe an 8).

And yes, I agree that Cassini should be extracting every last bit of data out of Titan that it can. May Cassini's mission be a long one.

[Mongo]

... since it's coming to be accepted that even if we do find evidence of present or fossil life on Mars, there's an excellent chance that we will never be able to determine whether it appeared there originally or whether it's just the descendants of Earth germs catapulted to Mars via meteorite. Or, perhaps, the opposite is true -- but in either case, we would still be unable to prove that life isn't after all the result of some incredibly unlikely chance event that just happened to occur on one world in this Solar System and then got transplanted to another. (Ths is a very real scientific possibility.)

This is becoming OT for this thread, but I thought that I would register my opinion that, assuming Earth-type life existed on both Earth and Mars, is is FAR more likely that it would have originated on Mars, and then was transported via meteorite to Earth, than the reverse. I think this for two reasons:

1) The mass of (possibly living microbe-carrying) rock transported from Mars to Earth would be several orders of magnitude greater than that from Earth to Mars, mainly due to the smaller gravity well of Mars compared to Earth. It's a double effect--rocks are much more likely to be blasted free of Mars's gravitational infuence (while avoiding being heated to lethal temperatures) than is the case from Earth's considerably larger gravitational influence, and then the meteoroids are more likely to be captured by the larger Earth than by the smaller Mars.

2) Mars apparently would have been inhabitable by some form of life considerably earlier than was the case on Earth--and of course would have soon become more-or-less uninhabitable, at least on its surface, while Earth remained inhabitable ever since.

So it seems to me that the most likely path that 'terrestrial' life took would be the following:

1) Mars becomes stably inhabitable (i.e. no more giant sterilizing impacts).

2) Life emerges on Mars. Large impacts scatter meteoroids containing live Martian bacteria or spores through the inner Solar System, but no other place is inhabitable. Life may have repeatedly established itself on Earth, only to be destroyed by giant impacts each time.

3) Earth becomes stably inhabitable.

4) Almost immediately after 3) happens, Martian life establishes itself on Earth. This fits the geologic record, which shows that life seems to already be established almost instantly (in geologic terms) after conditions on Earth's surface allow this.

5) The surface of Mars gradually becomes uninhabitable. Life may still, however, exist underground, producing the anamolous methane/ammonia readings.

6) A rock is blasted from Mars fairly recently (about 15 million years ago) and lands in Antarctica. It appears to contain evidence of life.

Bill

[ustrax]

"The Huygens Atmosphere Structure Instrument (HASI) provided the temperature of the atmosphere from 1600 kilometres altitude down to the surface. "This has helped put all the other data into context," says Coustenis. Huygens measured the composition profile of the atmosphere to be a mixture of nitrogen, methane and ethane. The methane and ethane provide humidity, as water does in Earth’s atmosphere. At the surface of Titan, Huygens measured the temperature to be 94 ºK (-179 ºC) with a humidity of 45 percent."

Not THAT cold...

2 years since the landing...How time flies!

[nprev]

Hmm...the argon-40 ratio seems to indicate current internal activity. That's kind of an unexpected finding for a body of Titan's gross composition, isn't it? Given that, maybe Enceladus really DOES have an unusual abundance of radionuclides after all...

[ustrax]

Hmm...the argon-40 ratio seems to indicate current internal activity. That's kind of an unexpected finding for a body of Titan's gross composition, isn't it? Given that, maybe Enceladus really DOES have an unusual abundance of radionuclides after all...

That's a good one, but what made me scratch the head was the wind no-velocity...Creepy stilness!
I'll be doing some questions to the mission manager during this weekend.
Do you want me to add yours?

[nprev]

If you think it's appropriate, then by all means please do. Thanks!

[JRehling]

I'm not sure the Argon 40 indicates current activity: It only indicates that there was a lot of post-primordial outgassing.

And radiogenic heat is not the only explanation. Titan has an elliptical orbit and its primary is 95 times the mass of the Earth.

As for the lack of surface wind, Titan receives only 1% the solar input the Earth does, it rotates 16 times more slowly, and much of the solar heating takes place at altitude.

So there's some explanation for all of these things. Not necessarily the right explanations...

[edstrick]

Argon 40 is a radioactive decay product of Potassium 40, a common isotope of a common element with a very long half life.

Argon 36 is primordial, not a decay product. An atmosphere could have primordial argon, which would be an approximately solar mix (whatevern the number is) of 36 and 40, together with outgassed decay product argon 40.

Over time, you HAVE to start out with 100% primordial argon, and then add outgassed argon 40 to whatever is there. While you're adding outgassed decay product, you can also outgass trapped primordial argon with it's original ratio, AND have the atmosphere lose whatever argon is there on a continuing (potentially/probably) varying rate.

Note that from the density of Titan, you can make a good estimate of the ice/rock fraction by mass, and assuming reasonably "solar" composition of the rock, calculate the amount of Potassium 40 in the moon. Given that, and the amount <or lack of amount> of Argon 36 in the atmosphere, you can model away to your heart's content the evolution of argon in the atmosphere as the moon outgasses and as atmosphere is stripped away.

[nprev]

Thanks for the great explanation (as usual! ), Ed. I just found it a bit puzzling that the argon-40 ratio was cited in this fashion in the press release. It almost seemed as if they were implying that the ratio was unexpectedly high, which in turn might indicate that Titan has more potassium-40 then expected with implications for the entire Saturnian satellite system.

I know that argon isotope measurements are often problematic; IIRC, some of the early Soviet Mars data was really jaw-dropping. Hopefully Ustrax will get some more detailed answers as part of his Q&A.

[edstrick]

Regarding Argon and soviet mars probes, as I recall it...

Mars 3 was warming up instrumentation during descent for taking a panorama immediately after landing. Warming up may have been literally true: vaccuum tubes in some circuitry.

Mars 6 was prepping a mass spectrometer during descent for atmosphere composition analysis immediately upon landing. One of the very limited telemetry parameters transmitted during descent was a voltage or current on the mass spec's "ion pump" or something like that, that functions to maintain the vaccum in the analysis chamber. The pump current (i think) was abnormally high and way above expected levels. Since the vehicle was never heard from again after retrofire (followed within a fraction of a second of first impact, like the MER rovers), all they had was this anomalous engineering data to interpret. It could be reproduced by having a very significant amount of Argon in the atmosphere, which would be hard for the ion pump to trap or remove, compared with C02 or N2, and they suggested the atmosphere could be 30% or some largish fraction Argon, in addition to the CO2.

This was of major interest to the Viking GCMS team as their instrument was potentially damagable by ion pump overloading in a high argon atmosphere. As it turned out, both argon and nitrogen are in the few percent range. 1.5 % and 2.5% or so for one and/or the other. <can't remember which is which>

[edstrick]

As I recall from press release stuff last year, Huygens saw little or no Argon36 and no Krypton and Xenon, despite a special concentrator for inert gasses that was to remove nitrogen and other reactive gasses from a sample for analysis during descent.

This means that Titan has *no* remnant of a primordial atmosphere of gasses trapped form the circum-saturn nebula.

This means that Titan either has *no* remnant of a primary outgassed atmosphere (formed immediately after accretion) that contained Argon, Krypton and Xenon that was physically trapped in the rocks and ices that form Titan, or that those gasses were never released from the interior <not likely, see below>.

This means that essentially 100% of the Argon 40 seen in Titan's atmosphere was outgassed progressively over it's history as a decay product of Potassium 40, and essentially zero % of the Argon 40 is primordial.

Thus, since the decay rate of K40 is known, and the total amount of K40 in Titan is approximately known (solar abundance in the rock component of Titan), we know that Titan has outgassed it's accumulated Ar40 with at least enough efficiency to put the observed amount in the atmosphere. This is ***NOT*** a trivial observation. The only things we don't know is (1) when the outgassing happened: efficiently and early, or more inefficently and later (when there was more Argon 40 produced) and (2) whether Titan has lost large amounts of Ar40 from it's atmosphere to space and has thus outgassed more than the observed amount.

[JRehling]

As it turned out, both argon and nitrogen are in the few percent range. 1.5 % and 2.5% or so for one and/or the other. <can't remember which is which>

It's nitrogen then argon.

They're in the same order on Venus, as well as Earth, as well as Titan. Argon's the bridesmaid.

CO2, SO2, H2O, and CH4 may come and go, but N2 and Ar show up everywhere.

[MarsIsImportant]

Just a clarification...The actual numbers for Mars are:

95.32% carbon dioxide
2.7% nitrogen
1.6% argon
0.13% oxygen
0.07% carbon monoxide
0.03% water vapor
trace of neon, krypton, xenon, ozone, methane

Surface pressure 1-9 millibars, depending on altitude;
average 7 mb

Viking measured the % amount of carbon dioxide, nitrogen, argon, oxygen, carbon monoxide, and water vapor. The trace gases were found later.

*****
IMHO
The argon 40 number is significant for Titan.

[edstrick]

"Viking measured the % amount of carbon dioxide, nitrogen, argon, oxygen, carbon monoxide, and water vapor. The trace gases were found later".

Without digging into inaccessible reports, I believe that Viking did measure the Krypton and Xenon isotopes with the GCMS. It was a difficult measurement and the results were a bit noisy and had poorly quantified systematic errors, but they got it. Nothing else has landed a precision Mass-Spec on Mars <and survived> since Viking.

You look on the reports of gasses evolved from SNC Mars Rock Meteorites from 15 or so years ago and they plot the rock-trapped gasses (from shock-glassified feldspar mineral grains) vs Viking measurements and it's almost a perfect straight line over many orders-of-magnitude abundance on a Log-Log graph. It was the damn-near-absolute-proof that SNC meteorites were Martian the people were asking for. Nobody had ANY PLAUSIBLE EXPLANATION for that measurement except "they came from Mars".