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Posted by: Richard Trigaux May 27 2005, 08:32 AM

Hello all


http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1163, obtained by Cassini's ion and neutral mass spectrometer, shows a lithium ray (ordinate= 7)

and also http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1498, a mass spectrogram from the same instrument, also show lithium (abscissa=7)

First of all, I wonder if these rays are real, or if they are intrument defects or artifacts. Otherwise only lithium could have a mass of 7 daltons.

The presence of lithium is not really astonishing by itself (there was sodium in Io's atmosphere) but it implies that Titan's atmosphere would be much more reducing than with hydrogen alone. Also there are no traces of oxygen or water in these spectrograms (oxygen and water are linked through oxydation and photo-dissociation)

In a medium where there is an excess of hydrogen, all the available oxygen is reduced, so that this medium becomes reducing. But lithium is much more reducing than hydrogen, so it would take the oxygen in priority. So the presence of lithium implies that there is a source of lithium in excess over Titan's atmospheric oxygen, and that Titan's atmosphere is much more reducing than with only the presence of hydrogen and hydrocarbons...

Where could this lithium come from? It looks as if Titan's hydrogen would be formed directly from an interstellar cloud, and thus would contain some lithium. But if so, it would also contain helium... which is nearby absent! Right on the countrary the mainstream hypothesis is that this hydrogen comes from the photo-dissociation of methane. Maybe there would be some volcanism, releasing together methane and lithium. But this is incompatible with the presence of an ice surface or underground ocean, as lithium and water react violently.

So, if these rays are not instrument artifacts, they are a complete mystery. As far as I know, there was no comment on this.

Anybody have some idea?

Richard

Posted by: Marcel May 27 2005, 09:40 AM

[But this is incompatible with the presence of an ice surface or underground ocean, as lithium and water react violently.


Richard

[/quote]
Is that also true in extremely low temperatures and high pressures ?

Posted by: Richard Trigaux May 27 2005, 10:00 AM

[quote=Marcel,May 27 2005, 09:40 AM]
[But this is incompatible with the presence of an ice surface or underground ocean, as lithium and water react violently.

Richard

[/quote]
Is that also true in extremely low temperatures and high pressures ?

[/quote]


I don't know. As far as I know, at Earth temperature, lithium and water react. At minus 150°C, they may not. But if the lithium has an underground origin, it can come to the surface only through ice volcanoes, understand into liquid water, which temperature would be not much below zero°C. So, even if metallic lithium could survive on the surface, it seems impossible that it could exist into an underground ocean, or transit through ice volcanoes.

But this lithium could accumulate in the upper atmosphere from extraplanetary origin. But why and how?

Posted by: volcanopele May 27 2005, 10:27 AM

Be very careful interpreting INMS data below 10 daltons. Much of the spectrum below that point is dominated by instrument noise (as you can see in the first graph). The numbers in the second graph are too low to really be meaningful, again, probably just noise.

Posted by: Richard Trigaux May 27 2005, 10:57 AM

Yes volcanopele, you may be true, and in this case there would be no lithium, or at least no evidence of it. But I would prefer to hear such a statement from persons knowing the instrument, manufacturer or owner. However the first graph also shows a clear hydrogen ray at 2 daltons, which, as far as I know, was considered meaningful in further comments.

The only thing I see is that a ray of similar magnitude, marked "diacetylene", is spread between 50 and 55 daltons, while a ray marked "background" is not. (this "background" ray being very likely an instrument defect or calibration system). Like the "background" ray, and unlike the "diacetylene" ray, the "lithium" ray is not spread, leading to think that it may be too an instrument defect. But its intensity over time follows the general law, while the "background" ray is constant...

So I think it is difficult to guess the instrument limitations from the figures. Our only hope here is that somebody knowing it could reply to this thread. If anybody have some info, please...

Posted by: volcanopele May 27 2005, 04:02 PM

The instrument issue I mentioned is my recollection of a conversation I had with the INMS PI a few months ago. I can ask Roger again about it but I think he is in Crete for the Titan meeting (why can't I go to the cool places )

Posted by: Richard Trigaux May 27 2005, 04:33 PM

Fine if you ask the question to somebody of the INMS, we could know if the "lithium" ray is for real or just an instrument artifact, if there is a real matter or not.



Me too! I would like to be on Titan observing. But, in space exploration, for one who steps on the Moon declaiming historical speech, there must be 100 000 just doing paperwork of dirtying their hands in slush. Better than nothing!

Posted by: Gsnorgathon May 28 2005, 02:56 AM

A water-ammonia eutectic mixture can be liquid as low as about -100°C. I don't know if that counts as "much below zero" or not. :@)

But I'm inclined to take Jason's word that the PI says it's noise.

Posted by: TheChemist May 28 2005, 03:40 PM

I don't believe this ! There is a Titan meeting right here in Crete, exactly on the dates I will be away for the EMRS meeting in France.

The universe is against me

Posted by: Richard Trigaux May 29 2005, 11:28 AM

Two possible explanations for this ray of lithium:

1) if the INMS instrument uses the velocity of particles, there can be another part of the spaceship nearby where some particles may bounce off and reach the INMS with a reduced velocity, giving it to "see" a lithium atom where there would be a more common atom such as carbon. Such bouncing was observed around the space shuttle as a tenuous orange light surrounding the shutle on a distance of some tens of centimetres.

2)On Earth, http://www.sahra.arizona.edu/programs/isotopes/beryllium.html by cosmic protons produces Beryllium 7, which http://environmentalchemistry.com/yogi/periodic/Be-pg2.html of an unusual manner (electron absorbsion) into lithium 7. On Earth the radioactivity of this beryllium 7 is measurable, but of course atmospheric beryllium and lithium are quickly oxydized and washed to the ground by rain.

On Titan there are cosmic rays and nitrogen too, even if there is no oxygen. So we can expect than berrylium 7 and lithium 7 are produced. This lithium would not be oxydized, so that it could accumulate in the higher atmosphere. But this is possible only if methane rains do not wash it to the ground. Also lithium is reactive toward hydrogen and hydrocarbons.

Posted by: alan May 29 2005, 04:31 PM

From the discription of the chart Richard mentions at the start of this thread a Dalton is an "atomic mass unit per elementary charge" I don't know how this mass spectrometer works but wouldn't doubly ionized atomic nitrogen have 7 (14/2) atomic mass units per elementary charge?

Posted by: Richard Trigaux May 30 2005, 05:23 AM

Well found, it would have be a good example of a basic ignorant mistake leading to much fuss for nothing!

But googling "Dalton mass" leads to many pages where Dalton is defined by only the mass of atoms, hydrogen being (in gross) 1 dalton. Especially look at http://en.wikipedia.org/wiki/Amu , http://www.answers.com/topic/atomic-mass-unit, http://www.physicsdaily.com/physics/Dalton. The electrical charge does not matter.

Anyway the instrument is named "ion neutral mass spectrometer" implying that it can detect ions and neutral atoms as well, and probably without distinguishing them. For instance if it detects shocks on a plate, the electrical charge does not matter. But I too do not know how it actually works.


I agree that the sentence in the caption of the second chart "The mass range covered goes from hydrogen at 1 atomic mass unit per elementary charge (Dalton) to 99 Daltons." is ambiguous: what charge are they talking about? For common people a charge is an electric charge, but for physicist it may be any kind of subatomic property, including mass. I think they meaned "1 atomic mass unit per detected particle". Anyway if there was only this second chart, the lithium ray is not very clear and it could be accounted as noise. Only on the fist chart the lithium ray is clearly visible, and must be explained by a distinct phenomenon, eventually artifact, but other than the general background noise.

Posted by: deglr6328 May 30 2005, 07:47 AM

I suspect Alan is absolutely correct. Don't make too much of the definition of a Dalton, its just another word for atomic mass unit. They are very nearly identical. There is no special meaning of "elementary particle charge" either, a single proton has an elementary charge of +1. The ion and neutral mass spectrometer is (to GROSSLY oversimplify) basically a fancy and I think directional (though maybe that's just http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-mimi.cfm) http://huygensgcms.gsfc.nasa.gov/MS_Analyzer_1.htm. It works by taking charged atoms (ions) and subjecting them to a varying electric field then "watching" how they're deflected. The more massive an atom is the less it will be deflected by a given electric field. However this assumes the atoms all have identical charge (eg. a loss of 1 electron so the ions are all charged +1) if an atom is doubly ionized to a +2 charge for instance, then it will experience twice the pull toward (or 2x the push away) from the electric field its in, thus giving the "illusion" that the atom weighs half as much as it actually does. (I'm pretty sure this is all right )

Posted by: Richard Trigaux May 30 2005, 08:22 AM

Thank you deglr6328 for your precisions.

Following your post I found some stetchy explanations of the INMR working on the http://saturn.jpl.nasa.gov/spacecraft/inst-cassini-inms-details.cfm#open. (Please note that this instrument is NOT the MIMI or Huygen's mass spectrometer).

This page confirms that the INMS works by first ionising the collected particles into positive ions (two methods can be selected). These ions are then deflected and detected by their electrical influence into a set of four electrodes. There are still many details left, but with such a system we can really expect that double ionisation may occur, so that we could find double ionized nitrogen atoms mistakenly detected as a mass of 7 in place of 14, as Alan indicated. But this depends of the overal geometry, this geometry could be set so that double ionization may have a different effect.


This could be most satisfying explanation of the lithium ray.

So we just have to wait for confirmation of Volcanopele's friend Roger (or other people well knowing the instrument).

What astonished me in this affair was that nobody commented this lithium ray. But if they reasonably knew it was just an artifact, there was no need to comment it.

Pity, my hypothesis of radiogenic lithium was nice, I think.