Science results from MESSENGER's first flyby, Science publication July 4 Options

[ChrisC]

Audio of the teleconference will be streamed live at: http://www.nasa.gov/newsaudio

Howdy all -

I've been lurking for a year or two, and have been a fan of unmanned spaceflight in general for decades. First post!

I was unable to get the audio stream above to work during the news conference on Thursday. Does anyone know where I might find a recording of it, like they've been doing for the Phoenix missions?

[jmjawors]

The audio from the Phoenix pressers has been archived by JPL. APL doesn't appear, at least at first glance, to have done the same. Unfortunately we're at the mercy of individual NASA centers and their online prowess rather than a unified agency policy regarding these audio archives.

Of course, I may also not be looking in the right places.

[elakdawalla]

At the end of the briefing they listed a toll-free number to call for a replay for up to a week, but I didn't write it down -- anybody else catch it?

--Emily

[nprev]

Just went hunting for it; no joy. You might have to contact their media coordinator on Monday, Emily. (I'll give it a shot as well because I'm working nights this week, but I ain't got your street cred! )

EDIT: Mike, Emily's article here goes into a lot of detail about the water-group interpretation for the 16-18 amu FIPS findings.

[Juramike]

EDIT: Mike, Emily's article here goes into a lot of detail about the water-group interpretation for the 16-18 amu FIPS findings.

Thanks! I'm still scratching my head trying to understand the graph.

The mass spec data gives out an m/z (mass/charge) ratio as nicely described in Emily's post.

[I always think of mass spec experiment as a "ping and fling" You need to electronically ionize a molecular/atomic species, then accelerate it with a magnetic field, then try to wrap it around an electric field and see where it splats. A heavier chunk won't be deflected as much. Sorta like two vehicles trying to make it around too tight a bend at the same speed: A Porcshe will almost make it around the bend before it hits the guardrail, while an 18-wheeler fully loaded with a ton of bricks will plow right into the guardrail without being able to make much of a turn: looking at the two splat marks on the guardrail you can figure that that the splat with the least deviation around the curve was the heavier one.]

This particular MESSENGER instrument is looking at the already-ionized species flying around in space. (So neutral radicals won't get accelerated or detected.)

The graph shown in the post seems to have peaks at 16 and 17 and 18. So, as mentioned in the post, this could be due to charged water (H2O radical cation?, 18 amu), hydroxyl radical (-OH, 17 amu) and singly charged oxygen (O+, 16 amu) [it could also be doubly charged oxygen molecule O2++ 32/2 = 16].

That's all cool. But where I'm confused is that there appears to be a peak at roughly the same intensity at 15 amu. So this implies either a charged radical at 15, doubly charged at 30, or triply charged at 45.
(As you go up in weight there are more possibilities for combinations of elements possible to give that mass)

[As an aside, when a mass spec instrument has to ionize species itself, there is a possibility of getting transient associations flying around and giving a signal. LC-MS combinations are notorious for these effects. Many larger organic molecules after travelling through an LC-MS will generate signals composed of parent molecule + sodium [M+23], even if sodium wasn't originally present in the sample (it only takes a trace). Also when you ionize large organic molecules (like in a GC-MS, they can frag up. M-CH3 (M-15) is a common observed fragment if the parent has an ionization-labile methyl group. Most organic laboratory MS systems don't detect below 44 amu to eliminate carrier gas and atmospheric signals.]

Noway would I think that a 15 amu signal detected by MESSENGER could came from breakup of a complex organic. BUT I'm curious to know if it could be from comet-delivered ionized methane.

Looks like I'll have to download the Science paper to gets me some learnin'.

-Mike

[nprev]

I actually wouldn't be surprised at all if there are some organic subspecies represented in that region. For one thing, it's VERY crowded in the graph, with no outstanding peaks. Secondly, if the source is indeed the cometary impact residue at the North Pole, then presumably there is an abundance of organics in the material.

There are some tantalizing concepts evident from this data; I can hardly wait until Messenger achieves orbit! One other remote possibility is that there just might be active vents on Mercury due to the continuing crustal compression...nothing big, but enough to occasionally belch out some anomalous compounds that subsequently decompose in the exosphere. We should look for spikes in abundances.

[ChrisC]

Reminder ... Can one of you find out what that number is for the teleconference replay?

[Juramike]

I read the Science article carefully and I think I've cleared up my misunderstanding.

From the released plot (available at: http://messenger.jhuapl.edu/news_room/imag...n/Figure5_5.jpg):



If I understand correctly, the instrument is detecting a range of m/q (m = mass; q = charge) numbers depending on the energy of the inbound ion. The "wiggle room" for the m/q number should be a gaussian distribution. The Gaussian distributions for key m/q ratios are shown as the thinner dotted line in the graph above. It is the combination of all these Gaussian distributions that will make up the observed lines (thick heavy blue line in the figure above.

Big peaks are most likely "real", these are points that most likely correspond to a distinct species. So the peak at 16 is most likely due to O+ (16 amu). [Oxygen was detected by Mariner 10; so we know it's in there]. The peak at 18 is most likely due to water radical cation H2O+ (18 amu).

Where Gaussian curves overlap or combine, you can get a smaller peak. So the data point at m/q=17 could be due to the combination of the Gaussian centered at m/q=16 and the Gaussian centered at m/q=18. And there could also be a component of hydroxyl cation (OH+; 17 amu) which makes it's own cute little Gaussian centered at 17. [Note that this is 2 electrons away from "normal" hydroxyl anion from ionic dissociation of water -OH-->OH(+) + 2e(-); it's not a happy cation]. The combination of the 16 Gaussian + the 17 Gaussian + hydroxyl cation (OH+) + doubly charged H2S++ (m/q = 34/2 = 17) could give rise to the counts we see in the m/q = 17 bin.

The data point at m/q=19 could be due to the 'heavyside' Gaussian tail of the m/q=18 peak + a small chunk of the really big peak (and Gaussian tail) for sodium ion (Na+ at 23 amu) + some hydronium ion H3O+ (19 amu) [Not that hydronium ion is "naturally" ionized in the form of a cation.]

The data point at m/z=15 is much lower. This signal could be due to the Gaussian from m/z = 16 (O+) and other "stuff" (N+=14 amu Gaussian tail).

The upshot is that the "peaks" in the graph above are real signals and can be used to infer ionic and atomic species; datapoints outside the peaks are confounded by Gaussian tails, etc.

So to answer my own question: Is there an evident signal for methane CH4+ (m/z=16) or ammonia NH3+? (m/z=17)? Nope, there isn't.

-Mike

[TheChemist]

Mike,
Are there any clues in the article about the structure of the multiply charged ions at 6, 8 and maybe 10 ?
There is only limited space for ionic charges in diatomic and triatomic species, and for (assumption) doubly charged ions we are looking for MWs of 12, 16 and 20. Nice and even numbers.
I am curious.
Are the collision energetics enough to provide double ionization of atoms, like O(2+) for 8 ?

[Juramike]

Are the collision energetics enough to provide double ionization of atoms, like O(2+) for 8 ?

From the article:
"Plasma sheet electrons are most likely to be the ionization source for the creation of multiply charged ions (such as O++) observed by FIPS from corresponding singly ionized atoms (such as O+). MESSENGER does not directly measure thermal and suprathermal electrons. However, our detection of multiply charged ions provides indirect evidence for the presence of a hot electron component near the planet and supports the indirect detection of [ca.] 1- to 10-keV electrons in the region by MESSENGER's x-ray spectrometer."

There is a pretty cool graphic in the article that shows how the measured energies and relative abundances by weight zone changed along MESSENGER's flyby track. The leading edge had the highest measured H+ ion energies (planetary motion hitting solar wind). Interestingly, this area also corresponded to the maximum of "chunks" in the m/q = 3 - 10 range. So higher electron energies also correlates to area with most multiple charged ions.

The closest approach to the Mercury had the highest weight particles (evidence that sodium and magenesium were coming from the surface and not the solar wind).

Aside from O++ specifically mentioned above(and C++ mentioned later in the article), the authors used the generic term "multiply charged ions" as a catch-all for stuff in the m/q range of 4.67-11. Outside this range, they do specify Fe++ (28), Na++ and Mg++ (11-12) and Si++ (14) as possible candidates in Table 1 of the article.

-Mike

[TheChemist]

Many many thanks Mike !

(That Na++ would freak most freshman general chemistry students )

[Juramike]

one atom to the other: "OMG, I just lost an electron!!"

other atom: "Are you sure?"

First atom: "Yes, I'm positive!"

[nprev]

Oh, dear Lord...a chemist channeling Henny Youngman!!! (Good one! )