MESSENGER News Thread, news, updates and discussion Options

[Explorer1]

So little 'herma incognita' left... (pardon my Latin):

http://messenger.jhuapl.edu/gallery/scienc..._ortho_90_0.png
If that big crater is any indication, there should be serious diminishing returns when it comes to the permanently shadowed areas....

[tanjent]

Not sure I understand the point about the diminishing returns. Where would we expect to find the permanently shadowed areas if not in the permanent shadows? One question I have looking at that map is whether the hole in the middle is bounded on all sides by the limits of photographic coverage, or whether it is partially bounded by topographic shadows. It's a funny shape with partly straight boundaries, partly curved boundaries, and some irregular boundaries.

[Phil Stooke]

Photo coverage limits almost exclusively.

One thing we've learned from the Moon is that the hydrogen is not located just in true 100% permanent shadow. Occasionally illuminated areas also show some hydrogen and some shadows have none. An LPSC presentation a year or two ago suggested that a small amount of heating was useful to help the ice diffuse downwards into the regolith. And radar shows us very clearly where the ice is on Mercury. It's odd that ground-based radar is less useful for revealing ice on the Moon.

Phil

[Explorer1]

Well what I meant was, since Mercury has practically no axial tilt, and thus no seasons, there are long shadows covering almost everything (compared with the Moon and its tilted axis and orbit, where only the deepest craters are in permanent darkness). So filling in these last sections is pretty slow, even over many Hermean years.

[Phil Stooke]

I see what you mean. Don't worry too much about that, there's lots of redundant coverage at the poles as every orbit crosses them and polar illumination is a big topic of interest. Also, light reflected off nearby peaks and crater walls will faintly illuminate some areas.

Phil Stooke

[scalbers]

Here are some videos of AGU conference presentations on MESSENGER.

First an overview talk by Sean Solomon: http://vimeo.com/33387373

Next is an entire session that includes a very well delivered presentation on volcanism by Jim Head that I saw in person (last talk): http://vimeo.com/33391425

Final Mercury session here (including some magnetosphere discussion): http://vimeo.com/33391931

[Phil Stooke]

Great! the Jim Head talk and many other excellent talks are in this presentation:

http://vimeo.com/33391425

(Head is right at the end)

See especially the talk by Blewett on hollows, the altimetry papers ... in fact they are all good.

Phil

[Phil Stooke]

Some great new pics in the MESSENGER picture of the day set recently! The volcanic vent on the SW edge of Caloris, hollows in a crater floor, and Pantheon Fossae in Caloris.

Phil


http://messenger.jhuapl.edu/

[Fran Ontanaya]

Those hollows are awesome. They are even on the central peak, not just around it.

[hendric]

They are so reminiscent of the cometary surfaces we've scene that I often wonder if they are caused by sublimation of light elements.

[Fran Ontanaya]

Or the martian Swiss cheese terrain. Some of the small hollows away from the peak could be from asteroids punching through a protective crust.

[Phil Stooke]

No - asteroids make craters. Hollows are very different. An internal process is more likely - volcanic or loss of volatiles, though the details are not understood yet. And there are a few on the Moon as well. In fact I think the lunar ones, which we will be able to look at close up or even sample one day, will be the key to understanding them.

Phil

[nprev]

Hmm. You made me think, Phil.

Mercury is ostensibly the most 'lunar' of the inner planets in many ways (mostly in appearance), but compositionally & environmentally it has a great many dissimilarities to the Moon. It will be interesting to compare the similarities & differences between hypotheses for the origin of many landforms between the two worlds.

[Paolo]

Or the martian Swiss cheese terrain. Some of the small hollows away from the peak could be from asteroids punching through a protective crust.

Mercurian hollows are compared to Swiss cheese terrain in the very Science paper Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity. The authors note that

The resemblance suggests that a sublimation process may be responsible for formation of the hollows.

moreover, they note that while CO2 sublimation causes Swiss cheese terrain to retreat by about 1 m/year, whatever volatile is involved in hollows formation on Mercury would retreat by 1 cm in about 70,000 years

[Phil Stooke]

I thought from the moment I saw that comparison with polar landforms on Mars that it was ridiculous... the geometry is different, the composition is about as different as it could get, and the mechanism is different. On Mars the ground itself (at the pole) is made of the volatile substance (CO2 ice) and it is being eaten away by scarp retreat from the points where it begins - so the small depressions get bigger and coalesce in expanding circles and composite multi-circle shapes.

On Mercury we have depressions looking more like Ina, the famous lunar example, but more widespread, and since then quite a lot of other examples have been found on the Moon. The sharp outlines are in places convex inwards, completely unlike the Mars south polar features. Often they are surrounded by smooth deposits apparently ejected from the hollow, completely unlike the Mars features. Yet the MESSENGER team have said several times that their hollows are not found on the Moon. I think we will be seeing a change of opinion on this pretty soon (especially after my LPSC poster!)

Phil