Mercury Flyby 2 Options

[Phil Stooke]

And a quick peek at the new image of the rayed crater near the limb, reprojected. Looks like another one with bright material inside it.

Phil



(I really should try to get a bit of work done...)

[SFJCody]

Just as Mercury has 'hot poles' does it also have, perhaps 'impact poles'; areas which (owing to Mercury's orbital and rotational properties) are likely to receive slightly more and/or slightly higher velocity impacts than other places?

[Phil Stooke]

No, it doesn't.

Phil

[elakdawalla]

That question is ringing a bell. Wasn't there a new paper out recently saying that Mercury doesn't have impact poles, but Mars does preferentially have more impacts near the poles?

Today's two three image releases appear to be from NAC departure mosaic #3. Updated context map is attached.


Key:

Inbound:
blue squares = NAC approach mosaic

Outbound:
Yellow = WAC Departure mosaic #1
Blue = NAC Departure mosaic #1
Purple = NAC Departure mosaic #2
Blue-green = NAC Departure mosaic #3

--Emily

Reason for edit: Added the third image release from today to the context map.

[SFJCody]

No, it doesn't.

Phil

Why not? It is true that Mercury is not tidally locked, like the moon (which does have cratering asymmetry for that very reason*), but it does have that 3:2 spin-orbit resonance resulting in a notable insolation asymmetry. Maybe I should crunch some numbers and find out.


*

Recent work has re-opened the question of lunar
cratering asymmetries. Examination of the spatial
distribution of rayed (and thus likely young) craters
gave evidence for a higher density on the Moon's
leading hemisphere[5][6], with an amplitude 150%
that of the trailing hemisphere. This seems to be in
agreement with a simple dynamical estimate [7], given
that the Moon orbits the Earth at 1.0 km/sec and thus
sweeps up more impactors (capable of generating a
crater larger than a given diameter) on its leading than
trailing hemisphere. A recent numerical study [8] re-
examined the hypothesis of an increased nearside
crater production due to gravitational focusing by the
Earth. It concluded that on average, the cratering rate
is approximately a factor of four greater on the near
side than on the far side.

Lunar Cratering Asymmetries. J. Gallant and B. Gladman LPSC XXXVI

[SFJCody]

That question is ringing a bell. Wasn't there a new paper out recently saying that Mercury doesn't have impact poles

It's been done? I really should pay more attention!

[Juramike]

Le Feuvre, M, and Wieczorek, M.A. Icarus 197 (2008) 291-306. "Non-uniform cratering of the terrestrial planets". doi: 10.1016/j.icarus.2008.04.011 (freely available here!)

From the article, the authors find about 10% increased cratering at Mercury's poles compared to the equator, and Mercury's cratering flux is about 1.9x that of our moon.

-Mike


[And Emily's right, this was in the September 2008 issue of Icarus]

[SFJCody]

From the article, the authors find about 10% increased cratering at Mercury's poles compared to the equator, and Mercury's cratering flux is about 1.9x that of our moon.

-

Ah, but they only consider cratering rates as a function of latitude, not as a function of longitude.

[Holder of the Tw...]

A news item worth noting in passing. Thanks to the solar sailing technique, the targeting for this flyby was awfully good. In fact, they are claiming it was the most accurate targeting of any planetary flyby other than earth. It came in just a wee bit under 200 km.

MESSENGER News

[Phil Stooke]

Going back to the question about longitudinal variations in cratering, people have been looking for that for decades. I think there are some claims from modelling and some from crater counts, applied to bodies all over the solar system. I don't find any of them convincing. As for Earth focussing impacts on one side of the moon or the other - or, even worse, shading the nearside from impacts - even less convincing. Whatever slight variations there might be are either overwhelmed by saturation cratering or by various geological effects, or they fall into that class of observations which exist only in the statistics, but not in any meaningful way in reality. Show me a visible impact gradient and I'll back down!

Phil

[Phil Stooke]

And here's the new limb view reprojected. There's quite a bit of relief distortion.

Phil

[volcanopele]

The only body I can think of is Triton, where there is a difference in the distribution of craters between the trailing and leading hemisphere.

[ElkGroveDan]

There's quite a bit of relief distortion.

I'll say. That crater on the left looks like a cake pan.

[Phil Stooke]

volcanopele: "The only body I can think of is Triton, where there is a difference in the distribution of craters between the trailing and leading hemisphere."

We have very limited coverage of Triton, not many craters, and an unknown effect from regional variations in geology (Canteloup terrain vs the plains units elsewhere) - it's not a very robust analysis.

Phil

[SFJCody]

Oh well...


Some new pics are up now, including a Mariner 10/Messenger comparison at different phase angles