Mercury Landers Options

[JRehling]

[quote=DDAVIS,Aug 16 2005, 03:30 PM]
[quote=BruceMoomaw,Aug 16 2005, 07:53 PM]

(2) Actually, it is thought likely that Mercury's regolith differs somehat in closeup appearance from the Moon's simply because meteoroids have been smashing into it at much higher speed. At a mnimum, there should be a lot more melted impact glass mixed into it. It's questionable, however, whether any such differences would be interesting enough to be worth a camera.

I cannot imagine sending a lander to a planetary body without a camera! It would be interesting to see if the character of the surface varies from the Moon, and if there is a lot more impact glass than the lunar surface one would expect a more pronounced heliogenshein effect than what one sees on the Moon. The higher gravity might cause slightly different crater details on the small scale.

Don Davis

[/quote]

Not to be a party-pooper, but the degree of specularity of Mercury's surface should be something that Messenger can characterize pretty well from orbit (even Earth-based observations have weighed in on this: Moon-Mercury light-curve comparisons are the subject of many papers -- but better resolution will say more). And I'm not sure if different surface g will alter *small* crater shape in unpredictable ways, or any at all -- the speed at which things move away from an explosion should be way, way faster than any planet's gravity would interfere with (very little outbursting material would fall back into a 10 meter crater, no?).

I could imagine there being some very subtle Moon-Mercury differences, but I bet a bit of adjustment (for hue, specularity) to a lunar landscape would be something we (I first wrote "you", but Don might be uniquely able!) couldn't tell from Mercury landscapes in a blind taste test -- of course, we'll have to get loads of Mercury landscapes to know! And there's always the off-chance that something we aren't even dreaming of will make a tangible difference. I don't think we'll see a Mercury lander without a camera anytime soon -- but then, we aren't going to see *any* Mercury lander anytime soon! If the flyby craft on a smash-and-grab mission had a rear-facing camera, however, with the next decades' technology providing quick "shutter speed", we could get something a heck of a lot like a surface panorama -- in fact, a large number of them.

[edstrick]

JRehling observed " We probably have, or will have, Mercury meteorites in collections "

Maybe but it's doubtful. They'll look, in all probability like lunar highlands breccias, but bulk compsition will be obviously non-lunar... a different pattern of volatile element depletion from what we think really heavily stripped the moon of "volatile" elements, including things like potassium, halogens, lead, zinc... in the mega impact that lead to our double planet.

More distinctive, and telltale, will be oxygen isotope fractionation patterns. The solar system was mixed thoroughally, but not completely, and martian, earth/moon, asteroidal (many different batches) and cometary isotopes are utterly distinctive in an 016/17 vs O16/18 plot.

There is *ONE* oddball meteorite I read about 2 or 3 years ago that resembles lunar meteorites but has an odd oxygen isotope pattern and it was being discussed as a possible mercurian ejecta sample, but I haven't heard a peep in the public reporting since.

Expectations, as I recall, from orbital dynamics stuff is that we should get about 1 merc meteorite for every 100 lunar ones <separate impacts?>.. and we dont' have 100 lunar ones.

Where we *should* put a lander down is on the mercury polar ice deposits in permanent shadows in craters. Keeping the lander warm will be the problem, not cool. This stuff is radar-bright and depolarizing.. the radar penetrates many wavelengths into the scattering ice without being absorbed and gets diffusely scattered back out with high reflectance.

Utterly unlike the marginally detected radar signature of lunar polar volatiles, if the detection claims aren't bogus anyway.

[Guest_Richard Trigaux_*]

What is astonishing with Mercury is that it closely ressembles the Moon, but it does not seems to have volcanic activity, despites the fact that Mercury is much larger than the Moon. The Moon hade huge lava flows about 3-3.2 Gyears ago, forming the "seas". But as fas as I know there are no traces of lava or volcanoes. A popular hypothesis about these lava flow is that they were produced by the cooling and closing of the inner liquid core. But I think the hypothesis of a tidal heating would be interesting too, while explaining why Mercury had not such a volcanic episode.

Another difference (perhaps explaining the lack of volcanoes) is the huge iron core. There is also the network of NE-SW and WN-ES fractures running all around it. A popular hypothesis is that these fractures are due to the shrinking of the iron core when it cooled, perhaps at the occasion of the large impact of Mare Caloris.

[JRehling]

JRehling observed " We probably have, or will have, Mercury meteorites in collections "

Maybe but it's doubtful.  They'll look, in all probability like lunar highlands breccias, but bulk compsition will be obviously non-lunar... a different pattern of volatile element depletion from what we think really heavily stripped the moon of "volatile" elements, including things like potassium, halogens, lead, zinc... in the mega impact that lead to our double planet.

More distinctive, and telltale, will be oxygen isotope fractionation patterns.  The solar system was mixed thoroughally, but not completely, and martian, earth/moon, asteroidal (many different batches) and cometary isotopes are utterly distinctive in an 016/17 vs O16/18 plot.

There is *ONE* oddball meteorite I read about 2 or 3 years ago that resembles lunar meteorites but has an odd oxygen isotope pattern and it was being discussed as a possible mercurian ejecta sample, but I haven't heard a peep in the public reporting since.

Expectations, as I recall, from orbital dynamics stuff is that we should get about 1 merc meteorite for every 100 lunar ones <separate impacts?>.. and we dont' have 100 lunar ones.

Studies have differed on how many Mercury ejecta would make it to Earth, with one study going as high as 0.5% of the total (how long a time span one allows for the travel is a factor, since complex orbital dynamics are part of the picture). The percentage of lunar and martian ejecta making it to Earth may be about 40% and 4%, respectively, according to one analysis. We have to assume that the analysis of collections for possible lunar and martian origin has only been done very incompletely, but about 1% of the recent large meteorite finds have been from Mars. We might expect a Mercury meteorite for every 20 or so Martian meteorites (the fact that Mercury has no atmosphere to slow ejecta on the rise is a big help), so with umpteen identified martian meteorites on the record books and others sure to have not yet been detected, I'll stick to my guns and say that it's a definite possibility that we have a Mercury meteorite somewhere, unidentified as such, although I'll grant that the most likely counting numbers for the sum total of them are 0, 1, and 2 -- in no particular order.

Doing my own reasoning here, I'll note that travel times for lunar meteorites is VERY short compared to planetary meteorites, so if an ejecta-launching impact hits any of these worlds every few million years, then we should be getting a steady trickle of impactors from Mercury and Mars, but nothing from the Moon in a typical year, until suddenly a huge number of lunar meteorites are sprung by an impact and they become a common occurrence for several tens of thousands of years. That is to say, the infall of lunar meteorites should be highly stochastic, while the long travel times from Mercury and Mars should put those "streams" into more of a steady state. So a count of lunar meteorites that are atop our current layers of topsoil and polar snows really tells us very little. In fact, lunar meteorite influx in a MEDIAN year may be lower than that from other planets because the sum is so heavily spiked into the times right after an appropriate impact.

NWA 011 was one meteorite that was identified as being possibly Mercurian, although it was high in iron, which goes a long way to nix that (Mercury is crunchy, that is to say iron, on the inside; chewy, that is to say silicate with very low iron, on the outside).

Where we *should* put a lander down is on the mercury polar ice deposits in permanent shadows in craters.  Keeping the lander warm will be the problem, not cool.  This stuff is radar-bright and depolarizing.. the radar penetrates many wavelengths into the scattering ice without being absorbed and gets diffusely scattered back out with high reflectance. 

Utterly unlike the marginally detected radar signature of lunar polar volatiles, if the detection claims aren't bogus anyway.

We still aren't sure just what the surface would be for those Mercury polar reflections. It may be dust atop ice, or sulfur, or something else. Of course, a mission would be interesting in any case, if it can poke through a possible dust cover to see what's beneath. It would be disappointing though if the reflective stuff were patchy and our lander missed it by 200 m.

[centsworth_II]

If getting a refector on Mercury is the objective, how about an impactor filled with thousands of tiny reflective devices -- like confetti -- that are released on impact to cover a large surface area? Or, a giant paint ball that would splat on the surface, coating it with reflective paint?

[tty]

Here is a recent study of the probability of finding a mercuriam meteorite:

http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1933.pdf


The conclusion is that probability-wise "we may be getting close..."

tty

[Guest_BruceMoomaw_*]

We don't want a reflector on Mercury for libration studies -- we need an actual, stationary surface beacon for reeally good measurements. (Remember that this was considered a worthwhile experiment even for the Mars Netlander -- and studies of Mercury's libration are thought to be more important.)

Certainly, when a Merury lander finally occurs, the first one will be in the polar regions (the BepiColombo lander would have landed at about 87 deg latitude). This isn't just because of the polar deposits; it's because that's the one part of Mercury where a lander can have a really long lifetime -- which is crucial for seismic studies.

[Guest_Myran_*]

Cant but agree with BruceMoomaw, libration studies is one good reason, the other is my own favourite, seismic. All we have about Mercurys interior currently is quesswork, informed guesswork perhaps -but still.
A lander might have to be assisted by a set of impactor probes, at least two.

If such a lander also would get some more toys to maximize the science return like a little shovel, a chemistry set and one Polaroid to imagine the polar-void - all the better.

[Guest_Richard Trigaux_*]

centsworth_II your idea is interesting, but it would not work as it: glass balls or paint drops arriving on Mercury surface at a cosmic speed would simply explode like meteorites. They need some kind of braking, if not an actual landing. For example they could be dropped on the ground from a low orbit.

But that would not be enough, says BruceMoomaw. So what do you need? an accurate reflector like the ones the astronauts installed on the Moon? I recall that these reflectors were made of an array of diamond-shaped pieces of glass, which has the property of sending back any ray of light right in the direction where it came. This is much more accurate and much more efficient (a much larger part of the light is reflected back to the emitter) than with just randomly scattered glass balls. But an already powerfull laser is required to shot at the Moon reflectors; 12 000 000 000 more power would be required to have Mercury with the same devices. This is perhaps the reason why there was no light reflectors on the top of any of the already numerous Martian landers.

So BruceMoomaw, what do you need? A radio beacon? It would be much more efficient in power, but less accurate. May an orbiter provide enough accuracy? After all, much info on Mars position may be infered from the radio wave emitted by Mars landers and orbiters, much more anyway than with nothing. So I understand that at least an orbiter around Mercury would provide much more data than nothing. And a lander still more.

The ideal would be active laser telemetry (a lander-based laser would reply to an Earth based one) but this would imply to send power lasers in spaceships. Let us have a dream: a powerfull laser geodesic network allowing to know the position of any planet with a micron accuracy...

[Guest_BruceMoomaw_*]

I honestly don't know why they had it in mind, but Stanton Peale's scheme would definitely have involved an active radio beacon. (This is more easily understandable for the Mars Netlanders, since laser reflectors on Mars would quickly get dust-coated.)

[JRehling]

I honestly don't know why they had it in mind, but Stanton Peale's scheme would definitely have involved an active radio beacon.  (This is more easily understandable for the Mars Netlanders, since laser reflectors on Mars would quickly get dust-coated.)

Do you mean "why" vs a laser reflector? Mercury is often 1.0 to 1.2 AU from the Earth, and only briefly within 0.7 AU. A laser big enough to be seen after the round-trip (d^4 dispersion of the beam) is either nonexistent or really expensive.

This reminds me of one "mission" plan I had: Send a big sheet of reflective foil to Pluto. It would (if it worked) be its own parachute, as light as it was, even in the thin Pluto atmosphere. No instruments, no power. It could be extremely wide. The idea is that it would settle somewhere on Pluto. Then hope that by the end of the long travel time (or later), lasers big enough to reflect off of it would be developed... or just monitor the dimming of the sheet as frosts accumulate on it.

[Guest_BruceMoomaw_*]

Well, I know that, John. I presumed that Richard Trigaux (like myself) was thinking about a surface reflector with an orbiter bouncing lidar off it -- just as the libration experiment on the Mars Netlanders would have involved a radio receiver on a Mars orbiter. But I still don't know why such surface laser reflectors (or radar reflectors) couldn't be used in association with an orbiter for Mercury libration measurements.

[JRehling]

Well, I know that, John.  I presumed that Richard Trigaux (like myself) was thinking about a surface reflector with an orbiter bouncing lidar off it -- just as the libration experiment on the Mars Netlanders would have involved a radio receiver on a Mars orbiter.  But I still don't know why such surface laser reflectors (or radar reflectors) couldn't be used in association with an orbiter for Mercury libration measurements.

I'd take a look at the sensitivity of the approach relative to the magnitude of the phenomenon. Mars gets a yank from Jupiter every 2.5 years or so, rotates every 24.6 hours, and has an axis inclined 23 degrees from its orbit. The last two factors decrease the magnitude of the Mercurian effect by a factor of a few hundred (I don't know about the yanking). The fact that Mercury's radius is smaller makes the problem an additional 30% harder. The approach would have to be *really* sensitive (at least three significant digits) in the martian case to work the same way and tell us about Mercury's libration. I don't have any facts telling me this is *the* answer, but it's gotta be part of the story.

[Stephen]

Where we *should* put a lander down is on the mercury polar ice deposits in permanent shadows in craters.  Keeping the lander warm will be the problem, not cool.  This stuff is radar-bright and depolarizing.. the radar penetrates many wavelengths into the scattering ice without being absorbed and gets diffusely scattered back out with high reflectance.

If the crater lay in "permanent shadow" wouldn't that cause problems if the lander was solar powered? (I guess you'd need an RTG lander. But that would presumably increase the weight & the expense of the thing. Not to mention political complications here on Earth.

Also how would that affect (direct) communications with Earth? If it shadowed from the Sun wouldn't that increase the risk that Earth would not be visible above the lip of the crater? If it wasnt, you would need some kind of orbital relay, either one already in place (as in the case of the MERs, who had Odyssey already conveniently in orbit around Mars) or one the lander brought along with them (eg Beagle 2 & Mars Express).

It would make for a complex and expensive mission, and so presumably one unlikely to be funded any time soon.

======
Stephen

[Guest_BruceMoomaw_*]

ESA was thinking about a lander only 3 degrees from the pole, but NOT in shadow. If I remember correctly, they hadn't quite ruled out powering (and heating) it with a small RTG, which of course WOULD make a shadowed-area landing possible. But the purpose of the lander was to get a better geological understanding of Mercury in general -- not of one of the polar deposits in particular.