Interesting article, although it's not a new theory. In fact, there is a Surveyor I image taken just after sunset which shows a rather sharply-defined, narrow horizontal cloud sitting at zero-phase to the setting sun and floating parallel to the horizon. Its height would be dependent on its distance from the camera, which was impossible to determine.

It was an extended exposure, and the photo interpreters of the time suspected it was some kind of lens artifact, shadowing the bright line of horizon below it. But no one ever came up with a decent tectnical explanation as to why that would have happened. So, the image seems to definitely show something real, not just a camera artifact.

I used to have a link to that image (I believe it appears in the NASA publication "Exploring Space with a Camera"). But this cloud is very apparent and obvious in the image.

-the other Doug

I'm sure NEAR's thrusters affected Eros' motion a very, very small (probably unmeasurable) amount. And you can't really fault the NEAR team -- they hadn't planned out this landing prior to the flight, and so hadn't prepared specific programming and command strings until actually rather late in the mission.

They had no idea whether or not they would survive the landing, so they didn't make any firm plans about what to do after touchdown. It was only after the touchdown, when they saw just how much data was left unreceived but still on the spacecraft's recorder (at least two and maybe three more pictures, the final taken from just a few feet above the surface, is what I've heard), that they decided to see if they couldn't hop up, re-point the high gain at Earth, and send that last little bit of data before they settled down again. And by the time they decided to go ahead and try, they discovered that NEAR's thrusters had continued firing -- to fuel depletion.

I did hear somewhere that the best spectroscopy readings they got during the mission were received after touchdown -- only at that point were they getting a really good spectrum of the surface material and not a suspected, misleading microns-thick layer of non-indigenous dust and gasses... in fact, if anyone has any hard data about that story, I'd love to hear it.

-the other Doug

The single most dangerous scenario for losing an RTG happened in 1970, when Apollo 13's LM-7 re-entered the Earth's atmosphere (which it was not supposed to do) and the plutonium fuel element for 13's ALSEP ended up in a deep trench in the southern Pacific.

However, the Apollo RTG fuel element (though not ceramic-encased pellets) survived intact (as far as anyone can tell), because it was encased in a hardened graphite cask that was designed to survive both entry heating and ground impact. The international AEC took a bunch of readings in the area around the LM impact point, and there was absolutely no plutonium release -- either during entry or at impact with the water. So the system worked as designed, even in the worst possible scenario.

My conclusion is that RTGs can, indeed, be flown safely. But these anti-RTG people aren't rational, so don't think we can appeal to them with rational arguments. Heck, they were arguing that Cassini *must* be deliberately impacted into Saturn (rather than insrted into Saturn orbit) because, of course, if the SOI maneuver had failed, Cassini was going to loop right back to Earth and spill its plutonium ALL OVER YOUR BACK YARD, SO BE VERY AFRAID...*sigh*...

-the other Doug

Mind? Of course not! That's what we do here.

By the way, I did a bit of research and got an explanation for the evaporation of black holes. It seems that there is a phenomenon known to exist in the cosmos called "vacuum fluctuations." Basically, what happens is that a pair of particles -- basically, a particle and an anti-particle, or in other words, matter and anti-matter -- can appear spontaneously in a vacuum. They immediately annihilate each other, so conservation of mass and energy is maintained. But for that instant, it is not. And it is that violation of the second law of thermodynamics that allows a black hole to evaporate.

You see, over the course of billions of billions of years, such a pair of particles will appear billions of times next to the event horizon of a black hole. One of the pair will be swallowed by the black hole, and the other will radiate away from the black hole. The effect is such that the mass of the particle that escapes is actually reduced from the mass of the black hole. Over billions of billions of years, this process will reduce the mass of a black hole down to zero.

But, as Richard says, that process takes many, many times longer than the cosmos has already existed. So, a vast majority of black holes haven't lost all that much mass, and it will take many billions of times longer than the Universe has already existed for most black holes to evaporate in this fashion. And since there is little data to constrain the upper or lower limits of the spontaneous particle creation/annihilation, it's hard to set an exact date by which all the black holes in the Universe will evaporate.

So -- in the final analysis, it's something that happens. But it happens so slowly, relatively speaking, that we don't have to worry too much about it.

-the other Doug

Well, yes -- it's all relative. The other Galileans pull Io out of an orbit that would otherwise get circularized, and thus makes it travel up and down through potential energy levels of Jupiter's gravitational field. And Io heats more than the other moons because the gravity gradients in Jupiter's field are greater closer in, and Io orbits closer to Jupiter.

So, yes, the primary source of the energy is Io's interaction with Jupiter's gravitational field. But the "pumping" that keeps Io moving up and down through Jupiter's gravity gradient is provided by the other three Galilean moons. It's the five-body tug-of-war that keeps Io hot... but yes, indeed, the energy is mostly drawn out of Jupiter's gravitation.

And Europa *is* tidally heated. Just not nearly as much as Io. All of the Galileans gain at least some internal heat from their complex interactions with the other three Galileans and Jupiter. But the effect grows less and less intense as you move away from Jupiter, and the gravity gradients between the extremes of the moons' orbits becomes less (thereby allowing for the drawing of much less energy from movement up and down the "well").

My point in re Enceladus is that there *must* be some kind of similar tug-of-war happening to produce the amount of tidal heating needed to melt interior ice. So, I was basically asking just how out-of-circular Enceladus' orbit might be, how much of that can be attributed to the influences of the other Saturnian moons, and what amount of tidal heating might come from that?

From some not-very-specific comments I've seen, it seems that the orbit is too circular, Enceladus is too small, and the distance from Saturn is too great for orbital eccentricities to account for what seem to be the observed heating effects. My sense is also that the other potential non-radiogenic sources of heating aren't enough to make the kind of heat we seem to be seeing.

So, the logic points me towards radiogenic heating. Now, what do we need to do to get supporting data that will help us actually limit this dataset? Is Cassini capable of discovering the why's, or do we need to design another probe to get to the bottom of it...?

-the other Doug

Thanks for doing the math -- I got confused with the whole discussion of that "10 times the density of Earth's atmosphere" line that had been discussed in another thread.

Still, 1.6 Bar and 4.35X Earth's air density lead to a lot more aeolian force than an Earth-based mind would ever expect from weak winds. Enough, I think, when taken with the relatively low density of the drifting particles, to cause the dune structures we've seen.

-the other Doug

Yeah -- on this subject, that's enough gab, bro...

-the other Doug

I assume the effects of the other moons have been taken into account when it comes to tidal heating of Enceladus? After all, Io isn't heated by its tidal interaction with Jupiter -- its internal heat is pumped by tides from the other three Galilean moons.

Also, if Enceladus has been regenerating the E ring for aeons, its mass has decreased significantly. Wouldn't this loss of mass tend to decrease tidal heating?

My bet is on radiogenic heating. It'll be interesting to see just what kind of model would make sense in terms of the type of composition you'd need for such heating.

Perhaps the Saturn system is so volatile (in more ways than one) because it somehow became anomalously Al-26 enriched *after* Saturn had gotten fairly far along in its accretion phase...?

-the other Doug

The air is *very* thick at Titan's surface. Pressure is 1.6 Bars and density is somewhere between 7 and ten times the density of the air at sea level on Earth.

The thicker the air, the more friction you have between the air and the surface, so very weak winds have a far greater aeolian force than they do on Earth. That's how a 2.5-mph windspeed (which, due to 1/7th G feels like a whopping 3.6-mph breeze on Titan) can entrain and shift a lot more material than you might think.

Also, the ice "sand" and the organic-smog-particle dark mantling are less dense than equivalent dust and sand particles on Earth. They're lighter, and so more easily moved by winds.

That's how such weak windspeeds can produce such large longitudinal dunes.

-the other Doug

There is, indeed, a vast difference between nuclear power systems and thermonuclear explosives. The former are used to generate electrical (or propulsive) power. The latter are simply designed to explode.

An explosive is not intrinsically a weapon. It's the old NRA ad slogan -- nuclear weapons don't kill people, people kill people. But if you build explosives, for whatever reason, someone will eventually at least *threaten* to use them as weapons.

I don't like the idea of deploying nuclear explosives in space for *any* reason, because until and unless they are used for some peaceful purpose (like deflecting an asteroid or blowing apart a comet to see what's inside), they are available for weapons use -- or at least, they are available to back up the threat of using them as weapons.

Even if I did trust an individual, an agency, or even a government to resist the temptation of threatening to use space-deployed nuclear explosives as weapons, things change. People die and are replaced, governments change (peacefully and otherwise).

So, don't expect me to sign a petition for maintaining a space-based fleet of nuclear explosives to quick-deploy against Earthbound impactors, or anything like that... I just don't think it would be a good idea.

-the other Doug

First, though it's likely redundant for most of the people here, let's talk some basics about black holes.

Black holes are all about the relationship between mass, energy and gravity. Gravity is a function of mass -- the more mass you have, the greater gravitational influence it has on surrounding masses.

Gravity even affects photons and other energy "particles." A black hole comes into being when a body has so much mass that any particle of mass or energy that approaches it would have to accelerate faster than the speed of light in order to escape it.

That would be relatively easy to understand, if it weren't for the predicted effects of special relativity. Einstein's classic E=MC^2 equation states the relationship between mass and energy -- that a given amount of energy (E) is equal to the square of a given amount of mass (M) traveling at the speed of light ©. That's been taken to mean, rather simplistically, that mass cannot be accelerated all the way to the speed of light, and if it were, it would transform entirely into energy.

More importantly, special relativity also states that the passage of time is relative to how fast you are traveling compared to another point in space. The faster you go, relative to a given point in space, the slower time passes for you -- again, relative to that other given point in space.

When any mass passes the point near a black hole where it would have to accelerate to lightspeed or beyond in order to escape, the black hole's gravity would theoretically accelerate any such mass beyond the speed of light. But since that's both impossible *and* the required effect of such strong gravity, what happens beyond that point is called a singularity -- it's a region of space where physics cannot describe or predict conditions. That line itself, beyond which no mass can escape, is called the event horizon -- since no information, not even photons, can come out from within that boundary and tell us anything about any events happening within.

As a mass approaches the event horizon and is suddenly accelerated to just short of lightspeed, the passage of time for that mass would slow to a near-stop. For us, observing from outside, time would pass normally and we would see time pass normally for the accelerated mass as it comes close to being swallowed -- but once again, if the matter is accelerated to or beyond lightspeed, then time passage for it ought to stop entirely, and our physics can't describe what happens to mass that's frozen in time (at least, relative to the outside universe). So, once again, we have a singularity -- we just cannot describe what happens to the mass, how it might behave, or anything.

For a long time, it was thought that black holes sucked everything in, and nothing, not even the smallest amount of information about the black hole within or the mass falling onto it, would be able to escape. But we've found out that there are basically two kinds of information that *can* escape from a back hole: whether or not it's rotating, and whether or not it's electrically charged (and what the charge is).

You can tell if a black hole is rotating because its gravity field rotates with it, and the gravity field is the only really major thing that extends beyond the event horizon. You can observe this gravity field by looking at the effect it has on objects near the black hole. A spinning gravitational field accelerates mass both towards the black hole and along the rotation vector, so you can see and measure the rate of spin. (Again, an interesting thing, since we're observing a time-passage-dependent effect, rotation, outside of a system within which Einsteinian physics states that the passage of time should have stopped.)

Charge is also detectable based on how masses behave near a black hole. I'm not as knowledgable as to how we can determine the charge, but I know it can be done.

Spinning black holes also radiate mass and energy -- well, the disk around them does, anyway. The spinning gravitational field smashes infalling matter and energy into an equatorial disk. As matter swirls down towards the black hole, some of it follows a trajectory that accelerates it away from the black hole. The way it works (I dont have the math to explain the effect, I've just seen the results), the mass sprayed away from the black hole is shot out in jets from the rotational poles. And so, a spinning black hole can have the appearance of a child's top -- a spinning, spiraling disk rotating around the vertical "stick" of polar jets. The black hole within, of course, is invisible.

There has got to be some way in which quantum physics and multi-dimensional physics will eventually be able to describe the conditions that exist within the event horizon of a black hole. But, as of right now, no theories exist that account for all the known facts -- or that answer most of the outstanding questions.

Oh, and one other thing -- black holes aren't forever. They lose mass very slowly over the course of time (something else that I don't have the math to explain properly), and after many billions of years, they can simply evaporate. However, the theories as to what happens when a black hole falls back below the mass required to maintain the singularity are pretty raw right now. It's possible that massive amounts of energy are released, but it's also possible that the singularity doesn't collapse immediately at the point where the mass falls below that critical level.

I'm sure there are others out there who can fill in anything I've missed, and correct me if I've made any faux pas, here... but I think that's a pretty good starting point for any discussion.

-the other Doug

Grooooaaaaaannnnnnnnnn....



-the other Doug

Oh, Spirit has her own live journal too:

spirit rover

But Spirit's journal isn't updated even as often as Oppy's -- and Spirit has always been the problem child. She had that schizophrenic episode back on Sol 18, remember. She's always writing dark, depressing poetry about dark and cold and such...

Actually, the whole thing is clever, and I appreciated someone taking the time to put together those live journals. And our MERs aren't the only probes with live journals at that site...

Hubble Space Telescope

Stardust

SOHO

GOES 9 and 10 (Sibling Rivalry, Satellite-Style)

Sojourner (aka Pathfindress)

And my personal favorite, that plucky Mars probe that doesn't yet know the Soviet Union has fallen and exists no more:

Mars 3

Unfortunately, whoever had the clever idea of setting up these live journals has obviously developed a life or something int he past year -- most of the journals have been pretty inactive for the past 12 to 18 months. But they're amusing, and the Mars 3 journal is outstandingly entertaining! His journal entries begin back in 1971...

-the other Doug

I've been watching the updated series and the original narration is entirely intact. There are, indeed, new computer-generated graphics, but more impressively, there are numerous images from the Voyagers, Galileo and Cassini that had not been acquired yet at the time Cosmos was originally produced.

For example, at one point Sagan is speaking of the outer planets, and his narration only discusses Jupiter and Saturn. But the images went on to show Voyager 2 and HST images of Uranus and Neptune, including the famous Voyager 2 Neptune rotation movie.

While there aren't any Saturn images from Cassini in the updated series, they do use Cassini's Jupiter encounter movies.

Oh, and there are quite a few Pathfinder and MER images of Mars in the appropriate segments.

The producers did a fine job of merging the new imagery into the existing narration.

-the other Doug

Thanks, Chris -- good link (the Jupiter image) to a really great desktop. My puny, six-year-old 15" monitor probably doesn't do it justice (can't even get the whole image on the screen as a desktop), but it makes a great desktop nonetheless. Sort of like looking out a window and seeing only part of this massive planet, just hanging out there...

-the other Doug

Yep - without clicking the links, I can tell you the names were Regor (Roger spelled backwards, for Roger Chaffee), Navi (Ivan spelled backwards, for Virgil Ivan Grissom) and Dnecos (for Edward H. White II -- i.e., the Second, which Dnecos is, backwards).

-the other Doug

Amazing, some of the details that you can bring out with that "special processing"...



-the other Doug

So, Oppy's endgame begins to play out... It's obvious that her IDD, if it can be made to work again, only has so many deploys left in it.

As Steve Squyres said in one of his many public speeches since the MERs landed, near the beginning of the mission, they would "whip out the arm" any time they got close to anything that looked even remotely interesting. But now that the rovers are getting older, the MER team is getting more cautious using things like the IDD, which could wear out and stop working at any time.

The arm can only be deployed a relatively few more times. The question now becomes, when and where?

I think the MER team needs to make some hard decisions. Do we continue to work up the interesting bedding and mineralogical differences we're seeing at the Erebus rim, or is there enough more to be gained by arriving at a locale like Victoria with a functioning IDD that would justify the risk of a mad sprint?

It would be very, very hard to sprint through the expanse of sporadically exposed bedrock that lies between us and Victoria without giving in to temptation and stopping to investigate. But, but, but -- can we deploy the arm 20 more times? Ten? Five? Two?

We just don't know. And we probably *won't* know until we rudely discover that the remaining number is zero.

IDD deploys may have just become the coin of the realm at Meridiani. Let's hope (and trust in a bit of luck) that we can spend it wisely.

-the other Doug

I know from the raw image caption that one of the moons (the one in the foreground, it looks like to me) is Rhea. What's the other one? Dione?

-the other Doug

I think your last sentence hits the nail on the head, Jon. Aeolian features on Titan are going to look more like fluvial features to Earth-trained eyes. However, you also note the difference between two different transport mechanisms, one with higher energy than the other -- it would seem to me we're looking at aeolian transport for fines and fluvial transport for pebbles.

Someone posted here that most of the ices present on the surface of Titan, and water ice in particular, would float on top of either liquid methane or liquid ethane. So we *may* be looking at a complex and very non-terrestrial process of crust/liquid interaction.

For as non-terrestrial as these processes must be, they certainly have created some hauntingly familiar-looking landscapes, though...

-the other Doug

I have to admit, my first reaction to using thermonuclear devices to punch holes in Europa's crust (thereby potentially harming any possible biota within their spheres of destruction) is... well... disgust.

Besides, liquid water wouldn't last for very long up against the vacuum of space that exists at the surface. An ice crust would develop awfully quickly -- you'd still need a way to punch through it with your submarine. Granted, it would be thinner than the natural crust, but it would still propose an obstacle.

Finally, the ice crust seems to be kilometers thick in most places. Just how deep of a hole you think you can dig with a nuclear weapon? Especially one that explodes above the surface? If you want maximum penetration from your nuclear hole, you'll need to bury the weapon about half as deep as the hole you want to make (the "hole" being a spherical void that would be vaporized by the blast and shock effects). That leads you back to ways of drilling or melting down into the crust to place your bomb.

All in all, while it's not the worst idea in the world, I think you'd need to work out all of these details before seriously proposing it...

-the other Doug

Camelopardalids? Don't ask me why, but that name just sounds so... absurd, somehow...

-the other Doug

Even better, you're not in the obituaries...



-the other Doug

Don't look now, guys, but your image, as reprinted at APOD, made Opportunity jealous! Take a look at her live journal.

Of course, Oppy does get jealous of her sister... there's a good amount of sibling rivalry going on there...

-the other Doug

Does Triton *really* have enough of an atmosphere to allow for efficient aerobraking? At least, without a gazillion passes before you're significantly slowed down?

-the other Doug