As many of you know, for a long time I've noted that the "continent levelling" dialogue is the most explicit reference for firepower in 40K. I mean, its so very straightfroward. Flotilla of battleships. Level a continent. A couple barrages of Lance batteries. Planet uninhabitable for generations. Even the "sterilize the planet" bit mentioned much later (page 240.)
You'd think that. You'd be surprised.
A few weeks back, I posted what I considered the "revised" and more accurate calcs for the incident. It got posted on Spacebattles. Lo and behold, some moron decides to nitpick the calc, calling it "excessive" and "unsubstantiated" (paraphrase, ,but that's the essential gist of it, as well as accusing me of over-inflating the calcs, etc.) Thread got locked after the moron in question started hurling a buncha logical fallacies at me (and I made a buncha calcs to further emphasize the point.) Semantical nitpicking of things like "sterilizing", claiming that the lance batteries weren't indicated to be capable of levelling a continent or rendering a planet uninhabitable (ignoring Occam's Razor, obviously.) And when referencing Mike's most excellent page dealign with planet killers (or Base Delta Zero, or the Edam debate) he nitpicks by saying sterilize might mean something else, or that you don't need a billion (or more) megatons to render a planet uninhabitable over the long term.
No doubt you could tell the idiot in question was an anti-40K wanker (much in the same vein that Darkstar constantly tries to refute the calcs for Star Wars), so his motivations should be clear, even if he acts like a frigging creationist. And despite the annoynace of dealing with the little troll, I *did* uncover some useful data while looking into the whole "sterilization" bit, and I realize I probably understated the calcs. I mean, I know I said they were consevative, but I quite likely understated them more than I thought.
So this thread, of course, which was planned to be the great rebuttal to the SB moron, will now be a revision of my previous calcs, incorporating the new data. I do this simply to have the data available. I will also probably incorporate further calcs, as well as try to describe (in more detail) what would be bound to happen in this incident, with regards to "sterilization".
Of course, I shouldn't have to reiterate the "barrage" and "flotilla" bits, they remain unchanged. Since this is an "order of magnitude" calc, I shouldn't have to run out calcs to account for these two variables, the values incorporating the permutations on these values (IE 2 salvos by 2 battleships, 2 salvos by 3 battleships, etc.) should easily fall within the "order of magnitude" range of the calcs.
It should also be noted that, while this is clearly an extinction level event (or rather a bit more than that.) it most probably was not an Exterimnatus. The fact it was not called such should intuitively tell us that it isn't (or at least, we have no reason to believe it is such.) But there are other matters to consider. Historically in many novels (Eisenhorn, Draco, Nightbringer and Warriors of Ultramar) Exterminatus is the ultimate, irrevocable act of destruction available to the Imperium (Similar in many respects to a Base Delta Zero attack in Star Wars.) The complete and permanant eradication of all life on a planet by various means (bombardment, virus bombs, cyclonics, etc.) Oceans are vaporized, atmospheres are strtipped away. The attack in Caves of Ice, however, was not permanant (Cain said that it would take "generations" to recover, but that's not permanant.) Moreover, Cain is a Guard Commissar, not an Inquisitor. To my knowledge, only Inquisitors (And high ranking Space Marine naval officers) are capable of calling down Exterminatus on a planet. There is a further consideration: the planet possesses a valuable resource (Promethium) so there is a very good motivation for the Imperium to try to eradicate the Necrons, while at the same time keeping the planet in such a state that it may eventually be reoccupied for resource extraction.
In any case, there are no real "sources" on planet killing weapons or ships, of course. The closest is Mike's page. But there are, if you look, plenty of analogues (which is where Mike did look, and which formed the basis of all that wonderful analysis I drew upon.) Most particularily, ,nuclear weapons and asteroid impacts, especially the latter.) Asteroids are the most useful because at the magnitudes they operate in are the ones dealing with extinction or sterilization of a planet, and all that good stuff. Of course, other weapons in theory could achieve similar destruction (although as Mike notes, modern technology such as nukes can't even come remotely close to mimicking the destruction a multi-mile asteroid can deliver.) It is also worth noting that while nukes and asteroids are useful analogies in doing this sort of analysis, there are differencecs. As Mike notes, nukes and asteroids differ dramatically. Asteroids are more concentrated in how they deliver energy, but they also carry substantial momentum. And, since it is a physial impactor, that same impactor can contribute much of its own mass (if not all of it) to the resulting ejecta. This makes it more efficient than nukes in some ways (dust loading and atmospheric heating due to friction, for example.) A nuke, though, can have some compensating factors (as an omnidirectional blast, it can both deliver energy into the ground and the air as it releases its yield into its surroundings, more effectively creating fireballs and other destructive effects.)
A beam weapon like a lance is not like a nuke OR a phyiscal impactor. In a way, its the least effective of the three. It does not contribute any of its own mass in the form of ejecta, it is a concentrated beam rather than an omnidirectional blast, so it is somewhat less "efficient" in distributing its damage via secondary mechanisms Mike's Planet Killers page describes. Of course, the sheer yields involved DO mitigate this, but it may require more enegy input to achieve similar effects (For example, a nuke or a beam weapon impact only create ejecta frrom bombarding the surface of the planet in question. So that they must bombard said surface more extensively to produce enough ejecta to match an asteroid impact. This of course means making bigger holes.)
So, bearing that in mind... we move onto the whole "sterilization" bit. Now, in the SB debate, the definition of "sterilization" got pretty heavily (and absurdly) nitpicked, but the context I used it in was fairly straightforward, pertaining strictly to an approximate range of energies (well a certain level and above, since higher yields that say, vaporise oceans or strip atmosphere will automatically fall into the "sterilize" category.) Mike's planet killer page uses it twice rather precisely (he does so as well in his creationism page in dealing with the Flood Myth), and it is attached to a rather precise figure "tens of billions" of megatons (or the 1e10-1e11 megaton range. The Flood myth debunking stated 100 billion megatons.) Some may think this is arbitray, but if you do some looking, its actually rather precise as far as planetary bombardment goes. There are many references to the "tens of billions" of megatons or more being needed to "Sterilize' a planet, whereas lesser amounts (hundreds of millions s or a billion-plus megatons) will at best partially do so.
Note that some of these artticles require PDF formats, so in addition to posting the links, I'll quote the relevant material. My specific commentary will follow after.
It has been estimated  that an impact of an asteroid of mass 1.3 x 1020kg (about 440km in
diameter) would completely evaporate the Earth’s oceans destroying any life. An asteroid of
1.1 x 1019kg (about 190km in diameter) would be sufficient to evaporate the photic zone (the
upper 200m of the ocean where photosynthesis would be possible). The mass of the impactors
responsible for the Orientale and Imbrium basins on the Moon are estimated to be 1.4 x 1018
and 2 x 1018kg . This corresponds to asteroid diameters ~100km. On a steady decline
model it is therefore highly likely that the Earth was hit by one or more of these sterilising
impactors in the period before 3.9 Gyr . This leads to the scenario of “impact frustration”
of the origin of life  where impacts of large objects limit the origin (or re-origin) of life to
near the end of the bombardment.
On the cataclysm hypothesis, however, the peak of the bombardment occurs at about 3.9 Gyr.
The largest impactors on the Moon at this time are the ~100km objects responsible for the
maria basins. While the larger size of the Earth, means that statistically it is likely to be hit by
somewhat larger objects at this time, it is unlikely that any of these would be the 50-100 times
more massive objects required to sterilize the planet, and quite possible that no impacts large
enough to sterilize the photic zone occurred. On the cataclysm hypothesis then the
environment would be relatively benign  and life could well have survived through the
cataclysm period. As there is evidence from analysis of Hadean zircons that oceans and
continents were present on Earth as far back as 4.4 Gyr [30, 31], life could have originated at
any time back to 4.4 Gyr ago. With good evidence for life only being found at 3.5 Gyr, the
time available for the emergence of life is ~0.9 Gyr, and life’s appearance may not be as rapid
as some previous studies have assumed [24, 25]
Doing some simple research (such as these most excellent asteroid calculators here and my favorite one here we can note that the minimum velocity required for asteroid impacts to be about 11 km/s. Average velocity seems to be around 17 to 20 km/s, but we will use the lower end of the scale.
The masses for a 100 km "sterilizing" asteroids are given as between 1e18 and 2e18 kilograms. This yields a KE of around 6e25 to 1.2e26 joules, minimum (11 km/s velocity. assuming 17 km/s velocity, the yields are 1.4e26 to 3e26 joules.) This is within the "tens of billions of megatons" range Mike postulated as sterilizing asteroids. But that's not all. There are other kinds of "sterilization" at higher asteroid masses (and thus yields) The "photic sterilizing" asteroid is 1.1e19 kg in mass, whereas an "ocean vaporizing" sterilization asteroid would be in the 1.3e20 kg range (the "50-100x more massive" asteroids mentioned later. The latter yields for the "photic zone" vaporisation would be around 6e26-1e27 joules, while the "ocean vaporization' range is in the 8e27-2e28 joule range, again depending on velocity.
Bear in mind that the "ocean vaporizing" bit probably does not apply in this case, given that a.) its an ice planet, and all the oceans are frozen (it would take more energy to first melt then vaporize the oceans) and b.) "ocean vaporizing" is part of an "exterminatus" event, which this is unlikely to be. Of course, if you want to annoy a moron being dishonest, there is no reason not to present this possibility as an "upper limit" Realistically though, the value probably falls into the 20-100 billion megaton range or so (maybe 2-300 billion, depending on circumstances.)
Artist's concept of a catastrophic asteroid impact with the early Earth. An impact with a 500-km-diameter asteroid would effectively sterilize the planet. The Earth may have experienced such gigantic impacts in its youth, but fortunately today there are no projectiles this large to threaten our planet.
The image link is on the site, so I won't reproduce it. As noted above, the 500 km asteroid is closer to the "ocean vaporizing" scale of things (1e20+ kg asteroid, e27-e28 joules yield.). It is worth noting that the statement indicates that the planet may have experienceed one or more such impacts, and that despite this, life managed to recover and flourish, albeit over a very long period of time (likely to be far longer than plausible for the Caves of Ice quote.)
Being clobbered by bolides is a fact of life for the
Solar System’s inner planets. During Earth’s Hadean
period (4.5 to ≈ 3.8 Ga) impacts were so severe that
some are calculated to have been large enough to cause
the oceans to boil, thereby sterilizing the planet of
any life except microbes existing deep underground.
Fortunately, major impact events decreased after the
"ocean boiling" falls into the 4e26 joule range (~100 billion megatons) although given partial vaporization and inefficiencies it could arguably fall into the low e27 range as well (a fact noted on Mike's BDZ page, I believe.) Nonetheless this still qualifies as the "tens of billions of megatons for sterilization" range as well. Note as well that the statement indicates multiple such impacts could have occured, so clearly life was not permanantly stamped out on the planet.
Another quote states:
In addition, between Jupiter and
Neptune are dozens of so-called planetoids astronomers
term"Centaurs." These are asteroid/comet type objects
such as 148 x 208 km Chiron. Centaurs are scary because
anything that size would sterilize the planet. However,
even bolides that are only one km (about one tenth the
size of the KT asteroid) are considered significant threats
to human life. And objects 10 km or more are believed
to be full-blown extinction threats. Hence the activity
seeking to identify large objects with potential Earth
Assuming silicon composition, the asteroid would mass around 8e18 kg roughly, falling into the 4e26-1e27 joule range. Again in that "tens of billions of megatons" benchmark. Isn't consistency fun?
Also of interest to note that "smaller" asteroids (10 km or so in diameter) are simply considered "extinction" threats (distinct from the above "sterilization") The "ten km or more" value would be in around the e7-e8 megaton range, larger (20 km or so) in the e8-e9 megaton range.
Planets are sometimes struck by asteroids so large that the energy imparted by the impact can boil oceans and sterilize worlds. Atmospheres escape into space.[/quote]
"boiling oceans" and "atmosphere escaping into space" is again in the e26-e27 joule range (at least), enough to "sterilize" a world, specifically. It does not specify whether ALL of the atmosphere goes into space (virttually any asteroid impact will involve at least some atmosphere being blown away, although within the established ranges for "sterilization" this is certainly possible. (although depending on how you deliver the energy, it could take alot more, especially if substantial ocean vaporization is involved.)
This is not really as "accurate" a sorucee as the others I have linked to, IMHO, but it is good supplementary proof.
That’s because asteroids were bombarding it, boiling the oceans and likely sterilizing the planet’s surface before about 3.8 billion years ago.
Again, not the most authoritarian of articles by itself, but it seems to be citing another source, so that does lend it somewhat more credibility. Again, mention of "boiling the oceans and sterilizing the planet", and around the 100 billion megaton mark.
It is generally agreed that the life have originated between 3500-3800 Myr ago (Orgel 1998). Before 3600 Myr, the oceans would probably have been repeatedly vaporized by large impacts, which could sterilize the planet eliminating small and large molecules and life itself (Maher & Stevenson 1988, Sleep et al. 1989, Zahnle & Sleep 1997, Lyons & Vasavada 1999). However, reconstruction of the last heavy bombardment indicates that large impacts up to 100-km in diameter occurred still 3600 Myr ago and generated excessive temperatures on Earth able to melt most of the planet´s mass (Lyons & Vasavada 1999). Though it is difficult to suppose that an accumulation of organic molecules could be possible before of this age, it would be feasible only during the intervals among each impactor fall (Marcano et al. 2000a), which would have surface temperatures between 85-110°C generated mainly by CO2 greenhouse effects (Kasting 1993).
Again, note the mention of a 100 km diameter asteroid bombrdment, the "vaporization" of oceans (repeatedly, suggesting parttial vaporization or large scale boiling instead of total vaporization, although of course that also an fall within the scale of "sterilization" too.) Also make note of the fact of "melting" much of its mass (presumably the crust, the mantle and core are already at/beyond melting point, as I recall.) Again, these effects quite logically fall in the "tens of billions/hundred billion" megaton range, approximately (depending on factors.), which we know from the KE of a 100 km diameter asteroid.
Byerly, along with a fellow researcher, two students at LSU and researchers at Stanford, UCLA and the University of California-San Diego, say samples collected from rocks in South Africa and Australia show an asteroid about 12 miles wide struck the Earth about 3.5 billion years ago.
That impact, among the oldest and largest documented, sent millions of tons of dust and vaporized rock into the atmosphere, the scientists said.
"The only things that would have survived would have been bacteria that lived below the surface of the ocean," Byerly said. "If it happened today, it would probably sterilize most of the planet."
A 12 mile asteroid would sterilize "most" of the planet, although how much "most" would mean is kind of vague (probably not much, given the earlier links.) This is only indicative of "partial" sterilization, as opposed to full sterilization by the other sourhces (a 12 mile asteroid would fall in the e8-e9 megaton range.)
The larger of these impacts would
have boiled off the early oceans into steam, sterilizing the planet of
any life forms that might have emerged.
Again, "boiling/evaporation" oceans is linked to "sterilization", which puts the firepower in the e26-e27 range.
Given the above, and the obvious fact that the aforementioned incident in Caves of Ice is CLEARLY a planetary bombardment/extinction level event, and that Cain (in reference to said bombardment) refers to "sterilizing" the planet, we have no reason to believe it means anything other than dumping enough energy into the planet to sterilize it, much in the same way the asteroids described above will "sterilize" an asteroid. It's all we need as far as the definition (and the event) are concerned, and further arguing (read nitpicking), in the absence of evidence to the contrary, is meaningless (and dishonest.)
There is more to the calc than just semantics wrangling, though. We are dealign with an ice planet. It's crust is covered quite extensively by kilometres thick ice, in addition to the crustal thickness itself. We know from the "uninhabitable" aspect of the quote that the bombardment produces substantial amounts of ejecta nad dust loading (and other such nasty effects.), so it also stands to reason that the bombardment reaches down into the crust (which is also consistent with the "however deeply" bit of the quote, to an extent. I mean, its unlikely that the Necron facilities are buried in the mantle, for exmaple.) But not only the crust must be penetrated, b ut the ice layers above it. Ice is of course easier to bust through, and this also means its unlikely to make very good ejecta (odds are it will melt/vaporise much more easily than silicon from friction.) Add to the fact the need to evacuate the layers of ice/rock on top of the Necron facilities belowground before they can be attacked (or reaching down deeply enough to collapse/melt the caverns they occupy, at least) we're definitely looking at least at fragmentation/vaporization of the area involved, ice and rock both.
The caves occupying the Necrons are said to be "city sized", and we know from other statements in caves of Ice that the caverns extended into the bedrock of the planet in places, so at least some crust (likely to a depth of tens or huhndreds of meters) had to be affected, in addition to the 3 or so kilometers of ice on top if it. The ice will almost certainly be blasted off in liquid or vapor form, and it doesn't matter how you do it. Even if the lance strikes don't directly vaporize it, the friction of hypervelocity ejecta (like in asteroid impacts) will almost certainly melt or vaporize it (even at a couple of km/s, nevermind the 5 km/s or so Mike notes.). This ice will also, it should be noted, heat up the atmosphere, but because it will likely melt and/or vaporize in the process, that moisture will also be kept in the atmosphere. (This will make it harder to blow off the atmosphere, at least completey, so the energy that one can input into the planet before removing the atmosphere will also be greater.)
The crust itself will probably melt and/or vaporize at least partly (both debris ejected out of and inside the crater, as well as the crater's sides.) The rest of the debris will probably be heated to some degree but also possess tremendous energy (hypervelocity ejecta, remember.)
Melting the ice to a depth of three kilometers (assuming a 1500 km radius continent) and 3 km depth requires at least. 8e24 joules of energy. Vaporisation would require somewhere in the 5e25 joule range. Melting the crust to a 1500 km radius and a depth of 300 meters (consevative) requires 9e24 joules. Total vaporisation would require 6e25 joules. Thus, the firepower requirement falls in the e25-e26 joule range, ignoring the conservative variables input and inefficiencies involved.
If its cratering, (which is probably more likely, although the above will probably play a not-insignificant role in the event) therea re one of two ways to estimate it. The first is for individual lance blasts to blanket the entire continent. Assuming a 1500 km radius continent, and 2 battleships with 50 individual lances apiece, requires roughly 5e25 joules of energy to crater.
Alternately, the battleships may concentrate their fire in a single, massive strike on a specific point (mimicking the effects of a single, large-scale impactor) Well, that would likely be equivlaent to "blasting" the entire continent at once. As we know frorm the "Planet Killers" page, that would require around 5e25 joules as well (conservative) Small wonder that the "searing continents" bit is so excessive - seriously fucking up the continent (and anything on it) requires ALOT of energy, no matter how you go about it.) destroying a continent requires ALOT of energy. (merely blasting stuff on the continent requires much less, doign things like blasting mountains and whatnot, way more. Look at the eclipse entry to see why, which incidentally is also well into the sterilization scale.
As we can see, the "sterilization" benchmark coincides greatly with the probable destructive effects from blasting the ice and crust on a continental scale.
(If you're curious, the "Planet Killers" page DOES note that "continental" level direct destruction can be achieved with much less firepower (1e7-1e8 megatons), and some MIGHT argue that "levelling" a continent is more consistent with this. However, we know that the planet is rendered uninhabitable for generations (which is impossible with that magnitude of firepower), that it is "sterilized" (which I have already dealt with) and that the bombardment was believed to be able to affect the Necrons to a substantial depth (which again, it won't do - Cain doesn' tknow how extensive the Necron facilities are, after all, and thus must err on the side of caution.) Most importantly, "levelling" can simply mean "destruction" or "demolishing" - interpreting the statement to "knock down" the continent or "make it flat/even" is absurd unless you melt the entire crust (which I already dealt with) - cratering will make it uneven unless you overalp the craters quite a bit (which will again increase the yield.)
Lastly, there's terraforming. After asking around among some of the more knowledgable 40Kers on the site, it does seem quite likely that even if the planet were "permanantly" sterilized (however you define that), the Imperium could repair the damage at least to some extent (unofrtunatley, I was too dumb to save all the references.) In any case, as long as there is atmosphere, the planet should be easy to make habitable - transplanting plant and animal life (and huumans) back to the world would be fairly easy and well within the established abilities. (Besides, atmosphere removal is more in line with Exterminatus) Given that the various links I posted above tend to rather obviously indicate that "sterilization" is not neccesarily permanant, the transplanting idea becomes even more probable an explanation. And as I was reminded, Hive Worlds (as mentioned in the 3rd edition rulebook) does make extensive use of atmosphere processing (because they're so bloody overcrowded, presumably) and related industries, so the fact alone they can maintain Hive Worlds as habitable should argue for them being able to clean up after the sort of attack Cain mentions, or at the very least artificially restore habitability (its a frigging ice ball, after all, and they only inhabit it for the facility. Only the really nasty effects like global firestorms and ejecta are likely to pose a problem.)
Even though the moron in question kept acting as if my sole purpose with the analysis was to inflate numbers, the aforementioned numbers ARE limited by other factors (Although in context you could also say it only "supports" the idea of petaton-range broadsides, but "consistency" isn't very strong among the idiot I dealt with.) Prime among them (as it usually is) is Abbadon's Planet Killer. We know its effects (I've calculated them) and I won't belabor the obvious. I will point out though, that my calcs DID allow for a fairly wide latitude in the Planetkiller calcs, even disallowing the planet-wide destruction. Taking into account all the variables, and the overall level of consistency (blowing a continent sized hole deep intot he planet, boiling off the oceans and atmosphere, etc.) within an hour of direct bombardment, it is quite safe to say that the planetkiller's brute force component probably delivers around e28-e30 joules to a planet, which can work out to the e25-e27 range, the upper end of which matches with the battleship firepower I'm arguing for (Abbadon's PK being 10x more powerful than a battleship would make sense - I mean its unlikely the Imperium could concentrate THAT many in one locale very easily.) This IS edging up on the upper end of the scale of the specific calcs I did, but its not really stretching things any (particularily given that the individual "effects" such as ocean vaporising, blasting a hole in the crust, and atmosphere removal are NOT distinct or individual factors, they are inter-related.)
Antoher limiting factor is, of course, Exterminatus. as I said, this likely isn't one, and it isn't a permamant destruction of a planet. And it probably doesn't vaporize the oceans (which an Exterminatus can do.) This fits comfortably with the e25-e26 joule (or e10-e11 megaton) range I am arguing for, of course, with still enough wiggle room should further revision become neccessary. I will deal with Exterminatus later on, but I will say the effects seem surprisingly consistent.
All in all, it seems quite reasonable to argue for the single or double digit petaton per salvo calcs. If anything, double digit seems even more plausible than I thought before (although triple digit, while possible, is really stretching the whole "consistency" thing, and I don't argue in favor of those.)