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Say I want a 100% effective mass extinction to take place on a planet similar to Earth. It should kill everything, even bacteria.
It doesn't need to happen overnight. I'm perfectly fine with some event/series of events that takes place on geological time-scales. Say 1-2 million years.

2 problems (that I can see) here:

  1. I need that planet later to be colonized/Terra-formed into a pleasant place for humans to live. It can't be fully destroyed/vaporized/turned into an asteroid belt. So its star going nova probably won't work.
    (It is fine to have the planet be suitable for Terra-forming much later, say 50-150 million years after the extinction event took place.)
  2. Killing all bacteria (or the local equivalent of bacteria) is going to be very hard. Consider that here on Earth we have bacteria that live many kilometers deep in the planetary crust. Would they even notice the planet being blasted into a gazillion little pieces by a planetary collision and coalescing back into a new planet later? Given that some species of bacteria are known to be able to withstand the rigors of space, this seems a possible way of, at least some, bacteria to survive.

Is is it possible to come up with something that has the desired effect?

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  • $\begingroup$ This is probably closest en.wikipedia.org/wiki/Late_Heavy_Bombardment , but note that it is speculated (apart from the event itself being speculative) that life survived in deep hydrothermal vents. It is hard to kill bacteria that evolved to tolerate high temperatures and pressures and live deep underground. $\endgroup$ – Ville Niemi Feb 4 '18 at 18:18
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Gamma Ray Burst

Have the planet and its star, traveling around the Galaxy, come within a couple light-years from a black hole that's eating away a star.

A nice sequence of gamma-ray bursts, near enough and long enough to sterilize the planet. Of course, an intensity sufficient to kill Archaea in deep mines or at the bottom of an ocean will wreck the atmosphere too.

"Home" sterilizer

As per comment, what if the GRB source is in the planetary system itself? I am not an astrophysicist, but it seems unlikely:

  • the secondary star of a binary system goes nova. Possible, but if the flash doesn't boil off oceans and atmosphere, sterilization down to bacteria won't happen. Even if the surface reaches 200 °C for several days, heat will seep only slowly inside the crust. Deep mines will probably remain habitable for insects, not just bacteria. And if we lose the atmosphere, the planet won't be viable afterwards.
  • the secondary star collapses into a neutron star or black hole. The problems now are: (a) the gamma radiation from neutron stars is apparently emitted along the rotation axis, which in most solar systems is normal to the ecliptic since both phenomena stem from the angular momentum of the original gas cloud which originated the solar system. So, the GRB will never hit the planets; (b) if we posit a different mechanism, e.g. X-ray emission from an accretion disk, said accretion disk would almost have to come from the primary star. Which means that the black hole's grasp somehow reaches the star's atmosphere; a fortiori, the planet in its orbit is a goner.

We could still have a gas-giant-massed black hole at cometary distances, eating away a superdense Kuiper belt or "smoke ring" (like the one around Tau Ceti). This would result in a very strong X-ray emission; will it be enough to sterilize a planet? Maybe.

Dark Death

A more handwaved explanation: dark matter exists and it weakly interacts with baryonic matter. The planet passed through a large and dense clump of dark matter, that seeped through everything from the stratosphere to the molten core, subtly altering electrochemical and nuclear properties of all matter. This is not too hard on most types of matter (some crystals shatter, some elements decay at slightly different rates, but that's all), but living matter is based on finely balanced energy levels and innumerable chemical reactions that have to blend together just so. All DNA and RNA based molecules simply broke up, killing all life within a few seconds. A lingering core contamination could still be detected from slightly skewed geoneutrino ratios.

Nano-Killers

These are biological machines, much more efficient and resistant than evolved bacteria. They will outcompete everything else, resisting to conditions more extreme than naturally evolved organisms can. Over a period of several thousand years, they'll infiltrate everything, and exterminate all competition. They'll not be DNA-based, but still have mechanisms to avoid random mutations, and be able to utilize different energy sources; and of course they will have some kind of count-down mechanism to have them die off after a certain time.

Just seed the whole planet with the beasties, and wait.

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  • $\begingroup$ I like the sterilization by gamma-ray bursts. Now I wonder... Suppose the planets sun is part of a binary system and the other star goes nova and turns in a (small) black hole or neutron star. Would that result in the same effect on the planet (and would the planet and its primary star remain stable) ? $\endgroup$ – Tonny Feb 4 '18 at 15:52
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    $\begingroup$ Deep sea vent life will laugh at your gamma ray burster. $\endgroup$ – Loren Pechtel Feb 4 '18 at 20:21
  • $\begingroup$ Yeah, I dunno about the gamma ray burst. Not only will a wide variety of ocean life survive, but lots of fish and invertebrates live in caves their entire lives, sometimes far underground. The gamma ray burst will have to be powerful to burn off the atmosphere to really affect them. $\endgroup$ – kingledion Feb 4 '18 at 20:35
  • $\begingroup$ they will have some kind of count-down mechanism to have them die off after a certain time. why bother? It's nanobots, so I'd say they don't qualify as "life". Plus, keeping them there will make sure that those pesky life-forms won't come again. $\endgroup$ – Hankrecords Feb 6 '18 at 8:06
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Raise the surface temperature to 125$^\circ$C

Life depends on protein synthesis. Most of the time this process begins to break down around 40C. (High fevers in humans are lethal because the body basically cooks itself.) Most foods are considered sterilized sufficient for eating at 55$^\circ$C as long as proper handling procedures are observed.

But, we don't care about fairly normal bacteria, we want to kill the extremophiles. Of the examples cited, the maximum discovered temperature is 121$^\circ$C. By increasing the temperature of the whole planet to 125$^\circ$, we will have boiled off the oceans completely killing all terrestrial and aquatic life. The extremophiles found at those deep ocean vents will be killed by exceeding their thermal tolerances and introducing them to lots and lots of air. That last part is important because we want to introduce them to as bizarre an environment as possible. They are used to high temps and high pressures. Let's give them really high temps and low pressures.

If this increase in temperature happens over a million years, that will be plenty of time for heat to seep down into the crust and crispy all the little bacteria that live down there.

Yeah but how?

You've mentioned terraforming so I'm going to assume that the civilization sterilizing the planet is also the civilization that will repopulate it later.

A big lens in orbit

Put a big lens in orbit to increase the amount of solar radiation that Earth receives. The bigger the lens, the faster the temperature increases. When the whole planet is cooked, just move the lens out of the way. Surface temperatures should start to fall.

The OP said its okay if the sterilization procedure takes 1 million years. I don't think this lens will take that long but that's plenty long enough to bake a planet.

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  • $\begingroup$ You need to maintain the temperature for some time, just raising it won't be enough. And once it's over you'll need to replace the lost water. $\endgroup$ – Loren Pechtel Feb 4 '18 at 20:24
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    $\begingroup$ For a comparison, I did an order of magnitude estimate for heat transfer from the atmosphere to the oceans. At 100 W/m^2; it will take about 4 years to heat the oceans by 1 degree C. I think it will take a long long time to sterilize the ocean, even with a very hot atmosphere. $\endgroup$ – kingledion Feb 4 '18 at 20:43
  • $\begingroup$ A problem is that the vapour tossed in the atmosphere will raise the albedo considerably. After that, the largest increases in temperatures will be in the high atmosphere, which means that you will end up essentially desiccating the planet. $\endgroup$ – LSerni Feb 5 '18 at 0:47
  • $\begingroup$ The OP says that a solution that takes a million years is okay. Even if it takes 10,000 years to reach the temperatures required, it satisfies the constraints. $\endgroup$ – Green Feb 5 '18 at 4:21
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As you say 100% is a very difficult number to get when target is planetary big.

There are, however, several possibilities to get very close to that, depending on the kind of catastrophe you are willing to pull unto the unsuspecting planet.

  • Physics: prevent, somehow, sun light from reaching planet (some kind of permanent eclipse; e.g.: planet is outside a Dyson sphere). This would turn planet in an iceball (notoriously harder than a snowball) rather quickly. Some bacteria might survive near geothermal vents, for a while, but that are not very stable and preventing life to migrate to follow might be enough, on the long run. Removing eclipsing "object" could revert conditions, despite albedo increase.
  • Chemistry: release in the atmosphere of some very toxic byproduct leading to poisoning of all life, possibly preserving a single species (the one producing the toxin, who uses it to gain dominance); this species is likely to die after all others are dead.
  • Radioactivity: deep ocean contamination with some kind of radioactive material with medium (hundred of years) decay time. This has highest probability to leave some "surviving pockets" around, along with some seriously radiation resistant bacteria (e.g.: Deinococcus radiodurans).

Much depends on the kind of event (natural or man-made) you are willing to pull and "how close" you need to be to the mythical 100%.

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  • $\begingroup$ Neither the chemical nor radioactive options have a chance of getting subterranean bacteria. I was thinking cold but I'm not sure there--deep in the rocks it might remain warm enough. $\endgroup$ – Loren Pechtel Feb 4 '18 at 20:22
  • $\begingroup$ @LorenPechtel: it depends on time frame. I (partially) agree about radiation, but pervasive chemical (think "Oxygen") is bound to percolate in whatever ecosystem permeable enough to have life in it in the first place. About cold that's another story: all life on earth is solar powered, directly or indirectly; if you take out sun light life will die, sooner or later, even if temperature won't drop to freezing food intake will stop trickling in. Life forms thriving in volcanic vents, do not form a stable ecosystems and depend on organic matter percolating down; if that stops they won't survive. $\endgroup$ – ZioByte Feb 4 '18 at 23:34
  • $\begingroup$ Sure, it could percolate eventually--if it didn't react with the environment and break down before then. $\endgroup$ – Loren Pechtel Feb 4 '18 at 23:41
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Depends on your definition of destroying the planet.

it doesn't need to happen overnight. I'm perfectly fine with some event/series of events that takes place on geological time-scales. Say 1-2 million years.

When you have millions of years you could chuck a mars sized body at the planet, which would convert the surface to magma, ( and you might get a nice moon out of it ).

Then wait a few million years and everything is cooled off and settled down. That will surely kill everything.

https://en.wikipedia.org/wiki/Giant-impact_hypothesis

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Your original idea is fine, a big enough rock works.

Another moon forming impact would do it, reducing the planets surface to magma is guaranteed to kill everything off.

planets don't shatter under large impact they liquefy under the massive deformation heating, there is actually an upper limit on how big an impact crater you can make in a planet nut becasue the planet breaks but becasue above that it liquifies destroying the crater. This is actually fairly easy to do to earth since the planet is near molten anyway.

since the planet is habitable now and it experienced this at least once the planet is definitely terraformable after suchan event, mostly you just have to wait for it to cool down. Depending on the size of the impact it might take more an a 100 millions years to cool off enough for liquid water however.

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