Cloud is made of hydrogen. Measurements give a density inside the Cloud a little greater than 10–10 gm. per cm3. I estimate that if the Earth moves through such a cloud for about one month the amount of hydrogen that will be added to our atmosphere will exceed a hundred grams for each square centimetre of the earth’s surface. Is this right, please?”
There was a silence as the implication of these remarks dawned on the meeting, or at any rate on some of the scientists.
“We’d better check that right away,” muttered Weichart. He figured on a pad of paper for perhaps five minutes.
“It’s right, I guess,” he announced.
Almost without comment the meeting broke up. Parkinson came up to Marlowe.
“But, Dr Marlowe, what does all this mean?”
“My God, isn’t it obvious? It means that enough hydrogen is going to come into the Earth’s atmosphere to combine with all the oxygen. Hydrogen and oxygen are a violently unstable chemical mixture. The whole atmosphere will blow sky-high. Trust a woman to spot that.”
Kingsley, Alexandrov, and Weichart spent the afternoon arguing. In the evening they collected Marlowe and Yvette Hedelfort and went to Parkinson’s room.
“Look, Parkinson,” began Kingsley, after drinks had been poured, “I think it’s up to you to decide what London, Washington, and all the other cities of sin are to be told. Things aren’t quite as simple as they seemed this morning. I’m afraid the hydrogen isn’t really as important as you thought, Yvette.”
“I didn’t say it was important, Chris. I simply asked a question.”
“And you were quite right to do so, Miss Hedelfort,” broke in Weichart. “We’ve been giving far too much attention to the temperature problem and overlooking the effect of the Cloud on the Earth’s atmosphere.”
“Not clear until Dr Marlowe finished work that Earth would be in Cloud,” grunted Alexandrov.
“That’s true enough,” agreed Weichart. “But now the decks are cleared we can get into action. The first point is one of energy. Each gram of hydrogen that enters the atmosphere can liberate energy in two ways, first by its impact with the atmosphere and second by combination with oxygen. Of these the first yields more energy and is therefore more important.”
“My God, this only makes it worse,” exclaimed Marlowe.
“Not necessarily. Think what’ll happen when the gas of the Cloud hits the atmosphere. The very outside of the atmosphere’ll become extremely hot, because it’s on the outside where the impact will take place. We’ve calculated that the temperature of the outer parts of the atmosphere’ll go racing up to hundreds of thousands of degrees, perhaps even to millions of degrees. The next point is that the Earth and the atmosphere are spinning round and that the Cloud will be hitting the atmosphere from one side only.”
“From what side?’ asked Parkinson.
“The Earth’s position in its orbit will be such that the Cloud will come at us from the approximate direction of the Sun,” explained Yvette Hedelfort.
“Although the Sun itself won’t be visible,” added Marlowe.
“So the Cloud will be hitting the atmosphere during what would normally be the daytime?”
“That’s right. And it will not be hitting the atmosphere during the night.”
“And that’s the crux of the matter,” continued Weichart.
“Because of the very high temperature I was talking about, the outer parts of the atmosphere will tend to blow outwards. This won’t happen during the “daytime” because the impact of the Cloud will hold it in, but at “night” the upper atmosphere will stream outwards into space.”
“Oh, I see what you’re meaning,” said Yvette Hedelfort. “Hydrogen will come into the atmosphere during the “daytime” but it will blow out again during the “night”. So there will not be any cumulative addition of hydrogen from day to day.”
“That’s exactly right.”
“But can we be sure that all the hydrogen will be evaporated off in this way, Dave?’ asked Marlowe. “If even a small proportion of it were retained, say one per cent or a tenth per cent, the effect would be disastrous. We’ve got to keep in mind how very small a disturbance — small from the astronomical point of view — could still wipe us out of existence.”
“I’d feel confident in predicting that effectively all the hydrogen will be evaporated away. The danger is rather the other way, that too much of the other gases of the atmosphere will also get evaporated into space.”
“How can that be? You said only the outer parts of the atmosphere would be heated.”
Kingsley took up the argument.
“The situation is this. To begin with, the top of the atmosphere will be hot, extremely hot. The bottom of the atmosphere, the part where we live, will be cool to start with. But there’ll be a gradual downward transfer of energy, tending to heat up the lower parts.”
Kingsley put down his glass of whisky.
“The whole point is to decide how fast the downward transference of energy will be. As you say, Geoff, only a very slight effect would be utterly disastrous. The lower atmosphere might be heated sufficiently to cook us, quite literally to cook us, all done to a turn quite slowly, politicians included, Parkinson!”
“You’re forgetting that we shall survive longest, because our skins are thickest.”
“Excellent, a point to you! Of course the downward transfer of energy might be fast enough to cause the whole of the atmosphere to be blown off into space.”
“Can this be decided?”
“Well, there are three ways of transferring energy, they’re just our old friends, conduction, convection, and radiation. We can be pretty sure already that conduction isn’t going to be important.”
“Nor convection either,” broke in Weichart. “There’ll be a stable atmosphere with a rising temperature as you go outwards. So there can be no convection.”
“So that leaves radiation,” concluded Marlowe.
“And what will the effect of radiation be?”
“We don’t know,” said Weichart. “It’ll have to be calculated.”
“You can do that?’ queried the persistent Parkinson.
Kingsley nodded.
“Can calculate,” affirmed Alexandrov. “Will be bloody great calculation.”
Three weeks later Kingsley asked Parkinson to see him.
“We’ve got the results from the electronic computer,” he said. “Good thing I insisted on having that computer. It looks as though we’re all right so far as radiation is concerned. We’ve got a factor of about ten in hand and that should be safe enough. There’s going to be an awful lot of lethal stuff coming downward from the top of the atmosphere though — X-rays and ultra-violet light. But it seems as if it won’t get through to the bottom of the atmosphere. We shall be pretty well shielded down at sea-level. But the situation won’t be so good in the high mountains. I think people will have to be brought down. Places like Tibet will be impossible.”
“But, by and large, you think we’ll be all right?”
“I just don’t know. Frankly, Parkinson, I’m worried. It’s not this radiation business. I think we’re all right there. But I don’t agree with Dave Weichart about convection, and I don’t think he’s as confident as he was. You remember his point about there being no convection because of the temperature increasing outwards. That’s all very well under ordinary conditions. Temperature inversions, as they’re called, are well known, particularly in Southern California, where Weichart comes from. And it’s quite true that there’s no vertical movement of the air in a temperature inversion.”
“Well then, what are you worried about?”
“The top of the atmosphere, the part that the Cloud is hitting. There must be convection at the top, because of the impact from outside. This convection certainly won’t penetrate through to the bottom of the atmosphere. Weichart’s right there. But it must penetrate downwards a little way. And in the region in which it does there’ll be a big transference of heat.”
“But so long as the heat doesn’t get to the bottom will that matter?”
“It may do. Consider things as they’ll occur day by day. The first day there’ll be a little penetration of the currents. Then at night we shall lose not only the hydrogen that has come in during the day but also the part of the atmosphere down to which the currents have penetrated. So in the first day and night we shall lose an outer skin of
