Venus and the End of the World
Before concluding the discussion on these aspects of comparative planetology on too happy a note, I should mention that the hypothesis that Venus once had oceans, as appealing and reasonable as it may be, is by no means universally accepted. For it depends crucially on the plausible reading of the ratio of deuterium to common hydrogen as evidence that Venus has lost a substantial amount of water. But this is not the only possible reading.
Some scientists have argued that the same ratio of deuterium to hydrogen could be caused by a continuous re-supply of water to the atmosphere of Venus. They find this hypothesis more plausible because, at the present rate of escape, water would disappear from Venus altogether in a few hundred million years. This would mean that right now we are witnessing the very end of a long process. Most of us would not be bothered by this prospect, but some people feel that we should treat with suspicion all lucky coincidences in science. Imagine, Grinspoon tells us, that
You show up at a house where no one seems to have been home for a long time, but you hear water running. You go upstairs and find a vigorously draining bathtub which has only an inch of water in it. Now, it is possible that you showed up just as the last bit of water was running out -- but isn’t it more likely that the tap has been left running?
The intuition behind this reasoning is a peculiar view of probability that sounds quite reasonable at first. Suppose that an urn contains two white balls and 98 black balls, while another contains two white balls and 9998 black balls. Without knowing which urn is which, you reach into one of them and come up with a white ball. You should conclude that in all probability you have reached into the smaller urn, for the chances of getting a white ball are two in a hundred, which are much higher than the two in ten thousand that you would find in the larger urn. It is not reasonable to believe that you have reached into the larger urn, since getting a white ball out of it is so much more unlikely.
These intuitions about probability come with their own problems, however. The first problem is that it turns scientific reasoning on its head. In the case of Venus, it leads us to expect that the amount of water we find is normal (that it has been like that for a long time). It leads us to assume that things are pretty much as we find them (a steady-state) because otherwise the situation would be very unusual and thus unlikely.
Notice how differently we reason in science. When we first observe any phenomenon, we take pains to determine whether our sample is representative. The history of science is littered with ideas that seemed promising but went nowhere because they were based on the assumption that we were looking at the normal state of things (this is the fallacy of induction). That is, normally we have to demonstrate that our sample of observations indeed represents the usual state of affairs. We are not allowed to take that for granted. Otherwise we may conclude, say, that we have discovered a new branch of the homo family based on one fossil with a peculiar skeleton, only to find out much later that it was the skeleton of an individual with a bone disease (a true case). We worry about whether the Viking and Venera landers ended up in representative locations in Mars and Venus, whether the Galileo probe went into a section of Jupiter that is like the rest of the atmosphere (it didn’t). Nature is rich and what we come to observe may actually be as unusual as flowers, birds, and bees are in the solar system: they exist in only one world – Earth – out of the many that orbit the sun.
The kind of reasoning that leads to a steady-state view of water in Venus also leads to some very strange views when applied elsewhere. John Leslie, for example, has concluded in The End of the World that the human species is likely to become extinct very soon. He reasons that if the human species were to live for a long time, let’s say millions of years, then the amount of people alive today would be an insignificant percentage of the total amount of human beings that will ever be alive. Thus, he thinks, belonging to such an unusual group of humans (those of today) would be extremely unlikely. On the other hand, if the world were to end within fifty years or so, we would be part of the largest group of humans who will ever be alive (the six billion or so alive today are far more than all the rest of the humans who have ever lived put together). And it is more likely, then, that if you were to pick a human at random he would belong to the overwhelming majority than to a very small minority. Therefore the human species is far more likely than not to become extinct very soon.
Many sensible people would consider Leslie’s argument a reductio ad absurdum of the kind of probabilistic reasoning under examination here. Nevertheless, he, like others, sticks to his probabilistic intuitions, despite counterarguments like the following. Suppose the devil places ten people in a room and tells them that he will kill them all if he gets double sixes in a roll of the dice. If they survive, he will then place 100 people in the room, and then 1,000, and so on, always multiplying by ten. And every time the devil will roll the dice in hopes of getting a double six. Now, most of us will think that the chances of any one group getting killed will be 1 in 36, but according to Leslie, the ill-fated chances of the group of ten thousand have to be far greater than those of the group of ten. Leslie concludes not that there is something very wrong with his reasoning, but that he has encountered a paradox of probability. And he remains worried about the end of humanity, just in case the paradox should resolve itself in the direction of his reasoning.
There is no paradox, however. The probability estimate that takes into account a causal mechanism has priority (in this case, that the devil will kill all the people in the room if he rolls double sixes), as do all probability estimates that clearly have more relevance. For example, suppose that an ordinary man goes to the hospital to have an appendectomy. He is informed by his doctors that the chances that the operation will go well are 98%, for their survey shows that that 98 out of every 100 patients who undergo the operation do well. But suppose now that the patient is 89 years old and suffering from cancer and its debilitating effects. Surely the previous estimate would not apply to him. Let’s say, for the sake of argument, that statistical records have been kept for people in his condition and that people in his condition survive less than 5% of the time. This latter estimate is the one that should guide his decision to undergo the operation, for it is based on the factors most relevant to him. It would be preposterous of him to say, “But my chances might still be 98%.”
It may turn out that the amount of water of Venus in a steady state, but to have confidence in that idea we need independent scientific reasons in its favor, not peculiar intuitions about probability. A candidate to keep the Venusian atmosphere re-supplied with water is volcanism. A good possible outside source of water is the combination of comets and their fragments, for they are basically a mixture of ice water and other compounds. As Grinspoon points out, however, comets collide with planets often but not continuously and, thus, we would not know whether that hypothesis is consistent with our present readings of Venusian water (comets would supply water in spurts). Furthermore, this approach needs to assume that the exceptionally large ratio of deuterium to hydrogen would exist in Venus for any given short period of time. Otherwise we would have to conclude again that Venus has been losing water for a long time. Now, some scientists say that in first coming up with the “oceans in Venus” hypothesis we assume the original Venusian deuterium-to-hydrogen ratio should be close to Earth’s, but that this assumption may be incorrect: after all, they are different planets. This point is fine as far as it goes. But the ratio is 120 times greater on Venus than on Earth! It is difficult to imagine what could account for such a phenomenal difference in natural ratios between the two planets. The original ocean hypothesis must continue to be considered most reasonable until further notice.
This proliferation of ideas is by no means restricted to planetary geologies. But perhaps we can see this point better by discussing several illustrations in connection with another point, namely that the variety we find in the solar system permits us to test our ideas of the Earth.
. D.H. Grinspoon, op. cit., p. 109.