A Europan Scenario Between living dirigibles on gas giants and potential organisms under the ice, we've had quite a week in terms of exotic life-forms. I didn't have space in yesterday's review of Unmasking Europa to talk about the book's chapter on biology, but here's an interesting glimpse of a not implausible biosphere on that moon, as presented by physicist Richard Greenberg: Brisk tidal water sweeps over creatures clawed into the ice, bearing a fleet of jellyfish and other floaters to the source of their nourishment. As the water reaches the limits of its flow, it picks oxygen up from the pores of the ice, oxygen formed by the breakdown of frozen H2O and by tiny plants that breath it out as they extract energy from the sun. The floating creatures absorb the ocygen and graze on the plants for a few hours. The water cools quickly, but before more than a thin layer can freeze, the ebbing tide drags the animals deep down through cracks in the ice to the warmer ocean below. Most of...

By Larry Klaes 'The Gasbags of Jupiter' sounds for all the world like the title of an early 1930s novel that would have run in a venue like Science Wonder Stories. In fact, as Larry Klaes tells us below, the idea grew out of Carl Sagan's speculations about free-floating life-forms that might populate the atmospheres of gas giant planets like Jupiter. Cornell physicist Edwin Salpeter had much to do with the evolution of that concept, helping Sagan produce a paper that was a classic of informed imagination (and one that led to numerous science fiction treatments as the idea gained currency). Larry's celebration of Salpeter's life gives a nod to that paper but also notes his deep involvement in the study of the most distant celestial objects. On March 14, the Department of Astronomy at Cornell University will commemorate the life of one of their most prestigious faculty members: Edwin E. Salpeter, the James Gilbert White Distinguished Professor of Physical Sciences Emeritus. Salpeter...

Just how many forms of life are there? We often speculate here about life on other worlds, but Paul Davies (Arizona State) is currently exploring the question from a different perspective entirely. Davies would like to know whether a 'second genesis' might have occurred, producing a fundamentally different form of life that would have evolved right here on Earth and might still occupy our planet. Life may, in other words, have started many times, perhaps with significantly different results we just haven't uncovered yet. Call it a 'shadow biosphere,' a concept the physicist calls for exploring: "...[It] is still just a theory. If someone discovers shadow life or weird life it will be the biggest sensation in biology since Darwin. We are simply saying, 'Why not let's take a look for it?' It doesn't cost much (compared to looking for weird life on Mars, say), and, it might be right under our noses." Finding these alternate life forms, if they exist, may be tricky, as they could be...

I promised a quick return to recent work on the Drake Equation, which helps us estimate the number of communicating civilizations in the galaxy, but a BBC story on Duncan Forgan has me back at it even sooner than I had intended. It's no surprise that the matters encapsulated in Drake's thinking should be in the news. After all, the era of Fermi and Drake was without firm knowledge of extrasolar worlds, of which we now know over three hundred. For that matter, the concepts of habitable zones around both stars and the galaxy itself had not come to fruition, nor had anyone ever heard of the 'rare Earth' hypothesis. We also work today with knowledge of Charles Lineweaver's studies of the median age of terrestrial planets in the Milky Way, which point to civilizations around other stars having had as much as two billion years-plus to emerge before our own Earth had even coalesced. Until we know more, I suspect we'll be adjusting Drake parameters for some time, as Duncan Forgan (University...

Centauri Dreams' recent post on the Drake Equation triggered a broad range of response, both in comments and back-channel e-mails, the latter of which produced a note from Kelvin Long quoting a rather controversial position on Drake by one leading scientist. Here it is. See if it raises your hackles: "I reject as worthless all attempts to calculate from theoretical principles the frequency of occurrence of intelligent life forms in the universe. Our ignorance of the chemical processes by which life arose on earth makes such calculations meaningless." The words are Freeman Dyson's, from his essay "Extraterrestrials" in Disturbing the Universe (Harper & Row, 1979), a book I re-read every few years as much to admire the author's rhetorical skills as to draw again on his insights. Kelvin has differing views on Drake and so do I, but I'm going to quote Marc Millis' reaction to the Dyson statement, reflecting as it does an approach toward scientific method that I share. Marc writes: "On...