Despite recent reports of possible fossils in Martian meteorites, Venus, not Mars, may hold the most promise for harboring life elsewhere in our solar system, according to a University of Colorado at Boulder professor.
Some four billion years ago when the sun was 40 percent cooler than today, Earth and Mars probably were frozen, said CU-Boulder Assistant Professor David Grinspoon of the astrophysical and planetary sciences department. But Venus, closer to the sun, may have had warm liquid oceans and a mild climate at the time. "There is some reason to believe Venus may have been the best haven for life in the early solar system," he said.
With 900 degree Fahrenheit surface temperatures and an atmosphere permeated by carbon dioxide, chlorine and sulfuric acid clouds today, Venus seems inhospitable to "our kind of life," he said. "But we really don't know much about life -- its requirements, it's differences and how to recognize it."
Humans on Earth "may have evolved from life forms provided by Venus," Grinspoon said. "Pieces of planets were blasting off of each other all the time early in the evolution of the solar system, and microbes from Venus could easily have wound up on Earth."
The standard scientific view is that life requires water and carbon-based molecules, he said. "We simply do not know if that is the only chemical system that can make life, because the only example of a biosphere we have is our own," said Grinspoon, who has been studying the surface, atmosphere and clouds of Venus for 10 years through NASA-sponsored programs.
Grinspoon is the author of "Venus Revealed," published by Addison-Wesley Publishing Co. of Reading, Mass., this month.
In some ways, Venus may have a better environment for nurturing life than Mars, he said. Like Earth, Venus has a "chemically lively surface and atmosphere" that could provide organisms with energy and nutrients.
"In my view, what makes Earth special is its atmospheric cycles that renew themselves like a garden tilling itself," he said. "It could well be that kind of an environment on Venus is just as important for life as carbon."
Because the surface and atmosphere of Venus are constantly renewing themselves through volcanic activity, there is "more potential for interesting chemical and even biochemical processes on Venus than on Mars."
It's possible, he said, that Venus could have tiny microbes in its cloud particles, or that some form of Venusian life could have developed by using ultraviolet light much like Earth's plants use sunlight to make food.
There could even be a non-carbon-based equivalent to lichens atop Venus' five-mile-high volcanoes, perhaps feeding on sulfur gases, he said.
The interactions of Earth's oceans, clouds, surface and biosphere has led some scientists to support the Gaia theory that Earth itself is a living system, he said. "By constantly exhaling sulfur gases that react with the clouds and surface minerals, Venus could be considered in that Gaia realm."
Although NASA's 1989 Magellan probe opened a new window on the planet using sophisticated radar mapping, there is still much to learn about Venus, said Grinspoon. One key is to keep an open mind about chemical and perhaps biological processes that may be occurring there and on other planets.
"Venus is the closest thing Earth has to a twin," he said. "Studying Venus is how we learned about the problem with our ozone layer, and it's a way for us to become wiser in taking care of our own planet."
Excerpts and images from "Venus Revealed" can be accessed on the World Wide Web at: http://sunra.colorado.edu/david/book.html
In an astounding and potentially exciting development, a team of top scientists has announced that studies of a meteorite indicate that life may have started on Mars billions of years ago. They examined a meteorite picked up in Antarctica in 1984, and find that shortly after its formation 4.5 billion years ago some complicated organic molecules formed that were, in their view, most plausibly been formed by early life. Further, from the ratios of isotopes of different elements, they think that this meteorite is from Mars. All these pieces of evidence are indirect, and have alternate explanations. And their most shocking conclusion is the least definite: that they have found microscopic fossils of primitive, bacteria-like organisms inside the meteorite. Their homepage contains much information about their arguments.
I myself am skeptical. I think the conclusion that there has been life on Mars is too important to be advanced on such uncertain evidence. Indeed, the draft of the paper had the word "possible" in the conclusions, but the editor of Science, where it appears in the August 16, 1996, issue, asked them to take it out. One of the arguments that the several meteorites thought to be of martian argument were indeed so was that they were "only" 1.3 billion years, much younger than the other meteorites, while this one is the same age as meteorites thought to be from the meteorite belt. The scientists argue, though, that gases they found in the meteorite's interior matches the composition of the martian atmosphere as measured by the Viking spacecraft. Another question is whether terrestrial contamination remains a possibility, though obviously the scientists considered that point carefully and have good arguments for ruling it out, especially that such contamination is not found in other meteorites. Still further, organic molecules this complicated are found in interstellar space and other places where they are thought to be created by non-living processes, so finding these molecules even in a martian meteorite does not require, to my mind, that life created them. The scientists argue that the mixture of these molecules found is not typical of terrestrial processes. They further argue that it is the totality of the arguments together that convince them that the molecules were formed by early life on mars, and that the proximity of the supposed fossil to the organic molecules makes the association very likely.
Anyway, certainly the discussion will be rich and full over the next years. One important question is whether any of the Mars space missions scheduled for 1996 or 1998 (or thereafter) should be changed to include instruments to search for such signs of life.
A Mars Meteorite homepage at JPL is also available. The article from Science is also on the Web.
J. Roger P. Angel and Neville J. Woolf have an excellent summary
article on "Searching for Life on Other Planets" in Scientific
American for April 1966, pp. 60-66. Angel recently received a
McArthur "Genius Grant" Fellowship for his work in this field.
The authors describe especially plans to build an optical
interferometer to image distant planets around other stars.
An "Institutional Profile" in SCIENCE, vol. 270, 22 December
1995, pp. 1925-6, by Jon Cohen describes "Novel Center Seeks to
Add Spark to Origins of Life." It describes an organization
in San Diego linking five major chemists interested in the origin
of life, and describes the status of their investigations.
Lists of extrasolar planets and of brown dwarfs, and a set of links to
other homepages, are maintained in the
Extrasolar Planets Encyclopedia
by Jean Schneider.
You can access information from the SETI Institute, including
information about the Drake Equation and about their Project Phoenix. Some
other SETI sites include:
Project META and BETA (Harvard/Planetary Society)
Project SERENDIP (U.C. Berkeley)
The report of a planet around the solar-type star 51 Pegasi spurred San Francisco State University astronomers Geoffrey Marcy and Paul Butler to analyze the data they had been collecting for years on the subject. They had thought that the discovery of planets would require subtle analysis of the data taken over many years, so they had been collecting this long span of data but had not yet run it through their processing computer programs. Out of the first 60 data sets they examined, half of their total, they found two other stars with planets around them.
Whereas the 51 Pegasi planet is astonishingly close to the sun, within the equivalent distance of the orbit of Mercury to our sun, the two new planetary systems have their planets at what we would think of more ordinary distances for giant planets. (The sensitivity is not enough to discover less massive planets like Earth.)
The first of the new discoveries, in Ursa Major, is larger than Jupiter. At its distance from the star, 47 UMa, the equilibrium calculation of energy in = energy out gives an average surface temperature of 1000 degrees Celsius (1800 degrees Fahrenheit).It orbits 47 UMa in 4.3 days from an orbit close in. Its period is about 1100 days (about 3 years) with an orbital radius about twice that of the Earth from the Sun. Its mass is about half that of Jupiter.
The second of the the planets, around the star 70 Virginis, orbits every 116 days in an elongated orbit. Its mass is about 9 times that of Jupiter. (Only lower limits to the mass can actually be measured, since the angle of inclination of the planet's orbit to our line of sight can be measured, so we measure the mass times the sine of the angle of inclination.) Its equilibrium temperature calculates to be 85 degrees Celsius (185 degrees Fahrenheit), enticingly below the boiling point of water. This temperature gives hope that conditions could be suitable for the evolution of life there.
All the observations were made at the Lick Observatory with the 3-m telescope there. The spectra of the stars are taken through a glass cell filled with iodine vapor, so the sharp spectral lines of iodine are impressed on the spectra of the stars, giving an accurate way of measuring the star's Doppler shifts over time. It is periodic oscillations of the Doppler shifts that reveal the presence of the planets.
Though planets were discovered in 1991 around a pulsar (pp. 317-8), conditions there are extreme. The first planet to be discovered about a sun-like star was found recently, and was reported in October 1995. This extraordinary discovery, by Swiss astronomers, was confirmed at the Lick Observatory by American astronomers. The planet was discovered by the wobble detected in the star's proper motion.
The planet orbits the star 51 Pegasi. It has a period of only 4 days, so is much closer to 51 Pegasi (1/6 the distance) than Mercury is to our Sun. It has a mass of one-half to twice that of Jupiter. The planet would be so close to the star that its gaseous envelope has probably been blown off; its temperature at its distance would be about 1000 degrees Celsius.
References: James Glanz, "Found: A Star Too Small to Shine," Science, vol. 270, pp. 1435-6, 1 December 1995; B. R. Oppenheimer, S. R. Kulkarni, K. Matthews, and T. Nakajima, "Infrared Spectrum of the Cool Brown Dwarf Gl 229B," Science, vol. 270, pp. 1478-9, 1 December 1995.
Reference: Jeffrey Winters, "The Planet at 51 Peg," Discover, pp. 86-7, January 1996.
(Section 19.2, p. 317)
So many reports of brown dwarfs being discovered have been made over the years that astronomers are prone to treat them with skepticism. But finally a brown dwarf has been reported that seems really to be one.
A Caltech/Johns Hopkins team reported in October 1995 the discovery of a small, cool companion to the star Gleise 229; the Gleise catalogue is of nearby stars. They were searching for such brown dwarfs by making near-infrared observations in the vicinity of nearby stars. Gleise 229 is only 19 light years from our Sun; the companion is called Gleise 229B. The clincher in making astronomers believe that it is a brown dwarf is the discovery of methane in its spectrum, something we see in Jupiter but not in stars; this molecule would be ripped apart by the higher temperatures of an actual star. For methane to appear as strong as it is in the spectrum, the star's temperature must be under 1000 kelvins.
In the near-infrared discovery observations at Palomar, the object is not cleanly separated from the image of Gleise 221 and would not have been detectable in the visible spectrum. But a Hubble Space Telescope image in the visible shows Gleise 229B clearly.
Gleise 221B is at least 20 times the mass of Jupiter. Thus it probably didn't form out as Jupiter did, as the result of an accretion of gases around a rock and ice core that started in a protoplanetary disk. It more likely formed at the center of its own collapsing nebula.
Since 100 stars were searched with only one brown-dwarf formed, the observations indicate that there are not enough brown dwarfs to solve the dark-matter problem.
There are many indications that life formed about 4 billion years ago not in lukewarm seas but in hot water, even above the boiling point. Such "thermophile" (heat lovers) have, over the past few years, to be astonishingly abundant on Earth. The thermofiles belong to "archaea," now a separate kingdom from "bacteria" and "eukarya." (See ETU p. 319)
"Clues to Fiery Origin of Life Sought in Hothouse Microbes," by William J. Broad, The New York Times, pp. C1 and C7, May 9, 1995]
Alex Wolszczan writes, as of 1/22/96, that there are still 3 known planets around PSR B1257+12 with the indication that there may be a fourth, much more distant one. The distances and masses of the 3 planets are:
| Planet | Distance (AU) | M Earth |
Porb(days) |
|---|---|---|---|
| A | 0.19 | 0.015 | 25.3 |
| B | 0.36 | 3.4 | 66.5 |
| C | 0.47 | 2.8 | 98.2 |
mail smartin@williams.edu with questions