The Rock From Mars (44 page)

In a later study of patterns of magnetization within the Allan Hills rock as it was heated and cooled during its journey, Benjamin Weiss and Joseph Kirschvink of the California Institute of Technology used Vanderbilt University’s SQUID (superconducting quantum interference device) to determine that the core of the meteorite never reached the killing temperatures felt by its exterior. The surface would have reached thousands of degrees at certain stages, but much of the melting shell would have blown away, never allowing the intense heat to reach more than a few millimeters inside. See B. Weiss, et al., “A Low Temperature Transfer of ALH84001 from Mars to Earth,”
Science
[Oct. 27, 2000]: pp. 791–795.) They reported that the rock’s heart got no hotter than 104 degrees Fahrenheit (40 degrees Celsius)—not much worse than a summer day in Houston, and without the humidity.

As Mittlefehldt fretted
• The total of known Martian meteorites has risen to thirty-four as of this writing, and scientists are evaluating a possible thirty-fifth.

A landmark report
• “Summer Study” report by the Space Science Board of the National Academy of Sciences at Iowa State University, 1962. Cited by science historian Steven Dick at George Washington University Space Policy Institute symposium, November 1996, in connection with events related in this book.

To the young scientist
• Carl Sagan,
Cosmos
(New York: Ballantine, 1985), pp. 99–101, 103, 106, 107.

With the two Viking
• Bruce Murray,
Journey into Space
(New York: Norton, 1989), pp. 68–73. See also Goldsmith and Owen,
Search for Life,
pp. 315, 337–47, and Sagan,
Cosmos,
pp. 101, 103–106. In order to prevent the Viking instruments from detecting Earth microbes that had hitchhiked aboard the lander, mission planners carefully sterilized the spacecraft with heat. They assumed that Martian life, like Earth life, would be based on carbon chemistry in water and that these microbes must take in food and give off waste gases, or they must take in and convert atmospheric gases into something they could use.

Each of the two landers carried an instrument that analyzed the atmosphere and found it to be 2 to 3 percent nitrogen (nitrogen exchange between living things and atmosphere on Earth is fundamental) but detected no telltale signs of life, such as methane. The soil-analysis instruments—gas chromatographs and mass spectrometers—found zero organic compounds at both sites (and they were capable of detecting even a few parts in a billion if they were there). The mere presence of such organics would not have proven the presence of life, because they could be made in nonbiological processes.

In addition to these indirect indicators, each of the two landers carried three biology instruments specifically designed to detect certain signs of life, if present. These were:

Each of the three biology experiments initially seemed to yield positive results—as if microbes were metabolizing the “soup,” or as if respiring, photosynthesizing microbes were turning atmospheric gases into organic matter. But the signals were too strong, and the scientific team was skeptical. Further study led them to believe that the results reflected exotic inorganic chemical reactions that only mimicked the effects of biological activity. For instance, it seemed, stable water was so completely alien to Martian surface materials that the addition of even a few drops produced reactions.

They’d found no organic
• In 1996, planetary geologist Bruce Murray, a veteran of Viking and other planetary missions and a former director of the Jet Propulsion Laboratory, would tell a symposium that Viking was “a complete success because it unexpectedly showed the surface of Mars is self-sterilizing. . . . Viking produced a powerful result.”

A few scientists
• In the mid-1990s, there was a resurgence of interest in reanalyzing some of the Viking data, but NASA officials preferred to focus on new instruments and conducting related research to “be better placed so that when data does come back from Mars, you have a better system to work with” (NASA chief exobiologist Michael Meyer interview with Steven Dick, Feb. 4, 1997, NASA archives).

In October 1993
• D. W. Mittlefehldt, “ALH84001, a Cumulate Orthopyroxenite Member of the SNC Meteorite Group,”
Meteoritics,
vol. 29 (1994a): pp. 214–21. When Mittlefehldt told the meteorite curator about his discovery, she urged him to get one more confirmation, from Bob Clayton, at the University of Chicago. Clayton was recognized as one of the leading experts on isotopic compositions of meteorites, and had developed key approaches for distinguishing Martian meteorites from all others. His laboratory confirmed that the rock had the unique Martian oxygen isotope fingerprint.

Mittlefehldt’s paper got processed with unusual speed: it was received by the journal on December 3, 1993, and accepted after revisions on December 21.

Score, working in
• Author interview with Score.

This was the first
• Ron Cowen, “The Case of the Misclassified Meteorite,”
Science News,
vol. 145 (1994): p. 206. See also Cassidy,
Meteorites,
p. 121. Mittlefehldt told
Science News
that the meteorite apparently incorporated its carbon dioxide at higher temperatures than the other SNCs, suggesting that it could have acquired the gas “from magma fluids. The higher pressure below the surface probably prevented more carbon dioxide from bubbling out of the rocky body; a significant fraction remained trapped” as carbonates.

Geologists in Germany
• Meyer,
Mars Meteorite Compendium,
p. 116. See also Donald Goldsmith,
The Hunt for Life on Mars
(New York: Dutton/Penguin, 1997), p. 53. Emil Jagoutz and colleagues at the Max Planck Institute for Chemistry in Mainz, Germany, determined the rock’s age. See E. Jagoutz et al., “ALH84001: Alien or Progenitor of the SNC Family?” (abstract),
Meteoritics
, vol. 29 (1994): pp. 478-79.

Isotopes, stable and unstable, would be important in the drama of the rock. The Germans’ “clock” was a variation on the familiar carbon-dating technique, a way of figuring out the age of certain materials based on the natural tendency of atoms, like people, to seek the most comfortable balance of forces within them—that is, to seek stability. If some traumatic event renders them unstable, atoms try to regain their balance by giving birth to offspring. Each type of unstable atom produces offspring at such a predictable rate that you can set a clock by them. Carbon dating is used to calculate the ages of archaeological finds, hair, cloth, bone, plant fibers, and other materials—but it only works as far back as 50,000 years or so. Other elements are required to measure cosmic time scales.

Each chemical element is defined by the number of protons in the nucleus of its atom. This number determines how it interacts chemically with other elements. The number of neutrons, on the other hand, determines atomic weight and which isotope of an element it is.

All the isotopes of a given element behave the same
chemically.
But they behave differently at the level of
nuclear
interactions, which occur under certain violent and usually intensely hot conditions. In those events, naked atomic nuclei actually collide and the type of isotope does make a difference.

Mittlefehldt’s Martian meteorite apparently hardened out of a hot flow of volcanic magma, whose intense heat would have sent inert gas bubbling out into the atmosphere while unstable (radioactive) isotopes formed in the cooling, hardening material. In the decay process, typically, the unstable “parent” isotope would give up part of its atomic nucleus in the form of energy (an alpha- or beta-ray emission, for instance) and form another isotope—the “daughter.” The rate of decay would be expressed in terms of the isotope’s half-life, a known unit of time. This is the number of years it takes for half the original radioactive isotope supply to turn into the daughter substance.

The clock in this case started when the rock crystallized on Mars, marking the last moment it harbored parents with no offspring. As the unstable parents decayed and produced stable daughters, the resulting gas would be trapped in the rock, unable to escape (until the rock was again heated to the melting point).

To pin the time the rock crystallized at 4.5 billion years ago, the German group compared the amount of rubidium 87, the parent isotope, with the amount of strontium 87, the daughter isotope. (This parent was known to decay into the daughter with the unimaginable half-life of 49 billion years. The age of the known universe is currently put at no more than about 14 billion years.) In this case, other isotopic data also indicated that the rock had been partly remelted some half a billion years after it first crystallized.

In contrast to unstable isotopes, stable isotopes stay the same over billions of years. They are prevalent in nature. People study stable isotopes not to find out the age of something but to pinpoint its birthplace. Various places, on Earth and in the universe, have different ratios of isotopes of a given element—an isotopic signature. This isotopic diversity occurs because minerals, water, and gases favor one isotope over another, or because living things can metabolize one type of isotope more efficiently. This is what tied all the Martian meteorites together—a common isotopic signature found in the Martian atmosphere and nowhere else. Meteorites from the moon have been identified using the same technique, based on Apollo lunar samples.

They estimated how
• See Meyer,
Meteorite Compendium,
p. 116. See also A. J. T. Jull et al., “
¹⁴
C Terrestrial Ages of Achondrites from Victoria Land, Antarctica,” (abstract),
Lunar and Planetary Science,
25 (1994a): pp. 647–48, and Jull et al., “Isotopic Composition of Carbonates in the SNC Meteorites Allan Hills 84001 and Nakhla,”
Meteoritics,
vol. 30 (1995): pp. 311–18.

Some researchers studied
• Cassidy,
Meteorites,
p. 127, citing the work of Nadine Barlow, University of Central Florida, “Identification of Possible Source Craters for Martian Meteorite ALH84001,”
Proceedings of the SPIE,
vol. 3111 (1997): pp. 26–35.

Analyses of how material got ejected off Mars had shown that near-vertical impacts producing craters larger than sixty-two miles (one hundred kilometers) in diameter and low-angle impacts (less than fifteen degrees relative to the horizon) that created elongated craters greater than about ten kilometers were the only ones that met the criteria. One such site was in the Sinus Sabaeus region of Mars, south of the Schiaparelli Basin, some fourteen degrees south of the equator, with the possible river channel on its northern side, according to Barlow. Another possibility was a sharp-rimmed crater east of the Hesperia Planitia region, twelve degrees south.

Specialists at the Jet Propulsion Lab in Pasadena estimated that the rock had to leave Mars at more than 11,000 miles per hour (about five kilometers per second) to escape Martian gravity.

And in time
• Cassidy,
Meteorites,
p. 126.

Mittlefehldt and others
• The carbonates in the rock were of several “types”:
calcium
carbonate,
iron
carbonate,
magnesium
carbonate, siderite, magnetite, and calcide. The researchers recognized the importance of the carbonates, at this point, as the first abundant substance anyone on Earth had ever seen that resulted from the interactions of rock with the Martian environment.

Various evidence indicated the age of the carbonates at between 3.6 billion and 1.3 billion years, but most investigators leaned toward the older age because Mars began to lose its water and its atmosphere around 3 billion years ago.

Like several others
• Author interview with Mittlefehldt.

Late one evening
• Author interviews with Mittlefehldt and Romanek.

(The intriguing carbonate
•
Mars Meteorite Compendium,
NASA/JSC (2001) on the Web at http://curator.jsc.nasa.gov/curator/antmet/mmc/mmc.htm, p. 123.

Slender, athletic
• Author interviews with Romanek.

It was only
• As noted earlier, Bogard and colleagues, in 1982, had been the first to identify a meteorite as Martian. See McSween,
Stardust to Planets,
pp. 99–100.

The Murchison meteorite
• Regarding the discovery of amino acids in the meteorite and its significance, see Christopher Wills and Jeffrey Bada,
The Spark of Life: Darwin and the Primeval Soup,
pp. 88–92 and 120–21; also Dick and Strick,
Living Universe,
pp. 75–79; and Goldsmith,
Hunt for Life on Mars,
pp. 148–49.

Amino acids come in left-handed and right-handed types, each a mirror image of the other. All life on Earth selects the left-handed variety exclusively—a state of affairs that allows chemical reactions to work more efficiently. This property of handedness is thought to provide a key signature for distinguishing biologically generated stuff from nonbiological. In theory, a living system would pick either right- or left-handed molecules to run on, but
nonliving
systems are just as happy with equal numbers of each. That’s what was found in the case of the Murchison meteorite—some of each. The discovery in Murchison showed, among other things, that these building blocks of life had been manufactured by purely chemical processes somewhere besides Earth.