Lunar Science: Light Amid the Heat

Before man’s first Lunar landing, most scientists thought of the Moon as a Rosetta stone: an untouched repository of precious clues that would help reveal its origin and history, to say nothing of providing new insights about the evolution of the Earth and other planets. After first successful landings, most of these fondest hopes have been realized. The Apollo missions brought back 594 lbs or lunar rocks and soil, thousands of photographs and a flood of data that have changed some of man’s basic concepts about the Moon. But many of the mysteries remain. Indeed, the very act of exploration has created new lunar puzzles. “The Moon”, said once Geophysicist Gerald Wasserburg, whose laboratory at Caltech dated many of the lunar rocks, “is giving us answers that we don’t even questions for”.

Apollo samples showed, for example, that the Moon and Earth have significantly different chemical compositions. That finding challenged the old idea that the Moon was ripped from the Earth. Yet scientists are still at a loss to explain how – or when – it was formed. Paleomagnetic studies of lunar rock indicate that the Moon once had an unexpectedly strong magnetic field – and thus a large molten iron core. Yet equally valid data suggest that a core of significant size could not have existed. Even the ages of the rock present new problems.  The oldest specimens show that the Moon’s surface underwent a violent event about 3, 9 billion years ago that remelted them, but scientists are still debating what might have caused that cataclysm.

For all the heat, Apollo’s missions shed considerable light on the Moon. It revealed that the Moon – and presumably the Earth – was under incredibly intense bombardment by great chunks of space debris in the first 600 million to 800 million years after its formation 4, 6 billion years ago. But 3, 1 billion years ago this bombardment stopped. The evidence returned by Apollo shows that the Moon’s surface has remained virtually unchanged through those eons of time. Perhaps most important of all, exploration of the Moon has shown that it is not a simple, uncomplicated sphere but a true planetary body with a complex history and evolution of its own. Like the Earth, the Moon was once at least partially molten, and thus became differentiated (many heavier elements sank toward its center, while lighter elements floated to the surface to form a crust). In the words of Apollo’s chief scientist in the 1970s, Noel Hinners “It is a piece of the solar pot from which all the inner planets are made. We had no idea of that before we went there.” Indeed, it is the rich load of Moon data already brought back by Apollo that makes the premature conclusion of the program such a bitter disappointment to many lunar scientists.

Nonetheless, all the missions were scientifically very productive and scientist-astronaut Harrison Schmitt was the first professional geologist on the Moon. The Taurus-Littrow landing site contained what may be small volcanically created cinder cones; they seem to be miniature versions of earthly features like Honolulu’s Diamond Head. The cones may well be remnants of what NASA Geochemist Robin Brett called “some of the last belches of lunar activity before the Moon turned off”. Apollo 17 planners scheduled a program of experiments and observation far more sophisticated than any of the earlier scientific efforts on the Moon. For Apollo 17 four wholly new instruments were included in the ALSEP package: a mass spectrometer to measure the Moon’s tenuous atmosphere; a detector that let earthbound scientists monitor the bombardment of cosmic dust particles and micrometeorites on the Moon’s surface; an array of four listening devices – geophones – that can pick up shock waves from explosive charges that were detonated after the astronauts had left and told much about the substructure of the landing site; an extremely sensitive gravimeter that was designed to pick up minuscule variations in lunar surface gravity.

Gravity Waves

Recording those tiny variations on the Moon went a long way toward settling an argument raging among physicists. In the 1960s, University of Maryland Physicist Joseph Weber astonished his colleagues with the announcement that he had detected gravity waves. Predicted by Einstein’s 1916 general theory of relativity, such waves are the vehicles presumed to transmit gravitational energy across space. Critics contended that Weber’s detectors probably sensed some of the Earth’s own rumblings. But if sudden variations in gravity are simultaneously picked up by a detector on the Moon and a comparable device on Earth, the sceptics may well be silenced.

During their travels across Taurus-Littrow, astronauts Cernan and Schmitt also performed several new “traverse” experiments. They took on the spot measurements to determine local fluctuations in the Moon’s gravitational field in hope of learning something about the density and structure of the material under the site. With data from a device called a “neutron probe”, scientists were able to calculate how long a particular sample has been lying on or near the lunar surface. The astronauts also sent penetrating microwaves into the lunar surface with a radio transmitting-receiving system. The pattern of the reflected signals could indicate, among other things, whether water is present up to a mile under the surface.

While his buddies worked on the Moon, Ron Evens marked his scientific contributions from high above in America. Besides intensive picture taking with both hands-held and automatic cameras, he examined the moon with a battery of experiments, including an ultraviolet spectrometer that made measurements of the thin lunar atmosphere that was used for comparison with those from the ground-based ASLEP spectrometer, and an infra-red scanner that took continuous temperature readings of the moon’s surface (its margin of accuracy within 2 degrees F). Evans did also his own detailed description of what he saw below him. Besides, it was just such “eyeball” observations by Apollo 15’s Al Worden that discovered the tantalizing cinder-cone-like features in the Taurus-Littrow region and played a key role in its selection as the landing site for the final lunar mission in the XXth century.


Light Amid the Heat. Illustration: Megan Jorgensen