Artist's depiction of a Mars-sized body hitting Earth in the solar system's early history.
42 years after the first moon rover transported the Apollo 15 astronauts over the lunar terrain, here are a selection of NASA photos taken by Apollo 15 commander David Scott and Lunar Module pilot James Irwin during their wheeled 1971 lunar adventure while Alfred Worden, command module pilot, remained in orbit about the moon.
Shown here, after three highly successful EVAs, Scott walks away from the first ever Lunar Roving Vehicle (LRV), a location where it remains to this day.
(All photos are sourced from NASA's excellent Human Spaceflight Gallery: http://spaceflight1.nasa.gov/gallery/index.html)
An artist's concept of the Apollo 15 Hadley-Apennine landing area showing the two moon-exploring crewmen, Scott and Irwin, driving on the lunar rover.
The lunar rover was attached to the lunar module and lowered to the surface and unfolded by the Apollo surface crew. When packed, the rover took up a volume of only four cubic feet.
Scott and Irwin drive the Lunar Roving Vehicle trainer called "Grover" during a simulation of lunar surface extravehicular activity in Taos, New Mexico.
Scott (right) and Irwin test out the lunar rover before the Apollo 15 mission to the moon at Kennedy Space Center (KSC), Fla., in May 1971.
Gover is driven up to the edge of a man-made crater in Cinder Lake crater field in Arizona to simulate the lunar landscape.
On July 31, 1971, the first lunar rover is unpacked during the first surface extravehicular activity (EVA) at the Hadley-Apennine landing site on the moon. The lunar module, "Falcon," is shown here with the rover and lunar module pilot James Irwin.
The US flag is unfolded and planted toward the end of the Apollo 15 mission; Irwin salutes.
The rover was an invaluable workhorse during the Apollo 15 mission, boosting the scope of how much of the lunar landscape around the Hadley-Apennine landing site the astronauts could explore.
Irwin stops the lunar rover from sliding downhill during the second Apollo 15 lunar EVA. Both of the rover's rear wheels appear to be off the ground. Scott was working on a fresh crater at the Apennine Front (Hadley Delta Mountain) when the vehicle started to slide down the 20 degree slope. Fortunately, the rover was stopped and the astronauts were able to continue their work.
Scientists have found telltale chemical fingerprints of the Mars-sized body that is believed to have crashed into baby Earth, pulverizing itself into debris that later formed the moon.
Evidence for the so-called giant impact theory comes from studies of oxygen isotopes in Apollo moon samples.
“We have developed a technique that guarantees perfect separation” of oxygen isotopes from other trace gases, lead researcher Daniel Herwartz, with the University of Cologne in Germany, wrote in an email to Discovery News.
The team studied several lunar meteorites and three basalt rock samples brought back by the crews of the Apollo 11, Apollo 12 and Apollo 16 missions, which took place between 1969 and 1972.
“We also had soil samples from NASA, but this material is not ideal for determining the bulk oxygen isotopic composition of the moon, as lunar soil may be contaminated by micrometeorites and the like,” Herwartz said.
The scientists were seeking evidence of Theia, a mysterious Mars-sized object that is believed to have crashed into Earth about 4.5 billion years ago, sending a cloud of debris into space that later reformed into the moon.
Most computer models predict that between 70 percent and 90 percent of the moon is Theia, which, like most planets in the solar system, should have a unique isotopic composition. If the moon was mostly Theia, scientists believe it would have a slightly different chemical makeup than Earth.
Other models predict as little as 8 percent of the moon is Theia, with the rest of the material coming from Earth.
Based on the slightly higher concentration of oxygen isotopes in the lunar samples, the actual mix of Theia and Earth in the moon may be closer to 50-50, but that has yet to be confirmed, the researchers wrote in a paper published in this week's Science.
Meanwhile, other teams of scientists have been looking at titanium, silicon, chromium, tungsten and other chemical elements, but so far the lunar samples show no detectable differences from Earth samples.
"This work is the first to claim to see such a difference in the isotopes of oxygen, and thus it may provide an important new constraint on the giant impact models, specifically on the size and composition of Theia," planetary scientist Robin Canup, with the Southwest Research Institute in Boulder, Colo., told Discovery News.
"The reported difference between the Earth and moon is extremely small -- small enough that I think there will be debate as to whether the difference is real or an artifact of how one interprets the data," she added. "Because this is such an important issue, this paper is likely to prompt additional work and critical debate on this topic, which will be great for the field no matter the final answer."
Herwartz and colleagues plan follow-on studies to analyze ancient Earth rocks in an attempt to figure out what material was added on to Earth after the moon’s formation to see if that accounts for the slight variation in oxygen isotopes.