This artist's conception of a planetary smashup whose debris was spotted by NASA's Spitzer Space Telescope in 2009 gives an impression of the carnage that would have been wrecked when a similar impact created Earth's moon.
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.
Billions of years ago, the Earth's atmosphere was opaque and the planet's surface was a vast magma ocean devoid of life.
This scenario, says Stanford University professor of geophysics Norman Sleep, was what the early Earth looked like just after a cataclysmic impact by a planet-size object that smashed into the infant Earth 4.5 billion years ago and formed the moon. The moon, once fully formed, which would have appeared much larger in the sky at the time, since it was closer to Earth.
Hundreds of millions of years later, he added, the first forms of life appeared, possibly having hitched a ride on a rock from Mars. The scenario is one presented by Sleep at a recent Royal Society conference here called Origin of the Moon. A paper detailing Sleep’s study was submitted to the symposium volume. [The Moon: 10 Surprising Lunar Facts]
Although many elements of the theory have been around for some time, Sleep's synthesis is "like putting together a jigsaw puzzle with some pieces already known and some that are speculative and have new aspects," said Dave Stevenson, a Caltech professor of planetary science who was not involved with Sleep's study.
One of these new aspects is how Earth cooled down to the temperatures necessary for life to evolve, following the — presumed — giant impact that formed the moon.
The processes Sleep discussed took place in the period called Hadean, about 4 billion to 4.5 billion years ago — before the first organisms came into being, and well before more complex life-forms, including dinosaurs, started roaming the Earth.
Back then, the Earth was nothing like the blue Earth we know today.
Instead, the entire Earth was hot and molten all the way to its inner core, a mixture of molten rock and liquid.
No life would have been able to survive these brutally high temperatures, which reached 2,000degrees Celsius (more than 3,600 degrees Fahrenheit). Liquid water had no chance to form.
The Earth's atmosphere at this time was also much heavier. Its mass was similar to that of today's oceans, and it pushed down on Earth's surface with a pressure of hundreds of bars. (For comparison, the average pressure at the Earth's surface today is 1 bar). It was also opaque — "you would not have been able to see much, just clouds covering everything," Stevenson said.
Beneath the clouds, a magma ocean swayed, with partially molten rock pushed around by tides, Sleep hypothesizes.
These tides were due to the mutual attraction of the Earth and the moon, and were much stronger than those in today’s watery oceans, as the moon was sitting much, much closer to the Earth back then.
The tides constantly stirred the ocean, causing the mantle to lose heat, similar to stirring and blowing on a bowl of soup. But once released from the Earth's depths, the heat was trapped at the surface, held back by the thick, opaque primordial atmosphere.
The heat could only escape the planet (and cool it down) at so-called cloud-top temperature levels — where it would be as cold as on a modern high mountain summit. But for the first 10 million years, the temperatures were much, much higher, Sleep said.
The energy loss caused by the mutual attraction of the Earth and the moon was also making the moon gradually pull away. This made the tides progressively weaker, so the molten rock was being stirred less and less, and the Earth's mantle began to solidify in stages.
"While at the top of the Earth there was still partially molten slurry with a bit of liquid left, in the middle there was a mushy layer, but the deep mantle was becoming solid,” Sleep said. "Lava was probably still coming up and erupting and freezing at the top, and then falling back in large, kilometer-size pieces that were sinking into the Earth."
Earth: Our Blue MarbleNASA Goddard Space Flight Center
Slowly, the internal heat flow ceased to dominate the climate, and the temperatures at the surface began to drop, with the heat being able to escape the atmosphere at last.
Life from Mars?
The sweltering temperatures and trapped heat were not the only obstacles for life to appear, Sleep said.
Another issue was overabundance of carbon dioxide in the primordial atmosphere. Carbon dioxide doesn't dissolve in molten rock, so it was bound to bubble up from the magma ocean, creating a so-called runaway greenhouse effect, Sleep said.
For the Earth to become habitable, most of this carbon dioxide had to vanish.
Sleep said this happened when the tectonic plates began to move in the late Hadean, some 4.4 billion years ago. With the plates moving, the carbon dioxide began to enter the mantle in a process called subduction, when one tectonic plate moves under another and sinks into the mantle.
Liquid water oceans had already begun to condense around that time, and once the Earth cooled sufficiently and most of the carbon dioxide was safely tucked away in the mantle, life did finally appear, Sleep said, adding that chances are that this life arrived on Earth from Mars.
"We know life was present on Earth about 3.9 billion years ago, but Mars was probably habitable for a long time before the Earth," Sleep said. "So you had hundreds of millions of years when Mars was not a particularly unpleasant place, with liquid water. If life evolved on Mars, rocks get knocked off by asteroids all the time — so at some point, a rock from Mars could've come in, trying to hit us on the head." [When Space Attacks: The 6 Craziest Impacts]
And if conditions on Earth were just right for life to start, this Martian rock could've been the beginning of everything we know today.
But it’s still only an idea, yet a testable one. Unlike Earth, though, the ancient geological record exists on Mars, Sleep said. It is just hard to examine.
10 Things You Didn't Know About the Moon
50 Interesting Facts About the Earth
The Search for Life on Mars (A Photo Timeline)
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