When a multiton meteorite strikes the ground at speeds of many miles per second, the local effect must be devastating, though the worldwide impact remains insignificant. Even the meteoroid that produced the famous Meteor Crater in Arizona, almost a mile across, when it struck the Earth about 50,000 years ago probably did nothing to affect our Neanderthal ancestors living in what we now call other states and foreign countries. This object probably was "only" the size of a bungalow and weighed a mere 20,000 tons; most of it fragmented upon impact or lies buried beneath the crater it produced. Throughout past aeons on Earth, similar objects have probably struck our planet at intervals measured in tens of thousands of years.
"Our planet undergoes a continuous bombardment by rocks from space. Although this fact has been well established, we can easily understand why men of good common sense long ago rejected this notion, despite tales from faraway lands of stones that had fallen from heaven. To this day, the holiest spot of Islam, the cubical building called the Ka'aba, houses the Black Stone of Mecca, whose veneration predates the origin of Islam in the seventh century A.D. The Black Stone's darkness suggests a meteroritic origin, though legend makes it a gift to Adam upon his expulsion from Eden, and explains it color as the result of a gradual change from white to black through the stone's absorption of the the sins of countless pilgrims.
Goldsmith 1997:33-34.
"I was interested in comparing different types of meteorites," Mittlefehldt recalls, "so I took a close look at a thin section of ALH 84001. What tipped me off was that this rock had significant amounts of trivalent iron oxides [iron-oxygen compounds in which the iron atoms each share three electrons with neighboring atoms]. Diogenites tend to have all their oxidized iron in divalent form [sharing only two electrons with their neighbors]. I got another thin section, and looked at the sulfides [the compounds that sulfur forms with other atoms]. The iron sulfides were disulfides [two sulfur atoms for each iron atom] instead of monosulfides [one sulfur per iron atom]. Then it all clicked."
As a meteorite expert, Mittlefehldt had in effect recognized a Mars rock by its gestalt. At that time, eleven meteorites had been identified as Martian; ALH 84001 would be the twelfth. What was missing was a fingerprint to clinch the identification. "After I found the iron disulfides, I found carbonates [compounds of oxygen and carbon, together with calcium, iron, or magnesium]. Then I told Marilyn Lindstrom, the meteorite curator, that I was sure the rock was Martian. She was excited, but she said we had to get an isotope analysis to be sure. So we sent a sample to Robert Clayton at the University of Chicago to get a count on the oxygen isotopes. He got results identical to those for other Martian meteorites."
Goldsmith 1997:49
Some isotopes are stable, capable of enduring without change for billions of years. Not surprisingly, these isotopes dominate the universe. Other isotopes are unstable, and undergo radioactive decay: They change into other types of nuclei, on time scales that vary from a fraction of a second to billions of years, depending on the type of unstable isotope. Carbon, for instance, has two stable isotopes, carbon-12 and carbon-13. The third isotope, carbon-14, is unstable: These nuclei are "radioactive," and decay to produce nuclei of nitrogen-14.
Although the radioactive decay of any individual nucleus cannot be accurately predicted, the results of these decays amoung large numbers of nuclei can be described both statistically and accurately. We know, for example, that carbon-14 nuclei decay with a half-life of 5,750 years."
Goldsmith 1997:51
Mars maintains a distance from the sun that varies between 130 and 160 million miles, while the Earth, moving along a more circular path, remains between 91 and 94 million miles from the sun.
Goldsmith 1997:160