by Laurance Doyle - Astronomer
To date we have looked at what makes a comfortable habitable zone around the Sun, a place where organics meet liquid water with some energy source. In our circumstellar habitable zone, Venus was a bit too close, and likely experienced a moist runaway greenhouse effect thereby losing its oceans. Mars was a bit too small, and so was not able to keep any plate tectonic activity going so that it could not recycle its atmosphere, and so lost its atmospheric blanket to its geology.
Other places in the solar system the tidally heated Jovian moon Europa, for example, and possibly comets (if they can be heated by residual radioactive elements) -- might also be promising places for habitability, although these liquid water oases would not be considered circumstellar, but rather tidally and radiogenically, heated habitable zones.
We have also seen that other stars circumstellar habitable zones might be quite different, but nonetheless potentially habitable. The small, cooler M-type red stars we say might allow a habitable zone, but the planets there would be tidally locked in their rotation. Nevertheless, a fairly reasonable atmosphere on the planet would be expected to distribute heat so that the day- and night-side temperatures might not be too extreme for biology to develop.
We also saw that the larger, hotter F-type white stars could also have circumstellar habitable zones since their production of ultraviolet light would actually create much more ozone than needed, apparently making planets in the circumstellar habitable zone around these hotter stars even safer from ultraviolet light than Earth is around our G-type yellow star.
But now lets take a look around the neighborhood to see what interesting things could demand our attention, technologically. After all, the dinosaurs had over 200 million years to develop methods to build a comet deflector and (apparently) never did so. Some were bipedal and had opposable "thumbs" (i.e. they could hitchhike). But Homo sapiens, in less than 5 million years, are already approaching the level of technology where we might be able to, indeed, deflect a comet on an impact course with our planet.
Just a few things we might look out for. In 1908 what appears to have been a stony meteor slammed at about 10 miles (16 kilometers) per second) into a remote region of Siberia; it is called the Tunguska impact event. It was small, 300 feet (91 meters) in diameter, but nevertheless affected forests (mostly by burning and flattening) for 100 miles (160 kilometers) around. The dust it threw into the air caused the Sun to appear greenish, and the Moon bluish over the next year. It also was twilight around the Northern Hemisphere even at midnight (due to the dust scattering sunlight high in the atmosphere). President Theodore Roosevelt declared it the "Year of Mystery." It would be several decades before Russian scientists could investigate it in more detail.
In Arizona there is a 1-mile- (1.6-kilometer-) across crater, now known popularly as "Meteor Crater" that was caused by a much larger meteor, about one-quarter mile (0.4 kilometer) across, that fell about 100,000 years ago, wiping out most biosystems in this region. Although it was at first thought to have been formed by a volcano, geologist Eugene Shoemaker recognized it as an impact crater, although the impactor was mostly vaporized upon impact. One can still visit the museum and buy a piece of this solidified vaporous iron that rained down from the skies shortly after the meteor hit.
Next Page: The Big Hits
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Larger impacts have also hit Earth, although they are more difficult to see. The larger the impact, the least often it occurs, and such "cratering equations" have been derived for the Moon, Mars, Mercury and various moons of the giant planets. The frequency of occurrence of such impacts is known accurately enough to allow what is known as "crater dating." For example, much of the terrain on Mars has been dated using the size distribution of craters evident on it. For example, a riverbed with more and larger craters is older than one with little or no cratering evident.
In Ontario, Canada there is a crater about 60 miles (97 kilometers) across that likely hit Earth more than 100 million years ago. The biggest known crater on Earth is actually in the ocean, just off the coast of Yucatan in Mexico. Shocked quartz (which is not made in volcanoes) is abundant there. There is evidence (a huge soot deposit), as well, of massive forest burning and also of a tidal wave perhaps a mile (1.6 kilometers) in height hitting all around southeastern North America.
The crater itself was found as a mass concentration that was detectable by satellite. This impact is thought to be the one that caused the extinction of, among many other species, the dinosaurs. "Space dust" (the element iridium, for example, which is much more abundant in space than on Earth) is found on "top" of most dinosaur bones around the world.
The largest mass extinction in Earth history is not, however, the one that caused the extinction of the dinosaurs (called the Cretaceous-Tertiary or C-T boundary event) but rather one that took place 200 million years before, at the end of what is known as the Permian Era. The Permian extinction led to the Age of the Dinosaurs, while the C-T impact cleared the way for the Age of Mammals. Recent evidence may indicate that the Permian extinction could also have been caused by a giant impact.
Rocks from the Permian period have constituents called "fullerenes" or colloquially, "Bucky Balls" tiny molecular balls made up of around 60 carbon atoms assembled into the shape of a soccer ball (i.e., in a geodesic dome shape, and hence named after the inventor of the geodesic dome, Buckminster Fuller). Sometimes, there is something inside the "ball" some molecules trapped there when the Bucky Ball formed. It looks like these form only in space, and so their presence at the Permian would indicate another impact. But scientists are still debating this new evidence.
A monk actually saw a giant impact into the Moon in the 14th century (and had the courage to record its effect as the dust cloud covered the crescent Moon). A very new impact crater, about in the right place, was noted by the Apollo astronauts as they rounded the Moon in 1968. Also, most astronomers (professional and amateur) saw the impact of the fragments of Comet Shoemaker-Levy hit Jupiters atmosphere a few years ago. So, the immediate terrestrial neighborhood includes lots of comets, asteroids and large meteors that occasionally impact Earth.
But this is still a somewhat newly accepted idea, as the term "meteor," which we still use, comes from a term related to meteorology -- the study of the atmosphere. It was actually not until the Danish astronomer Tycho Brahe could not measure the parallax (apparent shift in perspective caused by looking at an object from one position, then from another) of a comet that comets were then known to be farther than the Moon and not in Earths atmosphere as previously thought.
Fortunately our species has progressed quite some distance in toolmaking, even to the point of using our tools to be able to detect and deflect a comet heading toward Earth. It might be a good idea, actually, for us to consider funding, at some reasonable level, such efforts as they would surely tend to take priority in the event such a comet was ever headed directly our way. Such influences in the terrestrial neighborhood have certainly played a major role in the habitability of Earth.
If Homo sapiens actually managed to deflect a comet heading toward Earth someday, perhaps all the sacrifices the other animals and plants have made to accommodate us and our toolmaking ways to date might be somewhat forgiven. One could imagine the other inhabitants of Earth considering that although humans seemed to have wittingly or unwittingly devastated so many oceans, forests, landscapes, species, well they did manage to deflect that comet.
Perhaps we could start with a clean slate. It would be great to see, in the geologic record of our times, a cessation of the mass extinctions humans are causing. But perhaps we dont have to wait for the opportunity to deflect an impact to begin life anew.