Among the millions of striking images generated in the last century (when reporting the news became virtually identical with the showing of images) surely the most remarkable is the Earth as seen from the Moon, photographed (in 1969) by the NASA space travelers Neil Armstrong and Edwin Aldrin. Standing on the lifeless and inhospitable surface of our familiar and yet alien satellite, without water or air, fully exposed to the harshness of outer space, the Eagle crew contemplated their lovely home planet, a quarter of a million miles away.
A symphony in blue and white, with a bit of orange rust, the colors shown by our planet signal the presence of abundant water as ocean and cloud, and the presence of oxidized iron. It is our one sample of a planet with life. As long as it is our only sample, we shall not know whether it is typical for a life-bearing planet or whether it is some kind of freak phenomenon. Intuitively, one senses that the Moon's environment is likely to be more common than the Earth's. Requirements are modest: a big disorganized pile of rocks, driven by gravitational attraction, must coalesce to make one big spherical heap of rock. The mass is too small to sustain long-term internal heating and convection, and too small to hold on to an atmosphere. Apparently, Mercury has much the same origin. In contrast, Earth's history is much more complex, because of internal gravitational segregation, convection, and the buildup of an ocean and an atmosphere. Earth's siblings, Venus and Mars, are quite different because the complexity of development in the terrestrial planets allows for divergent evolution depending on differences in mass, distance from the Sun and rotation.
The face of Earth changes all the time. Swirling cloud systems run along ill-defined latitudinal tracks, more or less symmetrical about the equator. In the bands adjacent to the equator the motion is to the west, opposite to the sense of the overall rotation. In mid-latitudes, halfway between equator and poles, the motion is to the east, in the same sense as the rotation of the planet. The reason is simple: eastward motion of the planet's surface is fastest at the equator (40,000 km in 24 hours). When air parcels arrive from higher latitudes, they tend to lag behind this motion, coming from a region that moves less rapidly toward the east. This lag results in westward motion relative to the surface. Conversely, air parcels moving away from the equator will tend to get ahead of the local motion. Thus, they move eastward relative to the surface. The exchange of air masses between low and high latitudes results from winds set up by unequal solar heating.
Clouds over Africa and the Middle East (Source:NASA)
As the Earth rotates on its schedule of once every 23 hours and 56 minutes (as seen against the stars), different landmasses come into view. Their colors,hues of brown and green, change through the seasons, as does the extent of white areas near the poles. We note that there is an unequal distribution of land on the planet; there is a lot more of it on the northern hemisphere than on the southern one. Oddly, there also is a water hemisphere and a land-dominated hemisphere, with centers in the middle of the largest ocean (say, Christmas Island in the Pacific) and at the junction of Africa and Eurasia. A large rift separates the western-most landmasses (the Americas) from the rest, with western and eastern shorelines of the rift ocean (the Atlantic) running parallel to each other. From space, with sufficiently precise instruments, we can see that the rift is widening at the rate of roughly an inch per year (depending on latitude).
Thus the changing face of the Earth. Most recently, geologically speaking, the nature of the change itself has changed. Within the last two centuries, the human species entered a new phase of its history. Humans have become a major geologic agent, capturing vast amounts of water, changing the rates of erosion and the nature of geochemical cycles, and affecting the heat budget and the climate of the planet. Humans have set foot on a celestial body other than their home planet. And humans have begun advertising their presence in the solar system by making the home planet into a strong radio-emitter.
An alien observing station on one of the moons of Jupiter, monitoring the Earth, would have recorded large changes in the last hundred years, including global warming (since 1900), nuclear explosions, changes in forest cover, changes in the composition of the atmosphere, and a rapid increase in the intensity of radio waves. Finally, the Jupiter station would have recorded a rapid increase in small satellites circling the Earth. The unavoidable conclusion would be that something unprecedented is happening on Earth, as one of its life forms is changing the environment in fundamental ways. As the alien observer can tell, Intelligent Life is making its mark.
Satellite orbits over North America (Source: Bester Tracking Systems)
Of course, the alien observers would have known all along that Earth harbors Life even before these latest drastic changes set in. Over the last hundreds of millions of years Life has changed the Earth's environment, and living things have evolved along with this change, so that they were "in tune" with the environment. (This makes it look, to the naive observer, as though Life is changing the environment in a purposeful way, to make the environment more suitable for Life's continued existence.) The observers would have noticed that the Earth's atmosphere contains elementary nitrogen and oxygen (each as a molecule with two atoms) and they would have known that this combination is inherently unstable. As Hutchinson pointed out in the middle of the last century, bacteria need to keep renewing the nitrogen gas, and photosynthesizing plants need to keep making oxygen. Otherwise, after some time, the oxygen would be used up to oxidize carbon, iron and sulfides, and the nitrogen would end up as ammonia in the air and as dissolved ammonium, in the ocean.