The most easily-detectable structure from space

What is the human structure that is most easily distinguishable from space? What is striking, what stands out amongst the white, blue and brown, on the blue marble of our planet? If an alien spacecraft approached the Earth, what would it make out first? What human construction would stand out from everything else?

As we approach our planet, as the spatial resolution improves and we can see more details on its surface, the presence of oceans and continents is revealed, as well as changing clouds, multiple night lights mapping the profiles of the coasts and large nuclei of industrial and mining populations. However, what would we see during the day, what would stand out?

The Great Wall of China stands as an example of human tenacity. Or the Great Pyramids of ancient Egypt (there would also be some who would consider these constructions as a monument to misery and the darker side of human beings, as they were built on the shoulders of thousands of slaves). Nonetheless…

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Projection of the surface of our planet. What is the most easily-identifiable artificial structure on it? (NASA credit).

A long time ago, while I was sharing a table with Michael López-Alegría, NASA astronaut born in Madrid, I was surprised by the answer he gave us. Michael was the commander of the International Space Station (ISS) for Expedition 14. He was there from September 2006 until April of the following year, for nearly seven months. His answer was clear: the artificial structure that can be seen most easily from a low orbit is… the fields of greenhouses in Almeria, southeast of Spain.

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Europe, the Mediterranean, Middle East and Siberia, covered by snow. However, the white dot in the southeast of Spain corresponds to another highly-reflective substrate: the plastic greenhouses of Almería (NASA/Visible Earth credit).

In the image of Europe attached I would almost say that you can catch a glimpse of it. There is no doubt when you look more closely at the Iberian Peninsula. The fields are displayed as a bright spot at the bottom, on the Spanish coast.

There are tens of thousands of hectares (from 20,000 to 70,000, depending on the source) covered with plastic, where various crops of tropical fruit, peppers, tomatoes, zucchini, watermelons, melons and other horticultural products are produced in greenhouses, mainly bound for the Spanish and European market. The area is virtually a desert where it very seldom rains. In fact, a true desert is located nearby (this is one of the reasons why the German/Spanish Observatory Calar Alto is located in the vicinity). Production depends on groundwater, which is difficult to replace once depleted. This area was developed within very few years, mainly due to small plots being converted into greenhouses by their owners. The area is so easily distinguishable because plastic has a high reflectivity, or high albedo as us astronomers say.

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The Iberian Peninsula, with snow in the Pyrenees mountain range, which separates Spain from France (NASA/Visible Earth credit).

It is surprising to see the image, seeing how fast we can transform the environment and leave such a visible mark. It is largely very saddening. It is certainly true that thousands of people live from the economic activity in the area and many of the tomatoes or fruit that millions of Europeans eat out of season come from these greenhouses. However, such a development seems unsustainable, especially in a place where the most precious commodity, water, is so obviously scarce. And it will be far worse in the future, according to the predictions of the effects of climate change on Spain. To be fair, efforts are being made to optimize the use of water in order to optimize production, but where a resource is so scarce, even the most demanding technology cannot work miracles. Only rationally managing water, here and elsewhere, has a future. We must not forget that this element is truly essential.

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Fields of greenhouses in Almería (NASA/Visible Earth credit).

David Barrado Navascués

CAB, INTA-CSIC

European Space Astronomy Center (ESAC, Madrid)