If an advanced extraterrestrial civilization were to detect Earth and attempt to assess signs of intelligence, they would likely focus on several key indicators.
Technological Footprints: One of the most apparent signs would be the presence of satellite networks surrounding Earth. These artificial satellites form a dense layer of technology that would be noticeable through their orbits and radio emissions. Furthermore, the electromagnetic spectrum would reveal radio and television broadcasts, indicative of technologically advanced societies.
Urban Illuminations: Aliens could observe city lights at night. The widespread and organized patterns of illumination, especially in urban areas, would suggest a developed civilization with the capacity for large-scale infrastructure and energy management.
Atmospheric Composition: Aliens might analyze Earth’s atmosphere to understand its chemical composition. The presence of pollutants, such as CFCs (chlorofluorocarbons), could signal industrial activity. The complex mix of gases, including high levels of nitrogen dioxide (NO2) and carbon monoxide (CO), along with fluctuating levels of carbon dioxide (CO2) and methane (CH4) due to human activity, would imply active modification of the environment.
Global Changes and Structures: Large-scale modifications of the environment, such as deforestation visible via satellite imagery, crop patterns, dams, and other infrastructure, would show that humans have a significant impact on Earth’s ecosystems. Megastructures like the Great Wall of China or engineered landscapes, like those seen from space, could also serve as markers of intelligent species.
Communication Signals: Aside from radio and TV signals, any attempt at interstellar communication, such as directed, encoded transmissions—like those intentionally sent through projects aiming to contact extraterrestrial life—might be detectable by alien civilizations.
Space Exploration Efforts: Observations of objects leaving Earth’s atmosphere, such as spacecraft, would indicate an advanced stage of exploration and technological capabilities. Artifacts like the International Space Station provide evidence of sustained human presence in orbit.
Astronomical Phenomena Alterations: Aliens might notice slight changes—though these would be subtle—provoked by human exploration missions, such as satellites visiting the Moon, Mars, and beyond. Reflection properties of man-made structures on the Moon, or Earth-crossing spacecraft, could be detected with sensitive instruments.
Bio-signatures and Unnatural Patterns: Finally, they might study consumption patterns resembling those supporting large populations of intelligent beings. This includes vast agricultural fields or expansive marine harvests, indicating systematic approaches to sustaining large human populations.
In summary, through a combination of technological emissions, artificial illumination, altered atmospheric composition, signs of global engineering, and direct communication attempts, intelligent aliens would accumulate substantial evidence of human civilization’s presence and activities on Earth if they were to observe us first.