Our closest cosmic neighbor, Venus, is often referred to as the Morning Star because of its frequent appearance in the hours before dawn. This month, however, the planet, which lies roughly 85 million miles from Earth, dominates the early evening sky. When I point to it hanging prominently in the west, friends are often dismissive, insisting it must be an airplane or helicopter. Other than the sun and moon, no object in the night sky is brighter. So luminous is Venus, in fact, that on clear nights it is possible to see it a half hour or more before sunset, blazing through the deepening blue of approaching night.

And yet, Venus’s eye-catching brightness is deceptive. Even through large telescopes, it appears as little more than a bright white dot. This is because of a thick, highly reflective layer of cloud that obscures its surface. Under that dense atmosphere, 100 times thicker than our own, lies a rocky planet almost identical to Earth in size and mass. For these reasons, it is sometimes referred to as Earth’s Twin. But the similarities are superficial—Venus could hardly be more different. It’s also a fitting object of contemplation in this time of environmental collapse. I prefer to call it the Mourning Star. 

Let’s begin with Venus’s average surface temperature, which is a lead-melting 870°F. That temperature, the highest recorded on any planet in the solar system, is largely the result of tremendous pressure exerted by its smothering atmosphere—a force equivalent to what a submarine would experience under 3,000 feet of water. Venus is covered with numerous volcanoes that have also transformed the landscape and spewed enormous quantities of sulfur. Sulfuric acid vapor in the planet’s upper atmosphere gives Venus its characteristic yellow tinge. 

The first glimpse of the surface of Venus came from a Russian probe called Venera 9, which landed on the planet in 1975. It survived less than an hour, but in that short time it was able to transmit several grainy pictures of a rocky surface obscured in an ominous haze. Other satellites and Earth-based telescopes have used radar to penetrate Venus’s thick clouds, revealing a stunning topography of ragged peaks and sprawling plateaus. The summits of its highest mountain range, the Maxwell Montes, rise to an elevation of 35,000 feet, more than a mile taller than Mount Everest. 

But what makes Venus’s climate so deadly is the fact that 95 percent of its atmosphere is composed of CO2. Venus is a world ravaged by a runaway greenhouse effect. Scientists are still trying to piece together the geological events that led to this state, but researchers at NASA’s Goddard Institute for Space Studies have proposed a stunning hypothesis: In the distant past, Venus was covered in shallow oceans of liquid water and may even have been habitable. But over millennia, those oceans were blasted by intense sunlight. Because Venus is much closer to the sun and has an extremely slow rate of rotation (one day on Venus is equivalent to 117 of our own) the amount of solar energy reaching the planet’s surface is much greater than on Earth. This bombardment of solar radiation evaporated the oceans, turning them to water vapor. Once aloft in the atmosphere, these water molecules were broken apart by the sun’s ultraviolet rays, causing the light hydrogen atoms to escape into space. As water dwindled and eventually disappeared, carbon dioxide accumulated in the atmosphere, causing surface temperatures to rise and evaporation rates to increase. This is the pernicious feedback loop that scientists suspect changed Venus from a watery planet to an infernal hellscape.

At current rates of warming, do we risk a planetary cataclysm—a Venusification—of our own making? We are doing our best to see to it, rapidly infusing the atmosphere with carbon. The changes we are initiating are happening on the order of decades rather than millennia. Since the Industrial Revolution, we have increased the amount of carbon dioxide in the atmosphere by 48 percent. A World Meteorological Organization report from last year indicates that the planet is on track for 3° to 5°C warming by the end of the century. That sort of warming would result in the near complete melting of the polar ice caps and mid-latitude glaciers, the inundation of coastal cities, the loss of vast swaths of habitat and biodiversity and the forced migration northward of tens of millions of people fleeing regions ravaged by increasing temperatures.   

If there is one lesson that Venus, the Mourning Star, teaches us, it is that our atmosphere—the thin membrane that makes life on Earth possible—can, with enough perturbance, become an adversary, a poison shroud, a thick blanket snuffing out all life.