Cosmologically speaking a planet almost exactly like the earth is just next door. Venus, the second planet in our Solar system has about the same size, similar composition and is formed around the same star.
From a telescope light-years away, Venus would be virtually indistinguishable from our own planet. But We know that the surface conditions on the planet are anything but earth-like with temperatures similar to a self-cleaning oven, and an atmosphere completely saturated with carbon dioxide with sulfuric acid clouds.
The reason behind how two planets which are so similar in position, formation and even composition can end up being so different, has preoccupied an ever-growing number of planetary scientists and has motivated numerous Venus exploration efforts. Scientists believe that understanding why Venus turned out the way it did, will give them a better understanding of whether an Earth-like planet is the rule or actually the exception.
Venus has also interested planetary scientists as it offers a glimpse of a world that once might not have been so different from our own.
A Once-Blue Venus
The current scientific view of Venus proposes that at some point in the past, the planet contained much more water than its bone-dry atmosphere advocates today and perhaps even oceans. But as the sun grew hotter and brighter as a natural consequence of aging, surface temperatures started surging on Venus, eventually vaporizing all oceans and seas.
With ever more water vapor in the atmosphere, the planet passed into a runaway greenhouse condition from which it couldn’t recover. Whether earth-style plate tectonics with the outer layer of the planet broken into large, mobile pieces, ever operated on Venus is not known. Water is critical for the movement of plate tectonics and a runaway greenhouse effect would effectively shut down that process even if it did operate there.
But the ending of plate tectonics wouldn’t have completely ended all geological activity. The planet’s substantial internal heat must have continued to produce magma, which in turn poured out as voluminous lava flows and resurfaced most of the planet. Indeed, the average surface age of Venus is about 700 million years which is much younger than the multi-billion-year-old surfaces of our other cosmic neighbors- Mars, Mercury, and the Moon.
The world view of Venus-as-a-wet planet is just a hypothesis as planetary scientists don’t really know what caused this planet to differ so much from Earth, nor even if the two planets really did begin with the same conditions. We know less about Venus than we do about any other inner solar system planets, principally because the planet poses several exceptional challenges to its exploration.
For instance, radar is required to pierce the opaque, sulfuric acid clouds that encapsulate Venus making even viewing the surface much more trickier than the readily visible surfaces of the Moon or Mercury. And the high surface temperature of 470 degrees celsius (880 degrees Fahrenheit) implies that conventional electronics cannot last more than a few hours. That’s a major contrast from Mars, where our rovers have operated for more than a decade. The excruciating heat, acidity, and concealed surface have all deterred any sustained program of Venus exploration over the past couple of decades.
There have been two dedicated Venus missions in the 21st century: the European Space Agency’s Venus Express, which was operational from 2006 to 2014, and the Japan Aerospace Exploration Agency’s Akatsuki spacecraft that is currently in orbit.
Venus was once the primary choice of planetary exploration having some 35 missions between the 1960s and 1980s. The NASA mariner 2 mission was the first spacecraft to successfully have a planetary encounter when it flew past the second planet in 1962. The first images of Venus returned from the surface of another world were sent from the Soviet Venera 9 lander after it touched down in 1975. And the Venera 13 lander was the first spacecraft to transmit sounds from the surface of Venus. But the last Venus mission was the Magellan in 1989 launched by NASA which imaged nearly the entire surface with radar before its planned demise in the planet’s atmosphere in 1994.
In the last few years, several Venus missions have been proposed by NASA. The most recent planetary mission that selected by the space agency is a nuclear-powered craft called dragonfly, destined for Saturn’s moon Titan. Nearly 30 years after NASA set course for our neighbor, the future of Venus exploration finally looks promising.
But a sustained program of exploration instead of a single mission is desirable to bring our knowledge of Venus to where we understand it and be at par with Mars or the Moon. And, under an ever-brightening sun, Venus may even help us understand the fate of Earth itself.