The Venus Express spacecraft has found compelling evidence for active volcanoes on Earth’s sister planet. This will help us understand what makes planets like ours tick.
Data on distant worlds is hard to come by. In planetary science there are many important questions to which we can only answer, “We think so.” The question “Are there planets around other stars?” has recently gone from this category to the “Yes, definitely” column. Now “Are there Earth-sized planets around large numbers of stars?” is poised to follow suit.
Closer to home (much) is the question. “Is Venus geologically active today?” A recently published result seems to move the answer from “We think so” toward “Yes, we’ve found the evidence!” Venus is the only other Earth-sized planet we know. That should change soon, but we won’t be able to closely study the new extrasolar Earths, separated from us by the unspeakably deep interstellar divide. So Venus will be our only opportunity to learn the workings of another Earth-sized world for hundreds or thousands of years.
The surface of Venus is right there, beckoning and haunting us with its compelling answers so close and yet so hard to see. The closest planetary surface to us, and quite possibly the most relevant to the major questions about what makes our own home planet tick, it is also freakishly difficult to explore, thanks to both the crushing, searing surface conditions which destroy surface probes and keep NASA wary of mission proposals, and the thick, unbroken global cloud deck which frustrates direct orbital or telescopic inspection. Yet, just as the interstellar depths have finally been breached with technology and the relentless cleverness of planet-hunters, Venus is gradually giving up the mysteries of her surface. When Galileo flew by Venus in 1990 we discovered that the atmosphere is transparent in certain wavelengths of near-infrared light that allow a spacecraft to see the surface. In 2005 the European Space Agency launched Venus Express, the first orbiting spacecraft with the right instruments to exploit these spectral windows and make infrared surface maps. Now that Venus Express has been in orbit for four years, these maps have revealed that certain features, which resemble Hawaiian-type “hot spot”volcanoes on Earth, are covered with a different type of rock than the surrounding plains. A team of scientists analyzing these maps recently announced in Science that these areas are consistent with fresh volcanic minerals that have not yet chemically reacted with the hot, caustic atmosphere. They must have erupted recently, and this strongly implies that the volcanoes are active today.
Many scientists, myself included, have written that Venus today is probably volcanically active based on many pieces of circumstantial evidence, including likely volcanic gases in the atmosphere and the fresh appearance of the youngest volcanoes in radar images. Yet before this new result we lacked the “smoking gun” to move this question into the “definitely yes” column. Now we can study not just the structures, but also the active functioning, of a nearby almost-Earth.
Venus should be active, since size is the most important controlling factor in a planet’s internal cooling. For example Mars is smaller, cooler inside and mostly dead geologically. Yet, given their nearly identical sizes you would think that Venus and Earth should be more similar. Venus lacks plate tectonics, the system of interior motions and surface movements that organizes most geological activity on Earth and removes heat from our planet’s interior. Instead, Venus may be losing heat today mostly through these hot spot volcanoes. But smooth volcanic lavas, which are not made by hot spots, dominate most of Venus. So apparently Venus has changed her volcanic style radically over the eons. We don’t know why Venus changed but it may have to do with the runaway greenhouse that occurred on Venus billions of years ago, evaporating her oceans and leaving her hot and dry.
Water seems to be the key to the uniqueness of Earth, from life to climate to geology. Perhaps the drying out of Venus long ago changed the properties of the rocks in her interior, led to the cessation of plate tectonics, and caused the changes we see in volcanic style.
We need future missions to understand this history, but now we know there is an active planet next door from which we can learn about the possible lives and fates of Earth-like planets.