Lightning on Venus

When a spacecraft sweeps past a planet or satellite for the first time, we science reporters start clamoring for pictures and analyses of the results ASAP — what's become known as "instant science." Sometimes the mission teams oblige, and sometimes they just can't because the "results" must be collected carefully over weeks or months.

A good example of the latter occurred last week, when a series of 10 articles from the Venus Express science teams appeared in the journal Nature. Built by the European Space Agency, Venus Express has been studying the world next door since April 2006. Its primary mission — to study the planet for two full Venus "days," which equal 500 Earth days) — is now over, though the spacecraft is doing well and its mission has been extended until at least May 2009.

Lightning on Venus
Lightning is apparently a common occurrence in the hot, dense atmosphere of Venus, as this illustration depicts. But there's not much vertical convection, which is the primary reason that lightning happens on Earth.
European Space Agency
As the scientists report, Venus now seems even less like Earth than it did before. We've know for decades that it's a hellish place, with surface temperatures near 900°F (480°C) and oppressively dense carbon-dioxide atmosphere laced with sulfuric-acid hazes. Not high on my list of vacation spots.

Let me just touch on two of the most interesting results from the seven experiments carried by Venus Express. First, the spacecraft found "sound" evidence for the existence of lightning, a claim first made back in the 1970s but one that's remained controversial. What clinched the case for lightning was the unambiguous and frequent occurrence of "whistlers," which are electromagnetic pulses created by lightning strikes that sound like, well, whistles. They occur all the time after lightning flashes on Earth.

According to a team led by Christopher Russell (Univ. of California, Los Angeles), they're common on Venus as well. The spacecraft's magnetometer frequently picked up bursts of whistlers coming from the ionosphere, and the team concludes that lightning might be as commonplace on Venus as it is on Earth. Yet some scientists remain puzzled, because Venus's atmosphere lacks the vigorous vertical convection that leads to lightning on Earth.

Even though Venus has no magnetic field, electrical currents induced in its ionosphere keep the high-speed solar wind from entering the upper atmosphere as it races by.
European Space Agency
The other result to report is that hydrogen and oxygen are readily escaping from the planet, likely stripped away by the solar wind as it races by. Now, Venus hasn't had any water to speak of for billions of years. It probably boiled away when the planet's runaway greenhouse took hold early in its history. That catastrophic loss left the remaining airmass enriched in a heavier-than-normal isotope of hydrogen called deuterium. An atom of hydrogen can escape to space more easily than one of deuterium, and scientists realized in 1978 that the ratio of deuterium to hydrogen in Venus's atmosphere is 150 times greater than it is in terrestrial seawater.

But what's fascinating is that the D:H ratio is even higher (2½ greater) in the planet's upper atmosphere, 50 to 60 miles up. Why so? One possibility is that Venus is sweeping up water from passing (and colliding) comets, and cometary water has a huge excess of deuterium. The problem is that the atmosphere has far less water vapor on top than it does lower down — a puzzling result for which investigator Jean-Loup Bertaux and his experiment team have no explanation.

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