A debate: How vital is a planet's magnetic field?
Our nearest planetary neighbors, Mars and Venus, have no oceans or lakes or rivers. Some researchers have speculated that they were blown dry by the solar wind, and that our Earth escaped this fate because its strong magnetic field deflects the wind. However, a debate has arisen over whether a magnetic field is any kind of shield at all.
The controversy stems from recent observations that show Mars and Venus are losing oxygen ions from their atmospheres into space at about the same rate as Earth. This came as something of a surprise, since only Earth has a strong dipolar magnetic field that can prevent solar wind particles from slamming into the upper atmosphere and directly stripping away ions.
"My opinion is that the magnetic shield hypothesis is unproven," said Robert Strangeway of UCLA. "There's nothing in the contemporary data to warrant invoking magnetic fields."
Each of the three planets is losing roughly a ton of atmosphere to space every hour. Some of this lost material was originally in the form of water, so this begs the question: How did the planets end up with vastly different quantities of water if they are all "leaking" to space at similar rates?
"The problem is in taking today's rates and trying to guess what was happening billions of years ago," explained Janet Luhmann of the University of California, Berkeley. She believes Earth's magnetic field could have made the difference in the past when the solar wind was presumably stronger.
"People aren't putting all the cards on the table," Luhmann said. "We can't say that magnetic fields are unimportant from the current data."
Both Luhmann and Strangeway agree that sorting out what makes one planet wet while another is dry will require more data on how the atmospheric loss depends on the sun's output.
The main driver of ion escape from planetary atmospheres is the solar wind , which is a high-speed outflow from the sun consisting mostly of protons and electrons. Because these particles carry a charge, their paths bend when they encounter a magnetic field.For non-magnetized Mars and Venus, the solar wind basically barrels straight into the upper atmosphere and scoops up ions and carries them into space. Warth's magnetic field provides a barrier to the solar wind, called the magnetosphere, but ions still get stripped away through a circuitous route.
Essentially, the solar wind interacting with the Earth's magnetic field transfers some of its energy into the upper atmosphere in the polar regions. The auroras that are visible at high latitudes are one manifestation of this transfer. But it also heats up atmospheric ions enough that they escape up out of the poles, forming Earth's "polar ion outflows.
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