http://www.skyandtelescope.com.au 19the solar wind slowly erodes gas at high altitudes, especially
lighter atomic species such as hydrogen. Over the eons,
the solar wind has likely stripped away almost all of the
atmosphere that Mars once had.
The spacecraft has also measured changing atmospheric
escape rates over the southern hemisphere’s magnetic stripes.
As MAVEN project scientist Gina DiBraccio (NASA Goddard
Space Flight Center) notes, “We’re finding that these crustal
magnetic fields can both prevent and facilitate atmospheric
escape at Mars,” she says. The crustal field ‘bubbles’ prevent
the local atmosphere from being stripped away. But often
the solar wind’s magnetic field connects with these localised
Martian fields, providing a conduit for atmospheric particles
to escape (see https://is.gd/Mars_magnetism).
Like Mars and Earth, Venus probably once had abundant
water and, therefore, lots of atmospheric hydrogen, created
when solar ultraviolet light dissociated (broke apart) water
molecules. But because Venus lacked a global field, the solar
wind could penetrate the planet’s upper atmosphere and
eventually stripped away the loose hydrogen atoms. With
the hydrogen gone, any chance of reconstituting the planet’s
original cache of water was lost.
PROTECTIVE BARRIER
The bubble-shaped magnetosphere
produced by Earth’s magnetic field (orange
and blue lines at centre) plays a crucial role in
protecting our planet from the solar wind and
other forms of space radiation. Mercury and
Earth are the only terrestrial planets that still
have global magnetic fields.Some scientists have even explored whether a strong
magnetic field might do more harm than good. Earth’s
magnetosphere keeps the solar wind from slamming
directly into our atmosphere, which sounds beneficial. But
as Driscoll points out, there are competing effects: “The
stronger the magnetic field, the larger the magnetosphere.
This makes the magnetosphere a larger target to the solar
wind, and that could actually be funneling more solar wind
energy into the magnetosphere, which causes additional
atmospheric escape.”
Because strong dipolar fields can play both helpful and
potentially harmful roles, the importance of a magnetic field
for retaining an atmosphere — and for the rise of complex
life — remains an open question. Ultimately, we know of
one terrestrial-size planet with a rich abundance of life
forms, and that planet has maintained a strong dipolar
field for billions of years. As Stanley says, “Perhaps it’s just a
coincidence that life is on a planet with a magnetic field, but
it’s likely that the field is an important part.” ROBERT NAEYE is a former editor in chief of the US edition
of Sky & Telescope.