Sky & Telescope - December 2023 - 38

Interplanetary Sneezeguards
too. Even the magnetic field itself can dump energy into the
atmosphere, says Ofer Cohen (University of Massachusetts,
Lowell), if the space environment shakes the field up enough
to send waves through it in just the right way.
Against this onslaught, the magnetic field provides a limited
defense. The field only controls charged particles coming
in or going out, things like protons and ionized oxygen. It
cannot protect against radiation. Nor can it hold onto neutral
atoms and molecules. But planets larger than Mars are too
massive to let go of their neutral particles, which hunker
lower in the atmosphere, protected by gravity. For Venus and
Earth, ion-driven escape is the only option.
Even so, Earth's magnetosphere helps as much as it hurts.
In some cases, the field can spit energy from the solar wind
back out, but in others - particularly when the planet's field
and the solar wind's match up in just the right way - the
field stores the energy and deposits it in the atmosphere.
(Think aurorae.)
An intrinsic magnetosphere also makes the planet a much
larger target, diverting more energy from the solar wind into
the ionosphere. And the field funnels energy to specific regions
- on Earth, that's the poles - which can help heavier ions
escape. In fact, for our planet, the net effect may increase how
much energy the solar wind can dump into the atmosphere.
Simulations support this surprising conclusion. In 2018,
Herbert Gunell (now at Umeå University, Sweden) and others
explored what would happen to fictitious Venus, Earth, and
Mars analogs with different magnetic fields. They found that
the planets had similar loss rates whether unmagnetized or
strongly magnetized (as strongly as Jupiter, which has a field
20,000 times stronger than Earth's). But for fields similar to
the modern Earth's, the escape rate was a wee bit higher than
in the unmagnetized scenario.
Planetary Perm
Venus, Earth, and Mars all lose atmosphere differently. Earth
suffers a steady leak from its poles. At Venus, the atmospheric
loss looks more like your hair would if you stuck your head
out the car window while gunning it on the highway, Curry
says. And Mars has " party hair " - a big mohawk plume that
sometimes comes out of the pole and other times from closer
to the equator, depending on the orientation of the solar
wind's magnetic field.
The solar wind's properties matter a lot. The field it carries
is stronger at Venus's distance from the Sun (0.7 astronomical
unit) than it is at Earth (1 a.u.) or Mars (1.5 a.u.), and
it slicks Venus's atmospheric hair back as a result. (It's even
stronger at Mercury's roughly 0.38 a.u., but the pipsqueak
Iron Planet doesn't have an atmosphere.)
It's unclear how much Venus has suffered due to its
proximity to our star, but it has lost gas. Based on the ratio
of hydrogen to deuterium in its skies, we think nearly all of
the planet's water has fled to space. The likely reason is that
Venus was so hot under its greenhouse-gas blanket that the
water evaporated into the atmosphere, where sunlight could
break the molecules apart and leave the hydrogen atoms vulnerable
to space-weather robbery (S&T: Oct. 2021, p. 12).
" Escape has been really important at Venus, " Brain says.
Solar wind
Venus
SOLAR STYLIST Ions escape from Venus, Earth, and Mars in different
ways. At Venus, gas exits through a tail and a slick, pouf-like plume. At
Earth, tall polar fountains turn the planet into a garden gnome. And while
Mars loses three-quarters of its atmosphere through a tail, it also has a
tall plume, much larger than Venus's pouf when compared to the planet's
size. The plumes change latitude depending on the orientation of the solar
wind's magnetic field.
Earth
38 DECEMBER 2023 * SKY & TELESCOPE
BEATRIZ INGLESSIS / S&T
Sky & Telescope - December 2023

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