Sky & Telescope - December 2023 - 39

" It has been more important at Venus and Mars than it has
been at Earth, if you're interpreting the same isotope signatures
of all three planets. " That fact might be an argument in
favor of an intrinsic magnetosphere's importance.
Mars still provides the best case for a shield effect, but it's
a holey shield. The crustal fields do offer some protection.
The strongest ones lie in the southern hemisphere, and when
they're on the dayside - pointed at the Sun and the impinging
wind - there's a bit less atmosphere escaping than when
they're on the nightside, Curry says. On the other hand, she
adds, the crustal fields may make the atmosphere more vulnerable
by fluffing it up via magnetic pressure.
More important, though, is Mars's size: It's only half as
wide and one-tenth as massive as Earth. It therefore cannot
hold on to its atmosphere as strongly, and so what little
atmosphere exists is puffed up - nothing like Venus's slick
look. A diffuse, inflated atmosphere is a vulnerable atmosphere.
The solar wind hits this extended haze and easily
carries it away.
Mars's size likely doomed the planet at least as much as
the loss of its magnetic field. " I don't want to say it never
had a chance, " Curry says. But it's so small that it can lose
atmosphere in ways Venus and Earth can't. " Whether it had a
magnetic field or didn't, it already had different physics. "
How Close Is Too Close?
Not only does a magnetic field control where a planet's
atmosphere escapes from, it can also affect the atmosphere's
makeup: The field controls the escape of certain chemical
species more than others, as well as the influx of energy and
charged particles from the solar wind. But when it comes
to preserving an atmosphere, many other factors may have
a larger impact. The planet's size and distance from its host
TARGET PRACTICE Artist's concept of a solar eruption headed for
Mars. The sinuous white lines represent the solar magnetic field.
star, the orientation of its magnetic field, the space weather
it experiences, the type of atmosphere it has, and when (and
how quickly) that atmosphere builds up, are all important,
says Cohen.
A planet could replenish its atmosphere by coughing out
volcanic gases as quickly as it loses stuff to space. Venus may
have started its runaway greenhouse process so early that by
the time the global field shut down (assuming it had one),
the atmosphere was too large for the solar wind to remove
it. Plus, Venus is bigger than Mars. " It seems like yes, the
magnetic field did shield the atmosphere to some extent " on
Mars, Cohen says. " But we are not in a position to generalize
that statement to any planet. "
The space environment has a huge impact on atmospheric
loss, he emphasizes. Our terrestrial trio share a fairly clement
interplanetary climate - although solar spitballs can hit one
world and miss the others. But we find exoplanets in a range
of environments around other stars, some more harrowing
than others.
When close to its star, a planet will be baked in heat and
Mars
Mars has " party hair " -
a big mohawk plume that
sometimes comes out of the
pole and other times from
closer to the equator.
buffeted by stellar gusts. The planet Trappist-1e, for example,
ostensibly lies at the right distance from its star to sustain
liquid water on its surface, if it has the right atmosphere. But
Trappist-1 is an active red dwarf, unleashing searing radiation
and magnetic outbursts. Because the star is puny, its habitable
zone girds it tightly: 1e snuggles up at a distance less
than 8% the size of Mercury's orbit.
Astronomers have yet to detect an atmosphere on a terrestrial
world around a red dwarf, and Cohen suspects that
many of these exoplanets have no hope of keeping their
atmospheres. Some even orbit within their star's Alfvén
surface, which marks where the stellar wind is still connected
to the star - essentially, the planets are moving through the
star's outer atmosphere, the wispy but blisteringly hot corona.
Within this boundary, the stellar wind can overwhelm and
disrupt the planet's magnetosphere, constantly penetrating
the world's atmosphere.
The location of the Alfvén surface depends on the strength
sk yand tele scope .o r g * DECEMBER 2023 39
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Sky & Telescope - December 2023

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