Sky & Telescope - December 2023 - 36

Interplanetary Sneezeguards
MAGNETOSPHERES Venus, Earth, and Mars are
each sheathed in magnetic fields that stream out
behind their nightsides. The field carried by the
solar wind creates the small magnetospheres
around Venus and Mars. Mars also has local fields
locked in its crust from its long-gone global field,
and these can interact with the solar wind and
even pinch off. Earth, conversely, still generates
its own global field, encasing itself in a much
larger magnetic bubble. Here, Venus and Mars appear
to scale with each other, but Earth is 60% its
actual relative size, in order to fit it on the page.
Draped
interplanetary
magnetic field
Solar
wind
4
3
Venus radii
2
Solar
wind
Earth radii
15 14 13
12
10
Venus
Venus radius = 0.95 Earth radius
Mars radius = 0.53 Earth radius
locked into surface rocks. (Earth has these crustal fields, too,
although here we sometimes call them magnetic anomalies.)
We don't know whether Venus had a global field. Rocks
can preserve magnetic fossils so long as the rocks aren't
too hot, explains Shannon Curry (University of California,
Berkeley). Volcanism has flooded the Venusian surface, and
it can erase magnetism. But surface temperatures today don't
exceed the critical threshold to wipe away frozen-in fields.
Although spacecraft haven't detected crustal fields at Venus
36 DECEMBER 2023 * SKY & TELESCOPE
yet, our measurements are limited, she says - for instance,
we haven't flown a magnetometer close enough to the southern
hemisphere to have picked up any fields there.
Venus and Mars may not have intrinsic fields today, but
they do have a different kind of magnetic shield, one given
them by the Sun. A planet's upper atmosphere is largely ions,
which means it's full of charged particles. When the solar
wind hits the ionosphere, the magnetic field the wind carries
induces a current, making the charged particles flow. This
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BEATRIZ INGLESSIS / S&T

Sky & Telescope - December 2023

Table of Contents for the Digital Edition of Sky & Telescope - December 2023

Contents
Sky & Telescope - December 2023 - Cover1
Sky & Telescope - December 2023 - Cover2
Sky & Telescope - December 2023 - 1
Sky & Telescope - December 2023 - Contents
Sky & Telescope - December 2023 - 3
Sky & Telescope - December 2023 - 4
Sky & Telescope - December 2023 - 5
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