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Narrator: Today, Mars is a cold
desert surrounded by a thin wisp of air,

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but its dry lakebeds and empty
river channels point to a warmer,

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wetter past
maintained by a thicker atmosphere.

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Where did the ancient atmosphere go?

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And with it the water?

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To answer that question, NASA's
MAVEN orbiter

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has been studying
the upper atmosphere of Mars since 2014.

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Now, it has witnessed a rare phenomenon

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that was last seen
more than two decades ago at Earth.

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Among MAVEN’s suite of science instruments
is the Solar Wind Ion Analyzer,

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which measures electrically charged
particles or ions surrounding Mars.

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In this data visualization, yellow spikes
indicate the velocity

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of charged particles encountered by MAVEN
along one of its orbital tracks.

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The largest source of charged particles
in the solar system is the Sun,

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which constantly bombards the planets with
a stream of electrons and hydrogen ions.

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When this solar wind reaches
Mars, it interacts

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with heavier ions in the planet's
upper atmosphere.

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This creates a global magnetic field,
or magnetosphere,

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that deflects the solar wind around
Mars in a bow shock.

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MAVEN’s science orbit is designed
to probe these distinct regions in situ.

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With each pass, it crosses through
the magnetosphere, bow shock, and upstream

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solar wind, measuring changes in ion
velocity and density along the way.

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On December 25th, 2022, MAVEN

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encountered a sudden and dramatic decrease
in solar wind density.

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As the pressure of the solar wind dropped,
the Martian magnetosphere

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and bow shock ballooned outward,
engulfing MAVEN's orbit.

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From the spacecraft's
perspective, shielded beneath

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the bow shock,
the solar wind had disappeared.

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In 1999, NASA's ACE satellite
observed the same phenomenon at Earth.

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The solar wind density dropped
by more than 98%, causing our planet's

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magnetosphere to expand to over five times
its normal size.

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These rare events occur
when a fast-moving region of the solar

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wind overtakes a slower moving region,
leaving a low-density void in its wake.

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As quickly as the solar wind
had disappeared from Mars,

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it returned on December 27th
and squeezed the magnetosphere and the bow

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shock back to their usual proportions.

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MAVEN could once again feel the solar wind
blowing across its instruments,

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and it could continue to study
how Mars had evolved from a wet,

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hospitable planet
into the cold, dry world we see today.

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[Music]