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Early in its history, Mars had a far more hospitable climate than
the one that we see today, with a thick atmosphere and abundant
flowing water. So how did it evolve from warm and wet to cold
and dry? Since 2015, NASA's MAVEN mission has been
investigating this question by studying the red planet’s upper
atmosphere. Now, it has mapped high-altitude global wind
patterns at Mars for the first time. During the closest part of
its orbit, MAVEN skims through the upper atmosphere, ingesting
air molecules and determining their composition with an
instrument called NGIMS. MAVEN can also rotate NGIMS back and
forth to measure the air’s velocity. These measurements
provide a snapshot of windspeed and direction along a track of
MAVEN’s orbit. Because the orbit evolves over time, or precesses,
the spacecraft’s closest approach to Mars drifts across
the planet, allowing it to take snapshots at different
locations. Over the past four years, MAVEN’s tracks have added
up, crisscrossing Mars at many locations and at various seasons
and times of day, wrapping the planet in a web of observations.
The measurements reveal how the winds vary with location, and
evolve throughout the Martian year. Now, scientists have used
MAVEN data to build a global map wind currents roughly 200
kilometers above Mars: the first time that winds have been mapped
globally and at such high altitude anywhere beyond Earth.
Within the new map, scientists discovered an unexpected
feature: imprints of rugged terrain far below. As an
example, during two of its orbits MAVEN saw winds above the
Tharsis plateau diverting away from elevated terrain, driven by
giant shield volcanoes reaching twice the height of Mount
Everest. Here’s how the process works: Down at the Martian
surface, winds are forced around and over obstacles when they
encounter mountainous terrain. Disturbances in surface-level
winds also disturb the air above, propagating up through
the atmosphere as gravity waves. When they reach the upper
atmosphere, the gravity waves block the high-altitude winds,
causing them to alter course, and allowing MAVEN to sense the
presence of mountains and valleys on the surface, while
skimming the edge of space. High above Mars, MAVEN is studying
the upper atmosphere, and reaching out to feel the breeze.
It has now made the first measurement of high-altitude,
global circulation patterns, and discovered a surprising
connection to features on the surface. These measurements also
shed new light on ancient Mars, with its thick atmosphere, and
its warm, wet climate. By revealing how the climate of
Mars works today, MAVEN is helping scientists piece
together the early chapters of its dramatic history.
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