Sky & Telescope - February 2024 - 16

Mars Missions
EMM's global coverage and systematic, almost simultaneous
observations at infrared, visible, and ultraviolet wavelengths
are giving researchers a new, more detailed perspective. Mission
scientists like Al Matroushi think that scientists outside
the team are just realizing the possibilities that these integrated
atmospheric data offer, and more external collaborations
are starting to roll in. " That speaks of the gaps that we
had before, " she says.
Unexpected Aurorae
One of the most surprising EMM achievements so far is its
new insights into Martian aurorae, including the discovery of
a new kind. The mission wasn't planned with geomagnetism
in mind, so these observations come as a bonus.
On Earth, charged particles from the Sun are redirected
by our planet's magnetic field towards the poles, giving us
our beautiful high-latitude aurorae (S&T: Oct. 2023, p. 74).
Mars doesn't have a global magnetic field, but it still has an
atmosphere that can interact with the solar wind. It also
has crustal paleomagnetism, local magnetization engraved on
lava fields that solidified when the planet still had its global
magnetic cloak (S&T: Dec. 2023, p. 34). As a result, Martian
aurorae take many forms and can occur at any latitude.
Incoming protons are responsible for diffuse aurorae, which
occur on the nightside at low altitude, and patchy proton
aurorae, which appear on the dayside. Discrete aurorae, on the
other hand, are produced by electrons hitting the upper layers
of the atmosphere. Other orbiters have previously detected
several examples of them, but Hope is the first to provide a
global view.
" It's almost like a zoo, " says EMM science team member
and Martian aurora expert Robert Lillis (University of
California, Berkeley). " The last two years we've seen so many
different shapes, and sizes, and brightnesses, and patterns,
but they sort of fall roughly into three categories. "
The first type of discrete aurora is called crustal field aurorae.
These are bright and localized. They occur in areas where
the crustal magnetic field is strong and vertical and funnels
electrons into the thin atmosphere, increasing the flux and
therefore the aurora's brightness.
Then there are the patchy aurorae. These are the most
common type and appear as broad areas of emission, many
thousands of kilometers across on the nightside. They aren't
related to crustal fields.
Scientists knew of these two kinds of discrete aurora
before EMM, but the orbiter has revealed a third: snake-like
aurorae that extend for many thousands of kilometers from
the dayside in the northern hemisphere into the nightside.
These sinuous discrete aurorae, as researchers have called them,
seem to form away from crustal magnetic fields. They occur
both at dusk and dawn but are twice as frequent at dusk.
It isn't entirely clear if sinuous aurorae are bright enough
that a human standing on the surface of Mars could see
them. " If they were really obvious, the cameras on the rovers
might have seen them, " Lillis says. However, " human eyes
can become very accustomed to the dark, and therefore very
sensitive, so we think it would be possible, " he adds.
EMM has also captured new details of patchy proton aurorae.
NASA's MAVEN team first reported these aurorae in 2018
as a smooth glow. But EMM's EMUS has revealed that proton
aurorae can become irregular during solar storms, suggesting
that when the magnetic environment around Mars gets
uDUST IN THE AIR These images from the EXI instrument track the long, winding front of a dust cloud on Mars on September 10, 2021. The
images are each separated by two to three hours, and the cloud largely stayed put from beginning to end. The storm's large atmospheric front was
unusual for the location and season.
16 FEBRUARY 2024 * SKY & TELESCOPE
C. GEBHARDT ET AL. / GEOPHYSICAL RESEARCH LETTERS 2022 / CC BY 4.0 DEED (4)

Sky & Telescope - February 2024

Table of Contents for the Digital Edition of Sky & Telescope - February 2024