Conceptual image depicting the early Martian
environment (right) – believed to contain liquid water and a thicker
atmosphere – versus the cold, dry environment seen at Mars today (left). Credits: NASA’s Goddard Space Flight Center. Hi-res image
A type of Martian aurora first identified by NASA’s MAVEN spacecraft in 2016
is actually the most common form of aurora occurring on the Red Planet,
according to new results from the mission. The aurora is known as a
proton aurora and can help scientists track water loss from Mars’
atmosphere.
At Earth, aurora are commonly seen as colorful displays of light in
the night sky near the polar regions, where they are also known as the
northern and southern lights. However, the proton aurora on Mars happens
during the day and gives off ultraviolet light, so it is invisible to
the human eye but detectable to the Imaging UltraViolet Spectrograph
(IUVS) instrument on the MAVEN (Mars Atmosphere and Volatile EvolutioN)
spacecraft.
MAVEN’s mission
is to investigate how the Red Planet lost much of its atmosphere and
water, transforming its climate from one that might have supported life
to one that is cold, dry, and inhospitable. Since the proton aurora is
generated indirectly by hydrogen derived from Martian water that’s in
the process of being lost to space, this aurora could be used to help
track ongoing Martian water loss.
“In this new study using MAVEN/IUVS data from multiple Mars years,
the team has found that periods of increased atmospheric escape
correspond with increases in proton aurora occurrence and intensity,”
said Andréa Hughes of Embry-Riddle Aeronautical University in Daytona
Beach, Florida. Hughes is lead author of a paper on this research published December 12 in the Journal of Geophysical Research, Space Physics.
“Perhaps one day, when interplanetary travel becomes commonplace,
travelers arriving at Mars during southern summer will have front-row
seats to observe Martian proton aurora majestically dancing across the
dayside of the planet (while wearing ultraviolet-sensitive goggles, of
course). These travelers will witness firsthand the final stages of Mars
losing the remainder of its water to space.” Hughes is presenting the
research on December 12 at the American Geophysical Union meeting in San Francisco.
Different phenomena produce different kinds of aurora. However, all
aurora at Earth and Mars are powered by solar activity, whether it be
explosions of high-speed particles known as solar storms, eruptions of
gas and magnetic fields known as coronal mass ejections, or gusts in the
solar wind, a stream of electrically conducting gas that blows
continuously into space at around a million miles per hour. For example,
the northern and southern lights at Earth happen when violent solar
activity disturbs Earth’s magnetosphere, causing high velocity electrons
to slam into gas particles in Earth’s nightside upper atmosphere and
make them glow. Similar processes generate Mars’ discrete and diffuse
aurora – two types of aurora that were previously observed on the
Martian nightside.
This animation shows a proton aurora at Mars.
First, a solar wind proton approaches Mars at high speed and encounters a
cloud of hydrogen surrounding the planet. The proton steals an electron
from a Martian hydrogen atom, thereby becoming a neutral atom. The atom
passes through the bowshock, a magnetic obstacle surrounding Mars,
because neutral particles are not affected by magnetic fields. Finally,
the hydrogen atom enters Mars' atmosphere and collides with gas
molecules, causing the atom to emit ultraviolet light.Credits: NASA/MAVEN/Goddard Space Flight Center/Dan Gallagher. Download this graphic
Proton aurora form when solar wind protons (which are hydrogen atoms
stripped of their lone electrons by intense heat) interact with the
upper atmosphere on the dayside of Mars. As they approach Mars, the
protons coming in with the solar wind transform into neutral atoms by
stealing electrons from hydrogen atoms in the outer edge of the Martian
hydrogen corona, a huge cloud of hydrogen surrounding the planet. When
those high-speed incoming atoms hit the atmosphere, some of their energy
is emitted as ultraviolet light.
Images of Mars proton aurora. MAVEN’s Imaging
Ultraviolet Spectrograph observes the atmosphere of Mars, making images
of neutral hydrogen and proton aurora simultaneously (left).
Observations under normal conditions show hydrogen on the disk and in
the extended atmosphere of the planet from a vantage point on the
nightside (middle). Proton aurora is visible as a significant
brightening on the limb and disk (right); with the contribution of
neutral hydrogen subtracted, the distribution of proton aurora is
revealed, showing that it peaks in brightness just off the Martian disk
as energetic neutrals slam into the atmosphere. Credits: Embry-Riddle Aeronautical University/LASP, U. of Colorado
When the MAVEN team first observed the proton aurora, they thought it
was a relatively unusual occurrence. “At first, we believed that these
events were rather rare because we weren’t looking at the right times
and places,” said Mike Chaffin, research scientist at the University of
Colorado Boulder’s Laboratory for Atmospheric and Space Physics (LASP)
and second author of the study. “But after a closer look, we found that
proton aurora are occurring far more often in dayside southern summer
observations than we initially expected.” The team has found proton
aurora in about 14 percent of their dayside observations, which
increases to more than 80 percent of the time when only dayside southern
summer observations are considered. “By comparison, IUVS has detected
diffuse aurora on Mars in a few percent of orbits with favorable
geometry, and discrete aurora detections are rarer still in the
dataset,” said Nick Schneider, coauthor and lead of the IUVS team at
LASP.
The correlation with the southern summer gave a clue as to why proton
aurora are so common and how they could be used to track water loss.
During southern summer on Mars, the planet is also near its closest
distance to the Sun in its orbit and huge dust storms can occur. Summer
warming and dust activity appear to cause proton auroras by forcing
water vapor high in the atmosphere. Solar extreme ultraviolet light
breaks the water into its components, hydrogen and oxygen. The light
hydrogen is weakly bound by Mars’ gravity and enhances the hydrogen
corona surrounding Mars, increasing hydrogen loss to space. More
hydrogen in the corona makes interactions with solar-wind protons more
common, making proton aurora more frequent and brighter.div>
All the conditions necessary to create Martian proton aurora (e.g.,
solar wind protons, an extended hydrogen atmosphere, and the absence of
a global dipole magnetic field) are more commonly available at Mars
than those needed to create other types of aurora,” said Hughes. “Also,
the connection between MAVEN’s observations of increased atmospheric
escape and increases in proton aurora frequency and intensity means that
proton aurora can actually be used as a proxy for what’s happening in
the hydrogen corona surrounding Mars, and therefore, a proxy for times
of increased atmospheric escape and water loss.”
This research was funded by the MAVEN mission. MAVEN's principal
investigator is based at the University of Colorado's Laboratory for
Atmospheric and Space Physics in Boulder, Colorado, and NASA Goddard
manages the MAVEN project. NASA is exploring our Solar System and
beyond, uncovering worlds, stars, and cosmic mysteries near and far with
our powerful fleet of space and ground-based missions.
Bill Steigerwald / Nancy Jones
NASA Goddard Space Flight Center, Greenbelt, Md.
301-286-8955 / 301-286-0039
william.a.steigerwald@nasa.gov / nancy.n.jones@nasa.gov
Editor: Bill Steigerwald
Source: NASA/Mars
Bill Steigerwald / Nancy Jones
NASA Goddard Space Flight Center, Greenbelt, Md.
301-286-8955 / 301-286-0039
william.a.steigerwald@nasa.gov / nancy.n.jones@nasa.gov
Editor: Bill Steigerwald
