Early discoveries by NASA’s newest Mars orbiter are starting to reveal key
features about the loss of the planet’s atmosphere to space over time.
The findings are among the first returns from NASA’s Mars Atmosphere and
Volatile Evolution (MAVEN) mission, which entered its science phase on Nov. 16.
The observations reveal a new process by which the solar wind can penetrate deep
into a planetary atmosphere. They include the first comprehensive measurements
of the composition of Mars’ upper atmosphere and electrically charged
ionosphere. The results also offer an unprecedented view of ions as they gain
the energy that will lead to their to escape from the atmosphere.
“We are beginning to see the links in a chain that begins with solar-driven
processes acting on gas in the upper atmosphere and leads to atmospheric loss,”
said Bruce Jakosky, MAVEN principal investigator with the Laboratory for
Atmospheric and Space Physics at the University of Colorado, Boulder. “Over the
course of the full mission, we’ll be able to fill in this picture and really
understand the processes by which the atmosphere changed over time.”
On each orbit around Mars, MAVEN dips into the ionosphere – the layer of ions
and electrons extending from about 75 to 300 miles above the surface. This layer
serves as a kind of shield around the planet, deflecting the solar wind, an
intense stream of hot, high-energy particles from the sun.
Scientists have long thought that measurements of the solar wind could be
made only before these particles hit the invisible boundary of the ionosphere.
MAVEN’s Solar Wind Ion Analyzer, however, has discovered a stream of solar-wind
particles that are not deflected but penetrate deep into Mars’ upper atmosphere
and ionosphere.
Interactions in the upper atmosphere appear to transform this stream of ions
into a neutral form that can penetrate to surprisingly low altitudes. Deep in
the ionosphere, the stream emerges, almost Houdini-like, in ion form again. The
reappearance of these ions, which retain characteristics of the pristine solar
wind, provides a new way to track the properties of the solar wind and may make
it easier to link drivers of atmospheric loss directly to activity in the upper
atmosphere and ionosphere.
MAVEN’s Neutral Gas and Ion Mass Spectrometer is exploring the nature of the
reservoir from which gases are escaping by conducting the first comprehensive
analysis of the composition of the upper atmosphere and ionosphere. These
studies will help researchers make connections between the lower atmosphere,
which controls climate, and the upper atmosphere, where the loss is
occurring.
The instrument has measured the abundances of many gases in ion and neutral
forms, revealing well-defined structure in the upper atmosphere and ionosphere,
in contrast to the lower atmosphere, where gases are well-mixed. The variations
in these abundances over time will provide new insights into the physics and
chemistry of this region and have already provided evidence of significant
upper-atmospheric “weather” that has not been measured in detail before.
New insight into how gases leave the atmosphere is being provided by the
spacecraft’s Suprathermal and Thermal Ion Composition (STATIC) instrument.
Within hours after being turned on at Mars, STATIC detected the “polar plume” of
ions escaping from Mars. This measurement is important in determining the rate
of atmospheric loss.
As the satellite dips down into the atmosphere, STATIC identifies the cold
ionosphere at closest approach and subsequently measures the heating of this
charged gas to escape velocities as MAVEN rises in altitude. The energized ions
ultimately break free of the planet’s gravity as they move along a plume that
extends behind Mars.
The MAVEN spacecraft and its instruments have the full technical capability
proposed in 2007 and are on track to carry out the primary science mission. The
MAVEN team delivered the spacecraft to Mars on schedule, launching on the very
day in 2013 projected by the team 5 years earlier. MAVEN was also delivered well
under the confirmed budget established by NASA in 2010.
The team’s success can be attributed to a focused science mission that
matched the available funding and diligent management of resources. There were
also minimal changes in requirements on the hardware or science capabilities
that could have driven costs. It also reflects good coordination between the
principal investigator; the project management at NASA’s Goddard Space Flight
Center; the Mars Program Office at NASA’s Jet Propulsion Laboratory in Pasadena,
California; and the Mars Exploration Program at NASA Headquarters.
The entire project team contributed to MAVEN’s success to date, including the
management team, the spacecraft and science-instrument institutions, and the
launch-services provider.
“The MAVEN spacecraft and its instruments are fully operational and well on
their way to carrying out the primary science mission,” said Jim Green, director
of NASA’s Planetary Science Division at NASA Headquarters in Washington. “The
management team’s outstanding work enabled the project to be delivered on
schedule and under budget.”
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