MAVEN Science Instruments Explained – Why Mars Lost its Atmosphere


On November 18, 2013 NASA’s Mars Atmosphere
and Volatile EvolutioN or MAVEN Space craft was launched toward Mars with the goal of
scientific examination of Mars’ atmosphere. It is hoped that science experiments conducted
by MAVEN will offer scientists answers to questions about what happened to Mars’ atmosphere
and water. Around 3.5 billion years ago Mars had a magnetosphere
and an atmosphere which was thick enough and warm enough to sustain liquid water. Today,
the atmosphere is all but gone – as is the water. What was Mars atmosphere like 3.5 billion
years ago? How can scientists, examining today’s atmosphere on Mars, know what the atmosphere,
magnetic field, and water were like on a planet so many billions of years ago? Instead of
concentrating on the surface, all of the spacecraft’s instruments will concentrate on Mars atmosphere
in an effort to answer these questions. MAVEN carries a total of 8 scientific instruments
in 3 packages, the Particles and Fields Package, the Remote Sensing Package, and the Neutral
Gas and Ion Mass Spectrometer. There is no camera on MAVEN.
MAVEN will study the boundary between the planet’s current atmosphere and outer space.
MAVEN measures what’s coming into Mars atmosphere, what the current atmosphere looks like, and
finally what’s escaping from Mars atmosphere. The spacecraft flies in an elliptical orbit
from 150 km to over 6,000 km above the surface. It will make 5 deep dips down to where the
atmosphere is 30 times denser. This allows the spacecraft to sample several altitudes
of the atmosphere. MAVEN is a box 7.5 feet square by 5 feet high.
The hi-gain antenna is 6.5 feet in diameter. Solar panel wings span 37.5 feet wide. The
solar wings are bent at an angle of 20* because as MAVEN travels through the upper atmosphere,
the air pressure will increase to a point that could disrupt flight dynamics if the
panels were flat. 2 instruments on MAVEN measure the current
composition and structure of Mars atmosphere. They are the Imaging and Ultraviolet Spectrograph
and the Neutral Gas Ion Spectrometer. The Imaging and Ultraviolet Spectrograph (IUVS)
instrument measures the characteristics of the upper atmosphere and ionosphere from a
broad global view and is located on a movable platform arm extending down from the body
of the spacecraft. It is positioned here so that it can point in any direction regardless
of where the spacecraft itself is pointing. This also gives the instrument its full global
view of the planet. The purpose of the IUVS is to make three-dimensional
models of the upper atmosphere’s major molecules, atoms, ions, and their isotopes.
The spectrograph is an instrument that can tell us what gases are present in the atmosphere
by spreading an object’s light apart, revealing signatures of molecules so that we can tell
what they are. For example, the light wavelengths of Hydrogen and Deuterium (Heavy Hydrogen)
require a high resolution to separate them into wavelength lines for analysis in the
spectrograph, and the IUVS is capable of doing this. Scientists can look so closely that
they can tell the difference between Hydrogen and Deuterium. The lighter hydrogen is assumed
to have escaped into space leaving behind the heavier Deuterium.
Here’s how it works. Run electricity through a tube of gas and
it glows. If you run this through a prism you get the full spectrum of colors. Run it
through an instrument like the IUVS and what you see is a full spectrum of specific wavelengths
of light causing them to appear as different colors to the instrument. Run electricity
through a tube of Hydrogen gas and it glows pink. That is the fingerprint of Hydrogen
Gas. Every gas has its own unique fingerprint. So wherever we look we can tell how much Carbon
Dioxide, how much Oxygen, and how much Hydrogen or Argon there is.
And that is how the IUVS can tell us from a broad global view, what the current atmosphere
of Mars is made of. Located on the movable platform below the
spacecraft along with the IUVS is the Neutral Gas and Ion Mass Spectrometer (NGIMS). It
is located here to keep it away from its own gases and allowing it to face different directions.
Like IUVS, NGIMS determines the composition of the upper atmosphere, but rather than from
a global view like IUVS, NGIMS takes a closer more detailed view. NGIMS is the only instrument
on board that allows us to analyze the atmosphere immediately around the spacecraft, called
in situ. Since IUVS measures the atmosphere with a
broad global view, and NGIMS takes a closer more detailed local view, the two instruments
will complement each other. NGIMS studies how neutral gases and ions in
the Martian atmosphere interact with the solar wind. The red cap on the left is a protective
cap covering an ionization source found on the NGIMS instrument. It is fired off to remove
it when the space craft is in position around Mars. NGIMS uses this electron gun to bombard
the gas molecules into smaller, charged particles or ions.
Concentrating on the more detailed view, NGIMS is interested in how much space is between
atoms and molecules in the atmosphere and how that space changes with altitude. As the
spacecraft skims through the upper layer of the atmosphere it can sniff out atom by atom,
molecule by molecule to figure out what the composition of the atmosphere is. The very
top of the atmosphere is called the exosphere where the atmosphere is so thin that even
atoms don’t bump into each other. NGIMS measures the profiles of a number of
different chemicals including Argon. Measurements involving Argon will be particularly interesting.
Unlike other chemicals in the atmosphere, Argon doesn’t react with its surroundings.
So Argon is likely to be the most simple, straightforward indicator of atmospheric loss
today. Since lighter isotopes escape the planet more readily than heavier isotopes, atmospheric
loss will appear as a change in the number of heavy Argon isotopes compared to the number
of light Argon isotopes as altitude increases. Here’s how it works.
This is an illustration of an electron beam ionizing a neutral gas which is what NGIMS
will be doing. The gas first enters the spectrometer hitting a piece called the ion source. Here
a stream of electrons hits the molecules, breaks them into fragments, and giving each
fragment a charge, creates an ion from a neutral gas. That ion then gets focused into an analyzer
called a quadropole filter. It is called quadropole because of its 4 rods. Different electrical
voltages are applied to the 4 different rods. Each specific voltage isolates ions based
on each gas’ specific mass. As the ions spiral down the rods they get separated out
so only the fragments we want to see make it all the way through. Everything else flies
off in a different direction. They then hit a detector which counts not only what kinds
of ions come through it but also the number of ions of a particular mass that come through
it. What you have then is a mass spectrum, creating a picture of all the different gas
particles present in the Martian atmosphere. And that’s how NGIMS gives us a detailed
measurement the composition of the upper atmosphere. The Supra Thermal and Thermal Ion Composition
(STATIC) instrument is mounted on the articulating platform along with IUVS and NGIMS.
While IUVS and NGIMS are studying what is already in the atmosphere, STATIC concentrates
on what is going out of the atmosphere. Since MAVEN will be examining Mars atmosphere during
a period of relatively high solar activity, STATIC’s information will show how variations
in this solar activity relates to differences in atmospheric loss.
Thermal and Supra thermal ions mostly come from the Solar Wind, the usual suspect causing
the atmosphere to be swept away. The common theory is that because Mars lost its magnetosphere,
the solar wind swept away its atmosphere. Another thought however is that other properties
such as excess kinetic energy carried by the solar wind can transfer enough of this energy
to atmospheric particles that they can attain escape velocity regardless of the magnetic
field status. Our sun constantly emits high energy photons.
When one of these photons enters the atmosphere of a planet it can crash into a molecule,
knocking loose an electron and turning it into an ion. Ions by themselves don’t do
much, but when a magnetic field is nearby, the ion will spin around the field. Our sun
generates a gigantic magnetic field carried by the solar wind. As the sun’s magnetic
field sweeps past the planet, some ions will get carried away. Other ions, depending on
where they form, won’t get carried away but will hit the top of the atmosphere. These
ions can then crash into other molecules and fling atoms everywhere. Some of these atoms
can be knocked, or “sputtered” into space causing atmospheric loss.
STATIC captures these high speed ions, sends them through an analyzer and measures their
velocities and characteristics. Located on a dedicated boom mounted in between
the two LPW booms is the Solar Wind Electron Analyzer (SWEA).
SWEA measures electron properties of the incoming Solar Wind. SWEA can tell the difference between
electrons found in the solar wind and those in the Martian ionosphere by identifying their
different energies. By identifying where solar wind plasma ends and planetary plasma begins,
SWEA will be able to zero in on where the very top of the planet’s ionosphere is located.
Scientists are trying to understand this boundary layer between the solar wind and the planet’s
ionosphere because this is the region where planetary material is being lost.
It will also help scientists learn how these particles already in the atmosphere get enough
energy needed to boost them high enough into the atmosphere so they can escape.
The Solar Wind is packed with charged particles and magnetic field lines that can interact
with particles in Mars upper atmosphere. The ions must travel along magnetic field
lines in order to leave the atmosphere. On MAVEN the SWEA and MAG instruments work together
to map out the crustal magnetic fields and determine if these fields are ‘open’ or
‘closed’ due to changes in the Sun’s magnetic field and rotation of the planet.
An open magnetic field line allows particles to escape and a closed magnetic field line
will turn the particle back to the planet. SWEA measures these electrons flowing along
the magnetic line to find out if it is open or closed.
Electrons enter SWEA through 2 deflectors which are open to space. These electrons are
sorted and analyzed according to their energies, telling scientists which electrons are coming
from the solar wind and which electrons are coming from the planet’s atmosphere.
Positioned on the tips of either end of MAVEN’s solar panel wings are MAVEN’s Magnetometers
(MAG). They weigh less than one pound each. Since magnetic fields can emanate from other
components of the spacecraft, the magnetometers were placed as far away from the other instruments
as possible. These are extremely sensitive sensors that can detect a magnetic field that
is a million times weaker than the Earth’s magnetic field. The solar panels also contain
wiring which is laid out in a special pattern that cancels out any stray magnetic fields.
The magnetometer measures the strength, as well as the direction of magnetic fields.
Unlike Earth’s global magnetic field, which surrounds the entire planet, Mars only has
patches of magnetic field left in its crust. This has created pockets of atmosphere that
are protected against solar wind. The magnetometer helps us see where these crustal or “mini
magnetospheres” are located. These mini-magnetospheres influence how the Martian atmosphere interacts
with the Solar Wind. Here’s how they work.
A fluxgate magnetometer works essentially like a compass but instead of a spinning needle,
electromagnets are used to generate magnetic fields.
If you wrap a coil of wire around a bar of ferromagnetic metal and run a current through
it, the bar becomes magnetized and generates its own magnetic field. Reverse the current
and the field reverses direction. If you do this over and over again the 2 directions
cancel each other out. However, when an external magnetic field is present, like those on Mars,
the 2 directions are thrown out of balance allowing you to measure the external field.
By combining multiple fluxgate sensors you can measure a magnetic field in 3 dimensions.
The magnetometers help us see where the atmosphere is protected by magnetospheres and where it
is open to the solar wind. The job of MAG is to measure interplanetary,
ionospheric, and solar wind magnetic fields. The Langmuir Probe and Waves instrument (LPW)
consists of 2 parts. The first part is a pair of 2 cylindrical sensors that are installed
away from the spacecraft at the tips of two 7-meter long booms. This allows the sensors
to measure electrons without being influenced by the spacecraft itself.
The LPW instrument is designed to study what is coming into Mars atmosphere.
The pair of booms and sensors generate a variable voltage to the booms, which excite Langmuir
waves in plasma, measuring electron energy and density in the local area around the spacecraft.
This will help to more clearly define the density and boundary of the ionosphere.
The second part of the LPW instrument and sometimes referred to as the 9th instrument,
is the Extreme Ultraviolet Sensor (EUV) which is mounted directly onto the body of the spacecraft
facing the Sun. Extreme ultraviolet radiation is high energy
ultra violet radiation generated naturally by our Sun’s corona and brought to Mars
by the solar wind. LPW also works together with MAVEN’s magnetometers,
giving scientists a picture of the extreme ultraviolet radiation coming out of the sun
and flowing into the ionosphere. This energy can also heat up ions, which can also cause
escape. The job of LPW is to help scientists understand
the science of what is coming into Mars atmosphere, how the solar wind and ionosphere are structured
and how they interact with each other. The Solar Energetic Particle instrument (SEP)
consists of two identical instrument boxes, fix mounted at 90 degree angles from each
other and on opposite sides of the body of the spacecraft
SEP detects the highest energy ions coming into Mars by measuring the impact of the solar
wind on the atmosphere. The instrument is designed to study the energies of hydrogen
and helium ions emitted by our sun during solar storms, solar flares, and coronal mass
ejections. SEP will also help scientists understand what effect these particles have in ionizing
the upper atmosphere, as well as their role in the “sputtering” process.
The job of SEP is to determine the impact that solar energetic particles from solar
flares, solar storms and coronal mass ejections have on the Martian atmosphere.
Mounted on one corner of the spacecraft’s body and oriented toward the sun to ensure
good coverage of the solar wind is the Solar Wind Ion Analyzer (SWIA).
SWIA is a single sensor which measures the solar wind and magnetosheath ions, including
their density, temperature, and speed in order to determine the energy input to the upper
atmosphere. It takes these measurements in the undisturbed interplanetary medium at the
point that they encounter the Martian environment. It measures the speed of the solar wind as
it passes by the planet. SWIA measures solar wind ions, which are very, very fast protons,
and ionized hydrogen coming out of the sun. SWIA measures the speed of the Solar Wind
when it comes in contact with Mars upper atmosphere. These measurements will provide data on the
total solar wind input to Mars and how the solar wind and upper atmosphere react with
each other. SWIA and SEP both explore the solar wind.
SEP concentrates on studying the very highest energies of hydrogen and helium ions emitted
during solar flares, solar storms, and solar mass ejections. SWIA concentrates on measuring
the speed of the solar wind as it passes by the planet.
Although not one of the science instruments aboard MAVEN the Electra Communications Relay
plays a key role in the exploration of Mars. It will serve as a backup communications system
for the Curiosity and Opportunity Rovers currently exploring Mars surface. Since there are currently
three orbiters circling Mars and providing relay communication, MAVEN’s Electra won’t
be used immediately. However, as these orbiters reach the end of their expected life, or the
need arises, Electra will step in to communicate with and relay data back to scientists on
Earth. MAVEN meanwhile will communicate with Earth
using its high gain antenna. These signals will be received using NASA’s Deep Space
Network. The antenna is fixed on the spacecraft so the whole MAVEN spacecraft has to be moved
to point the antenna at Earth for its regular communications sessions that are expected
to be about five hours in duration, twice a week.
NASA’s MAVEN spacecraft will be studying the atmosphere of the Red Planet in amazing
new detail providing scientists with a comprehensive in-depth look at the Martian atmosphere. Using
all of the science provided by MAVEN will allow scientists to form models of Mars atmosphere
as it was 3.5 billion years ago and answer questions about how Mars lost it atmosphere
allowing the planet to dry out. These models may also allow scientists to answer questions
like whether the atmosphere was once substantial enough and warm enough to sustain liquid water
on the surface and possibly support life.