There’s an old saying that the clothes make the man. In planetary exploration, the instrument
suite makes the mission. Fewer and simpler
instruments can enable a lower cost mission but at the cost of restricting the
richness of the scientific investigations.
Jupiter’s moon Europa has been a priority to explore because there’s
good evidence that its vast ocean, hidden beneath an icy crust, may have the
conditions needed to enable life.
However, NASA’s managers have struggled to define a mission that is both
compelling and affordable. Over the last
several years, engineers at the Jet Propulsion Laboratory and Applied Physics
Laboratory have rethought the entire approach to exploring Europa. They started with a bare bones list of just
three must have instruments (with a longer list of optional desired
instruments). Their breakthrough was to
plan a mission that would orbit Jupiter and make many brief swoops past Europa
before swinging back out of the high radiation zone. NASA now has a concept that's affordable.
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| Current concept for the Europa Clipper spacecraft. Credit: NASA-JPL/Caltech. You can read a summary of the mission concept here, although current plans would replace the radioisotope power supplies discussed in the article with the solar panels shown above. |
What would ultimately define the mission, though, would be the suite of
instruments NASA’s managers would chose.
Designing instruments that can withstand the radiation has proven
difficult. NASA's managers could have decided on a
minimal instrument suite to reduce mission costs and risks, in effect to fly an
economy class mission to Europa.
As we learned last week (see here),
however, they announced that the selection of a rich instrument suite that will
make this a first class voyage. Not only
is the list long – and includes everything on that original desired list – the
instruments individually look to be highly capable. The resulting mission promises to be
incredible.
NASA’s announcement was widely reported on and by now I expect that
many of you have seen the instrument list.
In this blog post, I’ll discuss how these instruments will work together
to reveal Europa’s secrets. NASA did
little more than announce the names of the instruments and said little about
their capabilities. (This is standard;
we usually learn the details about the instruments in the next year or two as
their science team discuss them at scientific conferences.) Where possible, I’ve expanded upon the brief
list of instruments with previously published information or from information
published since NASA’s announcement. I’ve
also provided comparisons with the instrument suite for the European Space
Agency’s JUICE mission that will briefly study Europa but focus on the
neighboring moon Ganymede and Jupiter itself.
Ice and Ocean
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| The Europa Clipper mission will study the structure of the icy crusts, ocean, and the rocky world below. Credit: NASA-JPL/Caltech. |
Europa is an ocean world that likely hosts twice as much water as the
Earth, capped by an icy crust several kilometers to several tens of kilometers
thick. Several of the instruments will focus
on studying the ocean and the structure of the crust.
Europa is embedded within the powerful magnetosphere that surrounds
Jupiter. A salty ocean would interact
with the magnetic fields and reveal both its depth and salinity. NASA’s Galileo spacecraft all but proved the
existence of Europa’s ocean by measuring the induced magnetic field from this
interaction. Europa Clipper will refine
Galileo’s measurements with its own magnetometer (Interior Characterization of
Europa using Magnetometry (ICEMAG) – principal investigator Dr. Carol
Raymond of NASA’s Jet Propulsion Laboratory (JPL)). The plasma fields carried within Jupiter’s
magnetosphere locally modify the magnetic field, and the Europa Clipper will
carry a basic plasma instrument to allow the modifications to be accounted for
(Plasma Instrument for Magnetic
Sounding (PIMS) – principal investigator Dr. Joseph Westlake of
Johns Hopkins Applied Physics Laboratory (APL)).
NASA appears to have selected a core
instrument set focused on the investigation of Europa’s ocean. ESA’s JUICE spacecraft will carry a richer
set of investigations that will carry out broader investigations of Jupiter’s
magnetosphere. However, if the Europa
Clipper and JUICE operate at the same time, the Clipper’s instruments could
enhance JUICE’s investigations by providing a basic measurement of the magnetic
field at a second location. (JUICE will
arrive at Jupiter in 2030; a date for the Clipper’s arrival has yet to be set.)
Galileo’s camera and spectrometers revealed that the icy crust is
fractured and frequently covered with material that appears to have originated
in the ocean below. The Clipper’s radar
instrument (Radar for Europa Assessment
and Sounding: Ocean to Near-surface (REASON) – principal
investigator Dr. Donald Blankenship of the University of Texas, Austin) will
see below the surface to investigate the structure of the shell, potentially
all the way to the interface with the ocean below.
Ground
and ice penetrating radars face a fundamental tradeoff – do they focus on the
highest resolution to reveal fine structures (higher frequencies) beneath the
surface at the cost of shallow penetration or do they focus on maximizing the
depth of penetration (lower frequencies)?
Radar systems can focus on one or the other measurement based on the
frequency they operate at. The Clipper’s
REASON instrument will use dual frequencies to enable both fine resolution in
the upper layers of the icy shell and deep penetration. (JUICE’s radar system will use a single lower
frequency.) The frequencies that allow
deep measurements are subject to interference from radio wave emitted from
Jupiter, and these measurements will be made only on the hemisphere of Europa
that faces away from Jupiter where the bulk of the moon blocks Jupiter’s
emissions. (At Mars, two spacecraft
carry subsurface radar. NASA’s SHARAD
instrument uses higher frequencies while ESA’s MARSIS instrument uses lower
frequencies.)
The bulk of Europa is a
rocky world covered with an ocean that may be around 100 kilometers deep
covered by an icy shell that likely is kilometers thick. Measuring variations in Europa’s gravity
field can reveal this moon’s internal structure to its core. Differences in the gravity field would arise
from variations in the thickness of the icy shell, variations in the topography
of the ocean floor, or deep variations in the structure of the rocky body. The Clipper engineering team has put
considerable effort into enabling sensitive measurements of the gravity field
from multiple flybys by measuring slight differences in the Doppler shifts of
the radio. NASA did not announce a
science team for gravity measurements when it announced the instrument
suite. However, according to NASA’s
program manager for Europa instruments, Curt Niebur, “We have spent
considerable effort accommodating gravity science into the mission design. At
this point NASA is considering options as to how best to inject the necessary
expertise to the science team. But gravity science remains a key
investigation of the Europa mission.”
Composition
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| The Europa Clipper’s instruments will explore the composition of Europa’s surface, which includes darker material that may be salts or organics from the ocean below. Credit: NASA/JPL-Caltech/SETI Institute (See here for additional information about this image.) |
While Europa’s potentially life-bearing ocean lies hidden beneath the
icy shell, the surface of that shell is streaked and spotted with darker stains. Scientists believe that the staining
materials are likely materials such as salts or potentially organic molecules
brought to the surface when the ice shell fractures and ocean material erupts
to the surface. Three of the instruments
will examine the composition of these materials.
The composition of the surface will be mapped across the globe by the Mapping
Imaging Spectrometer for Europa (MISE) (principal investigator Dr. Diana
Blaney of JPL). Cameras are typically
optimized to provide sharp images but record only a selected few colors (or
specific spectral bands). Mapping
spectrometers trade imaging resolution for the fidelity of their spectral
measurements across many spectral bands.
MISE will use the spectra of light reflected from Europa to map the
distribution of “organics, salts, acid hydrates, water ice phases, and other
materials” across this moon’s surface.
Various processes such as sublimation and micrometeoroid impacts expel
material from the surface to form a tenuous cloud around Europa. The Europa mission’s two mass spectrometers
will directly sample this material and determine its composition by “weighing”
the atoms and molecules they encounter.
By doing so, they will be able to directly taste the ices, salts, and
any organic molecules present on the surface and will provide more sensitive
measurements of surface composition than MISE (but without MISE’s ability to
map composition across the surface).
The
MAss SPectrometer for Planetary
EXploration/Europa (MASPEX) (principal investigator Dr. Jack
(Hunter) Waite, of the Southwest Research Institute (SwRI)) will measure the composition
of gasses, ices, and organic molecules. (MASPEX
is the current state of the art and will be more sensitive than the equivalent
instruments that have been flown on the Cassini Saturn and Rosetta comet
missions. I’ve seen this instrument
included in a number of current proposals for new missions.)
The SUrface Dust Mass Analyzer (SUDA) (principal
investigator Dr. Sascha Kempf of the University of Colorado, Boulder) will
measure dust and salt particles ejected from the surface.
Previous generation equivalents to these two
instruments were included in the Cassini mission and in combination have
provided the data from measuring the composition of the plumes of Enceladus to
show that its internal ocean may provide a habitable environment.
Geology
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| The Europa Clipper’s moderate resolution camera will map the geology of this moon across its surface. Credit: NASA/JPL-Caltech. (See here for additional information about this image.) |
The surface of Europa’s icy shell records a history of fracturing and
interaction with the ocean below.
Globally mapping the surface will provide scientists with clues to what
processes shaped the surface and how material is exchanged between the ocean
and surface.
The wide angle camera in the Europa Imaging System (EIS) (principal
investigator Dr. Elizabeth Turtle of APL) will map the surface of Europa at 50
meter resolution in color to document the surface structure. While 50 meter resolution may seem to be coarse
compared to the high resolution images we have come to expect from Mars and the
moon, for a first dedicated mission to a world this will be detailed global
coverage. The best global maps of Venus
from the Magellan mission are approximately 300 m resolution. (Mars has better coverage, but it has been
the focus of many missions. At Mars, 75% of the surface has been mapped at 6
meter resolution (as of April 2012) in black and white by the Mars Context
Camera on the Mars Reconnaissance Orbiter while 61% has been mapped in color at
20 meters by the camera on the Mars Express orbiter (as of February 2013).)
During each of the planned 45 flybys, the spacecraft will travel close
to the surface of Europa. At each
encounter, the wide and narrow angle EIS cameras will record the surface
geology in high resolution recording details as small as one meter. The MISE spectrometer likely will provide
high resolution composition maps across the narrow strips of terrain that the
spacecraft will traverse during its close encounters.
Reconnaissance
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| The very highest resolution images of Europa taken by the Galileo spacecraft in the 1990s show rough terrain. (This image has a nine meter resolution.) NASA’s Europa Clipper mission will scout for safe landing zones using its suite of instruments. Credit: NASA/JPL. (See here for additional information about this image.) |
The ultimate goal for Europa exploration will be to directly sample
material from the ocean to determine whether it has the conditions likely to be
habitable and whether complex organic molecules indicative of life are
present. There are two ways to achieve
this goal. The first will be to find a
location where a future lander could sample material recently delivered from
Europa’s ocean to the surface. The
second would be to find plumes erupting from the surface that would be spewing
the contents of Europa’s subsurface water into space.
Two instruments are dedicated to scouting out landing sites for future
landers. The high resolution EIS camera
will image sites at resolutions as fine as 1 meter to search for landing zones
smooth enough for a safe landing. The Europa Thermal Emission Imaging System
(E-THEMIS) (principal investigator Dr. Philip Christensen of
Arizona State University) will map the surface in the thermal infrared to look
for locations warmer than the surrounding ice that may indicate the presence of
warmer water close to the surface. Locations
with water near the surface may be sites where a future lander could drill
beneath the to reach liquid water.
E-THEMIS appears to be based on the THEMIS instrument on the Mars
Odyssey orbiter, which provides thermal imaging in multiple spectral to map the
composition of the Red Planet. There’s
no mention that I have found for E-THEMIS being used for composition mapping
for Europa (the expected surface materials may not have characteristic spectra
in these bands at the frigid temperatures of Europa’s surface).
Researchers using the Hubble spacecraft appear to have observed a large
plume erupting from the surface. Repeat
observations so far have failed to observe subsequent plumes, which may mean
that these large plumes rarely erupt.
However, plumes that would be too small to be seen by the Hubble’s
telescope may be more common.
The Ultraviolet
Spectrograph/Europa (UVS) (principal investigator Dr. Kurt Retherford of
SwRI) will study the space above Europa’s surface to look for plumes and to
more generally study the composition and structure of the rarified atmosphere
surrounding Europa. (That near-vacuum
atmosphere will be the material that the MASPEX mass spectrometer will
sample.) The location of any plumes also
may be revealed by the E-THEMIS instrument by mapping very warm surface locations that could be the vent sources for plumes (as has been done for the plume sources for Saturn’s moon
Enceladus).
If any plumes are discovered, the Clipper almost certainly would be
retargeted to fly through them. The
MASPEX and SUDA mass spectrometers would then be able to directly sample and
analyze the composition of Europa’s subsurface water. (The source of any plumes could either be the
ocean itself in the case of a deep fracture or a lake trapped in the ice below
the surface but above the ocean.)
More to Come?
This instrument suite could be made even richer by future
announcements. NASA has requested and
received proposals for possible CubeSat spacecraft that the Europa Clipper
spacecraft would carry to Europa and release.
(See here.) These spacecraft, each likely the size of a
loaf of bread, might enhance the mission by providing, for example, additional
magnetometer measurements to study the ocean’s depth and salinity or by
providing high resolution imaging as they fly into the surface.
NASA has also asked the European Space Agency if it would like to
provide a daughter craft that might be a small lander or a probe that might fly
through and analyze and plumes. (See
here.) ESA’s managers have said that
any contribution to NASA’s mission would have to come from a competition that
would pit it against a number of excellent proposals for other science
missions.
Also, one US Congressman (who chairs the House subcommittee that funds
NASA) has said that he thinks NASA’s mission should be enhanced with a capable
lander. (See here.) While I appreciate his enthusiasm (which has
led to hundreds of millions of dollars being added to NASA’s planning for its
Europa mission), I’m skeptical that this will happen. Capable landers are expensive, and the
interesting places on Europa’s surface look to be exceeding rugged. I doubt that a credible design could be put
into place in time to launch with the Europa Clipper by the mid-2020’s even if
the funding were provided.
NASA’s Europa spacecraft will also be joined by Europe’s JUICE
spacecraft. The focus of the latter will
be Europa’s neighboring icy-ocean world, Ganymede. Because Ganymede lies outside the intense
Jovian radiation fields, the JUICE spacecraft will be able to orbit Ganymede
for an extended period of close up studies.
JUICE also will add to the Clipper’s studies of Europa by making two
Europa flybys of its own.
In many ways, the instrument list for NASA’s Europa spacecraft and
JUICE’s are similar. They both have
cameras, imaging spectrometers, mass spectrometers, a magnetometer, ice
penetrating radar, and a UV spectrometer.
JUICE, however, also will carry a laser altimeter to map the elevations of
Ganymede’s surface that won’t be in NASA’s suite. The European mission has a goal to study
Jupiter itself and therefore has a richer set of plasma instruments and a radio
and plasma wave instrument to study Jupiter’s magnetosphere along with a submillimetre
wave Instrument to study Jupiter’s atmosphere.
(It’s interesting to compare the estimated costs for NASA’s Clipper
mission (~$2B) with those for JUICE, which is a similarly complex mission. NASA’s mission will be approximately twice as
expensive, indicating how hard it is to design a spacecraft and instruments to
survive the radiation fields at Europa.)
When to Fly?
The next key question for NASA’s Europa mission will be when it will
launch. JPL’s design team are working
towards a 2022 launch, provided the money can be found. The funding is the rub, though. While Congress has pushed for an early flight
and backed that with generous funding, only this year has the President’s Office
of Management and Budget, which sets the administration’s budget policy, agreed
to make a trip to Europa an official NASA program and proposed a tiny down payment
towards the mission’s cost. However,
they and NASA’s managers, who ultimately work for the President and must
publicly support the administration’s position, only speak vaguely of a launch
in the mid-2020s or possibly later.
(This reminds me of the father who tells his children that, yes,
absolutely we will go to Disneyland someday (and means it), to get them to stop
pestering him about the trip now.) The
issue is that an earlier flight means either increasing NASA’s budget to pay
for the mission or trading it for other work that is on NASA’s plate. (See these good background pieces by Casey
Drier at the Planetary Society and Jeff Foust at the Space
Review. I’ve also written about this.)
Over the last twenty years, I’ve watched NASA struggle to find a Europa
mission that is both affordable and compelling.
The Europa Clipper mission design achieves the affordable and the
instrument suite NASA just announced provides the compelling. The
instruments that NASA selected will enable a suite of complimentary studies
that will allow us to understand Europa as an ocean world, judge whether it is
likely to have conditions that would make it habitable, and scout for locations
for the next logical mission, a lander.
This is possible because NASA’s managers took the gutsy move and decided
against an economy class mission that might have had just three or four
instruments and selected the full set of instruments needed to do the job
right.
