When
I wrote my blog post
on the New Frontiers mission proposals, I could identify eleven of the
twelve proposals. One, though, then had
no public trace. Shortly after my piece
was published, several people emailed me to tell me that the twelfth proposal
was a third Venus mission called the Venus Origins eXplorer (VOX). However, they offered no details on its
implementation.
Now
the team proposing the mission will present its implementation at an upcoming
scientific conference. Their abstract
for the talk reveals key facts about the proposal, and it’s a gutsy break with
the missions that have been assumed for New Frontiers Venus missions.
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| NASA's Magellan spacecraft mapped the surface and gravity field of Venus in the early 1990s. The VOX mission would return with a modern suite of instruments to remap the surface and study the lower atmosphere. Credit: NASA/JPL. |
To
understand the strategy the VOX team is following, it’s useful to look at the
history of the Juno Jupiter orbiter mission.
Every ten years, the planetary scientific community establishes its
priorities for the exploration of the solar system in process known as the
Decadal Survey. The 2003 Decadal Survey
report called for a Jupiter Polar Orbiter with Probes mission. The orbiter was to study global patterns and
the magnetosphere while three atmospheric probes were to examine the
composition and deep structure of the atmosphere. Unfortunately, since the Galileo mission with
its shallow atmospheric probe, the United States has lost the technology to
build probes capable of entering Jupiter’s atmosphere. The team that proposed the Juno mission focused
on the desired scientific goals and demonstrated that they could be met with
just an orbiter with the right instruments.
Based on that well-presented argument, the mission was selected and is
now orbiting Jupiter.
The
goals for a New Frontiers mission for Venus have assumed an atmospheric probe
that would also conduct surface measurements once it landed. The scientific requirements NASA’s managers
established for a mission appear tailored for that kind of a mission based on
the goals set out by the more recent 2013 Decadal Survey report. For example, the ratio of key gasses and
their isotopes can only be made by a probe that enters and directly samples the
atmosphere. With those measurements,
other questions such as past hydrologic cycles and the existence of now lost
oceans can be addressed. During its
descent the probe could measure the properties of the atmosphere, its clouds,
and its winds at different altitudes.
Measurements of the atmosphere at the surface could address the
weathering environment of the crust.
Chemical analysis of the soils and rocks at the landing site could
provide insight into the physics of and chemistry of the crust.
Two
of the proposed New Frontiers missions would fly either one or two entry
probe-landers to address these questions.
(You can see summaries of these at this blog
post.)
The
VOX proposal takes an entirely different approach. As the abstract states, “At the time of the Decadal Survey the ability
to map mineralogy from orbit and present-day radar techniques to detect active
[surface] deformation were not fully appreciated. VOX leverages these methods
and in-situ noble gases to answer [the key] New Frontiers science objectives.”
The VOX mission would
deploy a small, simple atmospheric probe to make the measurements of the key
gases and isotopes. The rest of the
measurements would be made from instruments on an orbiter, much as the Juno
mission uses orbital measurements to replace measurements from atmospheric
probes. (The composition measurements for
Jupiter that could only be made from within an atmosphere had already been made
by the Galileo mission’s atmospheric probe.)
The
key elements of the VOX proposal closely resemble the VERITAS mission that was
a finalist for the last Discovery program mission but not selected (losing out
to the Lucy and Psyche asteroid missions).
The principal investigator for VERITAS and VOX are the same, Suzanne
Smrekar at the Jet Propulsion laboratory.
The core of the VERITAS proposal was an orbiter that would carry a
modern radar instrument to remap Venus and a spectrometer to measure the surface
composition and study key aspects of the atmosphere. If selected, the VERITAS mission team was
considering proposing a small atmospheric probe call Cupid’s Arrow to toe-dip
into the atmosphere to make key composition measurements. A version of that probe now called the
Atmospheric Sample Vehicle (ASV) would be included in the VOX mission.
The
VOX orbiter would use three instruments to globally study Venus:
A
radar instrument would image the surface at 15- to 30-meter resolution, map
elevations to refine topography measurements, and search for minute changes in
surface elevations to look for evidence of current volcanic or tectonic
activity. The improvements in resolution
for each of these would be one to two orders of magnitude better than those
made by the Magellan orbiter in the 1990s.
The
orbiter’s radio signal would be used to map the gravity field at much higher
resolution than the Magellan mission to study the interior structure of the
planet
The
Venus Emissivity Mapper (VEM) would use spectral bands in five near-infrared wavelengths
where the atmosphere is transparent to map the surface composition. Measurements in additional bands would study
cloud structure and the presence of water vapor in the lowermost atmosphere.
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| These images, derived from radar imaging of Hawaii, show the improvement in resolution possible with modern radar systems compared to the data returned by the Magellan mission. This image is from a poster describing a 2012 JPL VERITAS mission that was proposed for the NASA Discovery program. The VOX mission would carry the same radar instrument. Credit: JPL |
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| These figures show the improvement in topographic resolution possible with a modern radar instrument using data from Iceland. The top image shows the equivalent Magellan resolution and the bottom image shows simulated resolution possible from a new Venus radar mission. Credit: Decadal Survey White Paper, NASA |
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| The Venus Express mission carried out the first mapping of Venus’ surface using thermal emissions. The VOX mission's Thermal Emissity Mapper instrument would create better composition maps than the Venus Express instrument. Red-orange colors in this Venus Express image of Venus’ Idunn Mons volcanic peak indicate warmer areas suggesting different surface compositions and younger ages than surrounding material. Credit: ESA |
The
ASV would be a small probe released from the orbiter. The probe would enter and briefly traverse
the upper atmosphere where it would collect a small sample of the gasses
present before its momentum carries it back into space. Once back above the atmosphere, a
miniaturized mass spectrometer would measure the composition of key gasses and
their isotopes. By only toe-dipping into
the atmosphere, the probe doesn’t need protection from the crushing pressures
and extremely high heat on the surface that the atmospheric probes of the other
New Frontiers Venus proposals require.
 |
| The VOX mission's Atmospheric Sample Vehicle would be based on the Cupid Arrow probe concept. Credit: NASA/JPL. |
While
the VOX mission is in competition with the two Venus entry probe-lander
proposals (and nine other proposals for other solar system destinations), in
many ways it is a compliment to those two missions. Within the column of air in which they
descend and in a square meter or so at their landing sites, the two proposed
entry probe-lander missions would make much more detailed measurements than
VOX’s radar and VEM instruments would from orbit for the same location.
The
VOX spacecraft’s instruments, however, would make those measurements over the
entire planet over at least three years (and possibly much longer in an
extended mission). The VEM instrument,
for example, could look at the surface of every volcano on Venus searching for
the presence of minerals suggesting recent lava flows. The radar images could show whether the
features on the surface are consistent with new flows. Radar interferometric measurements could look
for small surface deformations that could reflect surface movement from
earthquakes associated with eruptions or swelling from underground lava
movement. VEM measurements of the
atmosphere could look for changes in water vapor associated with volcanic
outgassing. And the gravity field
measurements would tie each volcano to the interior structure below the
surface.
The
VOX mission would make comparable studies for all the surface types on Venus as
well as monitor the lower atmosphere.
Perhaps most important of these would be observations of the tessera, which are continent-sized highlands
that likely are the oldest surviving surfaces on the planet. For these, the synergy of composition
measurements, surface imaging, and interior structure measurements could reveal
key aspects of the earliest history of Venus including the role of liquid water
and other volatiles in the then cooler times.
In
one set of measurements, the VOX and the two Venus entry probe-lander missions
are in direct competition. In much the
same way as strata of rock record geologic history, the presence and abundance
of key gasses and their isotopes record the origin and evolution of the
atmosphere. Because the Venusian atmosphere
is in contact with the surface, it also records key geological events. Measurements of xenon and its isotopes, which
have yet to be made at Venus, for example, would resolve key questions about
the origins of the atmosphere. Xenon
measurements also would reveal the total amount of degassing from the planet’s
interior, which reflects the cumulative volcanic activity. Similarly, a long-lost ocean at Venus would be
reflected in the ratio of hydrogen isotopes.
The
US Pioneer Venus and Soviet Venera entry probe and landers carried
now-antiquated mass spectrometers that could not make key composition
measurements such as the isotope ratios of xenon. The VOX mission’s atmospheric probe would
deliver a modern mass spectrometer to Venus that would answer the key first
order atmospheric composition questions.
The other two proposed Venus missions would carry much more sophisticated
instruments that could also address second and third order questions. By descending all the way to the surface, the
probes on these other missions would also study changes in composition at
different layers of the atmosphere and look for compounds that reflect the
current weathering at the surface.
(As
a side note, I’ve been following proposals for missions to explore the solar
system for several decades now. Cupid’s
Arrow, and its incarnation as VOX’s ASV, is in my opinion one of the cleverest
solutions I’ve seen to addressing a key set of planetary science questions at a
fraction of the cost of much more capable entry probes like those that would be
the core of the other two Venus proposals.)
To
understand Venus, eventually it’s not a question of a VOX-like mission versus
entry probe-lander missions. Their
capabilities are complimentary – in depth at one or a few locations versus
broader measurements across the entire globe across a number of years. The interpretations of measurements from orbit,
especially those of composition, would greatly benefit from ground truth
measurements on the surface. Eventually,
we need both types of missions to fly. (And
there’s also an entire branch of Venus science focused on global atmospheric
studies from orbit that neither of these types of mission addresses.)
In
the last New Frontiers competition, a Venus entry probe-lander was a finalist
but lost out the OSIRIS-REx mission now inflight to return a sample from the
asteroid Bennu. For the current
Frontiers competition, NASA will select just one mission from among three proposals
for Venus, one to return samples from the moon, three to return samples from a
comet, and four to study Saturn or its moons.
It’s a tough field. By the end of
this year, we’ll learn whether VOX is selected as a finalist in the current
competition with the final selection planned for 2019.
Appendix:
Here is the full list of studies proposed for the VOX mission from the
conference abstract:
1. Atmospheric physics/chemistry:
noble gases and isotopes to constrain atmospheric sources, escape processes,
and integrated volcanic outgassing; global search for current volcanically
outgassed water.
2. Past hydrological cycles: global tessera composition to determine the role of volatiles in crustal formation.
3. Crustal physics/chemistry: global crustal mineralogy/chemistry, tectonic processes, heat flow, resolve the catastrophic vs. equilibrium resurfacing debate, active geologic processes and possible crustal recycling.
4. Crustal weathering: surface-atmosphere weathering reactions from redox state and the chemical equilibrium of the near-surface atmosphere.
5. Atmospheric properties/winds: map cloud particle modes and their temporal variations, and track cloud-level winds in the polar vortices.
6. Surface-atmosphere interactions: chemical reactions from mineralogy; weathering state between new, recent and older flows; possible volcanically outgassed water.
2. Past hydrological cycles: global tessera composition to determine the role of volatiles in crustal formation.
3. Crustal physics/chemistry: global crustal mineralogy/chemistry, tectonic processes, heat flow, resolve the catastrophic vs. equilibrium resurfacing debate, active geologic processes and possible crustal recycling.
4. Crustal weathering: surface-atmosphere weathering reactions from redox state and the chemical equilibrium of the near-surface atmosphere.
5. Atmospheric properties/winds: map cloud particle modes and their temporal variations, and track cloud-level winds in the polar vortices.
6. Surface-atmosphere interactions: chemical reactions from mineralogy; weathering state between new, recent and older flows; possible volcanically outgassed water.
