2
Changes Since Origins, Worlds, and Life

The first item in the committee’s statement of task is repeated here:

This question requires an evaluation of the New Frontiers (NF)-5 draft announcement of opportunity (AO) and NF-6 mission themes included in Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023–2032 (NASEM 2023; hereafter, OWL) ahead of an AO for NF-5 that will be released no earlier than 2026. Recommended are 10 mission themes for NF-5 draft AO and NF-6 (from OWL), with 5 on both lists: Io Observer, Venus In Situ Sample Return, Saturn Probe, Comet Surface Sample Return, and Lunar Geophysical Network (see Table 1-1). In addition to the 6 mission themes recommended for NF-5, 3 additional mission theme targets were included in OWL: Centaur Orbiter and Lander, Ceres Sample Return, and Titan Orbiter. It is noted here that the Ocean Worlds mission theme in NF-5 evolved into the Enceladus Multiple Flyby mission theme in NF-6.

OWL did not reevaluate NF-5 mission themes because, at the time of commissioning of the decadal survey, it was expected that the NF-5 AO would be released prior to the completion of OWL, as stated in Chapter 1. Therefore, the last detailed review of the 5 NF-5 mission themes carried forward to NF-6 was performed in New Frontiers in the Solar System: An Integrated Exploration Strategy (NRC 2003; hereafter, NFSS—Venus In Situ Sample Return and Comet Surface Sample Return) or Vision and Voyages for Planetary Science in the Decade 2013–2022 (NRC 2011; hereafter, V&V— Saturn Probe and Lunar Geophysical Network).

In response to the first item of the statement of task, the Committee on Proposed Science Themes for NASA’s Fifth New Frontiers Mission reviewed each of these 10 mission themes, with a specific focus on advances in scientific or technological understanding, and/or programmatic developments. Each mission theme is reviewed in the following sections with respect to these topics. Additionally, the contributions of each mission theme to each priority science question as outlined in OWL are assessed.

LUNAR SCIENCE

Lunar South Pole–Aitken Basin Sample Return

South Pole–Aitken (SPA) Basin Sample Return was recommended as an NF mission in the NFSS (2003) and V&V (2011) decadal surveys.

The science goals of SPA Basin Sample Return outlined in V&V are as follows:

• Determine the chronology of basin-forming impacts and constrain the period of late heavy bombardment in the inner solar system and thus address fundamental questions of inner solar system impact processes and chronology;

• Elucidate the nature of the Moon’s lower crust and mantle by direct measurements of its composition and of sample ages;
• Characterize a large lunar impact basin through “ground truth” validation of global, regional, and local remotely sensed data of the sampled site;
• Elucidate the sources of thorium and other heat-producing elements in order to understand lunar differentiation and thermal evolution; and
• Determine ages and compositions of farside basalts to determine how mantle source regions on the far side of the Moon differ from regions sampled by Apollo and Luna.

OWL upheld the view of V&V that SPA Basin Sample Return addressed the highest priority lunar science. It was also noted that achieving all of the science goals outlined in the NF mission concept would be challenging with a fixed lander. OWL instead favored the new mission concept Endurance-A, a long-distance lunar rover that would deliver samples to Artemis astronauts for return to Earth. OWL recommended that “Endurance-A should be implemented as a strategic medium-class mission as the highest priority of the Lunar Discovery and Exploration Program (LDEP). Endurance-A would utilize the Commercial Lunar Payload Services Program (CLPS) to deliver the rover to the Moon, a long-range traverse to collect a substantial mass of high-value samples, and astronauts to return them to Earth.” As such, OWL also recommended removal of SPA Basin Sample Return from the NF-6 list.

The science goals of Endurance-A outlined in OWL are as follows:

• Determine the age of SPA Basin, to anchor the earliest impact history of the solar system;
• Test the giant planet migration/terminal cataclysm hypotheses, and to better constrain the inner solar system impact chronology;
• Determine the age and mineralogical and geochemical composition of deep and crustal materials exposed in SPA Basin to understand the bulk composition of the Moon, its primordial differentiation and geologic evolution;
• Determine the age and nature of volcanic features and compositional anomalies on the lunar farside to characterize the thermochemical evolution and the origin of the Moon’s nearside–farside asymmetry; and
• Determine the geologic diversity of the SPA Terrane to provide geologic context for returned samples and ground truth for orbital measurements.

In addition to OWL favoring Endurance-A, new developments warrant further reconsideration of SPA Basin Sample Return with a fixed lander. China’s Chang’e-4 mission landed in SPA and studied the landing area in situ using a rover in 2019. The Chang’e-6 fixed lander mission returned samples from SPA and conducted additional in situ science. In contrast, with its long-range rover and capacity to sample from multiple distant locations, Endurance-A could achieve the scientific goals outlined in OWL. The Chang’e-4 and Chang’e-6 missions have not achieved the goals of Endurance-A but provide complimentary science.

The Endurance-A science goals would address the three science themes for lunar exploration identified in OWL (see Box 22-1 in OWL). OWL also states: “If timelines or plans for Artemis render this partnership infeasible, NASA, with guidance from CAPS, could evaluate options for a robotic return of the minimum set of samples needed to accomplish the core science objectives, leveraging international partnerships and commercial capabilities as appropriate, while maintaining life cycle costs to NASA commensurate with a medium-class mission” (NASEM 2023, p. 574).

Lunar Geophysical Network

Lunar Geophysical Network (LGN) was recommended as an NF mission in V&V (2011) and OWL (2023). This mission includes deployment of a global network of geophysical instruments to monitor the Moon for a minimum of 6 years. As stated in OWL,

The science objectives of LGN outlined in OWL are as follows:

• Determine the internal structure and size of the crust, mantle, and core to constrain the composition, mineralogy, and lithologic variability of the Moon;
• Determine the distribution and origin of lunar seismic activity in order to better understand the origin of moonquakes and provide insights into the current dynamics of the lunar interior and the interplay with external phenomena such as tidal interactions with Earth; and
• Determine the global heat-flow budget for the Moon in order to constrain more precisely the distribution of heat-producing elements in the crust and mantle, the origin and nature of the Moon’s asymmetry, its thermal evolution, and the extent to which it was initially melted.

The LGN mission design includes at least four geophysical stations at different locations operating simultaneously for years, providing critical information on interior composition and structure, which, particularly in combination with global high-resolution gravity by the Gravity Recovery and Interior Laboratory (GRAIL) mission and topography by the Lunar Reconnaissance Orbiter (LRO) data, underlie the estimates of lunar formation, compensation mechanisms, heat distribution, and the history and generation of the lunar dynamo. LGN directly addresses at least 4 of OWL’s 12 priority science questions—Q3: “Origin of Earth and inner solar system bodies”; Q4: “Impacts and dynamics”; Q5: “Solid body interiors and surfaces”; and Q8: “Circumplanetary systems.” Fundamental questions about the lunar interior require in situ deployment of multiple seismic stations. Seismometers were deployed by astronauts at the Apollo 11, 12, 14, 15, and 16 landing sites. Analysis of these nearside data detected moonquakes that provide the best available constraint on the size and nature of the lunar core, mantle, and crust.

In 2023, India’s Chandrayaan-3 mission Vikram lander, carrying the Instrument for Lunar Seismic Activity (ILSA), may have detected a moonquake at the lunar south pole, but this detection has not yet been verified (Turner 2023). Soon, a Draper CLPS lander is expected to carry two seismometers to the lunar farside in 2026, named the Farside Seismic Suite (FSS). The Endurance-A nominal payload includes a magnetometer and laser reflectometer. However, the questions about the lunar interior included in the science objectives of LGN (e.g., cadence and location of seismic events and consequent location, detail and access to the interior) cannot be resolved with sporadic, infrequent, and spatially limited data alone (Sohl and Schubert 2007).

In 2017, Space Policy Directive-1 set a national goal of returning humans to the Moon (Artemis Program) and cooperation with commercial (via CLPS) and international partners. Lunar science objectives can now be met with a combination of robotic and human and government and commercial approaches. The responsibility of achieving decadal survey lunar science objectives falls under SMD’s Lunar Discovery and Exploration Program (LDEP), created in 2019. OWL emphasizes the need for synergistic partnerships between lunar programs, recommending that “the advancement of high-priority lunar science objectives, as defined by the Planetary Science Division based on inputs from this report and groups representing the scientific community, should be a key requirement of the Artemis human exploration program” (NASEM 2023, p. 573).

OWL also recognizes that “No single organizational chain has authority for executing lunar science and missions” (NASEM 2023, p. 571). Furthermore, there is as yet no overall strategy for lunar scientific exploration or a program director or chief scientist to lead such a plan. As a result, despite

substantial investment and tremendous potential for innovative lunar exploration, as stated by OWL, “LDEP activities are not optimized to accomplish high-priority planetary science goals at the Moon” (NASEM 2023, p. 571). This committee recognizes that one consequence of this approach is that currently the only path for LGN and its decadal survey science priority goals is through the NF Program.

SMALL BODIES

Comet Surface Sample Return

The Comet Surface Sample Return (CSSR) mission theme has been a part of the NF mission targets since NF-2 and has been confirmed on the target lists by the relevant decadal surveys (see Table 1-1).

Comets are the icy leftovers of the planetesimals that formed in the distant reaches of our solar system. These bodies, which are composed of dust, rock, organic materials, and ices such as methane, ammonia, and water, are thought to have experienced limited thermal evolution. This means that their compositions may provide powerful constraints on the nature of the solar nebula in the outer solar system. The most accessible comets are those with relatively short orbital periods that reside on unstable orbits within the giant planet zone. Often referred to as Jupiter-family comets, they are thought to have originated in the primordial Kuiper belt, with early dynamical processes sending them into cold storage within Neptune’s scattered disk. Many of these bodies are only a few kilometers in size, with some coming close enough to the Sun for their ices to sublimate.

As discussed in OWL, the CSSR mission theme

The science objectives of CSSR, as defined by OWL, are as follows:

• Determine the elemental, isotopic, and structural composition of the organic and inorganic components on a comet nucleus to understand early compositional reservoirs;
• Sample, preserve, and analyze cometary organic material to determine how complex organic molecules form and evolve in interstellar, nebular, and planetary environments;
• Determine the isotopic composition of cometary water to address the role of comets in delivering volatiles to Earth’s atmosphere and interior; and
• Determine if cometary organic matter contributed significantly to prebiotic chemistry and homochirality of life on Earth.

The CSSR mission theme addresses 8 of the 12 priority science questions from OWL see Box 1-1). It would yield transformative results on Q1: “Evolution of the protoplanetary disk,” and it would produce breakthroughs in Q2: “Accretion in the outer solar system”; Q3: “Origin of the Earth and inner solar system bodies”; Q4: “Impacts and dynamics”; and Q12: “Exoplanets.” It would also lead to more modest advances in Q5, Q9, and Q10.

The potential for CSSR to become a viable NF mission was demonstrated by the Comet Astrobiology Exploration Sample Return (CAESAR) mission concept, which was one of the two finalists

for NASA’s NF-4 call, the other being Dragonfly, which was selected. Its status as an NF-4 finalist suggests that CSSR can plausibly be achieved within the technical and budgetary constraints of the NF program. Furthermore, the successful return of samples by the Hayabusa, Hayabusa2, and OSIRIS-REx missions from near-Earth asteroids have demonstrated that sample return from small bodies is possible for missions whose budgets are NF class or smaller, although comets may require more advanced technology owing to the challenges that those small bodies present.

CAESAR would have targeted comet 67P/Churyumov-Gerasimenko, which was extensively studied in situ by the European Space Agency’s (ESA’s) Rosetta mission. The orbital location of 67P within the timeframe of NF-5, however, may not be optimum for sample return. If the orbital position is not ideal, this may require future CSSR mission proposals to consider other candidate comets.

Over the next several years, the commissioning of the Vera C. Rubin Observatory (previously known as the Large Synoptic Survey Telescope [LSST]) and NEO Surveyor, and the expected subsequent detection of undiscovered comets, may yield many new targets for CSSR. The concern would be whether the orbits of these new objects are known to sufficient precision to allow for future mission planning. In addition, the superb spectra derived from James Webb Space Telescope (JWST) observations may provide additional information on the composition of possible target bodies, which in turn could help optimize the choice of CSSR targets. The committee finds no significant practical or programmatic challenges that would change the recommendation of the CSSR mission theme.

Centaur Orbiter and Lander

The Centaur Orbiter and Lander (CORAL) mission was developed as a planetary mission concept study (PMCS) by OWL committee members. After science, technical, and cost assessments, it was recommended as one of several prospective NF-6 missions by the OWL steering committee.

Centaurs are small bodies with characteristics of both asteroids and comets that reside generally in unstable orbits between Jupiter and Neptune. They are a population of dynamically evolved but compositionally primitive small icy bodies, originally from the primordial Kuiper belt that typically display much less activity than Jupiter-family comets. Owing to their unstable orbits, Centaurs are considered transitory objects that may evolve into Jupiter-family comets, be ejected from the solar system, or collide with the Sun. As described by OWL, “CORAL investigates a Centaur from orbit and in situ … [conducting] a comprehensive study of the geochemical and physical properties of primordial ice-rich planetesimals, which trace the composition of nebular volatiles such as H₂O, CO₂, CO, and NH₃, revealing the nature of early solar system compositional reservoirs. The mission will map the surface and measure the ices and organics in situ” (NASEM 2023, p. 587).

The science objectives of CORAL, as defined by OWL, are as follows:

• Determine the chemical and physical properties of a Centaur to understand the nature of primitive planetesimals;
• Perform in situ elemental, isotopic, and organic analyses of a Centaur to develop a comprehensive understanding of the composition and initial conditions of the protoplanetary disk;
• Determine the shape, topography, geological landforms, and density of a Centaur to understand the evolutionary history of this population of objects; and
• Determine degree of aqueous alteration of a Centaur to investigate the biological potential of icy planetesimals and potential brine reservoirs.

The CORAL mission theme addresses 10 of the 12 priority science questions from OWL (see Box 1-1). It would produce substantial breakthroughs to Q1–Q3, which cover the Origins themes; modest advances to substantial breakthroughs for Q4–Q6 and Q8, which cover the Worlds and Processes themes; modest advances for Q9 and Q10, which cover the Life and Habitability themes; and modest advances for Q12, which is the Exoplanet theme.

Given the recent publication of OWL, there has been little scientific advancement on the CORAL objectives and no significant practical or programmatic challenges that would change the recommendation of the CORAL mission theme. However, there could be developments that might increase the number of possible targets. For example, the Centaur target list used in the CORAL PMCS was limited at the time of the study to known objects as of late 2020 through early 2021. Their orbits constrain the choice of the target Centaur by defining where a rendezvous would be possible within a 13-year interplanetary cruise. As discussed in the CSSR mission theme, the commissioning of the Vera C. Rubin Observatory and NEO Surveyor, as well as new data releases from existing surveys, such as ESA’s Gaia mission, and the subsequent expected detection of many new Centaurs, may allow CORAL to find rendezvous targets and cruise times superior to those suggested in the CORAL PMCS. However, it is noted that new Centaur detections would require long observation arcs to reduce orbital uncertainties to levels needed for mission planning. In addition, spectra derived from JWST observations, if available, may provide additional information on the composition of possible target bodies, which in turn could help optimize the choice of the target Centaur to visit.

Otherwise, from a programmatic standpoint, CORAL requires two radioisotope thermoelectric generators (RTGs), so availability of RTGs and plutonium may be a consideration in its selection.

Ceres Sample Return

The Ceres Sample Return mission theme was first discussed in OWL. It was developed as a PMCS by OWL committee members. After science, technical, and cost assessments, it was recommended as one of several new mission themes for the NF-6 list.

Ceres is a ~940-km-diameter dwarf planet in the asteroid belt that was revealed by NASA’s Dawn spacecraft to be a candidate ocean world owing to recent and potential ongoing geologic activity, in the form of cryovolcanism, such as at Ahuna Mons (Sori et al. 2018). Indeed, Ceres presents the feasible opportunity for a sample return from a candidate ocean world. Additionally, Dawn revealed evidence for a protracted history of water, rock, and potentially even organic compound interactions via measurements of the Cerean surface chemistry, such as observed within the 92-km-diameter Occator crater (Raponi et al. 2019). Although Dawn provided key information that transformed our understanding of Ceres, Dawn also provided the foundation for its future exploration because it revealed areas that require further detailed exploration. There are key areas, especially in regard to habitability, that require a future spacecraft; these areas include an assessment of chemical gradients and physico-chemical conditions (Castillo-Rogez et al. 2022).

The composition of Ceres suggests that it may not have formed within the asteroid belt, but rather it may have originated outside the orbit of Jupiter and migrated inward. As such, it presents the opportunity not only to better understand solar system migration mechanisms but also to obtain a sample from an object that formed in the giant planet zone (Burbine and Greenwood 2020).

As described in OWL, “Ceres Sample Return focuses on quantifying Ceres’s current habitability potential and its origin, which is important for understanding habitability of mid-sized planetary bodies.”

The science objectives of the Ceres Sample Return theme as defined by OWL are as follows:

• Characterize the depth and extent of potential deep brine layer(s) to determine whether liquid exists beneath Ceres today near hypothesized brine extrusion zones;
• Characterize the nature of Ceres’s brines from salt deposits to determine the chemistry of waters and their potential habitability;
• Determine the composition, structure, and isotopic composition of Ceres’s organics to understand processes of abiotic organic synthesis and evolution; and
• Determine the elemental abundances and isotopic ratios of Ceres’s materials via measurements on returned samples to determine its accretional environment.

The Ceres Sample Return mission theme addresses 7 of the 12 priority science questions identified by OWL (see Box 1-1). It would result in substantial breakthroughs to Q5 and Q10, which seek to answer questions related to the themes of “Solid body interiors and surfaces” and “Dynamic habitability.” Indeed, it is one of two NF mission themes that may result in substantial breakthroughs in Q10. Additionally, the Ceres Sample Return mission theme will result in breakthroughs in Q3 and Q4, which seek to answer questions related to the themes of the “Origin of Earth and inner solar system bodies” and “Impacts and dynamics,” respectively. It will additionally result in advances related to priority science questions Q1, Q2, and Q11, which seek to answer questions related to the themes of “Evolution of the protoplanetary disk,” “Accretion in the outer solar system,” and “Search for life elsewhere.”

Several summary reports of the findings of Dawn have been published since the publication of OWL; these can be used to inform mission requirements (see Castillo-Rogez et al. 2020; McCord et al. 2022). Additionally, several mission concepts have been published, demonstrating community interest (see Gassot et al. 2021). From a programmatic perspective, NASA and other space agencies have demonstrated the capability of sample return from small solar system bodies (e.g., Hayabusa, Hayabusa2, and OSIRIS-REx) and larger worlds like the Moon (Apollo 11, 12, 14, 15, 16, and 17; Luna 16, 20, and 24; and Chang’e 5 and 6).

OUTER SOLAR SYSTEM PLANETS AND MOONS

Enceladus Multiple Flyby/Ocean Worlds—Enceladus

The scientific importance of both Titan and Enceladus were discussed in detail in V&V; however, these were not specifically called out as NF mission themes. In the midterm review (NASEM 2018), Enceladus was identified as a potential target for a large mission if additional financial resources were made available to NASA by Congress. Similarly, Titan was identified as the destination for a high-priority, but deferred, large mission. Ultimately, in response to Congress, NASA added the Ocean Worlds theme to the NF-4 call, soliciting missions to explore Titan and/or Enceladus.

According to the NF-4 announcement of opportunity, “‘The Ocean Worlds’ mission theme is focused on the search for signs of extant life and/or characterizing the potential habitability of Titan and/or Enceladus. For Enceladus, the science objectives (listed without priority) of this mission theme are: a) Assess the habitability of Enceladus’s ocean; and b) Search for signs of biosignatures and/or evidence of extant life” (NASA 2016).

The science goals of Enceladus Multiple Flyby, studied in and included as a New Frontiers mission by OWL, are closely aligned with those of Ocean Worlds—Enceladus, which was not included in a decadal survey. Thus, the committee considers Enceladus Multiple Flyby and Ocean Worlds—Enceladus as one mission theme with the science goals of Enceladus Multiple Flyby superseding and encompassing those of Ocean Worlds—Enceladus.

Enceladus is an icy ocean world currently erupting cryovolcanic plumes of gas and ice grains from its South Polar Terrain. Cassini measurements of the temperature and salinity of Enceladus’s subsurface ocean paired with the detection of elements such as carbon, hydrogen, nitrogen, oxygen, and sulfur in the plume suggest that Enceladus could possess geochemical and geophysical conditions suitable for life (Thomas et al. 2016). As demonstrated by Cassini, a spacecraft can directly sample icy grain materials and molecular gasses erupted from Enceladus’s subsurface ocean to assess the potential for habitability.

As discussed in the OWL report, the Enceladus Multiple Flyby (EMF) mission theme:

The science objectives of EMF, as defined by OWL, are as follows:

• Search for and identify complex organic molecules in Enceladus plume materials, with velocities 4 km/s and sample volume >1 μl with appropriate contamination control to enable life-detection investigations;
• Determine the composition, energy sources, and physicochemical conditions of Enceladus’s ocean to assess its habitability; and
• Characterize Enceladus’s cryovolcanic activity to determine spatial and compositional variations in plume activity and the processes causing ocean material ejection and modification.

The EMF mission theme addresses 4 of the 12 priority science questions from OWL: Q5: “Solid body interior and surfaces”; Q10: “Dynamic habitability”; Q11: “Search for life elsewhere”; and Q12: “Exoplanets.” Given the recent publication of OWL, there has been little scientific advancement on the EMF objectives and no significant practical or programmatic challenges that would change the recommendation of the EMF mission theme.

Saturn Probe

The Saturn Probe mission concept was first included in the New Frontiers program in V&V. This mission would deploy a probe into Saturn’s atmosphere to determine the structure of the atmosphere as well as noble gas abundances and isotopic ratios of hydrogen, carbon, nitrogen, and oxygen. The key mission challenges identified in the V&V concept study were development of the entry probe and potential payload requirements growth (see Table ES.1 in V&V; NRC 2011). The Saturn Probe mission theme underwent independent cost and technical evaluation as part of V&V and was not studied further for OWL. Following V&V, the Saturn Probe mission theme was included in the NF-4 AO, proposed but not selected in response to the NF-4 call, and included in the NF-5 draft AO and the NF-6 list outlined in OWL.

Understanding the initial conditions in the protosolar nebula requires measurements of each of the giant planets’ elemental and isotopic compositions. Constraining giant planet formation mechanisms is particularly dependent on knowing when and where Saturn formed, over how long, and if its orbit has migrated over time to stop Jupiter’s inward movement. Noble gas abundances are also crucial for determining if helium rain has prolonged Saturn’s thermal evolution. Additionally, comparisons of what governs the diversity of giant planet climates, circulation, and meteorology require constraints on the vertical temperature and wind profiles, as well as vertical circulation.

The science objectives of the Saturn Probe mission from OWL include the following:

• Determine the in situ noble gas, elemental, and isotopic abundances to understand conditions in the protosolar nebula, as well as constrain Saturn’s formation, evolution, and migration;
• Determine the in situ tropospheric temperature–pressure profile to quantify Saturn’s heat transport and convective stability;
• Determine the in situ vertical wind shear to characterize Saturn’s tropospheric circulation and meteorology; and

• Constrain vertical mixing in Saturn’s troposphere to bound transport from the deeper interior.

The Saturn Probe mission theme addresses 5 of the 12 priority science questions from OWL: Q1: “Evolution of the protoplanetary disk”; Q2: “Accretion in the outer solar system”; Q7: “Giant planet structure and evolution”; Q8: “Circumplanetary systems”; and Q12: “Exoplanets.” It is the only NF mission theme to address Q7: “Giant planet structure and evolution” (NASEM 2023). Although some measurements may be obtained via remote sensing, many of the science objectives require in situ sampling.

The priority science questions addressed by Saturn Probe are still highly compelling and have not been significantly advanced in recent years because an atmospheric probe is required; thus, we find no significant practical or programmatic challenges that would change the recommendation of the Saturn Probe mission theme.

Io Observer

An Io Observer mission concept was first discussed in the 2003 decadal survey NFSS (NRC 2003). While this concept was deemed worthy of flight based on its science goals, it was not initially prioritized as a medium-class (NF) mission for that decade based on reasons of “mission sequencing, technological readiness, or budget.” However, the 2008 report Opening New Frontiers in Space: Choices for the Next New Frontiers Announcement of Opportunity (NRC 2008) recommended that the mission be added to the NF-3 candidate mission list, and it was subsequently included as a mission concept in NASA’s NF-3 AO released in 2009.

V&V reiterated the high scientific priority of the questions that would be addressed by Io Observer in the decade from 2013–2022, and the V&V committee commissioned an Io mission concept to be studied in detail. The subsequent cost and technical evaluation (CATE) analysis of this study demonstrated that the mission was a plausible candidate for the New Frontiers program. The report prioritized Io Observer as a New Frontiers candidate, “because of its compelling science and because it was the only outer planet satellite mission studied for which cost estimates placed it plausibly within the New Frontiers cost cap.” However, V&V restricted the NF-4 candidate mission list to only five concepts (a reduction from the eight that were solicited in the NF-3 AO), and Io Observer was not included on the NF-4 candidate list. V&V did recommend that Io Observer be added again as a candidate for the NF-5 opportunity.

Io is subject to intense tidal heating owing to its orbital resonance with Europa and Ganymede, making this world an unprecedented location to study tidal heating and its effects, including high heat flow, extreme volcanism, and active tectonics. These fundamental planetary processes are thought to have played a role in shaping the surfaces of young planets in our solar system, and their study at Io will lead to a better understanding of planetary evolution. Io’s dynamic atmosphere and its interactions with the Jovian system will also provide important insight into fundamental planetary physics. Additionally, the moon serves as an analog to extremely heated exoplanets like tidally heated worlds or planets with extreme solar insolation and provides an opportunity to understand worlds beyond our solar system.

The science goals of the Io Observer mission as defined in V&V are as follows:

• Study tectonic processes;
• Investigate interrelated volcanic, atmospheric, plasma-torus, and magnetospheric mass and energy exchange processes;
• Constrain the state of Io’s core via improved constraints on whether Io generates a magnetic field; and
• Investigate endogenic and exogenic processes controlling surface composition.

These science goals were responsive to priorities in V&V: “How did the giant planets and their satellite system accrete, and is there evidence that they migrated to new orbital positions?”—these can be

considered analogous to Q2: “Accretion in the outer solar system” and Q8: “Circumplanetary systems” in OWL.

During the 2019 Discovery Program call, the Io Volcano Observer (IVO) Mission was selected as one of four finalists for a Phase-A Concept Study but was ultimately not selected for flight. The primary goal of this mission would be to understand how tidal heat is generated, is lost, and drives the evolution of Io. IVO would address this goal through a series of 10 flybys of Io carrying a payload that included a camera, thermal mapper, magnetometers, plasma instrument for magnetic sounding, gravity science investigation, and ion and neutral mass spectrometer (NASA 2021a). This approach is the same as the Io Observer study commissioned in V&V, with a significant difference that IVO would have been solar powered and feasible under a Discovery budget.

In the 2020 study Options for the Fifth New Frontiers Announcement of Opportunity (NASEM 2020), CAPS recommended that Io Observer remain on the next NF AO in the event that IVO was not selected for flight. The study noted that “Discovery proposals have been a useful step in maturing mission concepts. For example, the missions that became OSIRIS-REx and Juno were proposed to the Discovery program twice and three times, respectively, before being revised, reproposed, and selected as New Frontiers missions.”

OWL reaffirmed the importance of the science that would be achieved by an Io Observer mission concept, and anticipated that Io Observer would have an opportunity to compete in the NF-5 selection. However, OWL did not include Io Observer in the NF-6 or NF-7 candidate mission list. The committee concluded that the selection of the IVO Discovery concept for a Phase A study demonstrated that fundamental Io science could now be achieved at a lower cost cap, and therefore this theme was given lower priority than other NF-6 and NF-7 themes that clearly demanded medium-class mission cost caps.

The Options for the Fifth New Frontiers Announcement of Opportunity report details many of the new scientific findings related to Io since V&V. This new information comes from Earth-based studies and continued analysis of data sets collected by the Voyager, Galileo, New Horizons, and Juno missions.

In December 2023 and then again in February 2024, the Juno spacecraft flew within ~1,500 km of Io, the closest approach of a spacecraft to the surface of the moon in more than 20 years. During the flyby, JunoCam collected color images of Io’s high northern latitudes at higher resolution than previous Galileo and New Horizons coverage, enabling new investigations of Io surface geology and an opportunity to identify surface changes that occurred over the past 2–4 decades (Ravine et al. 2024). The unique geometries of the Juno flybys compared to the Galileo mission also presented an opportunity to refine Io’s gravity field, and in particular, to improve constraints on the tidal potential Love number, k2, which is an important test of the presence of a shallow magma ocean beneath Io’s surface (Keane and Bolton 2024). Juno also observed Io with its microwave radiometer (MWR) to partially map the satellite’s surface and subsurface at six frequencies ranging from 600 MHz to 22 GHz, and analysis of these data is in progress (Zhang et al. 2024).

Despite the wealth of new data from Juno, the mission is limited in its ability to address many of the major gaps in our knowledge of Io due in part to its spin-stabilized orbit (which makes it difficult to point instruments at Io), resolution-limited data sets, and limitations of types of instruments carried in the Juno payload (Keane et al. 2022). Thus, we find no significant practical or programmatic challenges that would change the recommendation of the Io Observer mission theme.

Titan Orbiter

V&V considered a flagship-class Titan Saturn System Mission, of which the Titan Orbiter was one component (NRC 2011). Science objectives of the full mission were to “Explore Titan as an Earth-like system,” “Examine the organic chemistry of Titan’s atmosphere,” and “Explore Enceladus and Saturn’s magnetosphere for clues to Titan’s origin and evolution.” The Titan Orbiter underwent CATE cost evaluation in V&V, receiving an estimate of $6.7 billion. The key challenges identified in V&V for the full Titan Saturn System Mission were uncertainty in instrument mass, low launch margin, and power requirements (see Box C.11 in V&V; NRC 2011).

A less costly dedicated Titan orbiter concept first emerged in the mid-2010s, following the success of the Cassini mission (NASA 2021b). In response to NASA’s NF-4 call, a Titan orbiter “Oceanus” that included a sea probe was proposed. The sea probe element was de-scoped shortly before proposal submission. Oceanus was not selected for Phase A study in NF-4, and the selection ultimately went to the Dragonfly Titan mission instead.

In the CAPS 2020 report on New Frontiers 5 (NASEM 2020), it was stated that removing Titan Orbiter from the list of potential missions was appropriate based on programmatic considerations, given the selection of Dragonfly for New Frontiers 4. This theme was therefore not included in the NF-5 draft AO. This decision was revised in the OWL report, which stated that “Titan Orbiter provides important and complementary science to Dragonfly.” OWL further noted that Cassini was not a dedicated Titan orbiter, and that the upper atmospheric composition of Titan remains unknown.

The OWL mission concept study report for the Titan Orbiter included the sea probe that was de-scoped in the NF-4 proposal, on the basis that the required technology had matured since the Oceanus concept was first proposed. The Titan Orbiter was sent for TRACE analysis as part of OWL, with the sea probe once again de-scoped. TRACE analysis assessed the technical risk rating as medium-low, with the megaflex arrays and hypervelocity aerosampling technologies identified as key challenges. The TRACE cost estimate was $2.17 billion. The stand-alone version of the Titan orbiter was included in the NF-6 list outlined in OWL.

Titan is the only moon in the outer solar system with a thick atmosphere, and the only body in the solar system besides Earth with an ongoing hydrological cycle, including precipitation cycles, lakes, and surface fluvial erosion. This cycle involves liquid hydrocarbons, rather than water, making it a fascinating case study for comparative planetology. The long-standing question of how the methane cycle remains closed on geological timescales is a particularly important motivation for further exploration that links to geophysical questions about Titan’s interior. Titan’s climate is affected by bulk atmospheric properties and seasonal variations, but also by its complex hydrocarbon cycle, allowing further analogies with how Earth’s climate system operates. Despite significant advances, atmospheric chemistry and hydrocarbon haze formation in Titan’s upper atmosphere remain poorly understood. This chemistry has similarities to prebiotic chemistry on the early Earth, making Titan an important target for astrobiology research. From OWL:

The science objectives for Titan Orbiter from OWL are as follows:

• Determine Titan’s internal structure, the depth and thickness of the ice shell and subsurface ocean, and whether the former is convecting; and determine rates of interior–surface solid or gas interchange;
• Characterize Titan’s global geology and its landscape-shaping processes;
• Characterize Titan’s global methane hydrological and sedimentological system, including surface transport/flow rates and cloud distributions; and
• Quantify the production, transport, and fate of organic molecules in Titan’s upper atmosphere and atmospheric and climate evolution in general.

The Titan Orbiter mission theme addresses 6 of the 12 priority science questions from OWL (see Box 1-1): Q5, Q6, Q8, Q10, Q11, and Q12. (Note that the assessment of science questions addressed in the mission study report differed, in part because that report also included a sea probe element.) Titan Orbiter was described in OWL as having the potential to provide breakthrough advances in Q5: “Solid body interiors and surfaces” and Q6: “Solid body atmospheres, exospheres, magnetospheres, and climate evolution.”

Key breakthroughs that Titan Orbiter would enable in Q5 include a determination of the thickness and conductivity of the ice crust, an understanding of how material is transported across Titan’s surface, and a determination of the relative importance of aeolian, fluvial, cryovolcanic, and other processes in surface alteration. Advances in Q6 include determination of the 3D distribution of Titan’s aerosols and clouds, as well as detection of storms and rainfall events. In addition, the orbiter would allow advances in atmospheric chemistry, including the role of oxygen in atmospheric organics and the importance of ion neutral versus radical reaction pathways (NASA 2021b).

The science questions addressed by Titan Orbiter remain compelling and have not been significantly advanced since OWL was published. The results from new observations of Titan by JWST, as well as forthcoming observations, have not yet appeared in the peer-reviewed literature. Observations of Titan from JWST or Earth complement the science that Titan Orbiter would perform but are not a replacement for it.

VENUS

Venus In Situ Explorer

Venus In Situ Explorer (VISE) was recommended as an NF-class mission in three decadal surveys: NFSS (2003), V&V (2011), and OWL (2023). NASA removed VISE from the NF-5 Draft AO in 2022, citing programmatic balance (NASA n.d.-g). OWL included VISE in the NF-6 mission theme list, citing scientific importance (NASEM 2023).

Planetary scientists have prioritized VISE for its importance in understanding the evolution of terrestrial planets, writ large, and specifically why Venus and Earth are different. Venus also provides an opportunity to better understand Earth-sized exoplanets. OWL encapsulates this in Q3.4a: “Why are Earth and Venus so different?” Venus hosts a dynamic atmosphere and (likely) active surface, meaning that scientifically critical processes are complex, coupled, and occur across large spatial and temporal scales. Scientists will not fully understand these processes with data acquired only by orbiters or individual probes. An in situ platform that dwells in the atmosphere or reaches the surface requires an NF-class investment.

VISE was not subjected to the CATE process in V&V nor the TRACE process in OWL because “the survey committee concurred with Vision and Voyages’ decision that the phase-A study—conducted when a concept responsive to VISE was in the step-two competition for the third New Frontiers launch opportunity—was equivalent to or better than a CATE” (NASEM 2023). As noted previously, OWL did not comment on the NF-5 mission candidates because doing so would have created conflicts of interest. OWL recommended VISE for the NF-6 opportunity.

As most recently defined, VISE must address at least two of three science objectives:

• Characterize past or present large-scale spatial and temporal (global, longitudinal, and/or diurnal) processes within Venus’s atmosphere;
• Investigate past or present surface–atmosphere interactions at Venus; and
• Establish past or present physical and chemical properties of the Venus surface and/or interior.

These objectives would address at least five priority science questions from OWL (see Box 1-1): Q3: “Origin of Earth and inner solar system bodies”; Q5: “Solid body interiors and surfaces”; Q6: “Solid body atmospheres, exospheres, magnetospheres, and climate evolution”; Q10: “Dynamic habitability”; and Q12: “Exoplanets.”

OWL defined the newest NF-6 science objectives for VISE after the selection of three new missions to Venus in 2021 by NASA and ESA: VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy; Smrekar et al. 2022); DAVINCI (Deep Atmosphere Venus Investigation of Noble Gases, Chemistry, and Imaging; Garvin et al. 2022); and EnVision (European Space Agency n.d.). OWL acknowledged that these missions do not meet the science objectives of VISE, stating, “Venus In Situ Explorer (VISE) investigates the processes and properties of Venus that cannot be characterized from orbit or from a single descent profile.” For example, VERITAS and EnVision cannot directly measure atmospheric or surface chemistry, intrinsic magnetism, or seismic activity. DAVINCI will probe the atmosphere at one time and location on a planet with complex chemical gradients and dynamics in three dimensions and over time. No planned mission is designed to collect data at the surface, and the study of lunar, martian, and asteroidal samples attests to the value of direct geochemical and mineralogical analysis to understand the fundamental composition and evolution of planetary bodies. Ultimately, the VISE mission concept complements the newly selected missions. Because OWL recommended that VISE be included on the NF-6 mission list, when an NF-6 selection was expected within the current decade, the report recognized that the VISE science objectives were independent of EnVision, DAVINCI, and VERITAS and that VISE could be selected before those missions reach Venus.

Scientifically, recent work emphasizes that Venus is highly dynamic over the timescale of a spacecraft mission. Although data from the last NASA mission to Venus (Magellan) is now more than 30 years old, scientists continue to mine it for new insights. In 2023, new analyses revealed a large, volcanic vent that may have changed shape in the 8 months between two imaging passes, which would represent active volcanism (Herrick and Hensley 2023). Recent geophysical modeling makes the case for the formation of cratons on surface of Venus by mechanisms that may have operated on early Earth (Capitanio et al. 2024). The JAXA Akatsuki mission continues to operate as a meteorology-focused orbiter, returning spectacular images of the clouds in different wavelengths. The identity of the “unknown UV absorber” remains a mystery under active investigation (Jiang et al. 2024). The lack of knowledge about the physical and chemical processes in the clouds remains such that we cannot exclude the possibility of modern cloud life (Bains et al. 2024; Petkowski et al. 2024). During a Venus gravity assist, the Parker Solar Probe observed whistler waves, the origin of which has been debated for decades (George et al. 2023). The importance of Venus to the understanding of Earth-sized exoplanets only grows (Way et al. 2023). Investments in the Hot Operating Temperature Technology (HOTTech) program have shown promise and would critically enable a future VISE proposal.

Overall, ongoing scientific research and technological investments for this active and dynamic planet only bolsters the case for VISE’s in situ measurements.