A MESSENGER From Mercury—Principal Investigator Sean Solomon Returns

Episode Summary

Executive Summary: This Planetary Radio episode ties together Mars geology, spaceflight innovation, and Mercury science. Emily Lakdawalla explains Curiosity’s basalt-rich but sedimentary rocks and why they remain puzzling. Bill Nye highlights cost-cutting advances like reusable rockets and 3D-printed engines. Sean Solomon reviews MESSENGER’s landmark Mercury discoveries, especially polar ice and unexpected volatiles, as the mission nears its end.

Main Topics: Curiosity’s basaltic sedimentary rocks on Mars: Emily Lakdawalla describes the geological paradox that Curiosity’s rocks are sedimentary yet composed largely of basaltic material, suggesting Mars’ water-rock history is more complex than expected. MOLI imaging and rover field geology: The Mars Hand Lens Imager is presented as a versatile close-up and landscape-imaging workhorse that helps identify sediment types and supports field geology on Mars. Reusable rockets and 3D-printed engines: Bill Nye discusses SpaceX’s plans to land a reusable first stage on a floating platform and the broader cost reductions promised by additive manufacturing of rocket engines. MESSENGER’s Mercury discoveries: Sean Solomon reviews the mission’s major findings, including water ice at Mercury’s poles, dark organic-like surface coatings, and the planet’s surprising volatile inventory. Mercury’s composition and origin mystery: MESSENGER’s measurements challenged existing models for Mercury’s formation by finding volatile elements that were not depleted, undermining long-standing theories of the planet’s high density. Planetary Radio extras and skywatching: Bruce Betts covers current sky targets, a history note on Galileo’s asteroid flyby, the weekly trivia answer, and a Curiosity cooling-system factoid.

Key Arguments: Curiosity’s rocks are sedimentary, but their basaltic composition creates a geologic puzzle that may reflect Mars being fundamentally different from Earth. Water-related alteration on Mars appears localized and sparse, implying either limited water activity or chemistry we do not yet fully understand. MOLI is crucial because it can image grains up close, classify sedimentary structures, and also capture wide-angle self-portraits and landscapes. Reusable launch systems could lower access-to-orbit costs by allowing rocket recovery at sea and reducing the need for fixed launch-site constraints. 3D printing rocket engines and engine components could further reduce manufacturing costs and make deeper-space missions cheaper and more accessible. MESSENGER confirmed water ice in permanently shadowed Mercury craters, but also found darker material likely made of complex organics mixed with the ice. Mercury’s surface chemistry contradicted decades of formation models because sulfur, potassium, sodium, and chlorine were unexpectedly abundant rather than depleted. The pole deposits on Mercury appear relatively recent and pristine, offering a record of ongoing delivery of volatiles from comets and volatile-rich asteroids. MESSENGER’s planned crash will itself become a scientific experiment by exposing fresh material and creating a precisely known impact site for future missions to study.

Data Points: MESSENGER mission duration at Mercury: 3.5 years - Sean Solomon notes the orbiter had been circling Mercury for more than three and a half years. MESSENGER orbital count: more than 3,500 orbits - Sean cites the mission’s accumulated work returning data from Mercury. MESSENGER imaging output: more than 250,000 images - Total images returned by the spacecraft over the mission. MESSENGER data volume: more than 10 terabytes - Total publicly available science data from the mission. MESSENGER papers since 2009: nearly 200 peer-reviewed papers - Matt Kaplan references publications based on the spacecraft’s data. Mercury temperature contrast: 600 degrees - Sean describes Mercury’s extreme day-night equatorial temperature difference. Curiosity cooling/heating tubing: 57 meters - Bruce Betts shares a random space fact about the rover’s thermal management system. Calendar year with fewer than five active Mars spacecraft: 2011 - Bruce Betts’ trivia answer, accepted as the last year with only four working missions at Mars. Operational Mars missions in 2011: 4 - The missions listed were Mars Reconnaissance Orbiter, Mars Express, Mars Odyssey, and Opportunity. Galileo asteroid flyby year: 1991 - Bruce Betts notes Galileo’s flyby of asteroid Gaspra as the first spacecraft asteroid flyby. Trivia contest deadline: Tuesday, November 4th at 8 a.m. Pacific - Listeners were invited to submit the next contest answer by this time.

Pivotal Quotes: "I think that if you call it a paradox, it demonstrates that we don't actually understand what's going on." — Emily Lakdawalla: On the puzzling fact that Curiosity’s sedimentary rocks are basalt-rich. "We set out to do so in the expectation that we could distinguish among the theories... What we did not expect was that we would make measurements of the composition that would reject all the theories." — Sean Solomon: On MESSENGER’s surprising Mercury surface composition results. "One must be prepared for surprises." — Sean Solomon: On the broader lesson from planetary exploration and MESSENGER’s findings.

Implications: The episode underscores that planetary science remains full of surprises: Mars geology is still poorly explained, Mercury rewrites formation models, and cheaper launch tech could expand future exploration and access to space.