Mars Express Confirms Lots of Polar Ice! Where's the Rest?

Episode Summary

Executive Summary: The episode centers on Mars water: Marsis radar’s detection of a large, mostly pure south polar ice deposit, why it wasn’t surprising, and why it still leaves most Martian water unaccounted for. It also covers Enceladus’ possible subsurface ocean, New Horizons’ speed versus the Voyagers, and the week’s skywatching and trivia segments.

Main Topics: Mars south polar ice discovery: Jeff Plout explains that Mars Express/Marsis mapped a thick south polar ice deposit, refining earlier estimates and showing it is over 90% water ice. The remaining “missing” Martian water: The conversation shifts to the larger water inventory needed to carve Martian channels and where that water may have gone—into the atmosphere, space, or the subsurface. Searching for liquid water below the surface: Marsis and SHARAD are presented as tools that could detect shallow liquid water, though no clear subsurface liquid signal has been found yet. Mars mission synergy and future exploration: Odyssey’s ground-ice findings, Phoenix’s planned landing, and shared data among orbiters/landers illustrate discovery-driven Mars exploration. Enceladus and astrobiology: The host highlights studies suggesting a subsurface ocean and reactive plume chemistry on Enceladus, raising the possibility of life and future mission interest. Space trivia, skywatching, and mission updates: The episode includes Emily’s spacecraft-distance Q&A, Bruce Betts’ night-sky guide, a Mars meteorite fact, and the trivia contest winner and new question.

Key Arguments: The south polar ice estimate was not a surprise because topographic data from Mars Global Surveyor had already implied substantial polar ice inventories. Marsis improved the estimate by measuring the base of the deposit, producing a more accurate thickness map and indicating the ice is very clean (low dust content). The south polar layered deposits appear geologically older than the north, based on crater counts and surface stability over tens of millions of years. Mars still has a large unresolved water budget: the poles hold only a few tens of meters global equivalent, while erosion evidence implies hundreds of meters to a kilometer were once involved. Some Martian water may have escaped to space, been removed by impacts or the solar wind, or percolated back into the subsurface as ice or possibly liquid water. Radar instruments may detect shallow liquid water in theory, but practical limitations and rock properties may obscure any signature. Mars exploration is increasingly synergistic: one mission’s findings guide the next mission’s targets and instruments. New Horizons is faster than Voyager 1 in current speed, but it will never catch it because Voyager 1 had extra velocity from a Saturn assist and is farther from the Sun, so it slows more slowly.

Data Points: South polar ice thickness: about 36 feet (11 meters) globally equivalent - Estimated layer of water ice around Mars’ south pole from Marsis analysis Maximum thickness of deposit: 2.5 miles (over 3.5 kilometers) - Thickest south polar ice region measured by Marsis Water ice purity: over 90% water ice - Inference from strong radar reflections and minimal signal attenuation Voyager 1 distance: more than 100 astronomical units (AU) from Earth - Current distance stated during the New Horizons comparison segment Voyager 1 escape speed: about 3.6 AU per year - How fast Voyager 1 is moving away from the Sun/Earth region New Horizons speed: about 4.6 AU per year - Current heliocentric escape speed given in Emily’s Q&A Mars polar water inventory: roughly 20 meters global equivalent - Approximate combined amount locked at both poles Water needed for Martian erosion: hundreds of meters, maybe even 1 kilometer global layer - Estimate derived from catastrophic outflow channels and Valles Marineris erosion South polar crater age: tens of millions of years or more - Relative age inferred from many small impact craters on the south polar surface Mars impact meteorite estimate: about one Martian meteorite hits Earth every month - Random space fact mentioned by Bruce Betts

Pivotal Quotes: "The big question is, where's the other 99%?" — Matt Kaplan: Framing the unresolved Martian water inventory after discussing the polar ice discovery "We are still looking for the rest of the water." — Jeff Plout: Explaining that the polar ice is only a small fraction of the water needed to explain Martian erosion "Every time we learn something new, instead of reducing the number of questions, it actually raises two or three more." — Jeff Plout: On the nature of Martian geology and why new data create new mysteries

Implications: The episode underscores that Mars likely still hides most of its water below the surface or lost to space, making future radar, orbiters, and landers crucial. Enceladus also emerges as a compelling life-search target.