For years, the idea of water on Mars has captivated scientists and the public alike, fueling dreams of future human missions and the lingering question of past life. The planet’s history of liquid water on its surface billions of years ago has driven the hunt for hidden reservoirs. Recent excitement stemmed from an August 2024 study that analyzed data from NASA’s InSight mission, suggesting a significant amount of liquid water saturating Mars’ mid-crust—a region approximately 6 to 12 miles deep. This initial interpretation presented a vision of Mars harboring a substantial underground ocean, potentially enough water to cover the entire planet with a layer up to 1.2 miles deep if spread out evenly.
However, not everyone agrees with this optimistic assessment. Bruce Jakosky, a senior research scientist and former lead investigator for the MAVEN mission, has offered a more cautious perspective. In a recent letter to the Proceedings of the National Academy of Sciences (PNAS), Jakosky re-examined the same InSight data and concluded that a water-saturated mid-crust isn’t necessarily supported by the evidence. While he acknowledged the original study’s methods were “reasonable and appropriate”, Jakosky stated that “the results of their modeling suggest an alternative conclusion”. This means the data doesn’t require Mars to have as much hidden water as initially thought, implying the planet’s underground water reserves could be significantly smaller—perhaps even zero. This revised viewpoint significantly alters our understanding of Mars’ past, its potential for life, and the resources available for future human exploration.
Listening to Mars’ Inner Secrets
To understand how scientists are arriving at these varied conclusions, it helps to know how data from the InSight mission is used. Launched in 2018, InSight acted as a robotic ear, listening to the internal rumblings of Mars. Though the mission concluded in 2022, its collected data continues to be a rich source of information about the planet’s interior.
InSight gathered information primarily by measuring seismic waves, or “Marsquakes”. Just as earthquakes on Earth provide clues about our planet’s inner layers, Marsquakes reveal details about the composition and structure of Mars’ crust and deeper interior. Scientists combine this seismic data with gravity measurements to create “rock-physics models”. These models are complex simulations that help determine what types of rocks, how much empty space (porosity), and how much water (liquid water saturation) would best explain the seismic signals InSight detected from specific regions of the Martian crust. The initial 2024 PNAS study, led by geophysicist Vashan Wright, concluded that a mid-crust made of fractured igneous rocks filled with liquid water “best explains the existing data”.
The Probability Puzzle: Is There More or Less Water?
Jakosky’s re-evaluation doesn’t dispute the quality of the InSight data or the methods used to collect it. Instead, his argument centers on how the results, particularly regarding liquid water saturation (γw), are interpreted. The original study presented a “probability distribution” for γw, showing the likelihood of different levels of water filling the rock’s pore spaces. While the probability of γw being near 1 (meaning the pores are nearly full of water) did peak in the initial analysis, Jakosky emphasizes that “the distribution is very broad”. A broad distribution signifies that even if one value is the most probable, there’s still a significant chance that other values are correct.
To explain this, Jakosky references a key figure (Figure 1 in the paper) that illustrates both the incremental and cumulative probability distributions. The cumulative distribution helps understand the likelihood of γw being at or below a certain value. Based on the original analysis, Jakosky notes that there is a “50% probability of γw being above about ~0.63, but there also is a 50% probability of it being lower”. This indicates an equal chance that the mid-crust is more than 63% saturated with water as it is less than 63% saturated.
He further points out that the data shows “an equal 25% probability of being within each of the four quartile regions”. This means that dividing the possibilities into four equal segments, each segment has a 25% chance of containing the actual value of water saturation. Although higher values of water saturation are “slightly favored,” Jakosky argues it’s “not enough to discriminate substantially against the lower values”. What this means is that while a water-saturated mid-crust is one possibility that fits the data, it’s not the only possibility. Jakosky concludes that “any value of γw between 0 and 1 appears to be viable”. As a result, the InSight findings “provide minimal constraints on the actual crustal water content” and “do not require a water saturated mid crust”.
Reshaping Our Understanding of Martian Water
This difference in interpreting the data significantly impacts our estimates of Mars’ total water content. The original study estimated that if the mid-crust was water-saturated, it could hold 1 to 2 kilometers (0.6 to 1.2 miles) of water as a global equivalent layer (GEL). A GEL represents all the water on a planet spread evenly across its surface. For comparison, Earth’s GEL is about 3.6 kilometers, mostly due to its vast oceans.
However, if we consider Jakosky’s alternative viewpoint—that the data doesn’t rule out a much drier mid-crust—then the potential global water content could range “between 0 and 2 km GEL”. This broadened range means the mid-crust could be completely dry or contain a significant amount of water, and the InSight data alone cannot definitively distinguish between these scenarios.
The question of how much water is locked away in Mars’ crust is profoundly important. Scientists believe that more than 3 billion years ago, Mars had liquid water flowing on its surface. But as the planet lost much of its atmosphere, surface water couldn’t persist. The major scientific puzzle is what happened to all that water—did it become buried as ice, become confined in deep aquifers, get incorporated into minerals, or escape into space? If the crust contains a substantial amount of water—1 to 2 kilometers GEL—it would represent the largest known “sink” for water on the planet, indicating a major storage location that has influenced Mars’ history.
While the InSight data doesn’t definitively answer the total water content in the Martian mid-crust, it offers invaluable insights into the planet’s interior. Jakosky observes that the derived probability distribution for porosity (the amount of empty space in the rock) is much more focused, pointing to values between approximately 0.1 and 0.3. These figures are considered reasonable for porosity at that depth. This observation implies that a change in properties around 20 kilometers deep likely coincides with the depth where pore spaces in the crust close up.
Understanding the distribution of pore space and whether it’s filled with liquid water, solid ice, or is empty remains crucial for comprehending Mars’ water inventory and its long-term history. Future missions equipped with more advanced seismic profiling and geological analysis will be necessary to pinpoint the exact abundance of water in the Martian crust. This knowledge will not only refine our understanding of Mars’ past climate and its potential for life but also inform decisions about where to land future missions and how to access resources for human explorers.
Paper Summary
Methodology
This study re-examines seismic data from NASA’s InSight mission to interpret crustal properties, specifically focusing on the Martian mid-crust (approximately 10 to 20 km deep). Using rock-physics models, the research evaluates the liquid water saturation (γw), which is the fraction of pore spaces in the rock filled with liquid water, and analyzes its incremental and cumulative probability distributions.
Results
While initial interpretations of InSight data suggested the Martian mid-crust was nearly saturated with liquid water (leading to an estimate of 1 to 2 km global equivalent layer of water), this re-analysis indicates that the probability distribution for water saturation is broad. This means the data provides minimal constraints on the actual water content, with any value between 0 and 1 appearing viable. Therefore, the results do not necessitate a water-saturated mid-crust, and the global water equivalent could range from zero to 2 km.
Limitations
The study’s primary limitation is that the InSight data, despite its value, does not definitively confirm a water-saturated mid-crust. The broad probability distribution for liquid water saturation means that a dry mid-crust is also consistent with the observations. More precise future measurements will be needed to constrain the exact amount of water.
Funding and Disclosures
The author, Bruce M. Jakosky, declares no competing interest. This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
Publication Information
Title: Results from the inSight Mars mission do not require a water-saturated mid crust. Author: Bruce M. Jakosky. Journal: PNAS (Proceedings of the National Academy of Sciences). Volume/Number: 122, no. 11. Page: e2418978122. Publication Date: March 6, 2025. DOI: https://doi.org/10.1073/pnas.2418978122.
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