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What spacecraft actually measure
Neither method detects and counts individual dust grains in a polar ice sample. Radar instruments record returning radio echoes, while spectrometers record reflected sunlight. Scientists interpret those observations using models of how radio waves and light interact with ice, dust, and layers beneath the surface.
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The distinction matters: radar is most useful for mapping buried structure, while spectral analysis constrains properties of ice exposed at the surface. They examine different volumes and measure different signals, so their estimates need not describe the same material.
How radar reveals buried layers
SHARAD: echoes and reflectivity contrasts
NASA’s Mars Reconnaissance Orbiter carries SHARAD, a shallow subsurface radar. A radargram displays echoes returning at different times; those time delays help researchers map interfaces, layer geometry, and continuity within the polar deposits. Differences in echo reflectivity can indicate differences in electrical properties. Scientists interpret some contrasts as variations in the amount or type of material mixed with ice, including dust—but reflectivity is not a direct dust measurement. NASA/JPL’s explanation of SHARAD describes how the instrument maps subsurface layers.
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In a 2009 analysis, researchers considered 358 SHARAD observations. They reported alternating zones: some with high reflectivity and multiple contrasting layers, others more homogeneous and less reflective. The team compared that pattern with climate models to constrain possible explanations for how the deposits formed; radar reflectivity itself was not treated as a direct climate reading. The report described a stack reaching 2 kilometers (1.2 miles) thick. NASA/JPL reported the analysis in 2009.
As SHARAD science-team member Nathaniel Putzig put it: “Contrast in electrical properties between layers is what provides the reflectivity we observe with the radar.” That is why radar can distinguish layers without photographing their contents: the observed contrast is electrical, and its material cause must be interpreted.
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MARSIS: a second radar perspective
Mars Express carries MARSIS, a low-frequency radar sounder. NASA/JPL describes its main objective as searching for water from the surface to about 5 kilometers (3 miles) below it. But a radar echo does not identify ice automatically: ice can be difficult to distinguish because its electrical signal may resemble rock. Researchers therefore interpret echoes in context rather than treating a return as proof of a particular substance. NASA/JPL’s Mars Express overview describes MARSIS and its target depth.
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Visible and near-infrared light provides a different kind of evidence. The 2026 study by Pari Mohan and Aditya R. Khuller analyzes observations from OMEGA and CRISM, among other observations, by fitting them with a radiative-transfer model. The model estimates how light behaves in mixtures and layers of snow, firn, ice, and impurities such as Martian dust.
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Reflectance depends on several properties at once, including ice grain radius, dust content, and vertical layering. The researchers varied surface and subsurface grain radii, dust concentration, and surface-layer thickness, and used ensemble Monte Carlo sampling to characterize uncertainty. The method therefore estimates plausible ice properties from spectra; it does not directly assay dust.
The estimates depend on model choices and assumptions, including optical properties and atmospheric effects. The paper notes that earlier studies using different formulations and assumptions produced substantially different parameter estimates. Its figures should be read as model-derived results for the observations and scope analyzed, not as direct sample measurements or a universal value for every polar layer. Mohan and Khuller’s 2026 paper describes the analysis and its assumptions.
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Why surface and bulk estimates can differ
The 2026 study estimates less than 3% dust by mass in exposed north-polar ice generally, substantially below previous estimates reaching about 25%. The less-than-3% figure applies to exposed ice at the analyzed north-polar sites; it is not an established percentage for the full depth or every region of the polar deposit.
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To reconcile a relatively clean surface with radar-derived estimates of more impurity in bulk deposits, the authors propose a layered interpretation: relatively clean ice layers containing 0.05–0.5% dust by mass alternate with dust-rich marker beds containing 25–75% dust by mass. These are modeled endmember values for a proposed structure, not direct measurements from every layer. A surface spectrum can be dominated by clean exposed ice even when dust-rich beds occur below it, while radar responds to boundaries and contrasts within the buried stack. The study reports the surface estimate and layered interpretation.
| Method | What it observes | Best suited to | Key limitation |
|---|---|---|---|
| SHARAD and MARSIS radar sounding | Time-delayed radio echoes and reflectivity contrasts | Mapping buried layer boundaries, thickness, continuity, and subsurface structure | Reflectivity indicates electrical-property contrasts; attributing them to dust or another material requires interpretation. Ice and rock can have similar radar properties in some contexts. NASA/JPL, 2009; NASA/JPL. |
| OMEGA and CRISM spectral reflectance | Visible and near-infrared reflected light from exposed ice | Estimating exposed ice’s dust content, grain radius, and layering by fitting spectra | Results depend on radiative-transfer assumptions, dust and ice optical properties, atmospheric treatment, and the chosen layer model. Mohan and Khuller, 2026. |
What the numbers do—and do not—tell us
NASA describes the north polar layered deposits as dusty ice up to about 2 miles thick and approximately 620 miles in diameter. Their layers and surface textures can preserve clues to changes in Mars’s climate; texture may reflect differences in dust content or ice grain size. Those large-scale deposits are not compositionally uniform simply because one exposed-surface estimate is low. NASA/JPL’s polar-cap report discusses the layered deposits.
There is no direct polar-ice core analysis behind the percentages described here. The dust fractions come from interpreting remote spacecraft data with physical models. The most accurate shorthand is therefore: radar maps buried structure, spectroscopy estimates exposed ice properties, and the resulting dust values depend on which layers and observations are being described.
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