Between 2006 and 2016, the Cassini mission has conducted 13 downlink bistatic radar (BSR) radio science experiments of Titan’s surface. These experiments employ the High-Gain Antenna (HGA) onboard the Cassini spacecraft as transmitter and NASA’s Deep Space Network (DSN) antennas on Earth as receivers to establish a bistatic radio link bouncing off the surface of Titan. The distinct detection of X-band (λ=3.6 cm) returns from some of the observed Titan regions across different latitudes and longitudes allows to constrain surface roughness and near-surface composition based on the investigation of waveforms’ amplitude, frequency and polarization.Solid terrains probed by Cassini BSR experiments produce heterogeneous reflections ranging from broad and weak returns to narrower and more powerful echoes or a combination of both. This is indicative of different dominant scattering mechanisms. For purely specular returns, RMS slopes and dielectric constant values—connected to near-surface structure and composition—are retrieved using a Gaussian fit applied to echo spectra, as previously done in BSR data analysis. For weaker returns, contaminated or dominated by diffuse scattering, a full scattering-model-informed fitting approach that combines specular and diffuse reflection components is applied to decuple the two contributions and more accurately characterize surface properties.Herein, we present a progress update on the analysis of BSR experiments from flybys T14, T27, T34 and T124, highlighting regional variations in forward scattering and providing preliminary findings on surface roughness and near-surface dielectric constant of various regions on Titan. When possible, we exploit echo recordings from different, independently calibrated DSN antennas and discuss and compare BSR results with surface properties inferred from both Earth-based and Cassini (monostatic) RADAR observations.

Saturn's moon Titan was explored by the Cassini spacecraft from 2004 to 2017. While Cassini revealed a lot about this Earth-like world, its radar observations could only provide limited information about Titan's liquid hydrocarbons seas Kraken, Ligeia and Punga Mare. Here, we show the results of the analysis of the Cassini mission bistatic radar experiments data of Titan's polar seas. The dual-polarized nature of bistatic radar observations allow independent estimates of effective relative dielectric constant and small-scale roughness of sea surface, which were not possible via monostatic radar data. We find statistically significant variations in effective dielectric constant (i.e., liquid composition), consistent with a latitudinal dependence in the methane-ethane mixing-ratio. The results on estuaries suggest lower values than the open seas, compatible with methane-rich rivers entering seas with higher ethane content. We estimate small-scale roughness of a few millimeters from the almost purely coherent scattering from the sea surface, hinting at the presence of capillary waves. This roughness is concentrated near estuaries and inter-basin straits, perhaps indicating active tidal currents.

Mars' polar ice deposits are thought to preserve a record of climate throughout their evolution. In addition to the large north polar layered deposits (NPLD) at Mars' north pole, smaller ice deposits are preserved in craters nearby. These outlying deposits were potentially formed by the same mechanisms that drive NPLD formation, or may represent more local mechanisms. Distinguishing between these possibilities would help elucidate the spatial homogeneity of Martian climate processes. Here, we analyzed SHARAD radar depth profiles from 34 locations across the NPLD and 5 locations within the Korolev crater ice deposit using Fourier transform analysis and dynamic time warping to quantitatively assess the similarity between the internal layered stratigraphy of the two deposits. We identify broad stratigraphic similarities between the Korolev deposit and the NPLD, suggesting they likely formed due to the same climate forcing mechanism, with local variability also observed across the NPLD.

Abnormally bright radar reflections below the Martian south polar layered deposit were originally interpreted as evidence of subglacial liquid water. However, unlike on Earth, conditions beneath the Martian ice are too cold to create or maintain meltwater. In this work, we use radar reflectivity simulations to show that the strong reflections can instead be caused by constructive interference between dusty ice layers that are more closely spaced than the radar resolution. Unlike previous hypotheses, interference does not require anomalous subsurface conditions or exotic materials to be present beneath the ice. In addition, interference between thin layers can explain the variable power of radar returns beneath the entire ice sheet and does not require different mechanisms to be responsible for reflections in different regions.

Titan's labyrinth terrains are an organic-rich, topographically elevated, highly dissected and puzzling geomorphic unit. How these features came to be composed of organics and remain elevated may hold clues about Titan's complicated history, and in particular the dynamics and composition of Titan's crust. One subtype of labyrinth terrains, the radially networked labyrinth terrains, is found in Titan's mid-latitudes. They are dome-shaped with radial drainage patterns and appear to be a clustering of uplifted, organic-rich dissected plateaux. We use scaling relationships to determine whether they formed as elevated surfaces that were uplifted by solid-state diapirs or cryomagmatic laccoliths at depth. Based on the large, variable spacing between features, we find it unlikely that they formed via density-driven diapirism. Instead, their dimensions suggest that they are cryomagmatic intrusions that formed near the most prominent rheological contrast in the ice shell, the brittle-ductile transition (BDT). At that location, we argue that intrusions spread horizontally and inflate, forming large cryomagmatic laccoliths (upturned, large saucer-shaped sills). The intrusions would flex the overlying lithosphere and surficial sedimentary layers (likely undifferentiated plains), resulting in prominent domed features that are susceptible to erosion and incision by methane rain and wind. This process then leads to the highly dissected, dome-shaped labyrinth terrains. To determine whether the laccoliths formed near Titan's BDT, we calculate lithospheric strength envelopes for a pure water ice shell with and without an insulating methane clathrate crust using two conductive heat flows: 4 and 7 mW/m2. The plausible range for the BDT for an ice shell with a 1 km thick methane clathrate crust is 12-34 km for the 4 mW/m2 heat flow. This agrees well with the expected intrusion depths of the laccoliths (21-28 km) associated with a cluster of radial labyrinths in Titan's northern midlatitudes, as derived from a scaling relationship relating intrusion depth to dome width. We find that methane clathrate thicknesses of 2 km or greater result in a BDT that is generally too shallow (3-24 km) to match our observations. If we consider the higher heat flow, these BDTs are even shallower. Although methane clathrate is known to be stronger than ice, its low thermal conductivity significantly raises the underlying ice shell's temperature, which results in a significantly thinner lithosphere that is not able to support these large plateaux. We conclude that in the location of this radial labyrinth terrain cluster there may be intrusive cryomagmatic activity approximately 21 km deep within the water ice shell, with a putative surface methane clathrate crust, if it exists atop a conductive ice shell, that is constrained to be <2 km thick.

&lt;p&gt;Between 2006 and 2016, the Cassini spacecraft carried out 13 bistatic radar observations of the surface of Saturn&#039;s largest moon, Titan. Unmodulated right circularly polarized radio signals were transmitted by the spacecraft to the moon&amp;#8217;s surface. Cassini&amp;#8217;s high gain antenna was pointed so that specular reflections from Titan&amp;#8217;s surface were received on Earth. Proper processing of right (RCP) and left circularly polarized (LCP) echoes from the moon can provide information about surface roughness and near-surface relative dielectric constant (&amp;#603;&lt;sub&gt;r&lt;/sub&gt;) of the illuminated terrains.&lt;/p&gt; &lt;p&gt;During Titan flybys T101, T102, T106, and T124, the track of the bistatic observations crossed the main stable liquid bodies of the north pole of Titan: Ligeia, Kraken, Punga Mare, and their estuaries. Strong and narrowband X-band (&amp;#955;=3.6 cm) echoes were successfully detected from the seas at the Deep Space Network 70-meter station in Canberra.&lt;/p&gt; &lt;p&gt;Reflected spectra feature Dirac-like shapes, with a spectral broadening around 1 Hz and lower bounded by the processing time resolution. Compared to bistatic observations of other planets, this implies unprecedentedly low RMS slope values for Titan&amp;#8217;s seas on an effective length-scale of a few meters. Profiles of reflected LCP and RCP power are in general consistent with purely coherent reflections from the Fresnel area around the moving specular point, indicating a very flat surface.&lt;/p&gt; &lt;p&gt;In addition, from the circular polarization power ratio, the surface dielectric constant can be derived. This can enrich our current understanding of the chemistry of Titan&amp;#8217;s liquid hydrocarbon seas, further constraining their methane-ethane mixing ratio. From Cassini RADAR, VIMS, and ISS, Titan&amp;#8217;s seas are expected to be ternary mixtures of methane, ethane and nitrogen (&amp;#603;&lt;sub&gt;r&lt;/sub&gt; &amp;#8776; 1.6-1.9). From bistatic radar data, significant relative variations in liquid hydrocarbon composition are seen, and an unexpected correlation between the dielectric constant and incidence angle of observation seems to arise. The absolute values of permittivity are somewhat lower than expected.&lt;/p&gt; &lt;p&gt;From the computed dielectric constant values, physical optics models are used to constrain the RMS height of the surface. This analysis provides meaningful insights into the presence of small capillary waves in the order of millimeters over the liquid surfaces of Titan, as already detected by Cassini monostatic RADAR.&lt;/p&gt;

Mountain formation and evolution on Titan is poorly understood, due in part to a lack of high-resolution topographic data. By applying advanced processing techniques, we are able to increase the along-track spatial resolution of the Cassini RADAR altimeter by up to a factor of ten, enabling more detailed analysis. A survey of mountainous and hummocky terrain reveals a unique, characteristic waveform shape. By modeling reflections from different landscapes, we are able to show that these waveforms contain sub-resolution topographical information. We found that mountain elevation on Titan varies greatly over short distances, and evidence suggests that many mountain ranges on Titan have been eroded down to the plains level even within a single high-resolution radar footprint.

The Selk crater region is the future landing site of NASA’s Dragonfly mission to Titan. The region was imaged by the Cassini RADAR at incidence angles from 5° to 72° and at various polarization angles. Using this data set, we mapped six terrain units and assembled a backscatter curve for each, providing normalized backscatter cross section ( σ 0 ) as a function of incidence angle. By fitting these backscatter curves with a sum of a quasi-specular and diffuse terms and evaluating three alternative formulations of the first and two for the second, we extracted the best-fit surface effective dielectric constant, rms slope, and scattering albedo. Although the parameters’ absolute values are model dependent, relative values between terrains indicate real variations in surface properties. The results are consistent with the impact exposing and fracturing a low-loss tangent material such as the water-ice bedrock, which is likely also present in the hummocky terrains and to a lesser degree in the plains and interdune regions. The dunes and dark terrains are composed of smooth, uniform material with low dielectric constant (1.5–2.3 median values for all models) compatible with organic sand. A diffuse single-scattering model enabled independent derivation of the dielectric constant from high-incidence observations, leading to low values (<2) over all terrains, indicating a depolarizing (sub)surface. Finally, radarclinometry revealed lateral variations in rim height, which remains below 300 m along the SARTopo profile but reaches up to 600 m at other locations, hinting at a rim less eroded than previously thought.

Recent discoveries of anomalously bright radar reflections below the Mars South Polar Layered Deposit (SPLD) have sparked new speculation that liquid water may be present below the ice cap. The reflections, discovered in data acquired by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express orbiter, were interpreted as reflections from damp materials or even subsurface ponds and lakes similar to those found beneath Earth's ice sheets. Recent studies, however, have questioned the feasibility of melting and maintaining liquid water below the SPLD. Herein, we compare radar simulations to MARSIS observations in order to present an alternate hypothesis: that the bright reflections are the result of interference between multiple layer boundaries, with no liquid water present. This new interpretation is more consistent with known conditions on modern Mars.

Smith, Isaac; Hayne, Paul O.; Byrne, Shane; Becerra, Patricio; Kahre, Melinda; Calvin, Wendy; Hvidberg, Christine; Milkovich, Sarah; Buhler, Peter; Landis, Margaret; Horgan, Briony; Kleinböhl, Armin; Perry, Matthew R.; Obbard, Rachel; Stern, Jennifer; Piqueux, Sylvain; Thomas, Nicolas; Zacny, Kris; Carter, Lynn; Edgar, Lauren; Emmett, Jeremy; Navarro, Thomas; Hanley, Jennifer; Koutnik, Michelle; Putzig, Nathaniel; Henderson, Bryana L.; Holt, John W.; Ehlmann, Bethany; Parra, Sergio; Lalich, Daniel; Hansen, Candice; Hecht, Michael; Banfield, Don; Herkenhoff, Ken; Paige, David A.; Skidmore, Mark; Staehle, Robert L.; Siegler, Matthew; Soto, A.; Foss, F.; Lewis, S.; Whitten, J.; Vos, E.; Johnson, P. A.; Johnson, J. C.; Gallagher, C.; Brown, A.; Bertrand, T.; Phillips-Lander, C.; Oliveira, N.; Kite, E.; Thorsteinsson, T.; Tamppari, L.; Hauber, E.; Fanara, L.; Oberst, J.; Ulamec, S.; Cartwright, S.; Harrison, T.; Hibbard, S.; Portyankina, A. and Titus, T. N. (2021). Unlocking the Climate Record Stored within Mars’ Polar Layered Deposits. Bulletin of the AAS, 53(4)

Introduction: The origin and evolution of Titan’s labyrinth terrain remains an intriguing mystery, even after nearly two decades of study by the Cassini orbiter. Labyrinth terrain are highly dissected, locally elevated plateaus, believed to consist mostly of organic material similar to that of the undifferentiated plains [1][2]. In the past, labyrinth terrain has been subdivided into multiple types based on valley and upland width, valley or ridge geometry, unit margin geometry, or other characteristics [1]. In this work, we identify the earliest stages of radial labyrinth formation based on Cassini RADAR altimeter observations.

Introduction: Recent discoveries of anomalously bright radar reflections below the Mars South Polar Layered Deposits (SPLD) have sparked new speculation that liquid water may be present below the ice cap [1][2][3]. These reflections, discovered in radar data acquired by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), are far too powerful to be caused by a return from a simple boundary between water ice and other dry geologic materials. Instead, they are more consistent with reflections from damp materials or even something akin to the subsurface ponds and lakes sometimes found beneath Earth’s ice sheets [4][5]. While this possibility is exciting, it is also difficult to confirm. Through thermal modeling, it was shown that without something akin to a recently emplaced magma chamber present beneath the surface, the heat requirements simply cannot be met [6]. In addition, the location of the bright reflections does not seem to match any likely lake locations based on the inferred hydraulic potential beneath the SPLD [7]. In light of these inconsistencies, it is necessary to consider alternative hypotheses for the observed radar returns. Previous work involving data from the Shallow Radar (SHARAD) instrument has shown that radar reflections in layered deposits can be greatly affected by constructive and destructive interference [8][9][10]. Herein, we use a one-dimensional radar sounding model to show that interference patterns can produce reflections consistent with those observed by MARSIS without the need for any liquid water, using only materials already known to be present in the SPLD.

Introduction: Japan’s first exploration mission to the Moon, the 2007 KAGUYA SELenological and ENgineering Explorer (SELENE), was equipped with a 60 m wavelength Lunar Radar Sounder (LRS) [1]. This instrument successfully probed the Moon’s deep subsurface (>1 km). Detailed interpretation of the return waveforms, however, are challenging in the absence of a rigorous surface clutter analysis. Similar to what happened with ALSE, it is difficult to distinguish real subsurface echoes from synchronized returns received from surface features (“clutter”) away from and approximately parallel to the spacecraft ground track. Accordingly, we have developed and implemented an EM wave propagation simulator to identify clutter in LRS data and have used it in combination with an algorithm of range resolution enhancement. The resulting reduced data products are enlarging the number of regions where the inversion of radar echoes for the estimation of subsurface dielectric properties can be safely performed or sensibly improved. Methods and techniques: The LRS wave propagation simulator incorporates topography from the LRO Lunar Orbiter Laser Altimeter (LOLA) data grided at 512 pixel-per-degree. Our simulator uses elevation data to create a square and planar facet-based model of the surface with a scale of ~60-m/pixel both in latitude and longitude. Each resolution cell is represented by a rectangular area with along-track dimension equal to the LRS SAR resolution cell and a cross-track dimension of ~100 km. This tool enables a straightforward distinction between real subsurface echoes and off-nadir returns from surface features, like the mare’s scarps and ridges, both arriving at greater delay than the nadir surface echo. Figure 1 shows an example of a LRS radargram with its associated cluttergram. An ~300 m high ridge, barely visible in the optic images acquired by the SELENE Terrain Camera (TC), generates reflections in the radargram that correspond to synchronized recognizable reflectors in the cluttergram, thus revealing that they originate from surface features. It is worth to note that this clutter simulator is also able to show surface locations from which the radar returns originate or produce cluttergrams accounting only for the topography to the left and right side of the spacecraft. By comparing the radargrams and the simulated cluttergrams, we can perform a detailed analysis over regions of interest, e.g., candidate subsurface reflections that have been identified but not validated against the presence of rugged topography or where significant mass deficits have been identified within the high-resolution GRAIL gravity field determination [2].

Bright radar reflections observed beneath the south polar layered deposits (SPLD) by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument were interpreted to represent liquid water, but the required amounts of salt and heat to form and maintain liquids in this location are implausible given what is known about Mars. Here, we present another hypothesis that accounts for the bright reflections: hydrated and cold clay‐rich deposits at the base of the SPLD create the observed radar response. To support this hypothesis, we present experimental measurements and wave propagation modeling that show that smectites, cooled to 230 K, have real and imaginary parts of the dielectric permittivity large enough to cause the bright reflections, even when mixed with other materials. Further, we find that absorptions attributable to these minerals are present in south polar orbital visible‐near infrared reflectance spectra. Because these minerals are present at the south pole and can cause the reflections, we believe this to be a more viable scenario than the liquid water interpretation.

Mars Polar Science is an integrated, compelling system that serves as a nearby analogue to numerous other planets, supports human exploration, and habitability. Mars possesses the closest and most easily accessible layered ice deposits outside of Earth, and accessing those layers to read the climate record would be a triumph for planetary science.

Moray Sinus is an estuary located at the northern end of Titan's Kraken Mare. The Cassini RADAR altimeter acquired three segments over this mare during the T104 flyby of Titan, on August 21, 2014. Herein, we present a detailed analysis of the received echoes. Some of these waveforms exhibit a reflection from the seafloor, from up to 85-18+28 m of depth (1 sigma error). Monte Carlo simulations have been performed in order to assess the most probable values and estimation errors for the seafloor depth. Insights from this study, featuring the synergic use of the synthetic aperture radar images coupled to the altimetry and passive radiometry datasets, have been used to constrain the dielectric properties (i.e., absorptivity of the liquid) and roughness of this region of Kraken Mare. The resulting Ku-band specific attenuation of the liquid is 17-3+3 dB/mu s, corresponding to a loss tangent of 4.6-0.9+0.9x10-5, which is very similar to the loss tangent estimated at Ligeia Mare. The data in hand do not permit us to discern the most likely explanation for the lack of a seafloor reflection from the main body of Kraken Mare: either a very deep sea or a more absorbing liquid composition. However, if the main body of Kraken Mare is characterized by an absorption similar to Moray Sinus, then based on models of the response to altimetry mode observations we can conclude that it exceeds 100 m of depth, which is also compatible with radiometry observations. Plain Language Summary From 2013 to 2017, we have been probing the depth of Titan's methane-dominated seas by penetrating the liquid with Cassini's radar altimeter. The depth and composition of each of the seas had already been derived, except for Titan's largest sea Kraken Mare. Herein, we describe the final observation of this campaign before the end of Cassini in 2017. Our analysis reveals that the seafloor at the center of Moray Sinus-an estuary located at the northern end of Kraken Mare, is up to 85 m deep. The radar waves are absorbed to an extent such that the liquid composition is compatible with 70% methane, 16% nitrogen, and 14% ethane (assuming ideal mixing). The analysis of the altimetry data in the main body of Kraken Mare showed no evidence for signal returns from the sea floor, suggesting the liquid is either too deep or too absorptive for Cassini's radio waves to penetrate. However, if the liquid in the main body of Kraken Mare is similar in composition to Moray Sinus, as one would expect, then its depth exceeds 100 m. This is compatible with a separate estimate using the radar as a "radiometer," sensing thermal energy from the sea at radio wavelengths.

In its polar layered deposits (PLD), Mars possesses a record of its recent climate, analogous to terrestrial ice sheets containing climate records on Earth. Each PLD is greater than 2 km thick and contains thousands of layers, each containing information on the climatic and atmospheric state during its deposition, creating a climate archive. With detailed measurements of layer composition, it may be possible to extract age, accumulation rates, atmospheric conditions, and surface activity at the time of deposition, among other important parameters; gaining the information would allow us to "read" the climate record. Because Mars has fewer complicating factors than Earth (e.g. oceans, biology, and human-modified climate), the planet offers a unique opportunity to study the history of a terrestrial planet's climate, which in turn can teach us about our own planet and the thousands of terrestrial exoplanets waiting to be discovered. During a two-part workshop, the Keck Institute for Space Studies (KISS) hosted 38 Mars scientists and engineers who focused on determining the measurements needed to extract the climate record contained in the PLD. The group converged on four fundamental questions that must be answered with the goal of interpreting the climate record and finding its history based on the climate drivers. The group then proposed numerous measurements in order to answer these questions and detailed a sequence of missions and architecture to complete the measurements. In all, several missions are required, including an orbiter that can characterize the present climate and volatile reservoirs; a static reconnaissance lander capable of characterizing near surface atmospheric processes, annual accumulation, surface properties, and layer formation mechanism in the upper 50 cm of the PLD; a network of SmallSat landers focused on meteorology for ground truth of the low-altitude orbiter data; and finally, a second landed platform to access similar to 500 m of layers to measure layer variability through time. This mission architecture, with two landers, would meet the science goals and is designed to save costs compared to a single very capable landed mission. The rationale for this plan is presented below. In this paper we discuss numerous aspects, including our motivation, background of polar science, the climate science that drives polar layer formation, modeling of the atmosphere and climate to create hypotheses for what the layers mean, and terrestrial analogs to climatological studies. Finally, we present a list of measurements and missions required to answer the four major questions and read the climate record.

Introduction: The North Polar Layered Deposits (NPLD) are a formation of nearly pure water ice layers [1] up to 2 km thick and 1000 km across roughly centered on the north pole of Mars, in the Planum Boreum region. Although their precise age is unknown, it is likely no more than four million years old based on orbitally forced climate models [2, 3]. In addition to layering visible in outcrop imagery, the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter (MRO) has detected many subparallel reflectors within the NPLD [4]. Reflectors are organized into four groups or “packets,” separated by reflection-free zones. The exact source of these reflectors is a matter of debate, but they are generally thought to result from variations in dust content with depth [4, 5]. Previous work linked layers and reflectors to orbitally-forced insolation cycles, implying that reflectors could act as a climate proxy for late Amazonian Mars [6, 7, 8, 9]. One hypothesis for the source of radar reflectors is that they are caused by the so-called “marker beds” identified in outcrop stratigraphy [8, 10]. Marker beds are thin layers characterized primarily by their relative resistance to erosion, which implies that they have a different composition than the surrounding ice. Previous research has failed to conclusively link specific marker beds to radar reflectors, but has shown that some genetic link is likely [10]. By assuming SHARAD reflectors are caused by an enhancement in dust content within marker bed layers, Lalich et al. [11] were able to use reflectivity measurements to place constraints on layer composition. However, they were forced to make a number of simplifying assumptions and limited their analysis to ten small study sites around the NPLD. In this work we seek to extend that analysis through a combination of more extensive reflector mapping, the consideration of other types of reflector-causing stratigraphy, and the application of recently developed SHARAD processing techniques. Data and Study Area: Radar data were acquired using the SHARAD instrument on MRO. SHARAD is an orbital radar sounder that uses an 85 μs chirped pulse centered at 20 MHz with a 10 MHz bandwidth. SHARAD has a cross-track resolution of 3-6 km and an along-track resolution of 0.3-1 km achieved using synthetic aperture processing [12]. It has a nominal range resolution of 8.4 meters in water ice. In addition to standard radargrams, we also make use of data produced using a processing technique known as “super resolution,” which has recently been adapted for SHARAD [13]. Combined with targeted interference suppression, we are able to enhance the range resolution of SHARAD by a factor of three, and increase the signal-to-noise ratio by ~3 dB [13]. Previously, uncertainty in subsurface layer thickness hindered efforts to use SHARAD reflectivity as a proxy for ice composition [11], and analysis of “split chirp” radargrams suggested that what appeared as single reflectors in SHARAD data might instead be the result of multiple thin layers [14]. Using super resolution radargrams, we can place tighter constraints on layer thickness and more accurately discriminate between individual reflectors, dramatically increasing our ability to interpret reflectivity measurements. For this work we have selected the “saddle region” of the NPLD as our study area. The region’s flat topography virtually eliminates lateral clutter which can sometimes make SHARAD radargrams difficult to interpret. Our selection also facilitates comparisons to previous studies, which also focused on the saddle region [11, 15]. Unlike those previous studies, we aim to extend our analysis beyond the top packet of reflectors (~500 m depth) and therefore explore a longer period of time.

The stratigraphy of the north polar layered deposits (NPLD) of Mars is believed to contain a climate record of the recent Amazonian period. However, full utilization of this record is difficult without detailed information regarding the physical properties of the constituent layers. Here we present a method for determining the fractional dust content of individual layers using a combination of orbital radar reflectivity measurements and physical modeling. We apply this method to the upper 500 m of the NPLD at 10 study sites and compare the results to a cap‐wide radar‐mapped surface. Our results show that reflectivity can vary drastically both geographically and with depth, a result we attribute to changing dust content, though the impact of variable layer thickness cannot be totally discounted. These findings imply large‐scale regional patterns in ice and dust accumulation do not remain consistent through time. We also find that current models of Mars's dust cycle and polar ice accumulation consistently underpredict the dust content of layers, indicating that our understanding of dust transport, dust sequestration, or dust preservation remains incomplete. Comparisons of study sites on the NPLD also show that some locations contain fewer radar reflectors than others, meaning they may contain a less complete record of the planet's recent paleoclimate, and any future efforts to use the polar layered deposits as a climate proxy, including in situ measurements, should take this into account by choosing study sites wisely.