Radar Reflectivity as a Proxy for the Dust Content of Individual Layers in the Martian North Polar Layered Deposits. Issue 7 (2nd July 2019)

• Record Type: Journal Article
• Authors: Lalich, D. E.; Holt, J. W.; Smith, I. B.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 1690
• Page End: 1703
• Publication Date: 2019-07-02
• Subjects: Mars -- ice -- climate -- radar; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2018JE005787
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: 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. Plain Language Summary: The north polar cap of Mars is very similar to ice sheets on Earth. On Earth, scientists use the layers of ice to learn about how the climate has changed over time. It should be possible to do something similar on Mars, but we cannot drill ice cores on Mars, and so far, scientists have had difficulty pulling useful information out of the layers. In this paper, we show that it is possible to use radar reflections in the ice to get a rough idea of how much dust is in a certain type of layer. The brightness of a reflection is related to the composition of the layer that caused it, so after making a few assumptions, we can first measure that brightness and then use a model to estimate how much dust would have to be present in the ice to get a matching reflection. After doing this for many reflections at 10 different study sites, we found that some layers are a lot dustier than we previously believed, meaning our previous understanding of the Mars ice and dust cycles is incomplete. We also found that the composition of layers was very different from place to place, which means local conditions are playing a large role in ice and dust accumulation. Key Points: Radar reflections can be used to constrain the dust content of individual layers in the north polar layered deposits of Mars Layer dust contents are higher than predicted by models, indicating a gap in our understanding of how the global dust cycle varies with time Dust content and completeness of the climate record vary strongly by location and with depth