Temple University Geophysical Research Letter Paper i am attaching all necessary material to answer the question. please follow the instruction attached an

Temple University Geophysical Research Letter Paper i am attaching all necessary material to answer the question. please follow the instruction attached and use 2-5 pages to answer the question. RESEARCH LETTER
10.1029/2019GL082218
Key Points:
• Modern winds in Jezero crater come
from the east, but ancient winds
came from the southwest
• Southwesterly winds caused
significant erosion and likely
removed much of the delta deposit
• Wind streaks indicate that modern
winds vary by less than 10 degrees
around the westward mean with no
seasonal trends
Supporting Information:
• Supporting Information S1
Correspondence to:
M. Day,
daym@epss.ucla.edu
Citation:
Day, M., & Dorn, T. (2019). Wind in
Jezero crater, Mars. Geophysical
Research Letters, 46, 3099–3107. https://
doi.org/10.1029/2019GL082218
Received 25 JAN 2019
Accepted 11 MAR 2019
Accepted article online 13 MAR 2019
Published online 21 MAR 2019
Wind in Jezero Crater, Mars
Mackenzie Day1
and Taylor Dorn1
1
Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA, USA
Abstract Wind generates erosional and depositional morphologies across Mars, including in Jezero
crater, the planned landing site for the Mars 2020 rover. Known for subaqueously formed sedimentary
features, Jezero crater also hosts wind‐formed features that provide insights about the local wind regime. We
combine interpretations from dunes, wind streaks, transverse aeolian ridges, and yardangs to holistically
understand the recent history of wind in Jezero crater. Together, these features describe modern easterly
winds trending toward 263° ± 8° and older southwesterly winds that formed small yardangs. Previous
research has suggested that aeolian processes eroded the delta deposit in Jezero crater. We propose early
southwesterly winds removed the majority of material, exposing the more lithified units seen today. Modern
easterly winds continue to cause erosion, but lithologic heterogeneities remain the dominant control on
delta deposit evolution. Aeolian erosion has accentuated existing sedimentary structures, leaving the
striking delta remnant seen today.
Plain Language Summary Jezero crater on Mars will be the landing site for the Mars 2020 rover.
Jezero crater is known for being an ancient lakebed and for having a well‐preserved delta in the crater.
Jezero crater has also been exposed to high winds which left behind dunes and other evidence of windiness.
In this work, we focus on the wind‐generated parts of Jezero crater and use the geology to understand the
winds. Based on the geology, winds in Jezero crater blow from east to west but used to blow from the
southwest. These older winds were likely responsible for most of the erosion in the crater, because erosion
would have been easier when the lake first dried up.
1. Introduction
Aeolian activity pervades the surface of Mars, leaving behind signatures of wind in the local geology. From
dust devil tracks to migrating dunes, the surface of Mars shows abundant evidence of wind‐driven sediment
transport. Aeolian processes provide the dominant sediment transport mechanism on modern Mars
(Arvidson et al., 1979; McLennan et al., 2019), and aeolian sandstones provide evidence of ancient aeolian
activity (Banham et al., 2018; Day & Catling, 2018; Grotzinger et al., 2005; Milliken et al., 2014).
Wind‐landscape interactions can be depositional (e.g., forming dunes) or erosional (e.g., forming yardangs).
In either case, the morphology of the resulting features captures the direction of the formative wind (Goudie,
2008; Rubin & Hunter, 1987). Taken together, wind‐formed geologic features have been used to reconstruct
winds in modern and ancient systems on Earth and Mars alike (e.g., Bishop, 2011; Day & Kocurek, 2016;
Eastwood et al., 2012; Ewing et al., 2015; Peterson, 1988).
In this work we focus on the history of wind in Jezero crater (18.42° N, 77.67° E), the planned landing site for
the Mars 2020 rover. At ~45 km in diameter, Jezero crater sits in the Nili Fossae region of Mars, northwest of
Isidis basin (Figure 1; Wichman & Schultz, 1989). Once the site of a Martian lake (Fassett & Head, 2005;
Schon et al., 2012), Jezero crater hosts abundant evidence of ancient subaqueous deposition, including a
delta deposit emanating from the western crater rim (Goudge et al., 2017, 2018). Clay (Ehlmann, Mustard,
Fassett, et al., 2008) and carbonate (Ehlmann, Mustard, Murchie, et al., 2008) mineralogies in the region,
coupled with the lacustrine setting, make Jezero crater an attractive candidate for biosignature preservation.
©2019. American Geophysical Union.
All Rights Reserved.
DAY AND DORN
In addition to its aqueous history, Jezero crater also hosts evidence of aeolian processes. Previous work has
noted that Jezero is a highly active aeolian region with high sediment fluxes transported by wind from the
east (Chojnacki et al., 2018), and it has been suggested that the delta deposit was eroded by wind (Fassett
& Head, 2005; Schon et al., 2012). Dunes, ripples, yardangs, wind streaks, and transverse aeolian ridges
(TARs) are all present in the crater interior but do not reflect the same winds. In this study, we examine these
features as a record of aeolian activity in Jezero crater and discuss how the wind‐landscape interaction has
changed over time.
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Figure 1. Jezero crater. (a) Context Camera mosaic of Jezero crater (rim outlined in black). The locations of later figures are boxed. Red lines represent locations
and approximate trends of yardangs. (b) The same as (a) with the Goudge et al. (2015) geologic map units and aeolian features mapped in this work. White
lines show the wind streaks which are mostly confined to the Volcanic Floor unit. Black shaded regions show areas of dense TARs, and blue lines indicate where the
TAR wavelengths were measured. (c) Mars Orbiter Laser Altimeter (MOLA) color elevation map of the region around Jezero crater (star). North is up in all images
for this and later figures. TAR = transverse aeolian ridge.
2. Methods
We mapped locations and orientations of four aeolian feature types inside Jezero crater: dunes, yardangs,
TARs, and wind streaks. We refer to geologic units mapped by Goudge et al. (2015) for context associations
with aeolian features. Yardangs and TARs were mapped on a mosaic of images from the Mars
Reconnaissance Orbiter Context Camera (CTX) with resolution of ~6 m per pixel (see supporting information S1; Malin et al., 2007). Yardangs are erosional features that form fields of parallel ridges oriented in
the same direction as the dominant wind direction (Blackwelder, 1934; Ward, 1979). Yardangs tend to form
asymmetrically with slightly blunted upwind ends and tapered downwind ends. Because yardangs form via
the slow process of aeolian abrasion, they require long timescales to form. The exact magnitude of this timescale depends on the size of the yardang, the erodibility of the material, and the strength of the wind, but
from estimates of Martian aeolian abrasion rates (Bridges et al., 2004; Greeley et al., 1982; Kraft &
Greeley, 2000), we assume a formative timescale of at least hundreds of thousands of years.
TARs are common Martian bedforms of poorly understood origin (Balme et al., 2008). These light‐toned
symmetrical bedforms orient orthogonal to the wind but provide only a 180° ambiguous indicator of wind
direction (Zimbelman, 2010; Zimbelman & Scheidt, 2014). Rather than map each bedform individually,
DAY AND DORN
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we mapped regions where TAR coverage was estimated to be >70% (Figure 1b). Small TARs visible in
higher‐resolution images, but not clearly resolved in CTX, were ignored; however, subresolution TARs cover
a relatively small total area with respect to the mapped TARs. TAR wavelengths were measured along four
transects across large TAR fields (Figure 1b). We estimated the volume of unconsolidated aeolian material in
TARs by assuming a triangular TAR cross section with width equal to wavelength and a height/width ratio
of 0.12 (Shockey & Zimbelman, 2013). For TARs of average wavelength (λ) and areal coverage (A), the total
volume (V) of unconsolidated aeolian material is approximately
1
V ¼ Aλð0:12Þ:
2
(1)
Chojnacki et al. (2018) noted that TARs in Jezero have not migrated over the past decade of observation;
however, more recent observation has noted some movement in small TARs in the Nili Fossae region
(Silvestro et al., 2019). TARs are relatively modern features, but their age and reorientation times remain
poorly constrained (e.g., Berman et al., 2018; Silvestro et al., 2019).
Dunes and wind streaks in Jezero crater were studied using images from the High Resolution Imaging
Science Experiment (HiRISE) camera (McEwen et al., 2007). At ~25 cm per pixel, 44 HiRISE images covering
Jezero crater have been acquired over a period of ~10 years. Dunes are common on Mars (Hayward et al.,
2007) and have been mapped in the vicinity of Jezero crater (Chojnacki et al., 2018). Dunes reflect different
wind regimes with their crest morphology (McKee, 1979; Rubin & Hunter, 1987). Observed migration of
dunes on Mars suggests lee faces can be reworked, and dune morphology can record a distinct wind direction on timescales of a few years to single decades (e.g., Banks et al., 2018; Bridges et al., 2013; Chojnacki
et al., 2017). We used lee face orientations to infer wind directions and interpret them to represent wind integrated over several years in Jezero crater. Ripples have been discussed in previous work (Chojnacki et al.,
2018) and are not separately considered here.
Wind streaks can reorient on timescales of days to weeks (Sagan et al., 1972) and are the shortest‐timescale
forming features discussed in this work. The HiRISE images studied in this work are distributed across 12
Earth years, a much longer temporal range than the reorientation time of the streaks. Therefore, wind streak
orientation was measured separately in each HiRISE images using mapping software to record the approximate trend and length of each streak (see supporting information S1 for list of images).
Observations from the Mars 2020 rover will allow for more detailed analysis of local winds with the Mars
Environmental Dynamics Analyzer instrument (Pérez‐Izquierdo et al., 2018; Rodriguez‐Manfredi et al.,
2014) and with imaging of ventifacts (Bridges et al., 2014; Laity & Bridges, 2009), sand shadows, and abrasion
textures, as has been conducted for Gale crater with data from the Mars Science Laboratory rover, Curiosity
(e.g., Blake et al., 2013; Stack et al., 2014). In this work, we focus only on currently available observations
from orbit.
3. Results
3.1. Yardangs
Erosional bedrock lineations with a wavelength of ~100 m occur predominantly on the southwestern and
northeastern parts of the interior of Jezero crater (Figure 2b). The interpreted yardang erosional features
are oriented SW‐NE, and although lengthwise asymmetry is difficult to distinguish in many places, the features are dominantly blunted on the southwest side, suggesting formative winds from the southwest rather
than the northeast. Yardang textures are most distinct in the southwest of the crater on the Mottled Terrain
and Light‐Toned Floor units (Figure 1), with some additional examples in the aptly named Lineated Mottled
Terrain unit in the northeast. Erosional lineation directions are consistently SW‐NE and do not change
radially or along the rim. The delta deposit morphology is dominated by ancient fluvial morphologies; however, some subtle lineations on the top of the deposit, oriented normal to apparent channels, may have
resulted from southwesterly aeolian erosion (Figure 2d).
3.2. TARs
From the base of the rim, the Jezero crater interior measures ~42 km in diameter, for a total internal area of
~1,400 km2. Regions of dense TARs were mapped (Figure 1b) and cover ~286 km2 or ~20% of the crater
DAY AND DORN
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Figure 2. Aeolian features in Jezero crater. Dashed arrows show interpreted wind directions. (a) One of seven barchan dunes (1) in the northern channel. Dark
sands migrate over light‐toned transverse aeolian ridges (2). Both reflect easterly winds. (b) Erosional bedrock lineations (SW‐NE) interpreted as yardangs
formed by southwesterly winds. Dotted lines show the interpreted yardang trend. (c) A remnant of delta deposit streamlined by easterly winds. This is one of the few
features exhibiting E‐W erosional lineation. (d) The top of the western delta deposit. Subtle lineations (boxed) trending SW‐NE on the delta top are consistent
with elsewhere interpreted yardangs and are orthogonal to channel structures. Erosional textures from easterly winds were not observed, but transverse aeolian
ridge orientations in the large crater are consistent with E‐W winds.
interior. No clear correlation was observed between the previously mapped geologic units (Goudge et al.,
2015) and the distribution of TARs, except for an absence of TARs on the Volcanic Floor unit. This is in
contrast to the presence of wind streaks almost exclusively on the Volcanic Floor unit. The average
wavelength of mapped TARs in Jezero crater was determined from 149 measurements made across four
transects of the largest fields of TARs (Figure 1b). Mapped TARs averaged 32 m in wavelength, implying
an estimated average height of 3.8 m (assuming a height/width ratio of 0.12; Shockey & Zimbelman,
2013). Using equation (1), the volume of unconsolidated sediment in TARs in Jezero crater is ~ 0.5 km3.
TARs are dominantly oriented north‐south, with some deviation associated with flow channelization
around topographic obstacles (e.g., Figure 2c).
3.3. Dunes
Dunes are commonplace on Mars and have been mapped north of Jezero crater (Chojnacki et al., 2018);
however, only a few dunes are present in the crater interior. Seven dunes with distinct lee faces are present
in the inlet channel valley crossing the crater’s northern rim (Figure 2a). These ~250‐m‐long bedforms have
barchan morphologies that are elongated parallel to channel walls. The dip direction of the dune lee faces
ranges from northwest to southwest, indicating a dominance of transport and migration to the west overall.
Superimposed on the dune stoss slopes are smaller bedforms (possibly large Martian ripples; Lapotre et al.,
2016) that largely reflect the orientations of the dune lee faces, with some more complex patterning near the
dune crests. Both ripples and dunes have very low albedos and contrast starkly with the high albedo TARs.
The faster migration of dunes and ripples over TARs is evident in the northern inlet channel shown in
Figure 2a. Ripples form on dune stoss slopes but are also found in sand patches distributed mostly in the
DAY AND DORN
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Figure 3. Wind streak measurements. (a) Wind streak orientation across Martian seasons. Streaks consistently trended toward the west but varied around a mean
of 263°. No correlation between season and orientation is present. No High Resolution Imaging Science Experiment images of this area were taken here during
Mars year 29. (b) Wind streak orientation as a function of length. Shorter wind streaks seem to reflect higher variance and may be responding faster to changes in
wind regime than longer streaks which may have long reorientation times and tend toward the mean wind direction.
northwest of the crater interior in topographic lows and at the base of east‐facing delta scarps where
Chojnacki et al. (2018) measured ripple migration rates of 0.2 m/year.
3.4. Wind Streaks
Wind streaks were present in 26 of the 44 studied HiRISE images for a total of 334 measured wind streaks
(Figure 1b). The wind streaks were primarily found on the Volcanic Floor unit (331 streaks) with a few measured on the Light Toned Floor unit (three streaks). The orientation of the wind streaks was consistently
east‐to‐west (mean of 263° ± one standard deviation of 8°), indicating a dominance of easterly winds over
the ~10‐year coverage of HiRISE imaging. When compared over the Martian year, no significant trend
was observed in the wind streak orientation (Figure 3a). All of the studied HiRISE images were acquired
between 13:50 and 16:00 local Mars time, and no discernible trend was present in the data across this 2‐hr
period. The length of wind streaks varied from 35 m to 3 km. More variability was observed in the shortest
wind streaks (Figure 3b) with longer streaks tending to the mean. This suggests that the shortest streaks may
be responding to changes in the local winds, whereas longer streaks may be integrating a signal of the winds
over longer, but still subannual, times.
4. Discussion
Evidence of long‐lived active winds in Jezero crater is clear and abundant. Such evidence supports the widely
held interpretation that wind eroded the western delta deposit, but we suggest that such erosion may not
have come from a unimodal wind history.
4.1. Reorientation of Winds
The youngest aeolian features in Jezero crater show easterly wind with
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