46th Lunar and Planetary Science Conference (2015)
2442.pdf
LARGE (>1 KM) RAYED CRATERS IN HESPERIA PLANUM, MARS: WHAT’S THE EJECTA TRYING
TO SAY? Tracy K.P. Gregg1, 1Dept. of Geology, 411 Cooke Hall, University at Buffalo, Buffalo NY 14260
([email protected]).
Introduction: Rayed impact craters on Mars are
defined by the following characteristics [1, 2]: radial
to subradial “filamentous elements” with clear separation and constrast with the underlying surface in visible or thermal emission imagery that extend ≥ 25 crater
radii (and commonly >100 crater radii [2]) and diameters > 1 km. Tornabene and others [2] conducted a
global search for martian rayed craters located between
45°N and 45°S with diameters of 2 – 10 km, and discovered 4 “definite” along with 3 “probable” rayed
craters. Subsequently, Schon and others [3] identified
a Gasa crater, 7-km-diameter rayed crater in eastern
Promethei Terra, as a rayed crater. Rayed craters are
interpreted to represent the youngest large, primary
impacts on Mars [1 – 4] and so their identification has
important implications for crater retention ages, as well
as weathering processes and target material properties
[e.g., Schultz and Singer, 1980].
Tornabene and others [2] suggested that martian
rayed craters are confined to young lava plains (such as
surround Cerberus Fossae), possibly because crater
rays are likely ephemeral features, and because rays
are most easily identified where their albedo and thermal inertia values differ from those of the underlying
terrain. A preliminary search for rayed impact craters
meeting the above criteria was conducted specifically
within Hesperia Planum, because Hesperia Planum has
been interpreted to be composed of layered lava flows
that were emplaced at the base of the Early Hesperian
Epoch [5,6]. Identification of rayed craters within
Hesperia Planum may help to constrain the rate at
which crater rays weather away on Mars.
Approach: Using the JMars package [7] developed at ASU (see jmars.asu.edu), I relied primarily on
the nighttime global mosaic of Thermal Emission Imaging Spectrometer (THEMIS) [8] data to identify
impact craters with rays that appeared to extend several crater radii away from the impact. Where relationships were difficult to discern within the nighttime
THEMIS infrared (nTIR) images, Context Camera
(CTX) [9] images were used to confirm or deny the
presence of rays, exhibited as concentrations of secondary craters [2,4]. Using the JMars “measure” tool,
crater diameter was measured, and a distance equivalent to 25 crater radii was calculated for each possible
rayed crater. Again using the JMars measure tool,
crater rays were measured to determine if they extended at least 25 crater radii.
A dimensionless parameter, Rr, was calculated to
determine whether impact crater rays were sufficiently
long to allow the crater to be classified as “rayed,”
given by the ratio of 25 * (measured crater radius) /
(maximum measured crater ray length). Thus, if Rr ≥
1.0, the crater is considered to be “rayed.”
Note was made of whether the impact craters displayed lobate ejecta blankets in addition to rays (Figure 1). Furthermore, it was noted whether the lobate
ejecta blankets and the crater rays were arranged symmetrically around the impact crater. Rays are more
easily identified using nTIR images whereas lobate
ejecta blankets are more clearly observed using daytime THEMIS images (dTIR)
Preliminary Results: Two impact craters within
Hespera Planum have Rr > 1.0: Resen and Pál (Table
1). A total of 18 impact craters displaying rayed morphologies were identified. Of these, 11 displayed either asymmetric lobate ejecta or a “forbidden zone” [2]
where crater rays are not observed, indicative of
oblique impacts. The smallest 2 impact craters did not
display lobate ejecta blankets.
Discussion: The advantage of examining craters
within Hesperia Planum is that the target material is
likely lyered mafic lava flows throughout [6]. Thus,
variations in impact crater ray morphology (specifically, variations in Rr) within Hesperia Planum probably
reflect crater age, with older impacts having smaller Rr
values. This hypothesis can be tested by counting the
size-frequency distribution of small (100’s of meters)
impacts superposed on the lobate ejecta of the craters
discussed here.
The majority of rayed craters within Hesperia
Planum have asymmetric ejecta or ray patterns, indicating oblique impacts, similar to the results presented
by Tornabene and others [2]. More detailed examination is required to determine if the less-obviously rayed
craters were also produced by oblique impacts.
References: [1] McEwen, A.S. et al. (2005) Icarus,
176, 3351-381. [2] Tornabene, L.L. et al. (2006) JGR,
111, E10006. [3] Schon, S.C. et al. (2009) Geology,
37(3), 207 - 210. [4] Preblich et al. (2007) JGR, 112,
E05006. [5] Greeley, R. and Guest, J.E. (1987) USGS
Geol. Invest. Map I-1802-B. [6] Tanaka, K.L. et al.
(2014) USGS Sci. Invest. Map 3292. [7] Gorelick,
N.S. et al. (2003) LPSC 34th, Abstract #2057. [8]
Christensen, P.R. et al. (2004) Space Sci. Rev. 110, 85130. [9] Malin, M.C. et al. (2007) JGR 112, E05S04.
[10] Christensen et al., PDS, http://themis.asu.edu.
46th Lunar and Planetary Science Conference (2015)
2442.pdf
Figure 1. Rayed crater centered at 102.93°E, 16.35°S, shown in the THEMIS daytime infrared imagery in the insert
in the upper left; THEMIS nighttime imagery reveals crater rays in the main image. Note that both the lobate ejecta
and the crater ray distribution suggest an oblique impact, coming from the upper left. North is at the top of the image. Image courtesy of ASU/JPL/NASA [10].
Table 1. Impact craters displaying rays within Hesperia Planum.
Longitude Latitude Diameter 25 * Radius Measured Rr1
Notes
(km)
(km)
ray length
(km)
108.87
-27.93
7.4
92.5
100
1.08 Resen crater; asymmetric ejecta
108.69
-31.25
67.5
843.75
870
1.03 Pál crater; symmetric ejecta
109.95
-19.04
1.2
15
13.7
0.91 symmetric ejecta
98.72
-18.48
35.2
440
380
0.87 symmetric ejecta
102.93
-16.35
6.31
78.875
65
0.82 asymmetric ejecta
109.86
-19.87
1.2
15
11
0.73 asymmetric rays
110.37
-13.5
2.5
31.25
20
0.65 asymmetric rays
115.3
-19.83
7.2
90
57
0.63 asymmetric ejecta
109.6
-17.66
7.2
90
57
0.63 crater chain
118.76
-17.87
7.2
90
54
0.60 asymmetric ejecta
107.88
-9.37
7.7
96.25
57
0.59 somewhat asymmetric ejecta
116.62
-21.69
9.8
122.5
70
0.57 symmetric ejecta
93.23
-35.84
10.3
128.75
69
0.54 Cue crater; symmetric ejecta
110.94
-13.13
7.7
96.25
50
0.52 asymmetric ejecta
112.39
-20.29
6.6
82.5
41
0.50 asymmetric ejecta
94.99
-34.88
6.7
83.75
41
0.49 asymmetric ejecta
113.39
-18.83
7.5
93.75
46
0.49 Loon crater; wind streaks modify rays
110.64
-19.63
2.2
27.5
12
0.45 no lobate ejecta
110.96
-11.74
1.9
23.75
9.0
0.38 dark halo; no lobate ejecta
1
Rr is 25 times the crater radius divided by the measured maximum crater ray length.