SCIENCE IN MOTION
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The Current Martian Cratering Rate
I. J. Daubar, A. S. McEwen, S. Byrne, M. R. Kennedy, B. Ivanov
Abstract
The discovery of 248 dated impact sites known to have formed within the last few
decades allows us to refine the current cratering rate and slope of the production function
at Mars. We use a subset of 44 of these new craters that were imaged before and after
impact by Mars Reconnaissance Orbiter's Context Camera -- a thoroughly searched data
set that minimizes biases from variable image resolutions. We find the current impact rate
is 1.65×10-6 craters with an effective diameter ≥ 3.9 meters / km
2 / year, with a
differential slope (power-law exponent) of -2.45 ± 0.36. This results in model ages that
are factors of three to five below the Hartmann (2005) and Neukum et al. (2001)/Ivanov
(2001) model production functions where they overlap in diameter. The best-fit
production function we measure has a shallower slope than model functions at these
sizes, but model function slopes are within the statistical errors. More than half of the
impacts in this size range form clusters, which is another reason to use caution when
estimating surface ages using craters smaller than ~50 meters in diameter.
Article on ScienceDirect
See also:
JPL/NASA release
UANews release
Examples of craters listed in this paper (Click for larger version)
Observations of the northern seasonal polar cap on Mars: I. Spring sublimation activity and processes
C.J. Hansen, S. Byrne, G. Portyankina, M. Bourke, C. Dundas, A. McEwen, M. Mellon, A. Pommerol, N. Thomas
Abstract
Spring sublimation of the seasonal CO2 northern polar cap is a dynamic process in the current Mars climate. Phenomena include dark fans of dune material
propelled out onto the seasonal ice layer, polygonal cracks in the seasonal ice, sand flow down slipfaces, and outbreaks of gas and sand around the dune margins.
These phenomena are concentrated on the north polar erg that encircles the northern residual polar cap. The Mars Reconnaissance Orbiter
has been in orbit for three Mars years, allowing us to observe three northern spring seasons. Activity is consistent with and well described by the
Kieffer model of basal sublimation of the seasonal layer of ice applied originally in the southern hemisphere. Three typical weak spots have been
identified on the dunes for escape of gas sublimed from the bottom of the seasonal ice layer: the crest of the dune, the interface of the dune
with the interdune substrate, and through polygonal cracks in the ice. Pressurized gas flows through these vents and carries out material
entrained from the dune. Furrows in the dunes channel gas to outbreak points and may be the northern equivalent of southern radially-organized
channels (“araneiform” terrain), albeit not permanent. Properties of the seasonal CO2 ice layer are derived from timing of seasonal events such
as when final sublimation occurs. Modification of dune morphology shows that landscape evolution is occurring on Mars today, driven by
seasonal activity associated with sublimation of the seasonal CO2 polar cap.
Article on ScienceDirect
See also:
Observations of the northern seasonal polar cap on Mars II: HiRISE
photometric analysis of evolution of northern polar dunes in spring
Observations of the northern seasonal polar cap on Mars III:
CRISM/HiRISE observations of spring sublimation
Read the JPL press release
Partial List of Observations in this Paper
PSP_009122_2600
ESP_016032_2600
ESP_016388_2600
ESP_024537_2600
ESP_024669_2600
ESP_025104_2600
ESP_025315_2600
ESP_025737_2600
ESP_026080_2600
Earth-like sand fluxes on Mars
N.T. Bridges, F. Ayoub, J-P. Avouac, S. Leprince, A. Lucas & S. Mattson
Abstract
Strong and sustained winds on Mars have been considered rare, on the basis of surface meteorology measurements and global circulation models,
raising the question of whether the abundant dunes and evidence for wind erosion seen on the planet are a current process. Recent studies showed
sand activity, but could not determine whether entire dunes were moving—implying large sand fluxes—or whether more localized and surficial
changes had occurred. Here we present measurements of the migration rate of sand ripples and dune lee fronts at the Nili Patera dune field. We
show that the dunes are near steady state, with their entire volumes composed of mobile sand. The dunes have unexpectedly high sand fluxes, similar,
for example, to those in Victoria Valley, Antarctica, implying that rates of landscape modification on Mars and Earth are similar.
Read the UANews press release
Read the Nature article
Observations in this Paper
PSP_004339_1890
PSP_005684_1890 (captioned image)
ESP_017762_1890
ESP_018039_1890
Digital terrain model
Animated GIFs
PSP_004339_1890 and PSP_005684_1890
PSP_005684_1890 and ESP_017762_1890 (October 2007 and May 2010)
Seasonal Flows on Warm Martian Slopes
Alfred S. McEwen, Lujendra Ojha, Colin M. Dundas, Sarah S. Mattson, Shane Byrne, James J. Wray, Selby C. Cull, Scott L. Murchie, Nicolas Thomas, Virginia C. Gulick
Abstract
Water likely flowed across ancient Mars, but whether it ever exists as a liquid on the surface today remains debatable. Recurring slope lineae (RSL)
are narrow (0.5-5 m), relatively dark markings on steep (25°-40°) slopes; repeat MRO/HiRISE images show them to appear and incrementally grow during
warm seasons and fade in cold seasons. They extend downslope from bedrock outcrops, often associated with small channels, and hundreds of them
form in rare locations. RSL appear and lengthen in the late southern spring/summer from 48°S to 32°S latitudes favoring equator-facing slopes--times
and places with peak surface temperatures from ~250-300 K. Liquid brines near the surface might explain this activity, but the exact mechanism and source of water are not understood.
Science: Is Mars Weeping Salty Tears?
Read the University of Arizona press release
View our breakout page for this paper
Seasonal Erosion and Restoration of Mars’ Northern Polar Dunes
C. J. Hansen, M. Bourke, N. T. Bridges, S. Byrne, C. Colon, S. Diniega, C. Dundas, K. Herkenhoff, A. McEwen, M. Mellon, G. Portyankina, and N. Thomas
Abstract
Despite radically different environmental conditions, terrestrial and Martian dunes bear a strong resemblance, indicating that the basic processes of
saltation and grainfall (sand avalanching down the dune slipface) operate on both worlds. Here we show that Martian dunes are subject to an additional
modification process not found on the Earth: springtime sublimation of Mars’ CO2 seasonal polar caps. Numerous dunes in Mars’ north polar region have
experienced morphological changes within a Mars year, detected in images acquired by the High Resolution Imaging Science Experiment (HiRISE) on the
Mars Reconnaissance Orbiter (MRO). Dunes show new alcoves, gullies, and dune apron extension. This is followed by remobilization of the fresh deposits
by the wind, forming ripples and erasing gullies. The widespread nature of these rapid changes, and the pristine appearance of most dunes in the area,
implicates active sand transport in the vast polar erg in Mars’ current climate.
Full article in Science
Read the University of Arizona press release
Observations in this article
PSP_007962_2635
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PSP_008968_2650
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PSP_009105_2640
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PSP_009324_2650
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ESP_015935_2640
ESP_016256_2635
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ESP_016546_2635
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ESP_016836_2635
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ESP_017768_2640
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ESP_017974_2650
PSP_010019_2635
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ESP_018036_2635
Discovery of Columnar Jointing on Mars
M.P. Milazzo, L.P. Keszthelyi, W.L. Jaeger, M. Rosiek, S. Mattson, C. Verba, R.A. Beyer, P.E. Geissler,
A.S. McEwen, and the HiRISE Team
Abstract
We report on the discovery of columnar jointing in Marte Valles, Mars. These columnar lavas were discovered in the wall of a pristine,
16-km-diameter impact crater and exhibit the features of terrestrial columnar basalts. There are discontinuous outcrops along
the entire crater wall, suggesting that the columnar rocks covered a surface area of at least 200 sq. km, assuming that the rocks obliterated
by the impact event were similarly jointed. We also see columns in the walls of other fresh craters in the nearby volcanic plains
of Elysium Planitia-Amazonis Planitia, which include Marte Vallis, and in a well-preserved crater in northeast Hellas.
Full article in Geology
Read the USGS news release
Observation cited in this article
PSP_005917_2020
