GEOL 2920
Baker, 4/27/09
Summary of: Segura, T.L. B. Toon, and A. Colaprete (2008), Modeling the environmental effects of
moderate-sized impacts on Mars, J. Geophys. Res., 113, E11007, doi:10.1029/2008JE003147.
Goal of Paper: To simulate the climatic environment after moderate-sized impact events on Mars
“…we envision a cold and dry planet, an almost endless winter broken by episodes of scalding rains
followed by flash floods.” [Segura et al., 2002]
Initial Conditions: Impact and water sources
 An impact ejecta produces pulverized, melted and vaporized target material (including volatiles). The
melt and vaporized material are very hot (1000-3000 K) and are distributed globally. The melted and
vaporized ejecta radiates energy while in the atmosphere (thus heating exposed surface ice) and is still
hot (~1600 K) when it precipitates onto the surface. Water vapor remaining in the atmosphere is
precipitated later and is maintained by a brief hydrologic cycle.
 Three sources of injected water (i.e. water directly vaporized by impact): 1) The impactor, 2) target
crater material, 3) Exposed ice (i.e. polar caps)
 Also have water source from melted subsurface ice due to propagation of the thermal wave; assumed
that all melted ice could potentially be evaporated
Model: Used a 1-D radiative convective model coupled to a 1-D finite difference sub-surface model;
Surface pressures, gas amounts, temperature were updated at every time step.
 Latent heat release with water phase change, and hydrologic cycle with evaporation of water from
surface.
 Looked at effects of 30 km, 50 km, and 100 km impacts at 9 km/s (smaller diameters than the Segura
et al., 2002 Science paper)
 Two sets of runs: 1) radiative effects of water clouds are not considered, 2) radiative effects (due to
optical properties) of water clouds are considered.
 Kept track of: 1) Quantities of water precipitated, 2) Water evaporated from the ground, 3) Water
collected on the surface as a function of time
Assumptions:
 Early Mars conditions: Sun is at 75% luminosity and ambient atmospheric CO2 pressure at 150 mbar
(also model higher values of 1 to 2 bars)
 Globally emplaced ejecta blanket
 T = 0 is when all rock vapor has rained out but injected water remains in the hot atmosphere
 Sub-micron dust in atmosphere not considered
 Water contents: Asteroid water content 5% by mass; average regolith water content is 20% by mass
under a 40-cm dry cap.
 Energy sinks in models: 1) latent heat from injected water, 2) energy associated with heating the
atmosphere, 3) energy heating the debris layer to 1600 K. Altogether, these energies account for less
than ~15% of the total kinetic energy of the impactor. Only means of energy loss at the top of the
model (i.e. atm/space interface)
GEOL 2920
Baker, 4/27/09
Results:
Precipitation and temperatures >273 K may be sustained for many months or years in the early Martian
CO2 climate as a result of impactors >30 km
A 30 km object impacting under current Mars conditions (6.1 mbar CO2 pressure with no cloud radiative
effects) produces above freezing temperatures for only 1-2 days.
Estimates on total precipitation for all impactors:
 Fit a power law curve to the global precipitation totals of the modeled impacts to estimate the
precipitation totals for impactors > 10 km in diameter.
 Somehow derived impactor diameters >10 km from craters on Mars, and used these to integrate the
total rainfall caused by all impact events. Get about 650 m of precipitation.
Estimates on erosion rates:
 Used the Universal Soil Loss Equation (USLE), which was developed to estimate terrestrial topsoil
erosion: A = R K LS C P (where A=annual soil loss, R=erosivity index, K=soil erodibility factor,
LS=topographic factor (depends on slope), C=cropping factor, P=conservation factor)
 Assumed USLE can be applied to Mars, and C=P=1, K=0.12(bedrock) to 0.48(silt), LS=2.0-6.0 (for
large basins), soil density of 2200 kg/m3
 Total Erosion per Event = (Calculated Annual Erosion Rate from USLE) x (Time of Precipitation
for the Impact Event)
 Total erosion from all impactors >10 km diameter = 2.6-46 m (depending on values for LS and K;
for 1 bar CO2)
 Suggest that these erosion estimates are minima
GEOL 2920
Baker, 4/27/09
Implications:
 Geologically short hydrologic systems from impact events with brief but intense episodes of
precipitation. Enough precipitation appears to result to account for the erosion observed without
forming mature, well-developed fluvial systems. Brevity of precipitation events may also account for
the lack of ubiquitous presence of clays and carbonates.
 Channels need not to form near the impact site because the impactor ejecta is distributed globally.
Questions:
 How global are the warming events? How global are the precipitation totals? Effects of topography
and atmospheric circulation?
 How useful is the USLE for use in Martian erosion estimates? Issues with gravity scaling, slope
estimates (especially considering crater slopes), soil properties and heterogeneities, magnitude of
precipitation events, etc.
 How does this scenario fit in with the flow regimes, locations, and episodicity of water as implied by
the morphological evidence for water on Mars (e.g. fans/deltas, open/closed basin lakes, channel
morphologies, etc.)?
Class Notes:
 Water erosion is not a dominant process in the evolution of the Martian surface, therefore the total
amount of erosions implied by these models may be too great given the timescales
 It may be useful to consider the effect of varying the initial water content of the ground from 20%
down to 5% to see effects on total precipitation and erosion rates
 Questions on applying this 1-D model to a 3-D view. How global, or, alternatively, how local are
these effects being felt?