ASTROBIOLOGY
Volume 10, Number 6, 2010
ª Mary Ann Liebert, Inc.
DOI: 10.1089/ast.2010.0472
Dynamic Temperature Fields under Mars Landing Sites
and Implications for Supporting Microbial Life
Richard Ulrich,1 Tim Kral,2 Vincent Chevrier,3 Robert Pilgrim,3 and Larry Roe 4
Abstract
While average temperatures on Mars may be too low to support terrestrial life-forms or aqueous liquids, diurnal
peak temperatures over most of the planet can be high enough to provide for both, down to a few centimeters
beneath the surface for some fraction of the time. A thermal model was applied to the Viking 1, Viking 2,
Pathfinder, Spirit, and Opportunity landing sites to demonstrate the dynamic temperature fields under the
surface at these well-characterized locations. A benchmark temperature of 253 K was used as a lower limit for
possible metabolic activity, which corresponds to the minimum found for specific terrestrial microorganisms.
Aqueous solutions of salts known to exist on Mars can provide liquid solutions well below this temperature.
Thermal modeling has shown that 253 K is reached beneath the surface at diurnal peak heating for at least some
parts of the year at each of these landing sites. Within 408 of the equator, 253 K beneath the surface should occur
for at least some fraction of the year; and, within 208, it will be seen for most of the year. However, any life-form
that requires this temperature to thrive must also endure daily excursions to far colder temperatures as well as
periods of the year where 253 K is never reached at all. Key Words: Mars—Temperature—Lander—Liquid—
Brine. Astrobiology 10, 643–650.
1. Introduction
M
icrobial life requires aqueous solutions for growth
and replication; and, in the presence of liquids stabilized by dissolved ions or solid surfaces, it might be supported
well below 273 K. While many microorganisms can remain in
stasis in a fully frozen environment, with no liquid present
(Gherna, 1994), surrounding liquids provide transport of
nutrients to the cells and so are a requirement for active metabolism. A comparison of methanogens found in Siberian
permafrost with methanogens from non-permafrost habitats,
both of which were placed under martian conditions, showed
that the permafrost-derived methanogens exhibited much
greater survival (Morozova et al., 2006). Studies of methane
evolution in which 14C-labeled substrates were used demonstrated metabolic activity in the permafrost organisms down to
256.5 K (Rivkina et al., 2002). In another study, permafrost microorganisms were able to incorporate 14C-labeled acetate into
lipids at temperatures as low as 253 K (Rivkina et al., 2000).
Pure water will be solid below 273.16 K; but, as shown in
Table 1, concentrated ionic solutions can form thermodynamically stable liquids at much lower temperatures (Brass,
1980; Kargel, 1991), as low as 205–206 K for certain salts
(Chevrier and Altheide, 2008; Chevrier et al., 2009). A second
4
condition for liquid stability is related to its evaporation rate
into the dry martian atmosphere (Ingersoll, 1970; Sears and
Moore, 2005; Moore and Sears, 2006). Concentrated salt
solutions exhibit much lower evaporation rates than pure
liquid water, especially at low temperatures (Sears and
Chittenden, 2005; Chevrier and Altheide, 2008; Altheide et al.,
2009), which results in longer possible residence times due to
mass transfer limitations at or near the martian surface. The
two Mars rovers and Mars Express OMEGA detected considerable amounts of evaporitic deposits stemming from iron
and sulfur, so highly concentrated solutions appear to have
existed in the past (Squyres et al., 2004; Gendrin et al., 2005;
Nelson et al., 2005; Rao et al., 2005). Therefore, if stable
aqueous liquids are present on Mars today, they are probably concentrated brine solutions and could exist at temperatures supportive of life.
The planet-wide average temperature of Mars is about
220 K, which is too low to support the life-forms or liquid
solutions described above. However, the dynamic nature
of the near-surface temperatures enables the existence of
aqueous solutions at certain times and depths over a large
fraction of the planet. The purpose of this paper is to evaluate
the surface and subsurface temperature fields at the Viking 1,
Viking 2, Mars Pathfinder, Spirit, and Opportunity landing
1
Department of Chemical Engineering, 2Department of Biological Sciences, 3Arkansas Center for Space and Planetary Sciences, and
Department of Mechanical Engineering, University of Arkansas, Fayetteville, Arkansas.
643
644
ULRICH ET AL.
Table 1. Eutectic Concentrations and Temperatures of Salt Solutions Possibly Present on Mars
Salt
Saturation concentration (% wt)
Eutectic temperature (K)
Reference
25
48
23
21
30
44
270
205
252
239
223
206
Altheide et al., 2009
Chevrier and Altheide, 2008
Brass, 1980
Brass, 1980
Altheide et al., 2009
Chevrier et al., 2009
MgSO4
Fe2(SO4)3
NaCl
MgCl2
CaCl2
Mg2(ClO4)2
sites with regard to the conditions conducive for terrestrial
life. Certain temperatures were used as benchmarks, such as
253 K, described above as a demonstrated low-temperature
condition for metabolic activity in terrestrial methanogens;
273 K for pure water; and the eutectic temperatures of salt
solutions from Table 1. We are not making the claim that
253 K is a lower limit for metabolic activity on Mars; we are
using this benchmark because metabolic activity has been
demonstrated at least down to this temperature by terrestrial
organisms. These locations on Mars were chosen because
their thermal and physical properties were carefully measured during landing site selection and the vicinity of any
lander is of interest with regard to the possibility of biological contamination. The Mars Pathfinder (MPF) site was
considered in more detail than the others as a means to
validate our thermal model.
2. Modeling Surface and Subsurface
Martian Temperatures
A finite-difference procedure was used to predict regolith
temperature profiles from the surface down into the martian
subsurface as a function of time, latitude, thermal inertia,
surface albedo, surface emissivity, distance of Mars from the
Sun, and atmospheric opacity. Effects considered include
direct solar radiation to the surface (Rapp, 2004), radiation
up from the ground with use of an emissivity of 0.97
(Bandfield 2002), conduction through the near-surface region, and 30 mW/m2 of geothermal energy from below
(Schorghofer and Aharonson, 2005; Urquhart, 2005). The
atmospheric opacity was set at t ¼ 0.5, a commonly used
year-round average (Appelbaum et al., 1997; Smith et al.,
1997; Smith et al., 2006). Radiation from the atmosphere to
the surface was calculated with the method of Kieffer et al.
(1977) and Schorghofer and Edgett (2006) that involved setting this radiative flux to a set percentage of the noontime
solar heating and keeping it constant throughout the sol.
They recommend 2–4%, and we used 2%. Table 2 shows the
input data for the landing sites used in this study, which
were taken from some of the latest available sources. In addition, the model requires the product of regolith heat capacity and density. Heat capacities of basalts range from
about 600–1000 J/kg K, so a value of 800 was taken. Bulk
density was taken to be 2000 kg/m3, which takes into account the blocky nature of the landing sites.
Temperatures at and below the surface of Mars are driven
almost totally by radiation from the Sun, radiation of the
ground to the sky, a small amount of radiation from the sky
during both day and night, and conduction into and out of
the subsurface. Conduction and convection from the atmosphere, either free or forced, contributes relatively little to
surface and subsurface temperatures (Haberle and Jakosky,
1990). Omitting these effects can be justified on the bases of
available heat and the rate of heat transfer. There is a large
difference in thermal mass between the dense regolith and
the tenuous atmosphere. The column mass of the martian
atmosphere is 173 kg/m2 (compared to 10,300 kg/m2 for
Earth) and is equal to the mass of only about 9 cm of martian
regolith. Since the heat capacity of carbon dioxide at 200 K is
735 J/kg K, which is roughly the same as * 800 J/kg K for
basaltic materials, the thermal mass of less than 10 cm of
martian regolith is equal to the thermal mass of the 11 km
scale height for the atmosphere. Considering that the scale
height would be the total depth of the atmosphere if it had
the surface pressure and temperature throughout, only a
small portion of the total atmospheric column is in thermal
communication with the ground over the course of a sol.
Besides checks for internal consistency, the model was
compared to published results, such as the established
models used by Putzig and Mellon (2007), Kieffer (1976), and
Martin et al. (2003), and found to agree within about 5–10 K
over a wide range of conditions.
3. Validation of the Model to Temperature
Data at the MPF Site
We compared the model’s predictions of surface temperatures to surface and near-surface atmospheric temperatures
Table 2. Thermophysical Data, Locations, and Time of Year at Landing for the Sites
on Mars Modeled in This Study
Landing site
Thermal inertia
( J/m2 s1/2 K)
Albedo
Reference for thermal
inertia and albedo
Latitude
(north)
Longitude
(east)
Ls at landing
(degrees)
Viking 1
Viking 2
MPF
Spirit
Opportunity
283 14
234 11
386 10
250
220
0.216 0.004
0.283 0.003
0.186 0.003
0.23
0.20
Puzig et al., 2005
Puzig et al., 2005
Puzig et al., 2005
Jakosky et al., 2006
Jakosky et al., 2006
22.7
48.3
19.3
14.6
1.94
312
134
327
176
355
97
118
143
328
339
TEMPERATURE FIELDS UNDER MARS LANDING SITES
645
measured at the MPF site. Mars Pathfinder landed on July 4,
1997, at 19.38N, 326.58E and Ls ¼ 142.78. It did not measure
surface temperatures but transmitted back atmospheric
temperatures taken by three sensors on its meteorological
mast located 0.25, 0.50, and 1.0 m above the ground at various sampling rates for 76 sols (11% of the year) until the end
of its mission on September 27, 1997. Few sols had complete
coverage, but when all 76 sols are stacked together they
provide a set of temperature profiles just above the surface at
high temporal resolution, as shown by the points in Fig. 1.
The sunrise and sunset times indicated correspond to the
midpoint of this 76 sol period: Ls ¼ 1628. As expected, the
sensors closer to the ground reported slightly higher temperatures during the daytime and lower temperatures at
night with the near-surface atmospheric temperature inverting immediately after sunrise and sunset.
The model agrees well with these measured temperatures.
The slightly larger temperature amplitude is because the
model predicts surface temperatures and the thermocouples
measured atmospheric temperatures close to the surface. The
only simultaneous measurements of surface and near-surface
atmospheric temperatures made on Mars came from the two
Mars Exploration Rovers (Urquhart, 2005), which used their
Mini-Thermal Emission Spectrometers to make readings of
the surface and of the atmosphere about 1.1 m above the
surface. Opportunity data taken at Ls ¼ 3398 indicates that
the maximum difference occurred at 13:00 with the surface
being about 30 K warmer. For Spirit, the maximum was
about 20 K. In both cases, surface temperatures also dropped
below atmospheric temperatures at night. These temperature
differences are close to those we predict for the MPF site.
4. Surface Temperatures at the Landing Sites
Figure 2 shows the maximum and minimum daily temperatures at the surface for the landing sites. The dotted line
between them is the temperature at 1 m, which does not
change significantly during one sol but changes with the
seasonal cycle. The subfigures are sorted by latitude from
most northerly to most southerly landing sites. The Spirit
and Opportunity sites are the warmest of the five. Temperatures in excess of 273 K are reached at the surface for
about half the year at both locations. Opportunity is at
1.948S, which is considerably closer to the equator than Spirit
at 14.68S and results in less annual variation in the temperature field. Viking 1 and MPF are similar since they are
separated by only 835 km and 3.48 of latitude. At the surface
at Viking 2 (48.38N), 253 K is barely reached and then for
only a few sols.
Since the orbit’s eccentricity is significant and almost exactly out of phase with the northern seasons, this acts to
decrease the annual temperature variations north of the
FIG. 1. Comparison of MPF near-surface atmospheric temperature data (black from 0.25 m elevation on the meteorological
mast, gray from 0.5 m, open circles from 1.0 m) and our finite-difference surface temperature model (solid line) for a sol in the
middle of MPF’s mission, at Ls ¼ 1628.
646
ULRICH ET AL.
FIG. 2. Predicted maximum and minimum surface temperatures (black lines) and the temperature at 1 m deep (dotted line)
at the MPF landing site from our model. Black dots are calculated surface temperatures from the NASA Planetary Data
System (PDS), and open circles are the measured MPF 0.25 m elevation atmospheric temperatures.
TEMPERATURE FIELDS UNDER MARS LANDING SITES
647
equator measurably and reinforce the seasons in the southern hemisphere. In the northern hemisphere, temperature
variations will remain closer to their average, while at a
similar latitude in the south the average is about the same,
though the temperature swings more widely about this value.
This is made clear by comparing the panels in Fig. 2 for MPF
(19.38N) and Spirit (14.68S), which are at similar latitudes in
different hemispheres. Yearly amplitudes are larger for Spirit
because of the in-phase relationship of solar flux and axial tilt.
trated as deep as 1.2 cm at that time and is present at some
depth and at some time of sol for about three-quarters of the
year. Since the depths shown in Fig. 4 are those of the highest
daily temperatures, the warmest times of year are at the
troughs of these lines, where high temperatures penetrate the
deepest, and the coldest are at the peaks. Temperatures below the lowest line of each panel were not calculated; thus
there are blank areas for Viking 2. Temperatures in these
depths and times are below 200 K, which is too cold for almost any liquid solutions or life as we know it.
Four of the five landing sites see daily subsurface temperatures in excess of 253 K for most, or nearly all, of the
year. This temperature may occur briefly as deep as a couple
of centimeters. However, 253 K is not maintained on, or
under, the surface for a complete 24-martian-hour sol at any
location on the planet. Daily surface temperatures near the
equator can oscillate over 100 K around an average value.
These temperature cycles also occur with some time delay
under the surface but with decreasing amplitude with depth
that approaches the steady average for that time of year by a
few tens of centimeters down. Any subsurface location exhibiting above-average daytime temperatures will plunge to
correspondingly cold temperatures on the other side of average at night.
Figure 5 shows a situation contrary to that shown in the
previous figure: the depth to minimum diurnal temperatures
5. Subsurface Temperature Fields at the Landing Sites
The thermal model was used to estimate the surface and
subsurface temperatures at the MPF site for Ls ¼ 1628. The
model results, shown in Fig. 3, indicate that the surface
temperature reaches its minimum just before sunrise followed by a rise of about 80 K to a maximum of 268 K shortly
after noon. Subsurface diurnal temperature swings dampen
out rapidly with depth and become almost negligible at a
few tenths of a meter.
We calculated the maximum daily temperatures at various
depths as a function of time of year, as shown in Fig. 4 for all
five sites. With regard to the panel for MPF, the daily temperature, at the time of landing at Ls ¼ 1438 and at a depth of
0.5 cm, would have reached as high as 260 K at some time
during the sol. A temperature of 253 K would have pene-
FIG. 3. Surface and subsurface temperatures at the MPF landing site for a typical sol at Ls ¼ 1628, calculated from the model.
The decreasing temperature signal amplitude and the increasing time delay with depth are illustrated.
648
ULRICH ET AL.
FIG. 4. Depth to maximum diurnal temperatures for the five Mars landing sites. For example, at the Viking 1 site and
Ls ¼ 180, 270 K reaches a maximum of about 0.2 cm beneath the surface at some part of the sol, 260 K reaches 0.7 cm deep, and
253 K reaches 1.2 cm deep.
at the Opportunity site. The diurnal cycle at the warmest
time of year (Ls ¼ 2008) on the surface is 290–180 K for a 110 K
range around an average of 235 K. At this same season, 253 K
will reach 1.7 cm depth at peak heating during the day; but,
12 hours later, that same depth will be at 195 K for a diurnal
cycle of 58 K around roughly the same average.
Geothermal heating from within the planet creates a
temperature gradient that will result in warmer tempera-
tures appearing again, but much deeper underground
(Mellon and Phillips, 2001). This gradient is
dT q
¼
dz
k
where dT/dz is the temperature gradient in K/m, q is the
geothermal heat flux in W/m2, and k is the thermal conductivity of the subsurface in W/m K.
TEMPERATURE FIELDS UNDER MARS LANDING SITES
FIG. 5.
649
Minimum diurnal temperatures at various depths throughout the year for the Opportunity landing site.
The geothermal heat flux on Mars is highly uncertain but
has been estimated to be between 0.017 and 0.024 W/m2 by
extrapolating the somewhat better understood terrestrial
geothermal mechanisms to Mars with the use of various
weighted averages of planetary mass, density, and surface
areas (Solomon and Head, 1990; Hoffman, 2001). Some investigators have used values as high as 0.030 W/m2 (Schorghofer and Aharonson, 2005; Urquhart, 2005). Combining
this range with matching extremes for thermal conductivity
of 0.3 W/m K for loose sand and 2 W/m K for solid basaltic
rock gives a thermal gradient of 0.030/0.3 ¼ 0.100 and 0.017/
2 ¼ 0.009 K/m, respectively. These small gradients are not
enough to influence the temperature field within a few meters of the surface; and, although the geothermal flux is included in our model, omitting it has no distinguishable effect
on the results shown in the figures above. Deep underground,
however, the temperatures will be dominated by this heating
from below with no diurnal or annual heating effects present.
Starting from an average surface temperature of 220 K, these
two gradients would cause 253 K to be reached between 330
and 3700 m. The deeper end of this range is probably more
realistic, considering the bedrock structure expected on Mars.
If life exists in this environment, it will be a very long time
before such deep locales could be sampled.
6. Implications for Life in the Martian Subsurface
Requirements for subsurface life on Mars include temperatures sufficient to support metabolism and the presence
of aqueous liquids for nutrient transport. Metabolic activity
in permafrost bacteria has been demonstrated down to 253 K
(Rivkina et al., 2000), and studies in our laboratory show that
aqueous solutions containing salts known to exist on Mars
can remain liquid at this, and much lower, temperatures,
down to 205 K (Chevrier and Altheide, 2008). Thermal
modeling of the subsurface environment on Mars has indicated that 253 K is reached at diurnal peak heating for at least
some parts of the year at five of the six landing sites. Within
408 of the equator, a temperature of 253 K beneath the surface
will occur for at least some fraction of the year; and, within 208,
it will persist for most of the year. These peak temperatures
will penetrate down to 2–3 cm at most, which is within the
range of simple robotic scoops and deep enough to shield the
organisms from UV radiation. However, any life-form that
requires this temperature to thrive must also endure nightly
excursions to far colder temperatures and, away from the
equator, periods of the year where 253 K is never reached at all.
Abbreviation
MPF, Mars Pathfinder.
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Address correspondence to:
Richard Ulrich
Department of Chemical Engineering
Bell 3202
University of Arkansas
Fayetteville, AR 72701
E-mail: [email protected]
Submitted 7 February 2010
Accepted 4 May 2010