Lecture Notes for the Final
Climates and climate change
Martian geochemical cycles and climate
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The hydrological cycle
- This is very different from Earth.
- There is very, very little water vapor in the martian atmosphere
at any
one time (0.03% on average, by volume)
- This is in constant flux from one polar cap to the other, from the
summertime pole as a source of vapor from sublimation to the wintertime pole
as a place so cold that water vapor will freeze there.
- As Mars Express has found in certain craters, water will apparently form
glaciers in favorable locations away from the poles.
- It also forms frosts, as the landers have documented in their
locales and
as Mars Express and MOC have documented on many crater rims.
- Water vapor does freeze onto dust nuclei in the atmosphere to produce the
many clouds and fogs that have been recorded on Mars.
- There may be a lot of water ice also in the regolith (the impact-gardened
debris that passes for soil on Mars) and rocks
- Almost all of this will be permafrost
- There may be small, transient active layers above or even below the
permafrost, where water ice will change state into liquid, due to surface
warming or geothermal activity. This may be what accounts for the fresh
gullying documented by MOC and HiRISE. A geothermal disturbance might well
account for
the gigantic jökulhlaup-like outflow floods during catastrophic melting
of permafrost.
- At the present time, there may be almost no interaction between soil ice
and the atmosphere, so the permafrost may not actively participate in the
contemporary hydrological cycle on Mars.
-
The carbon cycle
- Carbon has a very active cycle on Mars.
- It moves from sublimation off the summertime polar cap to re-freezing on
the wintertime polar cap, much as water does.
- It, too, forms clouds and fogs upon freezing onto condensation nuclei
provided by the abundant dust on Mars.
- A very interesting aspect of the martian carbon dioxide cycle is the
effect that its seasonal fluxes have on air pressures on Mars.
- When the carbon dioxide ice on the poles sublimates (which can be rather
a dramatic, geysering phenomenon sometimes), it pushes the martian air
pressure up pretty drastically: remember that variation from about 6 hPa to
10 hPa, which is way more dramatic than we see on Earth.
- The effect is especially noticeable when it's the south polar cap that
sublimates during southern hemisphere summer: That cap is made up of far more
carbon dioxide ice than the lower, warmer north pole, which is dominated by
water ice.
-
The oxygen cycle
- Not much there: ~0.13%.
- What little oxygen there is seems derived from water and carbon dioxide
in the planet's regolith.
- This is based on the expectation that, in the exosphere, you would
disproportionately lose the lighter isotopes of oxygen (16O and
17 O, versus
18O) over the billions of years of martian history.
- On Mars, however, you find a more normal proportion of 18O,
which means
it must be getting replenished from somewhere: soil water, permafrost?
- In fact, you could calculate how much water you'd expect was lost from
Mars -- enough to create a global ocean on a smooth planet of something like
13 m in depth -- a lot of water.
- Because there is a little oxygen and it's being replenished somehow,
there is also a tiny bit of ozone, because of the intense UV exposure
of the
atmosphere: It breaks oxygen bonds and reforms them as ozone.
- Ozone tends to be eroded quickly by exposure to hydrogen, so ozone tends
to persist only in the very driest locations, as in the polar cap on the
winter side of the planet.
-
The nitrogen cycle and implications for the past atmosphere
- Also pretty puny on Mars: ~2.7%.
- But it's overrepresented in the heavy isotopes of nitrogen
(15N, instead
of 14N), which means that much of the lighter nitrogen snuck off
into space
through the exosphere.
- It's been estimated that fully 90% of Mars' primordial nitrogen escaped
this way.
- If that's the case, then that would mean the nitrogen content of the
early martian atmosphere, besides having a lot of nitrogen, would contribute a
lot to an increased atmospheric air pressure: maybe as high as 78 hPa
-
Remember the triple point of water?
-
Mars, with air pressure averaging around 6 or so hPa, typically sees water
changing state directly from ice to vapor (sublimating) whenever temperatures
rise above about 0° C or 273 K.
- If the air pressure got as high as 78 hPa, water would go from ice to
liquid at 0° C and then not evaporate until about 60 or 70° C!
- That would mean that water might well have persisted as a liquid
in
pressures that high, explaining the fluvial features we see on Noachian
landscapes.
- The argon cycle
-
Argon, as a noble gas, is highly unlikely to react with other elements and
compounds; while not impossible, it's just very unlikely
- It comprises about 1.6% of Mars' atmosphere by volume (which is higher
than on Earth, which only has about 0.9%)
- In 2007, the APXS instruments on the Mars Exploration Rovers, Spirit and
Opportunity, found that argon content varied, results corroborated from
orbit by the Mars Odyssey gamma-ray spectrometer.
- Argon content rises in winter and falls back in summer.
- It's not so much as argon reacting with anything, however: What happens
is that carbon dioxide fluctuates like mad, flowing between the two poles.
-
In winter, carbon dioxide precipitates or undergoes frost deposition on the
polar ice cap experiencing winter, which drastically drops air pressure.
-
In summer, it sublimates off the summer polar ice cap and raises air pressures
in that hemisphere.
- Argon just sits there, not reacting (and its freezing point is way below
carbon dioxide's, so it isn't doing the same sublimation/freezing thing), so
its constant presence in a fluctuating atmosphere creates the swings in
abundance.
- The magnitude is rather dramatic: the CO2:Ar ratio changes by
a factor of six over the course of the martian year.
Mars climates
- On Earth, the climate classification systems (e.g., Köppen
and
Thornthwaite) depend on measures of temperature, moisture/evaporation,
and precipitation. These may be modified to incorporate vegetation (which,
because of the rather sensitive environmental envelopes of many plant species,
covary with environmental conditions, which can be read from vegetation in the
absence of instrumental records).
- How can martian climates be classified? Basically, temperature, maybe
dustiness, wind, water vapor content, maybe snow or frost. Henryk Hargitai has attempted a
systematization of martian climates, inspired by Köppen's system but
allowing seasonal migration of the underlying factors:
- At the coarsest level, we could differentiate:
- North polar "frigid" climates north of the Arctic Circle at
64.9° N
- Northern transitional "temperate" climates south of the Arctic
Circle and north of the Tropic of Pisces (Mars' answer to our own Tropic of
Cancer) at
25.1° N
- Equatorial "tropical" climates between the Tropic of Pisces and
the Tropic of Virgo (Mars' version of the Tropic of Capricorn) at 25.1° S
- Southern transitional "temperate" climates from the Tropic of
Virgo to the Antarctic Circle
- South polar "frigid" climates south of the Antarctic Circle
- Just as on Earth, the whole system of climate factors would shift north
and
south with the shifting position of the thermal equator, or the
location of the "noon-overhead sun" or direct, vertical ray of the sun, caused
by the
obliquity of the planet's rotational axis.
- On Mars, the northern and southern hemisphere versions of these climates
would be much more different from one another than they would be on Earth, due
to the plot complications of Mars orbital ellipticity and the elevation
differences between the two hemispheres.
- The southern hemisphere summers would be "hotter" than the northern
hemisphere's summers because perihelion happens in the southern hemisphere
summer, leading to more CO2 sublimation off the South Polar Ice Cap
and higher air pressures, more wind, more dust storms.
- Southern hemisphere winters would be colder, too, because they
coïncide with aphelion, leading to greater deposition of CO2
ice in the seasonal cap, which extends further toward the equator than you see
in the northern hemisphere. Enhancing the effect is the greater elevation of
the Southern Highlands, leading to lower temperatures by normal and adiabatic
lapse rates (drop in temperature with a gain in elevation, which is
intensified if the air itself is moving vertically).
- Nested within the coarse bands, just as with the Köppen system, the
basic climate classes can be modifed by terrain extremes and by
persistent differences in albedo that create
smaller regional climates. On Earth, for example, we have the H or Highland
climates, and we can recognize that effect on Mars, too. We can see the need
to modify our climate belts for areas of unusually low elevation/high
pressure, too, and for
areas of unusually low albedo, which affects air pressure and heat retention.
So, here are the modifications to the main climate zones defined by Hargitai:
- Equatorial zone highlands
- Tharsis and Elysium
- Extremely cold due to lapse of temperature with elevation
- Air with tiny amounts of water vapor could rise and cool adiabatically
enough on such tall slopes as to induce the formation of clouds, and these
volcanoes often are topped with clouds.
- Low albedo equatorial zones
- Syrtis Major
- This would be an area of high thermal inertia, meaning it would more
slowly
heat up and cool down than surrounding terrain.
- This effect could produce "land and sea" breezes without an actual sea!
- Low elevation equatorial zones
-
Valles Marineris
- Extreme depth leads to increased air pressure.
- This might push local barometric pressure above the triple point of
water, enabling liquid water or brines to exist occasionally, depending on
temperatures.
- Temperatures would be relatively warm because of the equatorial location
and could be warmer on the floor of the canyons because of the lapse of
temperatures with elevation, making these transient excursions above the
triple point a bit likelier.
- The venturi effect of these aligned canyons, together with increased air
pressure, could result in increased wind flow.
- Greater warmth could also make for thermals coming up the canyon walls.
- Low elevation transitional zones
- Hellas and Argyre Planitia
- Even though temperatures would be colder this far south of the equator,
the floors of these two immense craters are so far below the areoid that they
would be warmer than the surrounding terrain.
- That and the increased air pressure could also lead to transient
excursions above the triple point, resulting in increased gullying.
- There could also be strong winds and updrafts, which, together with
Coriolis Effect, can produce a lot of dust devils in spring and summer.
- These two craters, particularly Hellas, do spawn a lot of dust devils, a
few of which spiral up into planet-covering dust storms.
- You can get to the Hargitai Mars climate page and a nice collection of
temperature, dust, and pressure diagrams at http://planetologia.elte.hu/mcdd/index.phtml?cim=climatemaps.html
Relatively recent climate change on Mars
- Geography of gullying
-
We saw that the presence of gullying is
concentrated on poleward-facing
slopes in the lower and lower-middle latitudes, which makes sense if Mars had
greater obliquity in its past.
- Increased obliquity
would position the sun during summer such that the poleward-facing slopes
would essentially be the adret slopes and the sunnier slope would melt soil
water, especially in the lower, warmer latitudes.
-
Mars' axis changes its
tilt, much as
Earth does, only over a more extreme range of values (~15° - ~45°)
over about 124,000 Earth years,
since it doesn't have our massive moon to stabilize it (Earth's obliquity
ranges from ~22° to ~24.5° by comparison). There's some speculation
that this could become as extreme as 0° to 60° over millions of years!
- Recent glaciation
-
We saw evidence of recent glaciation in the northern tropics near Elysium.
-
Like low and mid-latitude poleward-facing gullyng, mid-latitude glaciation
could also be expected from more extreme obliquity, especially if aphelion
coïncided with the northern hemisphere's winter.
- Recent accelerated polar cap sublimation
-
Interestingly, today, it seems that the martian polar caps are
sublimating away measurably year to year.
- Mars is apparently also
experiencing global warming and shrinking polar caps!
- Climate change deniers here on Earth are having a field day with this,
claiming that global warming here couldn't possibly be coming from human
actvities if it's also going on at Mars, that it's, therefore, merely the sun
increasing in irradiance. Never mind that the increasing radiance of the sun
over the course of its life plays out over billions of years and the secular
changes would be unnoticeable. They are also well within the range of the
feedback systems of our living planet to accommodate.
- This is something of an "apples and oranges" non sequitur.
-
On Mars, increased polar cap sublimation seems connected with increased
dustiness (Fenton et al. 2007, available at http://humbabe.arc.nasa.gov/~fenton/pdf/fenton/nature05718.pdf).
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Dust in the middle and upper troposphere absorbs solar radiation
and warms the atmosphere.
-
On Earth, you can evaluate the association among
solar irradiance, volcanic sulfates, carbon dioxide, methane, nitrous oxide,
and borehole temperature anomalies over the last four centuries by downloading
this file, https://home.csulb.edu/~rodrigue/geog400/tracegas.ods.
-
You could try scatterplotting various drivers and borehole temperatures, doing
correlations between them and borehole temperatures, or trying either multiple
regression or principal component analysis modeling to see which drivers
account for the borehole readings. If you have nothing to do.
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