Lecture Notes for the Final
Fifth order of relief
- This describes very small features, ranging from sub-centimeter in size
up to a few meters wide.
- This would be the scale of things observed or observable from something
like a lander or rover or a small section of a high resolution sensor, such as
MRO's HiRISE or MGS's MOC.
-
Boulders and rocks
- The Viking, Pathfinder, and Phoenix landers, and the Sojourner, Spirit,
Opportunity, and Curiosity rovers have imaged individual rocks, and the rovers
have
carried APXS spectrometers to help identify oxides, elements, and minerals and
constrain the kinds of surface the rovers traversed. The last three have also
carried Mössbauer spectrometers, which specifically target iron-bearing
minerals.
- Sojourner:
- identified basalt rocks with very little quartz (i.e., igneous
rocks not much fractionated), e.g., Yogi rock.
- also identified more andesitic rocks (i.e., igneous rocks that
had experienced some fractionation, some separation out of olivine and
calcium plagioclase in favor of minerals coming out of solution at cooler
temperatures and after reactions with the first minerals to "freeze" out at
the highest temperatures), e.g., Barnacle Bill.
- æolian fluting, pitting, and faceting are clearly visible
on rocks imaged by Sojourner (e.g., Moe)
- evidence of rock smoothing and rounding is also seen, and these
can be produced by fluvial transportation, wave action, and perhaps even
glacial transport. There's even evidence of relithification of smoothed
pebbles and cobbles into conglomerates, which very much implies marine
or lacustrine environments (e.g., Shark)
- Viking landers:
- caught wind erosion in flagrante delicto
- some circular feature on a rock is exposed downwind of a rock named
Whale Rock
- rocks are uncovered by sand next to Big Joe
- the APXS spectrometers shoot a stream of alpha particles
(basically, helium nuclei), which are a form of highly ionizing particle
radiation.
-
These
particles smack into substances in front of the APXS.
-
Some of these are reflected directly back into the spectrometer by the
heavier atomic nuclei they hit. These are unaltered in wavelength or electron
volts because they were not absorbed.
-
In other cases, the alpha particles knock electrons out of the inner
electron shells of an atom, which then allows electrons from outer shells to
pop down to fill the suddenly abandoned lower orbital places (highly
"desirable" electron real-estate).
-
To drop inward, however, the electrons have to pay out some of the energy they
needed to hang out at higher orbitals, which they do by releasing X-ray
photons.
-
The APXS then registers the distribution of these X-ray photons, generating
spectra (line graphs of energy levels measured in electron volts by
intensity measured in counts per second of reflected alpha particles or X-ray
photons).
-
These spectra have peaks and pits arranged in shapes typical of particular
substances.
- You can use reference spectral libraries to find characteristic
peaks and absorption lines and general shapes to figure out which elements and
oxides are in a given surface and then use that to constrain the range of
minerals present, which can tell you the kind of rock the spectrum was taken
from.
- This can be assisted by such statistical techniques as principal
components analysis.
-
Frost deposits
- Viking 2 caught images of frost forming on Utopia Planitia
- Phoenix was ultimately killed by the sheer amount of carbon dioxide ice
(dry ice) forming on its solar panels, which were, apparently, crushed by its
weight.
- Frost may have an interesting röle to play in explaining new
martian gullies, too!
-
Polygon patterned ground
- Phoenix caught images of polygon patterned ground that were at the right
scale for this permafrost-related ground reworking.
- Large polygons had been seen from orbiters but usually at such a large
scale that there was debate about whether this could be the same process that
produced the smaller patterning in Earth's permafrost and periglacial
environments.
-
Dust devil activities and tracks
- The Phoenix lander had an instrument called Telltale, which
allowed recording of the martian wind through observation of, basically, a
weight on a string. It took thousands of shots of the telltale, which were
put together into a movie. There was an interesting episode where it wasn't
just the weight and string moving all over but the base of the instrument as
well. This was a strong shaking, which is believed to have been a
direct hit by a dust devil on Phoenix.
- The Spirit rover also got some great movies of a swarm of dust
devils coursing across Gusev Crater.
- It's believed that dust devils extended the lives of Spirit and
Opportunity by cleaning the solar panels that were getting coated with
so much dust that they were losing the ability to recharge the rovers'
batteries.
-
Sand and dust streaks
- Wind deposits sand into moving heaps: Seen up close with Viking and
Pathfinder/Sojourner.
- Barchans are classic crescent-shaped dunes with a slip
face on leeward side and horns pointing downwind.
- Transverse dunes are related to barchans but ridges are
straigher.
- Wind carves out hollows in soil at base of rocks and creates
streaks and wind tails downwind.
-
Recurring slope lineæ (RSL)
- These are the annually repeating phenomena of narrow dark streaks that
form each summer on equator-facing slopes in the mid-latitudes and in
equatorial regions of Mars and lengthen over the course of the "warm" season.
- I showed you an animation of these, formed from repeated HiRISE images in
Terra Sirenum in the viewgraph presentation accompanying the lecture on
Noachian regions: https://home.csulb.edu/~rodrigue/geog441541/viewgraphs/marsthirdorder1a.ppt
(slide 22).
- McEwan et al (2011, in Science, and 2014, in Nature:
Geoscience) reported these from HiRISE imagery analysis and argued that
they are caused by the melting of brines on warm slopes in spring and summer,
which then seeps downslope, forming these gradually lengthening damp streaks.
- Water is not a plausible fluid, because, even at summer surface
temperatures, which can get up around 20° or 25° on Mars, the air
pressure is too low for liquid water. Extremely briny water, however, has a
far lower freezing point, allowing a liquid condition to persist at martian
temperatures and air pressures.
- Indeed, most RSL are found at temperatures as low as -43°, and water
couldn't exist as liquid at that temperature even if the air pressure were
quite high.
- In 2015 (Nature: Geoscience), Ojha, et al. presented data
from CRISM (like HiRISE, on the Mars Reconnaissance Orbiter, 2006-present),
showing that the spectral signatures of these streaks is most similar to
brines rich in magnesium perchlorate, magnesium chloride, and sodium
perchlorate. This pretty much clinches the brine argument, though what the
source of the brines is has not yet been worked out to anyone's satisfaction.
-
Gullies
- One of the surprises from high resolution orbital imagers that can
revisit and re-image small features has been the appearance of fresh new
gullies, complete with detachment alcoves, runout channels, and
depositional aprons.
- Malin Space Science Systems, which operates the MOC imager, on reporting
this finding back in 2000 or 2001, thought that this might be evidence that
water can still flow on Mars: They resemble Earth gullies point for
point.
- It didn't take too long for the Mars community to have a "yes, but ..."
moment and start arguing for alternative interpretations.
- Dr. Laity's collaborator, Dr. Nathan Bridges and another collaborator,
Dr. Claire Lackner, argued that water is involved, but it's not so
straightforward as seepage of liquid water down a slope.
-
They point out that the geography of these features isn't consistent with a
straightforward seepage of groundwater.
-
Gullies are found on steep slopes, most commonly crater rims and walls
(though some have been found on dunes), often starting at the very top (where
groundwater is apt to be scarcest, though it should be noted that, in a given
collection of gullies, the heads will be at pretty much the same elevation,
which would be consistent with an aquifer, so this is still a point of
confusion).
-
These are most commonly found between + ~30° and ~65°.
- Bridges and Lackner argue that the specific surfaces carved by these
gullies and the obliquity cycle need to be worked in.
- They note that gullies are often correlated with a kind of geological
unit called a mantling unit.
- This, they think, was the precipitation or condensation of water ice
around the abundant dust nuclei of the martian atmosphere, leading to a kind
of "dirty snow" or "filthy frost."
-
This would happen at mid-latitudes during high obliquity cycles,
when the concentration of solar energy in polar latitudes (higher sun angle
and longer day length) would cause the polar caps to sublimate and the water
(and carbon dioxide) vapor could then freeze out or snow out in lower
latitudes.
- This would build up a soft deposit of dust and ice, which seems to
mantle underlying terrain, kind of dulling its edges.
- Then, when Mars' obliquity went into a low phase, the fine ice in
these mantling deposits would sublimate out and desiccate them, and the water
and carbon dioxide vapor would freeze or snow out on the polar caps again.
The mantling deposits would sag and settle.
- Part of this might have included temperatures warm enough to allow
melting as well as sublimation, and the melt fluid would dribble out and
create gullies.
- As obliquity became lower and lower, more and more of the mantling
deposits would erode this way and this erosion could get pretty thorough in
the mid-latitudes and less so in the lagging higher latitudes.
-
A "plot complication" with both the Malin team's and the Bridges and Lackner
interpretations, though, is that gullies are more common in the Southern
Highlands than the Northern Lowlands, meaning at higher elevations.
- Higher elevations means lower air pressure and less likelihood of
water getting above its triple point.
- Higher elevations also means colder elevations and even less
likelihood of water being found in a liquid state.
-
Some people, such as Jon Pelletier (2008), have argued that, in fact, these
features, or at least some of them, are small dry avalanches,
essentially sand or gravel sloughing downslope. There have been several
elaborations on this basic idea:
-
Yolanda Cedillo-Flores and Héctor Durand-Manterola (2010) and Serina
Diniega argue that the sublimation of carbon dioxide or water ice in such
substrates as the mantling deposits or even sand undermines the support
of individual sand grains.
-
These then fall or roll down, entraining others that are precariously
supported.
-
In fact, these dust and sand materials may fluidize or flow as bone-dry
grains supported on the sublimating gas, more of which would sublimate as a
result of exposure by a flow once it gets going. The fluidization refers to
how the gas keeps the grains in buoyant suspension, so that their edges can't
lock and put an end to the movement through friction.
-
Sirena Diniega (2010) adds that carbon dioxide frost may itself get so thick
in the winter that it might start rolling down a steep slope, an avalanche
of dry ice grains, which then entrains other material in it.
-
Donald S. Musselwhite, Timothy D. Swindle and Jonathan I. Lunine went further
in 2001, arguing that we're looking at carbon dioxide vaporizing and, the
moment it escapes the soil, refreezing outside the soil as frost. The frost
grains then continue rolling downslope, buoyed up by more of the vaporizing
ice in a suspended flow, eroding soft soils as they go.
- The latest-breaking news on this front puts water back in the picture!
Boiling water to be specific. On 2 May 2016, Massé et
al. reported in Nature: Geoscience that water can be an erosive
agent right at the point of sublimation from ice to vapor.
- Water on Earth boils at 100° C, at sea level pressures. The boiling
point cools as you attempt to cook at higher elevations due to the exponential
drop in pressure, meaning you have to add cooking time to recipes if you're
trying your hand at the culinary arts in the Sierra. If you were boiling an
egg at the top of Mt. Everest (8,848 m, roughly 300 hPa), the water would be
only 72° C!
- On Mars, water boils at 0° C (0.6 hPa). So, warming of water ice
near the surface would induce boiling, and daytime highs often reach 20°
Cin the summer.
- Boiling would cause dry surface grains to spray upwards and settle just
below the junction between wet soil and dry, forming small ridges. These
ridges would become unstable as they steepened beyond the maximum angle of
repose and started sliding or rolling downslope, creating small avalanches of
dry grains.
- Ojha's team tested this idea by putting sand and ice blocks in a diving
compression chamber, dropping the pressure to martian levels, and then heating
the sand and ice to martian summer daytime temperatures. They were able to
video-record the process of spraying sand grains, formation of ridges, and
micro-avalanching.
- So, water, even pure water, cannot be dismissed as an agent of gully
formation, but it acts, not like water flows on Earth, but much like the dry
avalanches discussed above in connection with carbon dioxide ices. It's the
change of state of water to a turbulent gas that creates the dry avalanching
in this argument.
- Here's a nice press release on this project, with links to video clips
showing the process: http://www2.cnrs.fr/en/2753.html.
Blueberries
- Opportunity was sent to Meridiani Planum because of orbital spectral
evidence of hæmatite, which is an alteration product of olivine
in neutral water.
- Olivine is found in a range of types, dominated by iron or magnesium,
which sets it on somewhat different alteration courses, depending on exposure
to water and whether the water is neutral, acidic, or basic.
- It often goes through substitutions of elements to create iddingsite
(weird material that often preserves the shape of the original olivine).
- Iddingsite can be further altered into serpentine or, in other cases,
into gœthite.
- Gœthite can give way to hæmatite, which sometimes forms
concretions within the goethite.
- That appears to be what happened in Meridiani Planum: Hæmatite was
deposited by water within beds of gœthite.
- Further weathering weakened the sedimentary beds and the spherules
popped out and accumulated on the ground in great numbers.
- These spherules are a very clear evidence that Mars had water,
that that water had been neutral in pH at some Noachian times and
places.
- This amplifies the impression that Noachian Mars may have had water on
its surface and did have it in the form of groundwater, enough to alter
olivine in a few places all the way to hæmatite.
And so concludes our discussion of the regions of Mars organized by
scale.
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