Artist's Concept of Rover on Mars (credit: Maas Digital LLC)
n enormous amount of scientific information related to the
Martian geology and atmosphere, as well as providing some astronomical
observations from Mars. This article covers information gathered by the
Opportunity rover during the initial phase of its mission. Information on science gathered by Spirit can be found mostly in the Spirit rover article.
The ongoing unmanned Mars exploration mission, commenced in 2003 sent two robotic rovers, Spirit and Opportunity, to explore the Martian surface and geology. The mission was led by Project Manager Peter Theisinger of NASA's Jet Propulsion Laboratory and Principal Investigator Steven Squyres, professor of astronomy at Cornell University.
Primary among the mission's scientific goals is to search for and characterize a wide range of rocks and soils
that hold clues to past water activity on Mars. In recognition of the
vast amount of scientific information amassed by both rovers, two asteroids have been named in their honor: 37452 Spirit and 39382 Opportunity.
On January 24, 2014, NASA reported that current studies on the planet Mars by the Curiosity and Opportunity rovers will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.
Water hypothesis
On
March 2, 2004, NASA announced that "Opportunity has landed in an area
of Mars where liquid water once drenched the surface". Associate
administrator Ed Weiler told reporters that the area "would have been good habitable environment", although no traces of life have been found.
Larger grains suggest the presence of fluid.
This statement was made during a press conference, where mission
scientists listed a number of observations that strongly support this
view:
Distributions of spherules
- Hypothesis: Spherules are concretions created in water as a solvent.
- Competing hypothesis: Spherules are rehardened molten rock droplets, created by volcanoes or meteor strikes.
- Supporting data: Location of spherules in the rock matrix is random and evenly spread.
- Quote from Steve Squyres: "The little spherules like blueberries in a muffin are embedded in this rock and weathering out of it. Three ideas, lapilli, little volcanic hailstones, one possibility. Two, droplets of volcanic glass or impact. We've looked at these things very carefully. Probably concretions. If so, it's pointing towards water."
Detailed analysis of environmental, chemical, and mineralogical data
taken from the Opportunity rover led to the elimination of the competing
hypotheses, and the confirmation of the conclusion that the spherules
were formed in place as post-depositional sedimentary concretions from an aqueous source.
In the lower left, a spherule can be seen penetrating the interior of a vug.
Vugs
- Hypothesis: Rock was formed in water, for instance by precipitation.
- Competing hypothesis: Rock were formed by ash deposits.
- Supporting data: Voids found in bedrock resemble "vugs" which are left by eroded away, disk-shaped crystals, possibly dissolved in a watery environment.
- Quote from Steve Squyres: The second piece of evidence is that when we looked at it close-up, it was shot through with tabular holes. Familiar forms. When crystals grow within rocks, precipitated from water. If they're tabular, as they grow you can get tabular crystals and water chem changes and they go away or they weather away."
- Hypothesis: Water created tell-tale salt chemicals in the rock.
- Competing hypothesis: Chemistry of rocks is determined by volcanic processes.
- Supporting data: Sulfate salts and jarosite mineral were found in the rock. On Earth they are made in standing water (possibly during evaporation).
- Quote from Steve Squyres: "Next piece of evidence comes from APXS. We found it looked like a lot of sulfur. That was the outside of the rock. We brought with us a grinding tool, the RAT and we ground away 2-4 mm and found even more sulfur. Too much to explain by other than that this rock is full of sulfate salts. That's a telltale sign of liquid water. Mini-TES also found evidence of sulfate salts. Most compelling of all, the Mössbauer spectrometer in the RATted space showed compelling evidence of jarosite, an iron-(III) sulfate basic hydrate. Fairly rare, found on Earth and had been predicted that it might be found on Mars some day. This is a mineral that you got to have water around to make."
Crossbedding features in rock "Last Chance".
On March 23, 2004, NASA announced that they believe that Opportunity had not landed in a location merely "drenched in water", but on what was once a coastal area. "We think Opportunity is parked on what was once the shoreline of a salty sea on Mars," said Dr. Steve Squyres of Cornell University.
The announcement was based on evidence of sedimentary rocks
that are consistent with those formed by water and not wind. "Bedding
patterns in some finely layered rocks indicate the sand-sized grains of
sediment that eventually bonded together were shaped into ripples
by water at least five centimeters (two inches) deep, possibly much
deeper, and flowing at a speed of 10 to 50 centimeters (four to 20
inches) per second," said Dr. John Grotzinger, from MIT. The landing site was likely a salt flat on the edge of a large body of water that was covered by shallow water.
Other evidence includes findings of chlorine and bromine
in the rocks which indicates the rocks had at least soaked in
mineral-rich water, possibly from underground sources, after they
formed. Increased assurance of the bromine findings strengthens the case
that rock-forming particles precipitated from surface water as salt
concentrations climbed past saturation while water was evaporating.
The evidence for water was published in a series of scientific papers, with the initial results appearing in the journal Science and then with a detailed discussion of the sedimentary geology of the landing site appearing in a special issue of the journal Earth and Planetary Science Letters
Spherules and hematite
Early in the mission, mission scientists were able to prove that the abundant spherules at Eagle crater were the source of hematite in the area discovered from orbit.
Hematite
The distribution of hematite in Sinus Meridiani, where Meridiani Planum is located.
Geologists were eager to reach a hematite-rich
area (in the center of the picture at right) to closely examine the
soil, which may reveal secrets about how the hematite got to this
location. Knowing how the hematite on Mars was formed may help
scientists characterize the past environment and determine whether that
environment provided favorable conditions for life.
"Grey hematite is a mineral indicator of past water," said Dr. Joy Crisp, JPL project scientist. "It is not always associated with water, but it often is."
Scientists have wanted to find out which of these processes created grey hematite on Mars since 1998, when Mars Global Surveyor spotted large concentrations of the mineral near the planet's equator (seen in the right picture). This discovery provided the first mineral evidence that Mars' history may have included water.
"We want to know if the grains of hematite appear to be rounded
and cemented together by the action of liquid water or if they're
crystals that grew from a volcanic melt," said Crisp. "Is the hematite
in layers, which would suggest that it was laid down by water, or in
veins in the rock, which would be more characteristic of water having
flowed through the rocks."
The next picture shows a mineral map, the first ever made on the
surface of another planet, which was generated from a section of the
panorama picture overlaid with data taken from the rover's Mini-TES. The
Mini-TES spectral data was analyzed in a way that the concentration of
the mineral hematite was deduced and its level coded in color. Red and
orange mean high concentration, green and blue low concentration.
This spectral map of Eagle crater shows hematite.
The next picture shows a hematite abundance "index map" that helps geologists choose hematite-rich locations to visit around Opportunity's landing site. Blue dots equal areas low in hematite and red dots equal areas high in hematite.
The colored dots represent data collected by the miniature thermal emission spectrometer on Sol 11, after Opportunity
had rolled off of the lander and the rover was located at the center of
the blue semicircle (the spectrometer is located on the panoramic
camera mast).
A hematite abundance index map of Eagle crater.
The area to the left (with high concentration of hematite) was
selected by mission members for further investigation, and called Hematite Slope.
During Sol 23 (February 16) Opportunity successfully trenched the soil at Hematite Slope and started to investigate the details of the layering.
Spherules
This color-enhanced image shows spherical granules.
Microscopic images of the soil taken by Opportunity revealed small spherically shaped granules. They were first seen on pictures taken on Sol 10, right after the rover drove from the lander onto martian soil.
When Opportunity dug her first trench (Sol 23), pictures
of the lower layers showed similar round spherules. But this time they
had a very shiny surface that created strong glints and glares. "They
appear shiny or polished," said Albert Yen, science team member, during a
press conference on February 19. He said: "Data will hopefully help us
figure out what's altering them." At the same press briefing, Dr.
Squyres noted this as one of the main question: "Where did those
spherules come from, dropped from above or grown in place?"
Mission scientists reported on March 2 that they concluded a
survey of the distribution of spherules in the bedrock. They found that
they spread out evenly and randomly inside the rocks, and not in layers.
This supports the notion that they grew in place, since if their origin
was related to volcanic or meteoric episodes one would expect layers of
spherules as a "record in time" for each event. This observation was
added to the list of evidence for liquid water being present at this rock site, where it is thought the spherules formed.
Berry Bowl
The rock "Berry Bowl".
On March 18 the results of the investigation of the area called
"Berry Bowl" was announced. This site is a large rock with a small,
bowl-shaped depression, in which a large number of spherules had
accumulated. The MIMOS II Mössbauer spectrometer
was used to analyze the depression and then the area of the rock right
beside it. Any difference in the measured data was then attributed to
the material in the spherules. A large difference in the obtained
"spectra" was found. "This is the fingerprint of hematite,
so we conclude that the major iron-bearing mineral in the berries is
hematite," said Daniel Rodionov, a rover science team collaborator from
the University of Mainz, Germany. This discovery seems to strengthen the conclusion, that spherules are concretions, grown in wet condition with dissolved iron.
Rocks and minerals
The rocks on the plains of Gusev are a type of basalt. They contain the minerals olivine, pyroxene, plagioclase,
and magnetite, and they look like volcanic basalt as they are
fine-grained with irregular holes (geologists would say they have
vesicles and vugs).
Much of the soil on the plains came from the breakdown of the local rocks. Fairly high levels of nickel were found in some soils; probably from meteorites.
Analysis shows that the rocks have been slightly altered by tiny amounts
of water. Outside coatings and cracks inside the rocks suggest water
deposited minerals, maybe bromine
compounds. All the rocks contain a fine coating of dust and one or
more harder rinds of material. One type can be brushed off, while
another needed to be ground off by the Rock Abrasion Tool (RAT).
There are a variety of rocks in the Columbia Hills (Mars), some of which have been altered by water, but not by very much water.
The dust in Gusev Crater is the same as dust all around the
planet. All the dust was found to be magnetic. Moreover, Spirit found
the magnetism was caused by the mineral magnetite, especially magnetite that contained the element titanium. One magnet was able to completely divert all dust hence all Martian dust is thought to be magnetic. The spectra of the dust was similar to spectra of bright, low thermal inertia regions like Tharsis
and Arabia that have been detected by orbiting satellites. A thin
layer of dust, maybe less than one millimeter thick covers all surfaces.
Something in it contains a small amount of chemically bound water.
Plains
 
Above: An approximate true color view of Adirondack, taken by Spirit's pancam.
Below:Digital camera image (from Spirit's Pancam) of Adirondack after a RAT grind (Spirit's rock grinding tool) | |
| Feature type | Rock |
|---|---|
| Coordinates | 14.6°S 175.5°ECoordinates: |
Observations of rocks on the plains show they contain the minerals
pyroxene, olivine, plagioclase, and magnetite. These rocks can be
classified in different ways. The amounts and types of minerals make
the rocks primitive basalts—also called picritic basalts. The rocks are
similar to ancient terrestrial rocks called basaltic komatiites. Rocks of the plains also resemble the basaltic shergottites,
meteorites which came from Mars. One classification system compares
the amount of alkali elements to the amount of silica on a graph; in
this system, Gusev plains rocks lay near the junction of basalt, picrobasalt, and tephite. The Irvine-Barager classification calls them basalts.
Plain's rocks have been very slightly altered, probably by thin films of
water because they are softer and contain veins of light colored
material that may be bromine compounds, as well as coatings or rinds.
It is thought that small amounts of water may have gotten into cracks
inducing mineralization processes.
Coatings on the rocks may have occurred when rocks were buried and interacted with thin films of water and dust.
One sign that they were altered was that it was easier to grind these rocks compared to the same types of rocks found on Earth.
The first rock that Spirit studied was Adirondack. It turned out to be typical of the other rocks on the plains.
First color picture from Gusev crater. Rocks were found to be basalt. Everything was covered with a fine dust that Spirit determined was magnetic because of the mineral magnetite.
Cross-sectional drawing of a typical rock from the plains of Gusev crater. Most rocks contain a coating of dust and one or more harder coatings. Veins of water-deposited minerals are visible, along with crystals of olvine. Veins may contain bromine salts.
Columbia Hills
Scientists
found a variety of rock types in the Columbia Hills, and they placed
them into six different categories. The six are: Clovis, Wishbone,
Peace, Watchtower, Backstay, and Independence. They are named after a
prominent rock in each group. Their chemical compositions, as measured
by APXS, are significantly different from each other. Most importantly, all of the rocks in Columbia Hills show various degrees of alteration due to aqueous fluids.
They are enriched in the elements phosphorus, sulfur, chlorine, and
bromine—all of which can be carried around in water solutions. The
Columbia Hills' rocks contain basaltic glass, along with varying amounts
of olivine and sulfates.
The olivine abundance varies inversely with the amount of sulfates.
This is exactly what is expected because water destroys olivine but
helps to produce sulfates.
Acid fog is believed to have changed some of the Watchtower
rocks. This was in a 200 meter long section of Cumberland Ridge and the
Husband Hill summit. Certain places became less crystalline and more
amorphous. Acidic water vapor from volcanoes dissolved some minerals
forming a gel. When water evaporated a cement formed and produced small
bumps. This type of process has been observed in the lab when basalt
rocks are exposed to sulfuric and hydrochloric acids.
The Clovis group is especially interesting because the Mössbauer spectrometer(MB) detected goethite in it.
Goethite forms only in the presence of water, so its discovery is the
first direct evidence of past water in the Columbia Hills's rocks. In
addition, the MB spectra of rocks and outcrops displayed a strong
decline in olivine presence,
although the rocks probably once contained much olivine.
Olivine is a marker for the lack of water because it easily decomposes
in the presence of water. Sulfate was found, and it needs water to
form.
Wishstone contained a great deal of plagioclase, some olivine, and anhydrate (a sulfate). Peace rocks showed sulfur
and strong evidence for bound water, so hydrated sulfates are
suspected. Watchtower class rocks lack olivine consequently they may
have been altered by water. The Independence class showed some signs of
clay (perhaps montmorillonite a member of the smectite group). Clays
require fairly long term exposure to water to form.
One type of soil, called Paso Robles, from the Columbia Hills, may be an
evaporate deposit because it contains large amounts of sulfur, phosphorus, calcium, and iron.
Also, MB found that much of the iron in Paso Robles soil was of the oxidized, Fe+++ form, which would happen if water had been present.
Towards the middle of the six-year mission (a mission that was supposed to last only 90 days), large amounts of pure silica
were found in the soil. The silica could have come from the
interaction of soil with acid vapors produced by volcanic activity in
the presence of water or from water in a hot spring environment.
After Spirit stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or Mini-TES and confirmed the presence of large amounts of carbonate-rich
rocks, which means that regions of the planet may have once harbored
water. The carbonates were discovered in an outcrop of rocks called
"Comanche."
In summary, Spirit found evidence of slight weathering on the
plains of Gusev, but no evidence that a lake was there. However, in the
Columbia Hills there was clear evidence for a moderate amount of
aqueous weathering. The evidence included sulfates and the minerals
goethite and carbonates which only form in the presence of water. It is
believed that Gusev crater may have held a lake long ago, but it has
since been covered by igneous materials. All the dust contains a
magnetic component which was identified as magnetite with some titanium.
Furthermore, the thin coating of dust that covers everything on Mars
is the same in all parts of Mars.
First atmospheric temperature profile
Temperature profile taken by MGS over the MER-B site.
During a press conference on March 11, 2004, mission scientists
presented the first temperature profile of the martian atmosphere ever
measured. It was obtained by combining data taken from the Opportunity
Mini-TES infrared spectrometer with data from the TES instrument on
board the Mars Global Surveyor (MGS) orbiter. This was necessary because
Opportunity can only see up to 6 km high, and the MGS camera
could not measure data all the way down to the ground. The data was
acquired on February 15 (Sol 22) and is split into two data sets: Since
the orbiter is in motion, some data was taken while it was approaching
the Opportunity site, other when it was moving away. In the
graph, these sets are marked "inbound" (black color) and "outbound" (red
color). The dots represent Mini-TES (= rover) data and the straight
lines are TES (=orbiter) data.
Atmospheric science from the MER rovers has been published in a series of scientific papers in Science and Journal of Geophysical Research
Astronomical observations
Opportunity observed the eclipse, or transits of Phobos and transits of Deimos across the Sun, and photographed the Earth, which appeared as a bright celestial object in the Martian sky.
A transit of Mercury from Mars took place on January 12, 2005 from about 14:45 UTC to 23:05 UTC, but camera resolution did not permit seeing Mercury's 6.1" angular diameter.
Transits of Deimos across the Sun were seen, but at 2' angular diameter, Deimos is about 20 times larger than Mercury's 6.1" angular diameter.
