Terra Meridiani, Not Terra Firma

The first color postcard from the Opportunity landing site, at Terra Meridiani, was described as an alien, bizarre world by Principal Investigator, Steve Squyres. For someone who has anticipated these first pictures for sixteen years, he said: "This is exactly what it looked like in my wildest dreams. And they were pretty wild."


Outcrop or crater rim remnant thought to be the first bedrock ever seen from the surface of Mars.
Credit: NASA/JPL

The closest earth analog to the Gusev crater site, which now is home to the Spirit rover, has been offered as either an Antarctic dry lake, or the currently filled crater lake in Ghana, Africa.

But Terra Meridiani is on the opposite side of Mars, where it is pitch-dark at night when Gusev sunrise happens. Meridiani also has a different geologically history. Scientists have compared their Mars’ characterization challenge to what a few selected landing sites on Earth might reveal about our own history, if landers came to the Sahara, Oklahoma, Hawaii and the Amazon. If Gusev is most like a dry Antarctic lakebed, then Meridiani is as cold, higher in altitude by about a mile, and darker.

Sometimes also called Meridiani Planum, Opportunity’s site has one feature that its name implies: a flat plain (planum). Thought to be a volcanic region, the site is one of the few areas flat enough to land that also is near the equator and is rich in iron oxides (in particular, the mineral hematite). The iron-rich spot has been compared to the size of Oklahoma. Like Oklahoma and Texas clay, the red soil is the first clue that iron has oxidized or ‘rusted’.

Panorama of Terra Meridiani. Click for large view
Credit: NASA/JPL

Scientists said Sunday that there are many ways to oxidize iron in the hematite form: standing water, weathering from temporary exposure, and oxidation of volcanic basalts.

Which answer turns out to be correct for the history of Meridiani depends on what the rover’s sophisticated instruments eventually diagnose. A key diagnostic will come in the next two weeks, if all goes well with rover egress, when the soil reveals its first chemical spectra. A mineral type that will be ‘a slam dunk’ for a watery past is called goethite, or hydrated iron oxide.

Fine grain soil smoothed by airbag retraction
Credit: NASA/JPL

As far as definitively unravelling the water history on Mars, one can view Meridiani as being as much about goethite as hematite, since the two in combination will exclude a volcanic origin most likely. So far only hematite has been mapped from orbit, using the Thermal Emission Spectrometer, or TES instrument, on the Mars Global Surveyor.

Doug Ming, a geologist with Johnson Space Center in Houston, noted that if they find hematite, but mixed with a mineral called magnetite, or titanium magnetite, the volcanic origin of Meridiani would gain strength. These minerals combine with hematite when it is heated up to produce oxidation without water involvement. The instrument suite onboard can identify which of these mineral phases are present.

Besides hematite, which likely is predominantly in the red soil, the outcrop about 10 meters from the rover’s base station is the second objective for eventual expeditions. This lighter material may be the remnant of a crater rim, as the rover presently sits in a shallow, 20 meter wide crater. The color image shows just a portion of what has been seen as potentially layered stratigraphy–a priceless sampling opportunity for finding the geological records of newer material on top, and older material on the bottom.

The outcrop may hold another surprise, since it is the first martian bedrock ever seen from the surface. What has been boulders and hills in previous Pathfinder, Spirit and Viking images, now appears as a more permanent face on the local history. Unlike what wind, water or impact events may have carried to any of those sites from other places on Mars, the Meridiani bedrock has been a witness to billions of years of martian change. If the rover begins to put thermal cameras, chemical tests and microscopes to this section of Mars, a number of mysteries may suddenly become much less puzzling.

A final stand-out feature of the first day of images from Meridiani was the soil texture. Startling to Squyres, the first black and white images revealed the shear mark from when the airbags retracted. This shear experiment is not a piece of the general geology exploration since the rover is built to drive and trench the soil, not to shape it into a smooth top layer. But nevertheless, by not only scraping off the grey top layer to reveal a dark maroon color (the hematite clue), the airbag retraction has the unique ability to give a clue about the fineness of the soil.

Airbag imprint near Opportunity base station
Credit: NASA/JPL

The experiment can be compared to putting a hand in either soil or powdered sugar. The finer sugar will show a fidelity to the handprint unlike the coarser sand, even to the point of revealing, for instance, a wedding ring inscription or watch band. With this analogy, geologists were amazed to find a more talcum-powder-like consistency to Meridiani, since the fineness of the soil could preserve even a stitch seam of the original airbag’s imprint.

There are not many places to find fine, hematite-rich soil –only a few place even on Mars–but the history of Meridiani is likely in the outcrop and soil so exciting to Opportunity’s unique location.

Mars Time

While places like Mauna Kea volcano in Hawaii and the Tinto River in Spain may share some terrestrial geology and chemistry with Opportunity’s site, Squyres summarized that only Mars can answer the question: what on Earth might compare to Terra Meridiani?

It is not Terra Firma.

Related Web Pages

MER mission: JPL
Where on Mars is Opportunity?
A Bizarre New Mars
Second Opportunity, Safe on Mars
Water Signs
Microscopic Imager
Gusev Crater
Pancam– Surveying the Martian Scene
Mössbauer spectrometer
Alpha Proton X-ray Spectrometer
Mars Rover: The Owner’s Manual
Reverse Robotic Origami