Two For Two

Opportunity mission engineers reported at 9:06 pm PST Saturday night that the spacecraft had landed successfully on the Martian surface.

Opportunity descended through the martian atmosphere, then bounced and rolled to a stop exactly according to plan. Three weeks earlier, Opportunity’s twin sister, Spirit, also landed flawlessly.

Unlike Spirit, however, Opportunity came to rest on its side. Although the preferred landing orientation is right-side-up – Spirit landed this way – the rovers were designed to accommodate landing on any of their four sides.

While former Vice President Al Gore and California Governor, Arnold Schwartzenegger, looked on from the Jet Propulsion Center’s mission control room, flight controllers celebrated the passage of the Opportunity rover through its critical entry, descent and landing phase.

Local time on Mars was about one-thirty in the afternoon.

See gallery of Spirit’s Sol images and slideshow.

Simple tones received in the moments following this approximately six minute phase between entry and landing showed the lander in a no-fault condition.

The lander struck the surface of Mars at approximately two to three G’s (or earth-equivalent gravitational acceleration units). The landing site is the highest altitude (from the outer atmosphere to the surface) ever attempted by NASA, and approximately a mile higher (4500 feet) than the Spirit site inside a crater and potentially ancient lake bed.

Because it lands inside a protective shell shaped like a tetrahedron (or pyramid), it stands a one-in-four chance of landing on its base-petal. This was the case for both Pathfinder and Spirit, but Opportunity arrived on its side. This state is referred to as “side-down”.

By sequencing the order in which this tetrahedral shell opens, like a flower petal, the unfolding called reverse robotic origami eventually will right the lander to its nominal base-petal down configuration to drive off later in the mission. First events to happen then after retraction of the airbags is the opening of the rover’s solar panels, which allow the batteries to recharge.

Near real-time engineering data was collected using a constellation of orbital relay stations, which concluded their support for the landing phase with a sign-off, “Welcome to Mars, Again.”

The landing site, called Meridiani Planum, is a flat plain, without hills. The region is expected to be a good engineering landing site becaues the rock coverage is less than eight percent, which makes both landing and driving feasible. Meridiani is of scientific interest mainly because it is one of two areas on Mars with unusually high concentrations of an aqueous iron-containing mineral called hematite.

Opportunity’s landing site is on the opposite side of the planet from Spirit.

Meridiani Planum was chosen as the landing site for Opportunity because the hematite was detected there in large quanities. Hematite is an iron oxide that, on Earth, typically forms in the presence of water. The mineral was detected by the Thermal Emission Spectrometer (TES) onboard NASA’s Mars Global Surveyor (MGS) orbiter.

More than one process could have formed this hematite. It might have formed as the result of hydrothermal activity in association with volcanism, in hot water. Or it might have formed as sediment carried by water flowing across the martian surface.

The instruments carried on Opportunity will enable scientists to determine which process was responsible for the hematite at Meridian Planum, and thus to learn about the history of water on Mars.

The question scientists hope to answer, says Ray Arvidson is, “Has water been on the surface for a long period of time in the past, as lakes and rain systems and rivers, or has most of the [water-related] action been on the inside, a frozen surface but [hot] water circulating in the interior?” Arvidson is the deputy principal investigator for Spirit and Opportunity.

The science team will look for several clues to help unlock the landing site’s secrets. One important clue will be what other minerals are present.

Phil Christensen, the payload element lead for Opportunity’s Mini-TES instrument puts it this way: “The hematite itself is not particularly interesting. We know it’s there; we’ve mapped it. So what? I argue that it’s a beacon that says, water was here, okay? And so now if you’re looking for the most interesting places to go land, there’s a beacon that says, hey, there was mineral evidence of water here, go there. And you look in detail and see what else is there.”

One such mineral has already been detected in Meridiani Planum. Oddly, it was detected by two of the earliest missions ever flown to Mars, Mariner 6 and 7. These spacecraft, which flew past Mars in 1969, carried infrared spectrometers, instruments similar to TES and Mini-TES.

Wendy Calvin, a member of the MER science team, recently examined the data recorded by the Mariner spacecraft and found the signature of a water-bearing mineral other than hematite. She was unable to identify it, however.

It’s not a typical silicate or carbonate, says Calvin. If it were, TES would have been able to detect it. Calvin believes it is probably a type of clay known as ferrous silicate.

See Opportunity image gallery and slideshow
Ferrous silicates are clays that, on Earth, “were formed before there was a lot of oxygen in the atmosphere,” says Calvin. “They’ve got lots and lots of iron in them.”

This is consistent with what is known about Mars’s atmosphere. “Mars doesn’t have a lot of oxygen in the atmosphere – never did. Early Earth didn’t,” either, says Calvin.

According to Christensen, Opportunity’s Mini-TES instrument should be able to identify Calvin’s mystery mineral. This will help scientists determine how Meridiani Planum’s hematite formed.