Readers of Astrobiology Magazine frequently send in questions regarding stories, but no series has attracted as many inquiries as the progress of the twin rovers, Spirit and Opportunity, on Mars.

To be responsive, the questions have been assembled along with paraphrased answers culled from various mission interviews, published reports and images to illustrate the challenges and thrills of exploring our neighboring planet.

As the rovers head into the dead of martian winter, activity levels are being scaled down temporarily, so this snapshot of the mission will likely prevail until cold weather breaks in late December.

Question (Q): How long will the rovers last?

Answer (A): A mechanical breakdown can be worked around, things like a stuck wheel. An electronic glitch cannot be planned for. That kind of glitch can happen anytime and also shutdown the rover's dead. There are single points of failure in the electronics. Otherwise the dust buildup on the solar panels is the remaining limit.

The rovers are in the dead of winter now, particularly at Gusev from September to December this year. If the rovers survive the winter, then there are simulations that take the rovers out to 600-700 sols.

Q: With three successful airbag landings, should that mode be the first choice for future missions?

A: There is a mass limit to what airbags can cushion on Mars. If it is not mass-limited on the next lander, then that will likely be near the limit of what

Q: What is the chemistry of Opportunity showing?

A: The Meridiani plains stood out from orbit as a region the size of Oklahoma that was iron-rich with the often water-formed mineral, hematite. Opportunity found that concentration as predicted. One chemical surprise was very high levels of magnesium sulfates (Epsom salts).

At places, the Epsom salts compose 40% by weight, which makes some of the outcrops nearly all salt. That tells a story of water.

Q: But how much water and in what forms?

A: The particular ratios between bromine/chlorine with layered deposits supports first flowing water, and secondly evaporative pooling. What one sees with flowing water is a feature called cross-bedding in the geology, but the chemistry is mainly that chlorine will appear deeper in a layer than bromine when water evaporates. That is what happens at the Dead Sea on earth for instance.

Q: What is the geology of Opportunity showing?

A: Cross-bedding. Flowing water leads to these inclined layers, like "smiley faces" where the sand pushes forward in fits and starts, eventually cementing into layers that are not parallel to each other.

Q: What are the ages or epochs when water might have flowed at Opportunity?

A: One can only know relative, not absolute ages. One can say where there is intact stratigraphy that the deeper material is older than the top layers.

If one knew the annual cycles, one might estimate some age like tree rings of growth or sediments in a canyon, but there is no way to calibrate that cycle on Mars. So no one really has any idea of absolute ages. To get absolute ages, a sample return is needed (probably for isotope studies on sulfur or other elements).

Q: What was the uniqueness of landing in a crater (Eagle) at the Opportunity site?

A: Being 20 or so feet from bedrock on the lip of the crater. Not having to drive all across the plains to find intact stratigraphy.

Q: What are the blueberries?

A: They are thought to be concretions, or where briny water percolates through a softer rock and buds up from a nucleus into these tiny balls. These are actually smaller than blueberries you might find in a muffin, but they generally populate rock cracks and voids. As the soft rock weathers, the harder blueberries fall out and roll down a slope or are blown across the plains.

Q: Are there different shapes to the blueberries?

A: There are the round ones, mainly. But there are also twins, where a budding concretion comes out of another blueberry. And then there are striped ones, marked with a division like a croquet ball. The strip always runs parallel to the lines of the underlying crack where the blueberry is budding out from.

Q: What was unique about Bounce rock?

A: That is the only rock of any size on the Meridiani plains, perhaps for kilometers in the otherwise flat area as far as the cameras can see. It was called Bounce rock because the rover's airbags hit it and changed direction to land in Eagle crater.

The chemistry of Bounce is a near perfect overlay of the chemistry of one Mars meteorite picked up previously in Antarctica, so we went to Mars and found as expected, a Mars rock.

Q: What is the chemistry of the opposite side of the planet at Gusev Crater showing?

A: Basaltic lava fields ground to dust and blown by wind

Q: What is the geology of Gusev showing?

A: Much smoother rocks with no layering. Blocks of basalts, high in olivine which is easily decomposed if water were around. That is one mineralogical clue that water has not been as abundant at Gusev as Meridiani.

Q: Are the plains the same as the hills?

A: The hills are much older than the plains. The hills stand out as an island where the surrounding crater has eroded away from. One surprise is that the uniformly basaltic soil changes abruptly as one crosses a sharp boundary at the skirt of Columbia Hills. Maybe twenty feet can separate the lava fields from older outcrops and bedrock.

Q: Has hematite been found at Gusev, like Opportunity?

A: Not in the plains, which are nearly completely lava fields ground down to very fine dust. Near Columbia Hills, particularly the Pot of Gold rock at the base of the hills, those areas have hematite in the odd tendrils and spherical tops. This rock - Pot of Gold - is about the size of a potato with toothpick-like stems and jelly-bean-like protrusions at all angles.

Q: Does the stuck wheel on Spirit have anything to do with the trenching procedure? When five wheels are locked and the the sixth wheels digs down about half a foot into the soil.

A: No. Trenching is done with the front left wheel, while the stuck wheel is the front right. Spirit is now backing up Columbia Hills to compensate for the stuck wheel that is showing higher than expected currents and thus needs lubrication.

Q: What does trenching show about Gusev?

A: Nearer to Columbia Hills, one trench showed that beneath the basaltic top layer, about 5 cm, was a very rich Epsom salt layer. This is about 10-15% magnesium sulfate.

Unlike Opportunity's site, where water may have flowed, this Epsom salt layer is probably percolated through soil by capillary action, then the water evaporates. Gusev may have been moist at one time, but not drenched like Meridiani.

Q: Can the rovers look up in the sky and pick off the spectrum of methane, which seems to be observed from orbit by Mars Express and may be attributed to some decaying matter?

A: No. The rovers would need about five times greater resolution to see methane in the atmospheric spectrum

Q: What non-science images have been impressive?

A: The first solar eclipse seen from the surface of another planet. The first meteor seen streaking across the sky of another planet. Seeing earth from Mars as a tiny dot. Those three images come to the top of the non-science list.

The non-engineering image list is probably spare parts from the rover's descent, like the backshell, parachute and heat shield. And of course the landing pod itself is interesting when viewed looking back from the mobile rover.

Q: It was estimated before the mission that one day in three might be wasted logistically, and require some science re-doing. What has the actual ratio of unproductive-to-productive days on the surface?

A: About ten times better than the initial estimate. Only one sol in thirty has really been unproductive or required repeating. That is outside of when the Spirit firmware problem (sol 18) arose, which took the Spirit rover down to troubleshoot for about ten days straight.

Q: What lines of evidence contribute overall to the story of martian water?

A: First is the chemistry: hematite, salts, particularly the evaporative deposition of chlorine in deeper layers than bromine, which matches what one sees at the Dead Sea for instance.

This happens because chlorine is less soluble than bromine, and so during evaporation, the bromine will deposit last (on top). Secondly the geology and imagery shows evidence for cross-bedding, round balls or concretions, layering sedimentary rocks, and also newer evidence of polygonal cracking that may indicate freeze-thaw cycles that contract the deposits.

The rovers were designed to look at the geology and chemistry together and that has delivered the story of water history on Mars.

Q: How much more efficient is a trained geologist compared to a robotic one?

A: The rover team did a field test on Earth with an early model called FIDO. When trained geologists walked into the terrain, they could accomplish in ten minutes what it would take a day for a rover to do.

The basic tasks like finding an interesting rock, breaking it open, and examining it under a geologist's microscope. That man vs. machine comparison is excluding the specialized instruments, since a field investigator doesn't have a pocket Mossbauer spectrometer typically in the field, which these rovers do have. But the robots are much cheaper in this harsh environment.

Q: What are the four items on the next rover's wishlist, such as instruments or capabilities that were missing from the rover design?

A: A non-solar power source, more dowload bandwidth, more on-board navigational autonomy, and a Raman spectrometer. That latter is to cover a spectral band in the infrared that is not covered today by the thermal imagery (mini-TES) of rock and soil temperatures from the cameras.

Q: What is the bandwidth problem about?

A: The pancams can pump out far more data (ten to twenty times) what can actually be downloaded to Earth. The mission planners have had to select the pictures that they think offer the most reward for the bandwidth costs.

Q: Aren't there satellites overhead to store and transmit more images?

A: The mission team gets 100-200 megabits from the satellite constellation (Surveyor, Odyssey and to some extent, Express). But they are fundamentally science orbiters. They fly circular orbits to map as close to the surface as possible.

So from the rover viewpoint, the satellites fly over once a day for 15 minutes. What would help the download problem is a high orbit that is stationary overhead for long periods.

Q: How has it been living and working on Mars' time?

A: Mars' days are 39 minutes longer than the terrestrial 24 hours. So during the first four months, everyone - both engineers and scientists--lived on a rotating day. It was tolerable for the science team which was largely away from family and normal home lives. Most of them slept with blacked out windows and did not have to go to PTA meetings or drive children to school.

For the engineers, most of whom were living at home in Pasedena, those four months were difficult.

That is why on September first, the mission went to distributed science. So the Cornell team will run their operations from Ithaca, NY, not Pasedena.

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