For the past year, I've been a loyal follower of the USGS public lecture series in Menlo Park. On the 4th Thursday of every month, they put on a wide variety of scientific presentations, from the exploration of Mars to the effect of climate change on western forests. I'm just amazed at the breadth of scientific endeavors at the USGS: earthquakes & volcanoes, climate change, water quality & resources, marine geology, energy resources, and the San Francisco Bay ecosystem. All this from a facility I hardly knew existed.

This month brings a special treat. Every 3 years, the USGS opens its door for an Open House on May 16 and 17th with a wide variety of programs. There will be a number of interactive exhibits for kids & adults, along with a "Ask-a-Scientist" style lectures. Did I mention the live music? Bluegrass, Jazz, even a Celtic Scientist band (who knew such things existed?).

Here are my picks for the weekend:

Mapping California Coast State Waters

Examine maps of the sea floor off Half Moon Bay, CA, and learn how such maps are being made for the entire California coast. See a camera sled — a large metal frame holding video and still cameras that are towed just above the sea floor to collect images used to "groundtruth" the mapping data. View sea-floor images collected by the camera sled.

Where is my Copper Deposit? An Underground Mine Tour

Examine the geology of a copper deposit in an underground mine and help determine where to dig next.

Gold Panning*

Learn how to pan for gold and try your hand at panning at this "pan and release" exhibit.

Natural Science Geopardy*

A unique opportunity for you to test your natural science knowledge. Play a game of Geopardy and learn science facts of Geography, Hydrology, Geology, and more.

Do soils breathe? The role of soils and CO2 in a warming world

Did you know that the microbes in soil breathe out CO2, just like people do?  Come use some scientific equipment to measure how much CO2 can released by soils and see how plants take up some of that same CO2.  Then learn more about the role of CO2 in regulating the global climate and see what makes the soils of Alaska so interesting to USGS scientists.

* Designed for kids

USGS Menlo Park Open House 10 am - 4 PM on May 16th and 17th. For more information, visit the USGS Open House website.

New instruments hook to the underwater lab.
Credit: David Fierstein © 2005 MBARI

We heard about the Monterey Bay Aquarium Research Institute's new underwater laboratory in a radio story last fall. When that story aired, the lab (known as the Monterey Accelerated Research System, or MARS) was just getting going, with lots of neat experiments planned. Now, few of those have become a reality.

In case you missed the first story, the MARS is essentially an underwater data hub, perched on the ocean floor almost 3,000 feet below the surface of Monterey Bay. A 32-mile cable connects the system to land, acting as a power cord and data link. Several "underwater extension cords" allow a variety of instruments to plug into the hub, getting power from land and sending back data via the cable. That constant connection is a big step forward in undersea science; without it, researchers have had to use boats to stay physically close to their instruments (something hard to do for very long), or have sent the instruments off on their own, relying on batteries to keep them running and collecting data.

Until late February, earthquake scientists at the UC Berkeley Seismological Laboratory had been using that second method with their seafloor seismic station, the Monterey Ocean Bottom Broadband (MOBB). "We had to wait three months to even know if the instruments were alive," said Barbara Romanowicz, the lab's director. But the MOBB is now plugged in to the MARS system, and is transmitting its information about earthquakes in real-time.

That new stream of information could be especially valuable in California, because the MOBB provides a unique view of the main fault system, the San Andreas, which runs along the Northern California coast. Most seismometers are land-based, and therefore positioned on the east side of the fault. The MOBB is on the west side of the fault, offering a helpful perspective on the fault's shifts and shakes.

The researchers hope that the MOBB's new stream of real-time data will improve their earthquake models, and perhaps eventually help provide early warnings about impending quakes (for more on that topic, see the TV story, Earthquakes: Breaking New Ground).

The MOBB is just one instrument using the MARS hub. A tool that uses sound waves to track fish is currently attached, and within the next six months you can expect to see a robotic DNA lab and a robot that crawls along the seafloor, collecting data on animals that live in the mud.

Bad for the Lab, Good for the Country

Staff at Building Solutions, a home performance
company, install PV on a roof in Oakland. Next year, the renewable
and energy efficiency business will be even better.
Credit: Kate KenkeDr. Steven Chu, Noble-prize-winning physicist, and director of Lawrence Berkeley National Laboratory, was named as President-elect Barack Obama’s nominee for Secretary of Energy. Home Energy is a nonprofit magazine, but our offices are at Lawrence Berkeley Lab and the magazine was founded by Alan Meier, a lab scientist. People around here are saddened by the loss of Dr. Chu as director of the lab, but extremely excited about his nomination as Secretary of Energy. Dr. Chu believes in science and the important place of technology in helping us meet our energy goals and fight global warming—think cellulosic bio-fuels, nanotechnology, and yet undreamed of solutions to the present energy and environmental crisis.

Weatherization Works!

Word in energy efficiency circles is that the funding for Department of Energy (DOE’s) Weatherization Program will increase several-fold with President Obama’s proposed economic stimulus package. The Weatherization Program is managed state by state from money provided by DOE, and the funds pay to retrofit the homes of low-income families. Homes become healthier to live in, more energy efficient, and more comfortable for the occupants. For every one dollar the Weatherization Program spends, almost two dollars in energy savings results. Hundreds of thousands of homes have been retrofit so far, leaving about 99.5% of existing homes. Talk about green jobs potential! Many nonprofit and for profit organizations do weatherization work, and, basically, you retrofit the home of a low-income family the same way you retrofit a mansion. Lots more skilled people will be needed to do the work, and the jobs will provide a good income, benefits, and the possibility of future advancement. Community colleges, unions, professional training organizations, online trainers, and other players are gearing up to train the new green workforce.

How Many Btu Do You Do?

I promised in my last blog entry to explain the concept of heating-degree day and cooling-degree day. Sometimes you will hear that a home uses so many Btu or kWh per heating- or cooling-degree day, per square foot, per year. The degree days indicate the heating or cooling load on a building’s HVAC systems. A degree day is the rise or fall of one degree Fahrenheit for 24 hours. The rise or fall in temperature is measured from a baseline of 65F°. For example, if the average temperature tomorrow is 45F°, than the heating load on your heating system is 20 heating-degree days. If on a hot summer day the average temperature over a 24-hour period is 85F°, than the load on your air conditioner is 20 cooling-degree days. The number of heating-degree days for a winter in New York is around 5,000. Barrow, Alaska has about 20,000.

You can figure out how much energy you use to heat or cool your home by subtracting the baseline energy use. During a month when you are using neither your air conditioner or heater, such as in October or March (called the “shoulder” months), your gas and electric use represent your baseline. The baseline covers energy for lighting, appliances, hot water, and plug loads. Subtract out the baseline from your winter or summer energy use and you have the amount of energy to heat or cool your house. If you know the square footage of your home, and you have weather data for your area (go to www.degreeday.net to find out heating-degree days and cooling-degree days for your area), you are in a position to brag to your neighbors (or not) about your energy use.

At our house we used about 90 therms of natural gas from September 7 through December 7, 2008. There were about 480 heating-degree days (HDD) in our area during that time. Our baseline use of natural gas is about 10 therms per month, for heating water and cooking, leaving 60 therms for heating over the three-month period. Our house is about 1,200 square feet (ft²). Therefore, we used 60 therms/(480 HDD x 1,200 ft²), or about 0.0001 therms/HDD·ft². Since one therm of natural gas contains about 100,000 Btu of energy, that equals about 10 Btu/HDD·ft². That’s not bad, but not great either. How about you?

Geothermal power production could significantly add to the electric power generating capacity in the United States." That's the attention-grabber at the top of a September 2008 press release from the U.S. Geological Survey announcing the release of their first geothermal resource assessment in 30 years.

When I first began researching this story for QUEST, I was surprised that I hadn't heard more about geothermal power. It's never lumped into that renewable energy laundry list that's recited by politicians and journalists alike — you know, "…solar, wind, hydroelectric and biofuels". But it turns out that geothermal energy has really great potential.

To start, it's reliable. Geothermal is base load power, which means that the plants generate power at a constant rate around the clock. In fact, geothermal plants often have capacity factors of 86-95%, well above traditional base load generation such as coal.

It's clean. Geothermal power plants give off little or no sulfur compared to fossil fuel-fired power plants and they emit no nitrogen oxides. Emissions of CO2 per megawatt-hour are extremely low or absent for the newer flash plants. A typical geothermal plant may produce 1 lbs. of CO2 per MW hour. This figure compares with 1030 lbs. per MW hour of CO2 for a natural-gas fired plant, 1600 lbs. per hour of CO2 for an oil-fired plant, and 1820 lbs. per MW hour for a low grade coal-fired plant.

And, if the USGS assessment is accurate, and it probably is, geothermal power is abundant. According to the study:

"the power generation potential from identified geothermal systems range from 3,675 MWe (95% probability) to 16,457 MWe (5% probability); the power generation potential from undiscovered geothermal systems range from 7,917 MWe (95% probability) to 73,286 MWe (5% probability); and the power generation potential from Enhanced Geothermal Systems range from 345,100 MWe (95% probability) to 727,900 MWe (5% probability)."

So, what's wrong with it? As we touched on in the TV segment, there are several little drawbacks that no doubt should be considered. These include induced seismicity (little earthquakes that are triggered by geothermal developments), the initial expense of geothermal exploration and development, and the challenges of connecting the electricity generated by a geothermal plant to the grid at a point where there is sufficient available capacity to sell the electricity.

However, I was never really able to find a strong reason why geothermal energy should not be in everyone's renewables laundry list. And considering Obama included geothermal energy in his list during his last debate against John McCain, I would imagine we will all be hearing more and more about geothermal energy development in the months to come and beyond.

Watch the Geothermal Heats Up television story report online. And don't miss the steamy, behind-the-scenes photos for this story.

At Sonoma County State Beach, just south of the mouth of the Russian River, stand two seastack rock pillars surrounded by large boulders. The prominent blue schist rocks form something like an amphitheater above the coastal cliffs.

There is something about these rocks that draws you in. Maybe it's the way they jut out of the ground? Or perhaps it's the "Stonehenge" way they form an enclosed circle? Or maybe it's just a nice place to get out of the wind? Whatever it is, they seem to pull you towards them. And once you are there, they almost call out to be touched. The rocks, long known as the "Sunset Boulders", have attracted rock climbers for years. I've climbed these rocks before. But like so many other people, I had no idea I was touching history.

During the Pleistocene, 10 to 20,000 years ago, this place was very different than it is today, inhabited by massive mega-fauna; bigger elephants, lions, bears and wolves, than we see today. While those big animals went extinct thousands of years ago, they left their mark on this place.

Looking around these rocks it is easy for me to imagine the herds of Columbian Mammoths lumbering from the nearby wallow to rub against the boulders. I can picture huge herds of camel and horse grazing nearby. Yes, those animals evolved here in North America and then crossed into Asia where they thrived and survived. Weaving my way between the boulders, I can imagine how the predators could have used these rocks as an ambush site. I envision a huge saber-tooth cat slinking between the craggy rocks, looking to pounce on an unwary bison. I can see the prides of American Lion, similar but much larger than African Lions, basking on the tabletop boulders after a big kill. I can also picture the ultimate predator making their campsite here when that first hunting party foraged deeper inland. Yes, humans were here too. And I'm sure the same pull these rocks have today existed back then.

This seems like a sacred place to me. Sacred to history. So when you visit these rocks think about those who came before you. Think about the mammoth and the bison and the camel and the horse. Think about the lions, tigers, bears and wolves. And think about those first people. Tread lightly and respect this wonderful place. With care, these rocks will be here long after we all become part of history.

Special thanks to the San Diego Natural History Museum for contributing artwork and HD video to our story. Also, to see more artistic representations of Pleistocene mega fauna, by the artists who contributed to our segment, see:

Laura Cunningham's artwork

Artwork of Joseph Venus

William Stout's wonderful murals

Watch the Ice Age Bay Area television story online.

MESSENGER's color filter imaging capability reveals variations
in color on Mercury too subtle for the human eye.
Photo credit: NASA/MESSENGERLike a snow-bird relative vacationing in warmer climate localities and sending back picture postcards of their trip, NASA's MESSENGER spacecraft has made yet another swing past our Solar System's innermost planet, Mercury. But, like the traveler who just can't seem to get enough sightseeing in, this was another whirlwind flyby set to the furious tempo of a camera snapping pics–about 1200 in all…

Did MESSENGER find anything new, since its first flyby back in January? Here are a few highlights:

• Prominent "ejecta" rays streaking out from several large craters–previously revealed only by radar imaging from Earth, now photographed for the first time.

• 30% more of Mercury's largely unexplored surface than had been revealed by the Mariner 10 flybys in the 70's and MESSEGNER's own first flyby last January (spacecraft–namely Mariner 10 and MESSENGER–have now imaged 95% of Mercury's surface).

• "Hyper-color" (my own word) imaging of surface features that reveal variations in color too subtle for the human eye to notice, providing information on soil and rock composition.

I'm a planet junkie–and Mercury has always had a special place in my imagination. One might think of Mercury as the least interesting planet, in our Solar System as well as among dozens of "exoplanet" systems yet discovered. After all, it's a small, dry, cratered, and airless lump of rock and dust, resembling for the most part Earth's Moon. Consider, however, the point of view of someone who's favorite place on Earth is dry, dusty Death Valley, and my enamorment might not come as such a surprise.

In my imagination I see towering cliffs, enormous, deep crevasses, wide, flat dusty plains, bright brights in sunlit patches and dark darks in shadow….

But it's really its differences from Earth that make Mercury such an appealing exotic vision. Being where it is, 36 million miles from the Sun (about a third the Earth-Sun distance), the sunlight striking the Mercurian landscape is six times brighter–imagine that! And not just the visible light spectrum, but all the wavelengths of light the Sun puts out are free to impact Mercury's surface, unimpeded by an atmosphere: infrared, ultraviolet, X-rays, and potent burst of gamma rays rain down intensely on the planet's plains, mountains, and craters.

Speaking of the Sun, its behavior in Mercury's skies is, to say the least, zany. Mercury orbits the Sun in about 88 days (Earth days), but rotates so slowly that a single Mercurian day (the time from one high noon to the next) is about 115 Earth days. Not only does that mean sun-up to sun-down lasts roughly a couple of months, but that Mercury's orbital motion has a greater effect than its rotation on the Sun's apparent motion through its sky. The complicated relationship between Mercury's year and its day also causes the Sun to go "retrograde" at times–that is, periodically halt its progress from one horizon to the other and temporarily go in the opposite direction.

So, our prodigal vacationer MESSENGER has its itinerary straight: a climate with the brightest, warmest sunlight, pristine landscapes, long sunny days, and big skies that perform tricks for its amusement. Now, if only there was a beach…

It's been called the most dangerous fault in the U.S. The Hayward Fault runs 40 miles, from San Pablo Bay to Fremont, through some of the most densely populated areas in the country. Every 140 years for the past two thousand the Hayward Fault has jolted the East Bay. Geologists have figured out the regular history of these quakes by carbon dating trenches along the fault. A lesser known cousin of the San Andreas the Hayward fault is a creeper. Basically, it moves, slowly, along the surface but deep inside… it's locked until tension builds up and and it slips. It appears that it is time for the fault to slip again. The last major earthquake on the Hayward fault was 1868. Scientists believe that the temblor registered 7.0 in magnitude. Hayward and San Leandro were devastated. But if the quake were to happen today, it would be a much different story.

I met Mary Lou Zoback out at the Fremont Bart station, which sits right on top of the Hayward Fault. She pointed out cracks in the parking lot from the creeping fault. Zoback is a geophysicist who worked 28 years at the U.S. Geological Survey and who has done catastrophe modeling of risky residential buildings. Her company estimates that a 6.8 quake, or bigger, on the Hayward Fault could cause a disaster on par with Hurricane Katrina, causing 168 billion dollars in damage and leaving at least 200,000 homeless.

A number of public buildings in the east bay are undergoing retrofitting to make them more structurally sound. Area hospitals have until 2013 to meet seismic safety standards. There is a state inventory of public schools prone to collapse in a major quake, but no such list exists for private schools. And retrofitting standards for risky residences are confusing. I talked with Jim Cook, of Bay Area Retrofit. He says existing codes are unclear and there really is no specific licensing for seismic home retrofitters. Cook has been fighting local governments for years to improve seismic safety standards.

Homeowners can have their home evaluated but what if you are a renter? Many apartments and condos can collapse in earthquakes because they have parking or open commercial space on the first floor making this story weak or "soft." According to the Association of Bay Area Governments Earthquake and Hazards Program, soft-story apartment buildings were responsible for about two-thirds of the 46,000 uninhabitable housing units in the 1991 Northridge earthquake. In the Bay Area, unreinforced masonry (older buildings constructed of brick, stone or cement blocks) continues to be a threat.

The thought of a big earthquake is scary enough, never mind the chaos that can happen in the aftermath. But the damage from a large earthquake has repercussions that can last for a very long time. We can still see the scars from the 1989 Loma Prieta earthquake. Downtown Santa Cruz is not yet fully rebuilt and retrofitting continues on the Bay Bridge. We can prevent a lot of damage up front by shoring up our buildings and creating a family disaster plan and an earthquake kit. The Hayward Earthquake Alliance has put together some really helpful information on how to prepare for a major quake.

Listen to the Hayward Fault Radio Report and view the recent QUEST TV segment on the fault online.

I'm not a gambling man but I suppose living in the Bay Area is a gamble in and of itself, given that the likelihood of an earthquake here of magnitude 6.7 or greater in the next 30 years is 67 percent. As our QUEST TV segment on the Hayward Fault, produced by Amy Miller, and an upcoming QUEST radio segment produced by Andrea Kissack attest, the greatest seismic risk posed to Bay Area residents is the Hayward fault, which last ruptured 150 years ago. The fact that the fault ruptures on average every 140 years, offers a sober reminder of the seismic risk that people working and residing in the East Bay face every day, including Amy and Andrea, as well as several other QUEST colleagues who reside in Berkeley and Oakland. As Mary Lou Zoback stated during the interview, a major earthquake along the Hayward fault would be economically much more catastrophic than Hurricane Katrina, coupled with the difficulty of coordinating relief services in communities like Fremont, where more than 100 languages are spoken.

So we know – or should know – the seismic risks of living in one of the most vibrant, diverse places in the U.S. Short of leaving the region, what can we do?

Well, one of the most illuminating things about working on this story for me was learning a bit about retrofitting one’s home to make it withstand the lateral and vertical forces that accompany a strong earthquake. In short, you need to build shear walls – made of reinforced plywood and shear transfer ties – and bolt them to the walls in the foundation of your house. Suprisingly, there are no official codes as to what constitutes a proper seismic retrofit of a residential unit in California, nor is there a dearth of licensed contractors who will offer quotes and purport to retrofit your home but without any standards in place, homeowners are often at a loss to evaluate the quality of the retrofit which can easily exceed ten thousand dollars, depending on the size of the home and its location. Still, homeowners can avail themselves of a few retrofit resources online, such as Plan Set A, a guideline for retrofitting one's home that has been approved by building departments of several Bay Area municipalities such as Oakland and Hayward. Also on the Association of Bay Area Government's web site is a set of schematics illustrating shear wall construction. If you are interested in retrofitting your home, you should get quotes from several contractors, consult your city's building department to inquire about permits and possibly consult a structural engineer to perform a building analysis on your home.

If you're like me, though, and don’t own a home but want to prepare for "the big one," it's imperative to get an earthquake survival kit. The sells earthquake survival kits but why not make your own, provided that it has water, first aid supplies, a flashlight, food rations and other essentials for you to survive 72 hours while waiting for emergency help. If you want to make your own kit, try the USGS, the city and county of San Francisco, or helpful suggestions from the San Francisco Chronicle and LA Times.

Living in earthquake country, it pays to be vigilant. I applaud the 1868 Hayward Earthquake Alliance, a consortium of agencies that are raising awareness of the risk posed by the Hayward fault with a series of events aimed at educating the public about the importance of preparedness, including a city-wide drill in San Francisco on October 21st, the 140th anniversary of the 1868 Hayward earthquake. We may not be able to predict when exactly the next earthquake on the Hayward fault may occur but we can start planning today to mitigate its effects.

For those who aren't familiar with the Hayward fault, check out our this link to the USGS Google Earth tour over the fault.

The 1992 'Peekskill' meteorite and its point of
impact in Peekskill, New York. Credit: "Pierre ThomasWhy is it that meteorites are brought to me for identification in clusters? I don't mean that people bring clusters of meteorites-but it seems I get calls and visits from possessors of unknown rock samples, hopeful that they are of extraterrestrial origin, in bursts. This time I got two inquiries in two days!

The first thing I tell people is that I'm not a meteorite expert, but that I have a contact who is. This rarely discourages them from wanting to bring their rocks in for a look.

The first sample was brought in by a family who said they collected the chunk of iron from Lake Tahoe. This one actually looked promising to my mostly untrained eye: a fist-sized chuck of magnetic metal, with pits and holes and an overall melted look. I took some pictures to send off to our regional expert and told the family I'd call them to let them know what he said. The response to the pictures was pretty certain: it wasn't a meteorite, but a chunk of metallic slag. I was told that this is a common mistake; that often bits of slag from old foundries or other sources are taken for meteorites.

The second sample brought to me didn't really strike me as a meteorite, by appearance. It was metallic, but not magnetic; it was pretty heavy for its size; it didn't have any obvious signs of melting, and no real pits or holes-other than one, deep, tunnel-like hole the width of a finger. It didn't appear jagged or shrapnel-like, as fragments from an exploding metallic meteorite often do. Finally, it had wide, flat facets that looked much more like the result of natural rock cleaving as pieces of Earth's crust break apart.

I went ahead and performed a density measurement on the sample. It was pretty heavy, so our sensitive balance scales wouldn't handle the load. Instead, I resorted to our "learn your weight on other planets" scale-the one that tells you how much you would weigh on the Moon, Mars, and other planets, in addition to your Earth weight. (I found this scale useful when I had a package to mail and needed to know the weight; by selecting the Moon weight of the package, I would pay only one-sixth the normal Earth rate!)

The double-fist-sized sample was 11.3 pounds, which converted to 5126 grams. Then, I selected a graduated beaker from our lab, filled it with water and submerged the sample. Reading the difference in water level with and without the sample, I measured a volume displacement of 750 cc. So, the density-mass divided by volume-turned out to be about 6.83 grams/cc. That's twice the typical density of silicate-type rocks (stone), and fairly close to that of pure iron.

I sent the owner off with my appraisal that the rock didn't present the appearance of a meteorite, and though the density was in neighborhood of that of iron, the appearance (black, inside and out) and non-magnetic nature suggested some other metal or metal-stone mixture. As always, I encouraged him to seek an expert appraisal.

Let's face it, all rocks found on Earth are ultimately of extraterrestrial origin-though what we regard as Earth rock has been on Earth for many billions of years, and shaped, reshaped, and metamorphosed by eons of weathering and geological activity. Meteorites, then, are only the newcomers….

Forward camera view from Opportunity as the rover attempts to
climb up a slope toward the wall of Victoria Crater.
Photo by NASA/MER/Opportunity.Is there life on Mars? Well, that investigation is still ongoing–but from a cybernetic perspective, the surface of Mars is literally crawling with it: in the form of robots!

Four years after their planned three-month tour of duty began, NASA’s Mars Exploration Rovers (MER) Spirit and Opportunity roll doggedly on like a pair of aged, dusty desert prospectors looking for gold. In this case the "gold" is evidence for past water on Mars, and signs of that seem to abound.

What sparked this blog for me was the announcement of the plan to send Opportunity out of the depths of Victoria Crater, the half-mile impact crater that the rover has been exploring for almost a year now. Last September, when it was decided to send Opportunity into Victoria to get a close-up view of the sedimentary rock layers exposed in the crater walls, there was a lot of talk about this expedition possibly being the rover's last–it almost sounded like the robot was being sent into its own grave, its final resting place on Mars. After all, the rover had already operated ten times longer than what it was designed for!

What did Opportunity's year-long sojourn yield? By examining the multitude of exposed sedimentary layers, it is believed that those layers were probably originally laid down by wind (not a surprise on Mars, which even today is a world of wind-blown dust: dust devils, sand dunes, planet-wide dust storms). But there are also clues written in the rocks that the layers of sediment have been modified by the action of water.

One particular thing Opportunity has discovered are rock features dubbed "fins." These fins are raised edges around rock boundaries that are rich in the mineral hematite–a mineral that often forms in the presence of water. Opportunity found hematite on Mars early in its exploration, which supports the speculation that at least that rover’s region on Mars (Meridiani Planum) may have harbored at least shallow and intermittent bodies of water in the past.

The "fins" may have been formed when water dissolved away areas of sediment and then "filled in the holes" with deposited minerals–forming a kind of "fossil" of what was once an empty space.

When I lived in Northern Arizona, I remember driving across the plains east of Flagstaff and finding long, wide ridges of what looked like sandstone, snaking across the dusty desert like enormous gopher trails. I learned that these were the fossil remnants of what were stream beds: the streams formed deposits of sand and mud in their bed, which over time hardened into sandstone and mudstone. Later, the softer surrounding soils and sands eroded away, leaving the hardened stream beds as raised ridges of rock–dry evidence in a dry desert of past liquid water action. Though this is not the same process that formed the fins on Mars, it is analogous.

But now Opportunity's mission in Victoria Crater is done, and NASA is making plans to have the robot crawl back up the slope and exit the crater at the same place it entered last September. It will continue its mission by examining "cobbles"–small, loose stones on the surrounding planes, some of which were probably ejected by meteorite impacts in Mars' distant past.

Spirit, on the other side of the planet in Gusev Crater, is also still alive, and is making ready to do a bit more roving after a Martian winter of relative inactivity. With one of its six wheels no longer functioning, Spirit will limp along and continue prospecting–next stop: some white, silica-rich material that may have formed in hot water.