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.

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.

Victoria Crater on Mars, similar in size to the crater the
near-Mars asteroid 2007 WD 5 would have produced.
Credit: NASA/Mars Reconnaissance Orbiter

The possibility that a sizable asteroid would strike the planet Mars on January 30th temporarily raised the excitement level in the astronomical community to a pretty high level in the last couple of months. We were even toying with the idea of having a 3:00 AM Mars Bashing Party at Chabot that morning.

At one point astronomers had given odds of 1 in 25 that asteroid 2007 WD 5, newly discovered in November, would collide with Mars–which are astronomically great odds for this sort of thing. Alas, further observations refined our knowledge of the big rock’s trajectory, and the probability declined, hitting rock bottom (0.0%) by January 9th.

Why blog about a non-event? I see it as an opportunity to talk about big rocks bashing planets in general–specifically, the Earth.

While we haven't witnessed an event like this one (a big impact on a solid, Earth-like planet), we have examined the remains of past events, on Earth as well as other planets and moons—such as the hole in the Arizona desert called "Meteor Crater," an impact basin roughly the size of what might have been gouged out on Mars by 2007 WD 5. And compared to the asteroid that is believed to have caused the extinction of the dinosaurs, the Meteor Crater impact was a pipsqueak!

Smaller objects hit the Earth, or its atmosphere, all the time: meteors and meteorites. Fortunately we haven’t experienced a larger impact for a very long time. There was a significant impact of some kind in 1908, over Siberia–but luckily that wasn't a major catastrophe.

Nevertheless, the possibility of a big impact on Earth is something to take seriously. NASA certainly does. They even have a program for it: the Near Earth Object Program, whose goal is to detect and track Near Earth Objects (NEOs) in order to warn of those that might eventually collide with the Earth. A NEO is defined as an asteroid or comet whose orbit carries it close to Earth. The program searches for NEOs that are 1 kilometer in size or larger–objects that would cause catastrophic local devastation and "severe global consequences."

Thus far, over 5,000 NEOs have been found, almost 800 of them 1 kilometer across or larger–and it is expected that there are plenty more out there that we haven't found.

So, is this a good idea? Do we really want to know that the end of the world is going to occur on such and such a date in the near future–or would it be better not to know, living our daily lives in blissful ignorance right up to the last, Earth-shattering day?

Well, whatever your philosophical approach to that question might be, there is a practical side to the NEO Program. If we can predict a NEO collision with enough advance warning, there may be something we can do to avert disaster. For example, we could send Bruce Willis out to destroy it… .

Seriously, though, NASA is working on methods of diverting the course of a NEO, possibly with a spaceship that acts as a sort of tug boat, gently nudging the NEO off course far enough in advance of the impact to make it eventually miss the Earth.

This month, however, Mars 1, asteroid 0. The Martians are quite relieved…

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.

latitude: 37.8768, longitude: -122.251

Picture of the edge of Victoria Crater superimposed with
image of the rover Opportunity.
Credit: NASA/JPL

Mars reaches "opposition" on December 24th. This is the time when Earth crosses directly between the Sun and Mars–in other words, when Mars is at the opposite end of the sky from the Sun and at its closest distance from Earth–this time about 55 million miles. You can see Mars yourself in the evening hours if you face east and look high: it's that steady, bright, orange dot right between Gemini and Taurus.

So what's been happening on Mars, exploration-wise? Here's a quick summary on that score:

NASA's Mars Exploration Rovers, Spirit and Opportunity, have had their tours of duty extended a fifth time, which should keep the rovers going–their health willing–possibly through 2009. Having landed on Mars in January of 2004 for a nominal 90 day mission, the robot pair has now lasted almost four years.

Spirit, which landed in the huge Gusev Crater, has traveled four and a half miles from its landing point and is now exploring a range of hills on a volcanic plateau. Probably topping the list of scientific evidence it has turned up is that water, in some form, has altered the chemistry in the environment, sometime in the past.

Opportunity, on the opposite side of the planet from Spirit, is currently exploring the half-mile-wide Victoria Crater. Exposed rock layers in the walls of the crater are expected to be an excellent "book" of Mars' geologic history for Opportunity's various instruments to read.

In its more than seven mile journey, Opportunity has revealed even stronger evidence that Mars' distant past may have been warmer and wetter, and that, at least in Opportunity's neck of the woods (Meridiani Planum), there may have been extended periods with liquid surface water.

The Mars Reconnaissance Orbiter spacecraft, with its array of instruments and super-powerful camera, has produced the most discerning orbital imagery of Mars' surface to date, giving us aerial views of the Martian deserts, canyons, ice caps, plateaus, volcanoes, craters, drainage channels, sand dunes, and so on, that look like they could have been taken from the window of a small airplane flying at very low altitude.

Even as Spirit and Opportunity send back postcard after postcard from the ground, like a pair of camera-happy tourists, that tantalize us with evidence of possible lakes, seas, and oceans in Mars' past, Mars Reconnaissance Orbiter with its more global viewpoint has revealed evidence that suggest another possibility: that the apparently periodic "bursts" of water activity might have been the work of large meteoroid impacts blasting through layers of ice and creating temporary episodes of water melt…

To round out the role-call, NASA's 2001 Mars Odyssey and Europe's Mars Express orbiters are also still in business and contributing to our already huge–but nowhere near complete–body of knowledge of that wandering orange dot in the sky…

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.

latitude: 37.8148, longitude: -122.178