April 12, 2004 | David F. Coppedge

Mars Rovers Continue to Surprise Scientists

The Mars Exploration Rovers are still going strong, with many sols ahead for RATting rocks and rolling the plains [RAT, v., to use the Rock Abrasion Tool; sol, n., a Martian day].  The navigators are happy to be back on Earth time, and are poised for more thrilling discoveries as they enter the extended mission phase with no hardware or software problems.  But the science results are already puzzling to planetary scientists.
    Richard Kerr reported in the April 9 issue of Science on a gathering of more than 1000 researchers at the Lunar and Planetary Science Conference at Houston last week.  Mars exploration was the centerpiece of the show.1  Scientists were amazed to find a kind of desert varnish on many of the rocks examined by rover Spirit, indicating a possible moistening of the surface that must have occurred relatively recently and did not require much time.  (Desert varnish, a process that coats rock walls, is still poorly understood on earth.)  As announced weeks ago, the Meridiani site being explored by Opportunity seems to have been soaking in water in the past.  But another surprise, this one disappointing to some, is that Spirit did not find the expected evidence of lake deposits in Gusev Crater.2  In fact, the area appears quite dry; the presence of olivine in some of the rocks rules out any soaking of the rocks scoured by Spirit.  If a river ever flowed down the channel and flooded this crater, the deposits were evidently later buried in dry volcanic debris.
    Evidence for past liquid water on Mars still seems contradictory; there seems to be a dichotomy in the data between evidence for warm and wet vs. cold and dry.  A surprising image came from the European orbiter Mars Express3: alluvial fans in “a small area that appears to have been scoured by torrential rainstorms during the planet’s latter days of extreme cold and ice.”  It looks like a postcard from the Mojave desert in California, with “gullies on steroids,” Kerr writes.  “The problem is, Mars is not the Mojave Desert, not now and presumably not in the past few billion years since Mars entered its extremely cold and dry later years.”
    Spirit is now headed on an epic two-month rocky roll to the Columbia Hills, where scientists hope to find more clues to Mars’ past.  Opportunity is on a vast, crusty plain of soil with very few rocks to examine, but lots of room to rove.  The MER Website is loaded with interesting facts, anecdotes, pictures and animations, including a jazzy time-lapse composite of Spirit’s 90 sols of travel and scientific investigation compressed into 90 seconds, demonstrating how much work the rovers have already accomplished.  Watching it makes you feel like you’re along for a thrill ride.
Update 04/15/2004: A news item in the Apr. 15 issue of Nature4 emphasizes the implications of finding sulfates instead of carbonates in the rocks at Meridiani.  It means that any liquid ocean would have had sulfuric acid (at least 0.1%) that prevented the precipitation of CO2 into the rocks.  (The source of the sulfur might have been sulfur dioxide, SO2, from volcanos.)  “Once the abundance of SO2 dropped below the critical level to suppress carbonate formation,” Jeffrey M. Moore writes, “the atmosphere would have rapidly collapsed to near its present size, leaving carbonates very little time to form as layered marine deposits.”  The “blueberries” of hematite, he thinks, formed at the boiling point of water (that is, at the atmospheric pressure at earth sea level).  For pictures of the blueberries, see the MER Opportunity Press Release Images from March 18 and March 26.

1Richard Kerr, “Mars Rock Crud Gets in the Way,” Science, Vol 304, Issue 5668, 196-197, 9 April 2004, [DOI: 10.1126/science.304.5668.196b].
2Richard Kerr, “Spirit Coming Up Dry at Gusev,” Science, Vol 304, Issue 5668, 197, 9 April 2004, [DOI: 10.1126/science.304.5668.197].
3Richard Kerr, “’Mind-Boggling’ Martian Gullies Raise Climate Conundrum,” Science, Vol 304, Issue 5668, 196, 9 April 2004, [DOI: 10.1126/science.304.5668.196a].
4Jeffrey M. Moore, “Mars: Blueberry fields for ever,” Nature 428, 711 – 712 (15 April 2004); doi:10.1038/428711a.

It’s too early to draw conclusions.  The science papers will follow in about a year or two.  For now, let’s enjoy this rare Opportunity to share in the Spirit of discovery.
    Geologists looking for vast ages in the data cannot resist weaving their tales already, though.  Jeffrey Moore takes a little data and stretches it into millions of years: “The data from the Thermal Emission Spectrometer also hint that the sulphates formed before much of the present-day surface of the ancient southern highlands was carved out.  The specular haematite deposits at Terra Meridiani probably formed at a temperature of about 100 �C, converted from goethite.  This conversion temperature implies that the blueberries on the lake beds of Terra Meridiani were once buried to a depth of more than a kilometre [was he there?]  There they might have lain for aeons, before the rock layers above were eroded away, revealing a field of blueberries that are too large to be transported or destroyed by the wind.  The blueberry fields may have existed on the martian surface for millions of years.”  Then again, they may have not.  Hop into your time machine and get us some observations.

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