History

The spots in question were observed on images made by the Mars orbiter camera (MOC) on board the Mars Global Surveyor (MGS) spacecraft between 1998 and 1999 (images are credited to NASA/JPL/Malin Space Science Systems). These features appear in the southern and northern polar regions of the planet in the spring, and range in diameter from a few dozen to a few hundred metres. Malin and Edgett published their first observation on the Internet in 1999, and then in print (Malin and

Planetary Systems and the Origins of Life, eds. Ralph E. Pudritz, Paul G. Higgs, and Jonathon R. Stone. Published by Cambridge University Press. © Cambridge University Press 2007.

Fig. 13.1. DDSs on a southern dune covered with white frost, photographed by the MOC onboard MGS at Ls = 185 (1999.08.09., image no. M03-07564, M03-07565). Subset images: a. the observed crater, b. the MOC image strip, c. the dark dune d. the magnified image of the spots with high resolution. (Collegium Budapest, Mars Astrobiology Group.)

Fig. 13.1. DDSs on a southern dune covered with white frost, photographed by the MOC onboard MGS at Ls = 185 (1999.08.09., image no. M03-07564, M03-07565). Subset images: a. the observed crater, b. the MOC image strip, c. the dark dune d. the magnified image of the spots with high resolution. (Collegium Budapest, Mars Astrobiology Group.)

Edgett, 2000). One of us (A. H.) began to analyse the images in the summer of2000, based on images from the south that were clear and freely downloadable from the Internet (http://www.msss.com/moc_gallery/). It became clear that one group of these spots was strictly localized to dark dunes, clearly distinguishable from the usual rusty terrain of Mars. Thus, we coined the term dark dune spots (DDSs; Figure 13.1). Nobody can deny that the DDSs are striking or that they call for an explanation. As we shall see, the first explanation (Malin and Edgett, 2000), based on simple frosting and defrosting of the dry ice (carbon dioxide) cover, simply does not work. Noting that the majority of the spots are circular, the suspicion arose that some biological activity may also be involved in spot formation. Indeed, without a scale and the source, the images can be mistaken for pictures of a bacterial culture in a Petri dish. However, one cannot base a hypothesis on an analogy that is so much out of place and scale. Nevertheless, years of work have led to a detailed hypothesis involving biological phenomena that is consistent with all of the observed features. Conversely, we are not aware of any abiotic hypothesis that could explain the full set of observations. This, of course, does not mean that the biological hypothesis is right. Nevertheless, we feel encouraged by the fact that since 2001, when the biological hypothesis was published (Horvath et al., 2001), all new observations and data have made the hypothesis more plausible rather than less so. The main point is that the hypothesis is testable and we are sure to know the answer in the foreseeable future if astrobiological activity continues.

Perhaps the most important intellectual precedent of our hypothesis is the suggestion put forward by Lynn Rothschild (1995), that cryptic photosynthetic microbial mats are a potential basis for life on Mars. These mats can be found on Earth, sometimes under permanent dust cover. She pointed out that such a cover would be very important on Mars as protection from an aggressive environment. Needless to say, some think that proposing extant life on Mars is an extraordinary claim, requiring extraordinary evidence. We shall come back to this issue in the discussion. First, the claim may not be so extraordinary after all, but this is a matter of individual judgement. Second, we do not have extraordinary evidence yet, but we see clearly how such evidence can be obtained.

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