On the sedimentary rocks of early Mars
The term "early Mars" refers to the first 600 to 1000 million years after the planet first formed, and corresponds to the time of intense impact cratering in the Solar System and the emergence of life on Earth. Martian geochronology is divided into 3 periods: the Noachian period, the earliest, is defined as the period dominated by heavy impact cratering and widespread degradation of the cratered terrains; the Hesperian period, generally thought to be a short transitional time as the impact rate and other geomorphic processes tapered off and major ridged plains formed to cover considerable tracts of cratered terrain; and the Amazonian period, the recent period that includes Mars as it is seen today. The absolute ages of these periods are not known, but attempts have been made to estimate the ages by assuming that the Martian cratering rate is some function of the lunar cratering rate. In the present context, the Noachian period is taken as the period prior to 3.5 billion years ago, and the Amazonian period is taken to be the period up to the present from the time-frame of approximately 3.5 to 1.8 billion years ago. The Mars Global Surveyor spacecraft was launched in November 1996 and achieved orbit about Mars in September 1997. The spacecraft carries the Mars Orbiter Camera, which consists of three cameras: a narrow-angle system that obtains high spatial resolution images (1.5 to 12 meters per pixel), and red and blue wide-angle cameras to acquire regional and global views (0.24 to 7.5 kilometers per pixel).
The term "subaerial processes" refers to processes "under the atmosphere"; the term "eolian (aeolian) processes" refers to processes involving wind; the term "lacustrine" refers to lakes. ... ... M.C. Malin and K.S. Edgett (Malin Space Science Systems, US) present an analysis of apparent sedimentary rocks of early Mars, the analysis based on photographs obtained by the Mars Orbiter Camera. The authors make the following points:
- The authors point out that one of the primary questions concerning the Noachian period is whether it was warmer and wetter than the cold and arid Mars we see today, such that liquid water could persist on the surface of Mars for thousands to millions of years. Similarly, a key question regarding the Amazonian period is whether there were climate excursions after the present cold and dry conditions were established, climate excursions that again allowed liquid water to persist on the surface long enough for lakes or seas to have occupied certain geological chasms and many impact craters around the surface of the planet. Speculative affirmative answers to both of these questions are widely cited to support the idea that Mars may have had conditions favorable to the development and persistence of life.
- The authors present geologic evidence that the above two hypotheses are linked, that the materials in craters in chasms considered for 20 years to be Amazonian in age were instead formed in the Noachian period, that there are many more outcrops of these materials than previously known, that they could indeed represent sediment deposited in lakes, and that they are a small part of a substantially more complex and previously unanticipated Martian history.
- Specifically, the authors present evidence that layered and massive outcrops on Mars, some as thick as 4 kilometers, display the geomorphic attibutes and stratigraphic relations of sedimentary rock. Repeated beds in some locations imply a dynamic depositional environment during early Martian history. Subaerial (e.g., eolian, impact, and volcaniclastic) and subaqueous processes may have contributed to the formation of the layers. The apparent affinity of these layers for impact craters suggests dominance of lacustrine deposition; alternatively, the materials may have been deposited in a dry subaerial setting in which atmospheric density and variations of atmospheric density mimicked a subaqueous depositional environment. The source regions and transport paths for the material have apparently not been preserved.
- The authors conclude: "When applied to the two questions posed at the outset of this research article, our results show no evidence for climate excursions in the Amazonian but do provide evidence that can be used to support the contention that Mars in the Noachian was warm enough to be wet enough to sustain bodies of liquid water on its surface. But [our results] can also be used to argue that Mars was very different from any of our
M.C. Malin and K.S. Edgett: Sedimentary rocks of early Mars. (Science 8 Dec 00 290:1927)
QY: Michael C. Malin: Malin Space Science Systems, PO Box 910148, San Diego, CA 92191-0148 US.
Summary by SCIENCE-WEEK 22Dec00
For more information: http://scienceweek.com/swfr.htm
NEW EVIDENCE FOR RECENT UNDERGROUND WATER ON MARS
Life as we know it requires water, so the presence or absence of water on a planet or other astronomical body is an important issue. As our nearest neighbor, Mars is of most interest, and sooner or later Mars will be explored, will be visited by astronauts and researchers who will be able to examine the surface of Mars as the surface of the Earth has been examined. Meanwhile, our major geological studies of Mars are photographic, and as the photographic technology improves, new information concerning Mars continues to become available. ... ... M.C. Malin and K.S. Edgett (Malin Space Science Systems, US) report a high-resolution (2 to 8 meters/pixel) analysis of more than 20,000 images relayed by the Mars Orbiter Camera since 1997. The authors make the following points:
- Mars is now a desert world on which liquid water, because of ambient conditions, is not likely to be found at the surface: average temperatures are below 273 degrees kelvin and atmospheric pressures are at or below water's triple-point vapor pressure of 6.1 millibars. However, in 1972 the Mariner 9 orbiter mission photographed evidence -- in the form of apparent giant flood channels and arborized networks of small valleys -- that liquid water might have been stable in the surface environment at some time in the past. Analysis of Mars 4 and Mars 5 data, Viking orbiter images (1976-1980), and observations of flood terrain by Mars Pathfinder in 1997 supported this conclusion.
- The Mars Global Surveyor orbiter reached the planet in 1997, and one of the most important early results of the Mars Orbiter Camera investigation was the absence of evidence for precipitation-fed overland flow of water. For example, there are no contributory rills, gullies, and/or small channels associated with the Martian valley networks. Whatever the explanation for the absence of these features, the possibility that liquid water flowed across the Martian surface in a sizable volume for an extended period of time, and especially in the recent past, now seems quite remote.
- The authors, however, report that relatively young landforms on Mars, seen in high-resolution images acquired by the Mars Global Surveyor Mars Orbiter Camera since March 1999, suggest the presence of sources of liquid water at shallow depths beneath the Martian surface. Found at middle and high Martian latitudes (particularly in the southern hemisphere), gullies within the walls of a very small number of impact craters, south polar pits, and two of the larger Martian valleys display geomorphic features that can be explained by processes associated with ground-water seepage and surface runoff. The relative youth of the landforms is indicated by the superposition of the gullies on otherwise geologically young surfaces, and by the absence of superimposed landforms or cross-cutting features, including impact craters, small polygons, and wind-formed (eolian) dunes. The limited size and geographic distribution of the features argue for constrained source reservoirs.
- The authors conclude: "Although the available evidence suggests that the processes that created these landforms acted in the relatively recent past and could even be contemporary, the absence of old, degraded, or cratered examples remains a mystery."
M.C. Malin and K.S. Edgett: Evidence for recent groundwater seepage and surface runoff on Mars.
(Science 30 Jun 00 288:2330)
QY: Michael C. Malin, Malin Space Science Systems, PO Box 910148, San Diego, CA 92191-0148 US.
Summary by SCIENCE-WEEK 18Aug00
IN FOCUS: ON THE SEARCH FOR LIFE ON MARS
"There will be people on Mars long before the end of the twenty- first century. It's inevitable, and irresistible. It might happen before 2020. It could happen by 2011. Mars is our next frontier. The plans are being laid now, the missions designed. The technology exists. The latter-day equivalents of Magellan, Columbus and Cook, and the other explorers of the age of European expansion, are preparing themselves. The motives are many, as always, but central for scientists is the search for life, or former life. At stake is the issue, are we alone in the Universe?... The first people on Mars will find the sky pink from suspended bright red dust. The sunrises and sunsets may be even more beautiful than ours, though they might resemble those that follow wildfires in the Australian bush. Just before dawn on some nights there will be water-ice clouds, high in the sky; these will dissipate shortly after sunrise. The ground will be dry and barren, and strewn with grey rocks. Instead of soil there will be red dust. On many days, light winds will ripple the ground, and during the northern winter, great storms may develop, enveloping the planet in dust. In summer, temperatures near the equator will rise as high as 17 degrees centigrade during the day, but at night will plummet to minus 80 degrees, or less. Perhaps somewhere, on the flank of a volcano, or deep in Valles Marineris, our explorers will find a place where the ground is damp, maybe in a fresh landslip, in the heat of summer. On that summer's day, in that Martian spring, they might just find the microbes that will show that Earth is not the only inhabited planet. And you and I could still be alive to contemplate that moment."
Malcolm Walter: _The Search for Life on Mars_(Perseus Books, Cambridge MA 1999, p. 1 and 154)
[Malcolm Walter is at the University of Sydney (AU), and is associated with the US National Aeronautics and Space Agency (NASA) program directed to searching for life on Mars.]