Click for next page ( 9


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright Β© National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 8
~ - ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ / ~ ~ ~ ~ ~ ~ ~ ~ ~ l l / l i i 1 ~ ~ ~ ~ l l 1 , , , ~ ~ ~ ~ l~ , , \ ~ l i i ~ ~ ~ ~ l l \ ~ ~ ~ ~ - ~ - ~ ~ ~ l l - ~ ~ ~ l l he first high-resolution martian images acquired by the Mariner 4 ~ spacecraft in 1965 shattered pop- ular notions of Mars. Far from being an oasis, the surface of Mars appeared instead to be as battered and barren as the Moon. With its thin atmosphere and bitterly cold temperatures, Mars seemed more parched than the driest places on Earth. The prospect that life could have evolved there seemed dim. Each subsequent mission to Mars has changed that impression in sur- prising ways. Mariner 9 revealed ~.......................................... UNIX ,,,,,, .............. -. Of, ~~-.-.-.-.-.-.-.-........... an.. ancient towering volcanoes, extensive polar caps, and immense canyons apparently cut by water. Systematic observations of the surface and atmos- phere by Viking led to a dramatic increase in our knowledge of the breadth of martian geological history and the dynamics of the current cli- mate. The recent Mars Global Surveyor (MGS) mission has again rev- olutionized our understanding of mar- tian evolution. Far from having no magnetic field as previously believed, MGS discovered that large portions of the surface of Mars were strongly mag- netized early in martian history. Moreover, MGS has seen indications of recent or ongoing climate change, and has found small gullies with character- istics suggesting that they were recent- ly carved by fluid flow. Additionally, fundamental information has been derived from the study of martian meteorites. Detailed analysis of these samples has invigorated the debate over whether life ever existed on Mars. Despite studies to date, we still do not know fully where water exists on Mars today. There are direct observa- tions of four exposed martian water reservoirs, which include water vapor in the atmosphere, water ice in the atmosphere, seasonal water ice deposits at the surface, and perma- nent water ice deposits at the north and south poles. Of the four reser- voirs, the martian polar caps are by far the most massive. Recent MGS data sets indicate that the mass of water ice contained within the mar- tian north and south polar caps is equivalent to a global ocean some 22 to 33 meters deep. Recent observa- tions from the Mars Odyssey space- craft also suggest a patchy reservoir of water ice beneath the martian surface. At increasing depth, where the rock is warmer, liquid water may be present in pore spaces. 5..................................................... ; ............................................ $~x-:-:-::::::::::::: The Mars Pathfinder landing site (above), like much if not all of the martian surface, is drier than Earth's driest desert. But this may not always have been the case. Gullies on canyon and crater walls, such as these (top) in Sirenum Terra seen by Mars Global Surveyor, are possible evidence that water has flowed on or near to the martian surface in the geologically recent past.

OCR for page 8
- ~ - ~ :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-::::::::::::::::::::::::::::::::::::::::::::::::: .......................................... ~ ~~ · _ ~ · _· · ~ 1 ~ ~~ awn— —— ~ J to ~ ~ ~ ~ ~ ~~ se an tne—~~—n— ~ ~ ~ ~~ ~ ~ ~ ~ · ~ ~ · ~ ~ ~ ~ ·— · · _ · · · · _ ~ ~ ~ ·—~ ~ _ _ ~ ~ ~ ~ ~ _ ~ E ............................................... -.-.- -. :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:::::::::::::::::::::::::::::-:-:-:-:-:-:-:-:-:-:-:::::::::: :.:.:.:.:,:,:,:,:,:,:.:.:.:.:.:,:,:,:,:,:.:.:.:.:.:.:,:,:,:,:,:,:,:,:,:,:,:,:,:::: :2:2:2:2:2:2:2:2:2:2:2:~:~:~:~:~:~:~:~:~: :-:-:-:-:-:-:-:-:-:-:-:-:-:-:-:::::::::::::::::::::::::::::-:-:-:-:-:-:-:-:-:-:-:::::::::: _ On Earth, life is found wherever there is liquid water. On Mars, although the peak daytime surface temperature near the equator can rise above the freezing point of water, the average surface temperature is about -55°C. The surface of Mars today is cold, dry, oxidized, and exposed to an intense amount of solar ultraviolet radiation. These factors are likely to limit or even to prohibit life at or near the surface of the martian soil. The surface environment of Mars, however, may not always have been as hostile to life as it is today. The geo- logical evidence, especially in the val- ley networks, indicates that the mar- tian climate could have been apprecia- bly more hospitable to life about 3 bil- lion years ago the atmosphere appears to have been warmer and more dense, and liquid water existed on the surface. In such a climate, life could have developed, possibly leav- ing behind fossil evidence in mineral deposits created by surface water. To date, a single set of robotic stud- ies has searched directly for existing life on Mars: the Viking life-detection experiments, which were designed to test for organisms using carbon dioxide or organic molecules in a manner anal- ogous with terrestrial organisms. The results of the Viking experiments very strongly suggest that the materials test- ed were devoid of organic compounds or other signs of life, but this conclu- sion has been debated. The lack of unanimity in the scientific community highlights the difficulties inherent in the detection of microorganisms by robotic means. Indeed, even if it were generally acknowledged that the Viking experiments did not show the presence of life, the experiments could still be criticized as being overly geocentric in showing only a lack of evidence for lifeforms on or near the surface of Mars that were similar to life on Earth. The pace of Mars exploration is currently breathtaking! NASA current- ly has two spacecraft Mars Global Surveyor and Mars Odyssey operat- ing in orbit about the Red Planet, and two more the twin Mars Exploration Rovers, Spirit and Opportunity are en route. The NASA missions are At 3000 km long and up to 8 km deep, Mars's Valles Marineris canyon system dwarfs any such feature on Earth. Mosaic constructed from Viking 1 orbiter images. scheduled to be joined by a Japanese orbiter, Nozomi, and Europe's Mars Express orbiter and Beagle 2 lander. Moreover, NASA has well-defined plans to launch additional Mars mis- sions at each launch opportunity for the remainder of this decade (see the diagram on page 10~. Since Mars exploration activities for the rest of this decade are well in hand, the SSE Survey concentrated on identifying gaps in the existing pro- gram and laying the groundwork for activities in the decade beyond 2013. The SSE Survey concluded that, with our present state of knowledge and technological expertise, it is unlikely that robotic techniques will be able to conclusively prove whether there is or has been life on Mars. Results obtained from life-detection experi- ments carried out by robotic means on the martian surface can be challenged as ambiguous for the following reasons: · Results that show an absence of life may not be accepted because the experiments yielding them were too geocentric or otherwise limited; · Results consistent with, but not definitive of, the existence of life (e.g., the detection of organic compounds of unknown, either biological or non- biological, origin) may be regarded as incapable of providing a clear-cut answer; and · Results interpreted as showing the existence of life will be regarded as necessarily suspect, since they might reflect the presence of terrestrial con- taminants instead of true martian life. Definitive answers about the exis- tence of martian life will require labo- ratory analysis of Mars samples returned to Earth. Samples provide the ultimate ground truth for the wealth of data returned from tele- scopes, orbiting sensors, and in situ missions thoughout the solar system. g

OCR for page 8
NASA, together with other national and international space agencies, has detailed plans for missions (above) to follow on from the current Mars Global Surveyor and Mars Odyssey. These missions include the Mars Exploration Rovers (right), Mars Express, and Nozomi currently en route to Mars. Missions beyond 2009 are being planned. The SSE Survey recommends that NASA should spend the next 10 years preparing for a Mars Sample Return campaign near 2015. Sample return should be conducted to obtain rocks from a variety of geological settings. Moreover, to best assess Mars's potential for life, robotic techniques should be devised to collect samples from beneath the martian surface where conditions are more hos- pitable for living organisms. (For more discussion of sample-return missions, see the Technology Development section on page 25.) Faze {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~

OCR for page 8
CI! ~ ~ ~ ~ ~ ~ ~ ~ ~ _ . . r ~ ~ _ ~ · _ · _ _ · · r ~ ~ ~ - ~ - ~ - ~^ - ^t~ 8~ - - ~ - ~ 8~8 8~ - ~ ~ 8~ - r ~ ........... r ~ ~ ~ ~ ~ ~u' u ~u'~ ~u'. ~ .. ~u' ~ ~ - ' ~ - ~ ~ - - u' .~' ~ ~ ~ ~.u' - '~. ~ I r r ~ i ~ ~ ~ 1 ~ ~ ~ ~ 1 Prolile ~ I Mars SampleReturn and ~ | __ ~ Precursor Missions ~i~ .... .... . ~ . 1 _ _ Mission Type: Continuing program | of landers and orbiters leading toward I ,i. ~ . . . . . ~ sample-return missions I 92"""' 1 .... I ~ ~ Cost Class: Small, Medium, and Large ~ | .... .. .. ~ ki:.:.:,:X ,:.: :.:. 1 ~i -- ~ ~.:,:i.'; ,:.: "815~8155~. ~81~. :.:, I ~ ~ ~ ~u. ~ .e .~oaou. a.~..~. I :::: ~ :::: ~ :::: - ~ , .. .. ~ .:, ~ ,:.: :.:. 1 .. ~ .. .. ~ . ~ .. ~ 1 1 ~ 1 1 .. 1 W:i :: ~ I ~II^^T ~n~ r^T~ Irn O^I^^T^^ t%~l^~^l~t~ ~: ~ ~ tJUllblJL al IU I bLUI I I ObiblJLbU Oal I IVlbO . I :::- :: 1 :: ~ ~:.: ~_ ,:.: :.:. 1 ~ · ~ of martian soil and rock to Earth I .... ~ .. . .. ~ | ~ ~ ~ ~ .. .. 1 ... ~ .... .... I\ A ~ h h ' ~ ~1 ' ~ ' . .... ,,~ ~ ~v~easure`~'ec''emlca~anulso~oplc .... , composition of the atmosphere at I th~ ~ cf ~A~ ~ gases from the m~ddle end upper I ... - ... ; .. .. ~ ::.:: ~ -:.:< .:.: :.:. 1 ~ ,.:: - - :~ ~ ~ ~ ~ ~ ~ h ~ -~ - ~ ~ - ~ a L I I I U ~ I J I I ~ I ... - - ~x r ~,, . . ~ ~ ~ ., ~ , . . . 1 · -:4XC0CC;;W~-:-:: :-:-: :-: :-:~ ' :: :: ~ :~8~::.:.:.:.:.:::~ ~:~:.:.: .:.: :.:. 1 == =~ -~ ~- <—~—~~~——— . ~ ~nnrd ~t ~ Innr~ I ~rd c~ ~ nf . I _ ~ _ vul ~~llOL Cl IVI Iy- VbU O11rvvy vl _~ ~4 K~ Although technically challenging, collection and return of martian samples to Earth for intensive study in terrestrial laboratories | is a key scientific priority for the decade beginning in 2013. Advanced rovers may be used to collect samples. I Guiding Themes Addressed Important Planetary Science Questions Addressed | 1 _ I _ I I _~ = ;~ ~_ I ~ ~ ~r~ I ~)J ~ t’] t] t~ I _ I _ I ~ I _ I _ I =~ _ 1 _ I ~ ~ a.I'§ I'~ j4~ j`~ ~ I ~ I _ ~ ~ ~ ~_ I ~1~-3~-3~ _ I ~ , _ I ~#~ I _~ I _ 1 11 —