Editing Viking 2

{{Short description|Space orbiter and lander sent to Mars}}
{{Use American English|date=May 2024}}
{{about|the mission to Mars|other uses|Viking Two (disambiguation){{!}}Viking Two}}
{{Use mdy dates|date=November 2022}}
{{italic title}}
{{Infobox spaceflight
| name                  = ''Viking 2''

<!--image of the spacecraft/mission-->
| image                 = Viking spacecraft.jpg
| image_caption         = Lander shell (top) and orbiter
| image_size            = 300px

<!--Basic details-->
| mission_type          = [[Mars]] Orbiter/lander
| operator              = [[NASA]]
| COSPAR_ID             = {{plainlist|
* Orbiter: {{COSPAR|1975-083A}}
* Lander: {{COSPAR|1975-083C}}
}}
| SATCAT                = {{plainlist|
* Orbiter: 8199
* Lander: 9408
}}
| website               = [http://nssdc.gsfc.nasa.gov/planetary/viking.html Viking Project Information]
| mission_duration      = Orbiter: {{age in days|1975|09|09|1978|07|25}}&nbsp;days&nbsp; ({{age in sols|1975|09|09|1978|07|25}}&nbsp;sol)<ref name="NASA-20061218" /><br />Lander: {{age in days|1976|09|03|1980|04|11}}&nbsp;days&nbsp; ({{age in sols|1976|09|03|1980|04|11}}&nbsp;sol)<ref name="NASA-20061218" /><br />Launch to last contact: 1,676 days

<!--Spacecraft properties-->
| manufacturer          = Orbiter: [[Jet Propulsion Laboratory|JPL]]<br />Lander: [[Martin Marietta]]
| launch_mass           = 3,530 kg{{efn|"fully fueled orbiter-lander pair"<ref name="SSDC" />}}
| landing_mass          = <!--Mass after landing (recovered spacecraft only)-->
| dry_mass              = Orbiter: {{convert|883|kg|abbr=on}}<br />Lander: {{convert|572|kg|abbr=on}}<ref name="SSDC">{{NSSDC|id=1975-083C|title=Viking 2 Lander}}</ref>
| dimensions            = <!--body dimensions and solar array span-->
| power                 = Orbiter: 620&nbsp;W<br />Lander: 70&nbsp;W

<!--Launch details-->
| launch_date           = {{start date|1975|09|09|18|39|7=Z}}<ref name="NASA-20061218" /><ref name="NASA-Viking2">{{cite web |last=Nelson |first=Jon |title=Viking 2 |url=http://www.jpl.nasa.gov/missions/viking-2/ |work=[[NASA]] |access-date=February 2, 2014 }}</ref>
| launch_rocket         = [[Titan IIIE]]
| launch_site           = [[Cape Canaveral Air Force Station|Cape Canaveral]] [[Cape Canaveral Air Force Station Space Launch Complex 41|LC-41]]
| launch_contractor     = [[Martin Marietta]]

<!--end of mission-->
| disposal_type         = <!--Whether the spacecraft was deorbited, decommissioned, placed in a graveyard orbit, etc-->
| last_contact          = {{end date|1980|04|12}}<ref>[https://solarsystem.nasa.gov/resources/1060/beyond-earth-a-chronicle-of-deep-space-exploration/ NASA.gov]</ref>
<!--
    The following template should be used for ONE of the three above fields "end_of_mission", "decay" or "landing" if the spacecraft is no longer operational.
    If it landed intact, use it for the landing time, otherwise for the date it ceased operations, or the decay date if it was still operational when it re-entered.
    {{end date|YYYY|MM|DD|hh|mm|ss|TZ=Z}} (for Zulu/UTC) or {{end date|YYYY|MM|DD}} (if time unknown) 
-->

<!--orbit parameters-->
<!--as science-related articles, SI units should be the principal units of measurement, however, we usually use {{convert}} to display imperial units in parentheses after the initial values-->
| orbit_reference       = [[Areocentric orbit|Areocentric]]
| orbit_eccentricity    = 0.81630
| orbit_periapsis       = {{cvt|300|km|mi}}
| orbit_apoapsis        = {{cvt|33176|km|mi}}
| orbit_inclination     = 80.5°
| orbit_period          = 24.08 hours
| orbit_epoch           = July 24, 1980
| apsis                 = areion

|interplanetary         = <!--Infobox spaceflight/IP can be called multiple times for missions with multiple targets or combined orbiter/lander missions, etc-->
 {{Infobox spaceflight/IP
   |type                = orbiter
   |object              = [[Mars]]
   |orbits              = <!--number of orbits completed at target body (if applicable and known)-->
   |component           = ''Viking 2'' Orbiter
   |arrival_date        = August 7, 1976{{r|NASA-20061218|NASA-Viking2}}
 }}

 {{Infobox spaceflight/IP
   |type                = lander
   |object              = [[Mars]]
   |component           = ''Viking 2'' Lander
   |arrival_date        = September 3, 1976<br />22:37:50 [[UTC]]&nbsp;([[Timekeeping on Mars|MSD]] 36500 00:13 [[Airy Mean Time|AMT]])<ref name=NASA-20061218/>
   |departure_date      = <!--Date of leaving orbit for orbiters, date of launch for landers which took off again-->
   |location            = {{Coord|47.64|N|225.71|W|globe:Mars|name=Viking 2 lander}}<ref name=NASA-20061218/>
 }}

<!--transponder parameters-->
| trans_band            = <!--Transponder frequency bands-->
| trans_frequency       = <!--specific frequencies-->
| trans_bandwidth       = <!--bandwidth-->
| trans_capacity        = <!--capacity of the transponders-->
| trans_coverage        = <!--area covered-->
| trans_TWTA            = <!--TWTA output power-->
| trans_EIRP            = <!--equivalent isotropic power-->
| trans_HPBW            = <!--half-power beam width-->

<!--Only use where a spacecraft/mission is part of a clear program of sequential missions. 
If in doubt, leave it out-->
| programme             = '''[[Large Strategic Science Missions]]'''<br><small>''Planetary Science Division''</small>
| previous_mission      = [[Viking 1]]
| next_mission          = [[Voyager 2]]
| programme2            = '''[[Viking program]]'''

<!--mission insignia or patch-->
| insignia              = Project Viking Logo - Patch Style 1974-L-01916.jpg
}}

The '''''Viking 2''''' mission was part of the American [[Viking program]] to [[Mars]], and consisted of an orbiter and a lander essentially identical to that of the ''[[Viking 1]]'' mission.<ref name="NASA-20061218">{{cite web |last=Williams |first=David R. |title=Viking Mission to Mars |url=http://nssdc.gsfc.nasa.gov/planetary/viking.html |date=December 18, 2006 |work=[[NASA]] |access-date=February 2, 2014 }}</ref> ''Viking 2'' was operational on Mars for {{age in sols|1976|09|03|1980|04|11|format=commas}} [[Timekeeping on Mars#Sols|sols]] ({{age in days|1976|09|03|1980|04|11|format=commas}} [[days]]; <SMALL>''{{age in years and days|3 September 1976|11 April 1980|format=commas}}''</SMALL>). The ''Viking 2'' lander operated on the surface for {{age in days|1976|09|03|1980|04|11|format=commas}} days, or {{age in sols|1976|09|03|1980|04|11|format=commas}} sols, and was turned off on April 12, 1980, when its batteries eventually failed. The orbiter worked until July 25, 1978,<ref name="NASA-20061218" /> returning almost 16,000 images in 706 orbits around Mars.<ref>{{NSSDC|id=1975-083A|access-date=August 16, 2019|title=Viking 2 Orbiter}}</ref>

==Mission profile==
The craft was launched on September 9, 1975. Following launch using a [[Titan IIIE|Titan]]/[[Centaur (rocket stage)|Centaur]] launch vehicle and a 333-day cruise to Mars, the ''Viking 2'' [[Orbiter]] began returning global images of Mars prior to orbit insertion. The orbiter was inserted into a 1,500 x 33,000&nbsp;km, 24.6 h [[Mars orbit]] on August 7, 1976, and trimmed to a 27.3 h site certification orbit with a [[Apsis|periapsis]] of 1,499&nbsp;km and an inclination of 55.2 degrees on August 9. The orbiter then began taking photographs of candidate landing sites, which were used to select the final landing site.<ref name="nasa-solar-system" />

The lander separated from the orbiter on September 3, 1976, at 22:37:50 [[Universal Time|UT]] and landed at [[Utopia Planitia]].<ref>{{Cite web |title=Viking 1 and 2, NASA's first Mars landers |url=https://www.planetary.org/space-missions/viking |access-date=2024-06-14 |website=The Planetary Society |language=en}}</ref> The normal procedure called for the structure connecting the orbiter and lander (the bioshield) to be ejected after separation. However, due to problems with the separation process, the bioshield remained attached to the orbiter. The orbit inclination was raised to 75 degrees on September 30, 1976.<ref>{{Cite web |title=Viking 2 |url=https://www.jpl.nasa.gov/missions/viking-2/ |access-date=2024-10-09 |website=NASA Jet Propulsion Laboratory (JPL) |language=en-US}}</ref>

===Orbiter===
The orbiter's primary mission ended on October 5, 1976, at the beginning of [[solar conjunction]]. The extended mission commenced on December 14, 1976, after the solar conjunction. On December 20, 1976, the periapsis was lowered to 778&nbsp;km, and the inclination raised to 80 degrees.

Operations included close approaches to [[Deimos (moon)|Deimos]] in October 1977, and the periapsis was lowered to 300&nbsp;km and the period changed to 24 hours on October 23, 1977. The orbiter developed a leak in its propulsion system that vented its [[Spacecraft attitude control|attitude control]] gas. It was placed in a 302 × 33,176&nbsp;km orbit and turned off on July 25, 1978, after returning almost 16,000 images in about 700–706 orbits around Mars.<ref>{{Cite web |title=Viking 2 - NASA Science |url=https://science.nasa.gov/mission/viking-2/ |access-date=2024-10-09 |website=science.nasa.gov |language=en-US}}</ref>

===Lander===
[[Image:NASM-A19790215000-NASM2016-02690.jpg|thumb|left|Proof test article of the Viking Mars Lander]]
The lander and its [[aeroshell]] separated from the orbiter on September 3, 1976, at 19:39:59 UT. At the time of separation, the lander was orbiting at about 4&nbsp;km/s. After separation, rockets fired to begin lander [[deorbit]]. After a few hours, at about 300&nbsp;km attitude, the lander was reoriented for entry. The aeroshell with its ablative [[heat shield]] slowed the craft as it plunged through the atmosphere.
[[File:PSP 001501 2280 RED VL-2 lander.png|left|thumb|Photo of the Viking 2 lander taken by the Mars Reconnaissance Orbiter in 2006]]
The ''Viking 2'' lander touched down about 200&nbsp;km west of the crater Mie in [[Utopia Planitia]] at {{Coord|48.269|N|225.990|W|globe:mars_type:landmark_dim:100|display=inline,title}} at an altitude of -4.23&nbsp;km relative to a reference ellipsoid with an equatorial radius of 3,397.2&nbsp;km and a [[flattening]] of 0.0105 ({{Coord|47.967|N|225.737|W|globe:mars|name=Viking 2 landing site planetographic}} [[planetographic longitude]]) at 22:58:20 UT (9:49:05&nbsp;a.m. local Mars time).

Approximately {{convert|22|kg|abbr=on}} of propellants were left at landing. Due to radar misidentification of a rock or highly reflective surface, the thrusters fired an extra time 0.4 seconds before landing, cracking the surface and raising dust. The lander settled down with one leg on a rock, tilted at 8.2 degrees. The cameras began taking images immediately after landing.

The ''Viking 2'' lander was powered by radioisotope generators and operated on the surface until its batteries failed on April 12, 1980.

In July 2001, the ''Viking 2'' lander was renamed the '''Gerald Soffen Memorial Station''' after [[Gerald Soffen]] (1926–2000), the project scientist of the ''Viking'' program.<ref name="nasa-solar-system">{{cite web |url=https://solarsystem.nasa.gov/missions/viking-2/in-depth/ |title=In Depth: Viking 2 |website=NASA Science – Solar System Exporation |publisher=[[NASA]]}}</ref><ref>{{cite web |url=https://www.space.com/17238-mars-rover-curiosity-ray-bradbury-memorial.html |title=Mars Rover Landing Site Named for Sci-Fi Icon Ray Bradbury |date=August 22, 2012 |website=Space.com |last=Malik |first=Tariq}}</ref>

==Results from the ''Viking 2'' mission==

===Landing site soil analysis===
The regolith, referred to often as "soil," resembled those produced from the weathering of basaltic [[lava]]s.  The tested soil contained abundant [[silicon]] and [[iron]], along with significant amounts of [[magnesium]], [[aluminum]], [[sulfur]], [[calcium]], and [[titanium]]. Trace elements, [[strontium]] and [[yttrium]], were detected.

The amount of [[potassium]] was one-fifth of the average for the Earth's crust. Some chemicals in the soil contained sulfur and [[chlorine]] that were like those remaining after the evaporation of seawater. Sulfur was more concentrated in the crust on top of the soil than in the bulk soil beneath.

The sulfur may be present as [[sulfate]]s of [[sodium]], magnesium, calcium, or iron. A [[sulfide]] of iron is also possible.<ref>Clark, B. et al.  1976.  Inorganic Analysis of Martian Samples at the Viking Landing Sites.  Science:  194. 1283–1288.</ref> The [[Spirit Rover|''Spirit'' rover]] and the [[Opportunity Rover|''Opportunity'' rover]] both found sulfates on Mars.<ref>[https://web.archive.org/web/20040626074013/http://marsrovers.nasa.gov/gallery/press/opportunity/20040625a.html Mars Exploration Rover Mission: Press Release Images: Opportunity<!-- Bot generated title -->]</ref>

Minerals typical weathering products of [[mafic]] [[igneous rocks]] were found.<ref>Baird, A.  et al.  1976.  [https://www.science.org/doi/abs/10.1126/science.194.4271.1288 Mineralogic and Petrologic Implications of Viking Geochemical Results From Mars: Interim Report.]  Science: 194. 1288–1293.</ref>  All samples heated in the [[Gas chromatography–mass spectrometry|gas chromatograph-mass spectrometer]] (GCMS) gave off water.

However, the way the samples were handled prohibited an exact measurement of the amount of water. But, it was around 1%.<ref>Arvidson, R et al.  1989.  The Martian surface as Imaged, Sampled, and Analyzed by the Viking Landers.  Reviews of Geophysics:27. 39-60.</ref> Studies with [[magnets]] aboard the landers indicated that the soil is between 3 and 7 percent magnetic materials by weight.  The magnetic chemicals could be [[magnetite]] and [[maghemite]], which could come from the weathering of [[basalt]] rock.<ref>Hargraves, R. et al. 1976.  Viking Magnetic Properties Investigation: Further Results. ''Science'': 194.  1303–1309.</ref><ref>Arvidson, R, A. Binder, and K. Jones.  The Surface of Mars. ''Scientific American''</ref>  Subsequent experiments carried out by the Mars ''Spirit'' rover (landed in 2004) suggest that magnetite could explain the magnetic nature of the dust and soil on Mars.<ref>Bertelsen, P. et al.  2004.  Magnetic Properties Experiments on the Mars Exploration rover ''Spirit'' at Gusev Crater.  ''Science'': 305. 827–829.</ref>
{{Wide image|Mars Viking 22a002.png|1500px|''Viking 2'' lander image of Utopia Planitia.}}

===Search for life===
''Viking 2'' carried a biology experiment whose purpose was to look for life.  The ''Viking 2'' biology experiment weighed {{convert|15.5|kg|lbs|abbr=on}} and consisted of three subsystems: the [[Pyrolytic Release]] experiment (PR), the Labeled Release experiment (LR), and the Gas Exchange experiment (GEX).  In addition, independent of the biology experiments, ''Viking 2'' carried a Gas Chromatograph/Mass Spectrometer (GCMS) that could measure the composition and abundance of organic compounds in the Martian soil.<ref>[http://www.msss.com/http/ps/life/life.html Life on Mars<!-- Bot generated title -->] {{webarchive |url=https://web.archive.org/web/20141020034248/http://www.msss.com/http/ps/life/life.html |date=October 20, 2014 }}</ref>

The results were unusual and conflicting: the GCMS and GEX gave negative results, while the PR and LR gave positive results.<ref>[http://www.spacedaily.com/news/mars-life-00g.html Viking Data May Hide New Evidence For Life.] Barry E. DiGregorio, July 16, 2000.</ref>  Viking scientist Patricia Straat stated in 2009, "Our (LR) experiment was a definite positive response for life, but a lot of people have claimed that it was a false positive for a variety of reasons."<ref>[http://dsc.discovery.com/news/2009/09/28/viking-lander-mars.html ''Viking 2'' Likely Came Close to Finding H2O.] {{webarchive |url=https://web.archive.org/web/20090930025048/http://dsc.discovery.com/news/2009/09/28/viking-lander-mars.html |date=September 30, 2009 }}</ref>

Many scientists believe that the data results were attributed to inorganic chemical reactions in the soil. However, this view may be changing due to a variety of discoveries and studies since Viking. These include the discovery of near-surface ice near the Viking landing zone, the possibility of perchlorate destruction of organic matter, and the reanalysis of GCMS data by scientists in 2018.<ref>{{Cite journal|last1=Guzman|first1=Melissa|last2=Mckay|first2=Christopher|last3=Quinn|first3=Richard|last4=Szopa|first4=Cyril|last5=Davila|first5=Alfonso|last6=Navarro-Gonzalez|first6=Rafael|last7=Freissinet|first7=Caroline|date=2018|title=Identification of chlorobenzene in the Viking gas chromatograph-mass spectrometer data sets: Reanalysis of Viking mission data consistent with aromatic organic compounds on Mars|url=https://hal-insu.archives-ouvertes.fr/insu-01820363|journal= Journal of Geophysical Research: Planets|language=en|volume=123|issue=7|pages=1674–1683|doi=10.1029/2018JE005544|bibcode=2018JGRE..123.1674G|s2cid=133854625 }}</ref> Some scientists still believe the results were due to living reactions.  The formal declaration at the time of the mission was that the discovery of organic chemicals was inconclusive.{{citation needed|date=September 2015}}

Mars has almost no ozone layer, unlike the Earth, so UV light sterilizes the surface and produces highly reactive chemicals such as peroxides that would oxidize any organic chemicals.<ref name="Hartmann, W 2003">Hartmann, W.  2003.  A Traveler's Guide to Mars.  Workman Publishing.  NY NY.</ref>  The [[Phoenix Lander|''Phoenix'' Lander]] discovered the chemical [[perchlorate]] in the Martian soil.  Perchlorate is a powerful oxidizing agent, which could have eradicated any organic material on the surface.<ref>[http://www.planetary.org/news/2008/0806_Alien_Rumors_Quelled_as_NASA_Announces.html Alien Rumors Quelled as NASA Announces Phoenix Perchlorate Discovery.] {{webarchive |url=https://web.archive.org/web/20100904132551/http://www.planetary.org/news/2008/0806_Alien_Rumors_Quelled_as_NASA_Announces.html |date=September 4, 2010 }} A.J.S. Rayl, August 6, 2008.</ref>  Perchlorate is now considered widespread on Mars, making it hard to detect any organic compounds on the Martian surface.<ref name="NYT-20131001">{{cite news |last=Chang |first=Kenneth |title=Hitting Pay Dirt on Mars |url=https://www.nytimes.com/2013/10/01/science/space/hitting-pay-dirt-on-mars.html |date=October 1, 2013 |work=[[New York Times]] |access-date=October 10, 2013 }}</ref>

===''Viking 2'' lander image gallery===
{{Wide image|Viking Lander 2 Camera 1 NOON HIGH RESOLUTION COLOR MOSAIC.jpg|1500px|align-cap=center|''Viking 2'' lander Camera 1 NOON HIGH RESOLUTION MOSAIC (With Low Resolution Color).}}
{{Wide image|22i103-104-105-109 FROST.jpg|1500px|align-cap=center|''Viking 2'' Lander Camera 2 FROST (Low Resolution Color) Sol 1028, 1030 and 1050 between 11:34 and 12:40.}}
<gallery class="center">
File:First Color Image of the Viking Lander 2 Site.jpg|First color image (''Viking 2'' lander Camera 2 sol {{age in sols|1976|09|03|1976|09|05}}, September 5, 1976) 14:36
File:22g144 netoyee.jpg|''Viking 2'' lander Camera 2 22G144 (Low Resolution Color) Sol 552 19:16
File:Mars Viking 22e169.png|Frost on Mars.
File:22i067 FROST.png|''Viking 2'' lander Camera 2 FROST (Low Resolution Color) Sol 955 12:13
File:Mars Viking 21i093.png|Frost at the landing site. (false color)
File:21i090 21i093 21d224-225-226 LOW RESOLUTION COLOR.jpg|''Viking 2'' lander Camera 1 FROST (Low Resolution Color) Sol 960 14:14
File:21i091 HIGH RESOLUTION COLOR.jpg|''Viking 2'' lander Camera 1 FROST HIGH RESOLUTION (With Low Resolution Color) Sol 959 14:39
File:SKY-AT-SUNRISE-(22b023).jpg|''Viking 2 '' lander Camera 2 SKY AT SUNRISE (Low Resolution Color) Sol 34 04:22
File:SKY-AT-SUNRISE(22h093).jpg|''Viking 2'' lander Camera 2 SKY AT SUNRISE (Low Resolution Color) Sol 631 04:00
</gallery>

==Orbiter results==

===Viking program===

The Viking Orbiters led to significant discoveries about the presence of water on Mars. Huge river valleys were found in many areas. They showed that water floods carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. In the southern hemisphere, the presence of branched stream areas suggests that there was once rainfall.<ref>{{Cite book|isbn = 978-0-8165-1257-7|title = Mars: Maps|last1 = Kieffer|first1 = Hugh H.|date = October 1992|url-access = registration|url = https://archive.org/details/mars0000unse}}</ref><ref>Raeburn, P.  1998.  Uncovering the Secrets of the Red Planet Mars.  National Geographic Society.  Washington D.C.</ref><ref>Moore, P. et al.  1990. ''The Atlas of the Solar System''. Mitchell Beazley Publishers NY, NY.</ref>

The images below are mosaics of many small, high-resolution images.

<gallery>
Image:Streamlined_Islands_in_Maja_Valles.jpg|Streamlined islands seen by Viking showed that large floods occurred on Mars.  Image is located in [[Lunae Palus quadrangle]].
Image:Viking Teardrop Islands.jpg|Tear-drop shaped islands caused by flood waters from [[Maja Valles]], as seen by Viking Orbiter.  Image is located in [[Oxia Palus quadrangle]].  The islands are formed in the ejecta of [[Lod (crater)|Lod]], [[Bok (Martian crater)|Bok]], and [[Gold (crater)|Gold]] craters.
Image:Chryse Planitia Scour Patterns.jpg|Scour patterns, located in [[Lunae Palus quadrangle]], were produced by flowing water from Maja Valles, which lies just to the left of this mosaic.  Detail of flow around [[Dromore (crater)|Dromore]] crater is shown on the next image.
Image:Detail of Maja Valles Flow.jpg|Great amounts of water were required to carry out the erosion shown in this Viking image.  Image is located in [[Lunae Palus quadrangle]].  The erosion shaped the ejecta around [[Dromore (crater)|Dromore]].
Image:Vedra,_Maumee,_and_Maja_valles.jpg|Waters from [[Vedra Valles]], [[Maumee Valles]], and Maja Valles flowed from Lunae Planum on the left, to [[Chryse Planitia]] on the right.  Image is located in [[Lunae Palus quadrangle]] and was taken by Viking Orbiter.
Image:Kasei Valles topolabled.JPG|Area around northern Kasei Valles, showing relationships among [[Kasei Valles]], [[Bahram Vallis]], [[Vedra Valles]], [[Maumee Valles]], and [[Maja Valles]].  Map location is in [[Lunae Palus quadrangle]] and includes parts of Lunae Planum and [[Chryse Planitia]].
Image:Flow from Arandas Crater.jpg|The ejecta from [[Arandas (crater)|Arandas]] crater acts like mud.  It moves around small craters (indicated by arrows), instead of just falling down on them.  Craters like this suggest that large amounts of frozen water were melted when the impact crater was produced.  Image is located in [[Mare Acidalium quadrangle]] and was taken by Viking Orbiter.
Image:Alba Patera Channels.jpg|This view of the flank of [[Alba Patera]] shows several channels/troughs.  Some channels are associated with lava flows; others are probably caused by running water.  A large trough or graben turns into a line of collapse pits.  Image is located in [[Arcadia quadrangle]] and was taken by Viking Orbiter.
Image:Branched Channels from Viking.jpg|Branched channels in [[Thaumasia quadrangle]], as seen by Viking Orbiter.  Networks of channels like this are strong evidence for rain on Mars in the past.
Image:Dissected Channels, as seen by Viking.jpg|The branched channels seen by Viking from orbit strongly suggested that it rained on Mars in the past.  Image is located in [[Margaritifer Sinus quadrangle]].
Image:Ravi Vallis.jpg|Ravi Vallis, as seen by Viking Orbiter. [[Ravi Vallis]] was probably formed when catastrophic floods came out of the ground to the right (chaotic terrain).  Image located in [[Margaritifer Sinus quadrangle]].
File:Deimos-viking1.jpg|[[Deimos (moon)|Deimos]], photo taken in 1977.
</gallery>

==See also==
* [[Exploration of Mars]]
* [[List of missions to Mars]]
* [[List of Mars orbiters]]
* [[Timeline of artificial satellites and space probes]]
* [[U.S. Space Exploration History on U.S. Stamps]]

== Notes ==
{{notelist}}

==References==
{{Reflist|30em}}

==External links==
*[https://artsandculture.google.com/exhibit/the-viking-mars-mission-the-viking-mars-missions-education-and-preservation-project/RQKiJsUJbOktIw?hl=en The Viking Mars Missions Education & Preservation Project, VMMEPP] online exhibit.
*[https://web.archive.org/web/20070802044655/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Viking_02 ''Viking 2'' Mission Profile by NASA's Solar System Exploration]
* [https://www.nasa.gov/feature/45-years-ago-viking-1-touches-down-on-mars 45 years ago: ''Viking'' 1 Touches Down on Mars]

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