Editing Terraforming (section)

===Mars===
{{main|Terraforming of Mars}}

[[File:TerraformedMars.jpg|thumb|right|210px|Artist's conception of a terraformed Mars]]

In many respects, [[Mars]] is the most Earth-like planet in the Solar System.<ref>Read and Lewis 2004, p.16</ref><ref>Kargel 2004, pp. 185–6.</ref> It is thought that Mars once had a more Earth-like environment early in its history, with a thicker atmosphere and abundant water that was lost over the course of hundreds of millions of years.<ref>Kargel 2004, 99''ff''</ref>

The exact mechanism of this loss is still unclear, though three mechanisms, in particular, seem likely: First, whenever surface water is present, [[carbon dioxide]] ({{chem|C|O|2}}) reacts with rocks to form [[carbonate]]s, thus drawing atmosphere off and binding it to the planetary surface. On Earth, this process is counteracted when [[plate tectonics]] works to cause volcanic eruptions that vent carbon dioxide back to the atmosphere. On Mars, the lack of such tectonic activity worked to prevent the recycling of gases locked up in sediments.<ref name="forgetmars81">Forget, Costard &amp; Lognonné 2007, pp. 80–2.</ref>

Second, the lack of a [[magnetosphere]] around Mars may have allowed the [[solar wind]] to gradually erode the atmosphere.<ref name="forgetmars81" /><ref>{{Cite web |archive-url=https://web.archive.org/web/20120427034520/http://www.cosmosmagazine.com/news/2369/solar-wind-ripping-chunks-mars |archive-date=2012-04-27 |title=Solar wind ripping chunks off Mars |work=[[Cosmos (magazine)|Cosmos]] |date=25 November 2008 |url=http://www.cosmosmagazine.com/news/2369/solar-wind-ripping-chunks-mars |access-date=2009-06-18}}</ref> [[Convection]] within the core of Mars, which is made mostly of [[iron]],<ref>{{cite news |author = Dave Jacqué |url = http://www.anl.gov/Media_Center/News/2003/030926mars.htm |title = APS X-rays reveal secrets of Mars' core | publisher = Argonne National Laboratory | date = 2003-09-26 | access-date = 2009-06-10 }}</ref> originally generated a [[magnetic field]]. However the [[dynamo theory|dynamo]] ceased to function long ago,<ref>Schubert, Turcotte &amp; Olson 2001, p. 692</ref> and the magnetic field of Mars has largely disappeared, probably due to "loss of core heat, solidification of most of the core, and/or changes in the mantle convection regime."<ref name="Carr & Bell 2007">{{cite book |doi=10.1016/B978-0-12-415845-0.00017-7 |chapter=Mars |title=Encyclopedia of the Solar System |pages=359–377 |year=2014 |last1=Carr |first1=Michael H. |last2=Bell |first2=James F. |isbn=978-0-12-415845-0 }}</ref> Results from the NASA [[MAVEN]] mission show that the atmosphere is removed primarily due to [[coronal mass ejection]] events, where outbursts of high-velocity protons from the Sun impact the atmosphere. Mars does still retain a limited [[magnetosphere]] that covers approximately 40% of its surface. Rather than uniformly covering and protecting the atmosphere from solar wind, however, the magnetic field takes the form of a collection of smaller, umbrella-shaped fields, mainly clustered together around the planet's southern hemisphere.<ref name="SOLWIND">Solar Wind, 2008</ref>

Finally, between approximately 4.1 and 3.8 billion years ago, [[asteroid]] impacts during the [[Late Heavy Bombardment]] caused significant changes to the surface environment of objects in the Solar System. The low [[gravity]] of Mars suggests that these impacts could have ejected much of the Martian atmosphere into deep space.<ref name="forgetmars80">Forget, Costard &amp; Lognonné 2007, pp. 80.</ref>

Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it.<ref>Faure &amp; Mensing 2007, p. 252.</ref> A thicker atmosphere of greenhouse gases such as [[carbon dioxide]] would trap incoming [[solar radiation]]. Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other.<ref>{{cite book |doi=10.2514/6.1993-2005 |chapter=Technological requirements for terraforming Mars |title=29th Joint Propulsion Conference and Exhibit |year=1993 |last1=Zubrin |first1=Robert |last2=McKay |first2=Christopher }}</ref> Carbon dioxide alone would not suffice to sustain a temperature above the freezing point of water, so a mixture of specialized greenhouse molecules might be manufactured.<ref>{{cite journal |last1=Gerstell |first1=M. F. |last2=Francisco |first2=J. S. |last3=Yung |first3=Y. L. |last4=Boxe |first4=C. |last5=Aaltonee |first5=E. T. |title=Keeping Mars warm with new super greenhouse gases |journal=Proceedings of the National Academy of Sciences |date=27 February 2001 |volume=98 |issue=5 |pages=2154–2157 |doi=10.1073/pnas.051511598 |pmid=11226208 |pmc=30108 |bibcode=2001PNAS...98.2154G |doi-access=free }}</ref>