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. 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. Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it. 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. 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. Venus Terraforming Venus requires two major changes: removing most of the planet's dense carbon dioxide atmosphere, and reducing the planet's surface temperature. These goals are closely interrelated because Venus's extreme temperature may result from the greenhouse effect caused by its dense atmosphere. Venus’s atmosphere currently contains little oxygen, so an additional step would be to inject breathable O2 into the atmosphere. An early proposal for such a process comes from Carl Sagan, who suggested the injection of floating, photosynthetic bacteria into the Venusian atmosphere to reduce CO2 to organic form, and increase the atmospheric concentration of O2 in the atmosphere. This concept, however, was based in a flawed 1960s understanding of Venus’s atmosphere as much lower pressure; in reality, the Venusian atmospheric pressure (93 bars) is far higher than early estimates. Sagan’s idea is therefore untenable, as he later conceded. An additional step noted by Martin Beech includes the injection of water and/or hydrogen into the planetary atmosphere; this step follows after sequestering CO2 and reducing the mass of the atmosphere. In order to combine hydrogen with O2 produced by other means, an estimated 4*1019 kg of hydrogen is necessary; this may need to be mined from another source, such as Uranus or Neptune. Mercury Although usually disregarded as being too hot, Mercury may in fact be one of the easiest bodies in the solar system to terraform. Mercury's magnetic field is only 1.1% that of Earth's but it is thought that Mercury's magnetic field should be much stronger, up to 30% of Earth's, if