Editing Venus (section)

=== Magnetic field and core ===

In 1967, ''[[Venera 4]]'' found Venus's [[magnetic field]] to be much weaker than that of Earth. This magnetic field is induced by an interaction between the [[ionosphere]] and the [[solar wind]],<ref name=Eroshenko_et_al_1969/><ref name=Kivelson_Russell_1995/>{{Page needed|date=January 2023}} rather than by an internal [[dynamo theory|dynamo]] as in the Earth's [[Planetary core|core]]. [[Magnetosphere of Venus|Venus's small induced magnetosphere]] provides negligible protection to the atmosphere against [[solar radiation|solar]] and [[cosmic radiation]].

The lack of an intrinsic magnetic field on Venus was surprising, given that it is similar to Earth in size and was expected to contain a dynamo at its core. A dynamo requires three things: a [[Electrical conductor|conducting]] liquid, rotation, and [[convection]]. The core is thought to be electrically conductive and, although its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo.<ref name=Luhmann_Russell_2006/><ref name=Stevenson_2003/> This implies that the dynamo is missing because of a lack of convection in Venus's core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much higher in temperature than the top. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced [[heat flux]] through the crust. This [[Thermal insulation|insulating]] effect would cause the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead, the heat from the core is reheating the crust.<ref name="nimmo02"/>

One possibility is that Venus has no solid inner core,<ref name=Konopliv_Yoder_1996/> or that its core is not cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already been completely solidified. The state of the core is highly dependent on the concentration of [[sulphur]], which is unknown at present.<ref name="nimmo02" />

Another possibility is that the absence of a large impact on Venus (''contra'' the Earth's "Moon-forming" impact) left the core of Venus stratified from the core's incremental formation, and without the forces to initiate/sustain convection, and thus a "geodynamo".<ref name="Jacobsen2017">{{cite journal | last1=Jacobson | first1=Seth A. | last2=Rubie | first2=David C. | last3=Hernlund | first3=John | last4=Morbidelli | first4=Alessandro | last5=Nakajima | first5=Miki | title=Formation, stratification, and mixing of the cores of Earth and Venus | journal=Earth and Planetary Science Letters | publisher=Elsevier BV | volume=474 | year=2017 | doi=10.1016/j.epsl.2017.06.023 | page=375| arxiv=1710.01770 | bibcode=2017E&PSL.474..375J | s2cid=119487513 }}</ref>

The weak magnetosphere around Venus means that the solar wind interacts directly with its outer atmosphere. Here, ions of hydrogen and oxygen are being created by the [[Dissociation (chemistry)|dissociation]] of water molecules due to [[ultraviolet]] radiation. The solar wind then supplies energy that gives some of these ions sufficient speed to escape Venus's gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind could have led to the loss of most of Venus's water during the first billion years after it formed.<ref name="nature450_7170_629"/> However, the planet may have retained a dynamo for its first 2–3 billion years, so the water loss may have occurred more recently.<ref name="O'Rourke_et_al_2019"/> The erosion has increased the ratio of higher-mass [[deuterium]] to lower-mass hydrogen in the atmosphere 100 times compared to the rest of the solar system.<ref name=Donahue_et_al_1982/>