Question

At present, which one of the following planets does NOT have a magnetic field of internal origin produced by an active dynamo?

• A. Mercury
• B. Venus
• C. Earth
• D. Uranus

Hint:

A simple trick: Planets with slow or no significant rotation (like Venus) usually cannot sustain a magnetic dynamo, while planets with molten metallic cores and rapid rotation (Earth, Jupiter, Mercury, Uranus) do.

Answer 1

The Correct Option is B

Step 1: Understanding magnetic fields of planets.
Planetary magnetic fields are generally produced by an active dynamo mechanism in the liquid metallic cores of planets. This requires both: 1. A conducting fluid (such as molten iron/nickel).
2. Sufficient rotation to sustain convection currents. Step 2: Evaluating each option.
- Mercury: Despite its small size, Mercury has a weak but measurable magnetic field generated by a partially molten iron core.
- Venus: Venus rotates extremely slowly (one rotation = 243 Earth days), and convection in its core is not vigorous enough to sustain a dynamo. As a result, Venus lacks a significant internally generated magnetic field.
- Earth: Earth has a strong global magnetic field due to its active liquid outer core and rapid rotation.
- Uranus: Uranus also has a global magnetic field generated by dynamo action in a conducting water-ammonia ocean layer.
Step 3: Conclusion.
Among the given planets, the only one without an internally generated magnetic field is Venus. \[ \boxed{\text{Therefore, the correct answer is Venus.}} \]