So, lets see what Mercury can do in 10th house without any conjunction or aspect from any other planet. 1st lets see what these 2 things represent --
Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
10th house almost represents the same things like Government, Father, Authority, Fame etc. Here, Authority should not be understood in literal sense, like someone got a Government job or became a Manager or CEO of a Firm. Here Authority means doing any work with complete perfection and your opinion matters when comes to making any decision related to that work, so even a good Hacker can be a person of authority as he is master of that particular work.
So, it is a very difficult position to guess someone's career. Why? Because with the advent of computers, internet and social media, Mercury rules over so many career, but common thing would be every career would somehow relate with communications. So here in 10th house, Mercury can make a person teacher, researcher, counsellor, sales man, marketing person, blogger, columnist, journalist and what not?
So, in this situation how we can decide the career of a person is by looking at Zodiac Sign it is placed and Nakshatra it is in. In the sign of Gemini, it can make someone journalist, but in the sign of Virgo, a career in corporate world is on, in sign of Aquarius the person takes up scientific work but in sign of Pisces, it makes a person Spiritual Counselor. So, we can see how different careers can be seen from one placement of Mercury and for every profession, communication is becoming an importance. Also, the houses which Mercury rules through Gemini and Virgo signs give an indication regarding a person's work environment.
Other factors, Mercury is communication and 10th house is father, so this person has very communicative relation with his father. Likewise, from 10th house it aspects the 4th house of Mother and Home Environment. So, person is very communicative with his mother and in home environment.
Hope this helps. Tomorrow - Mercury in 11th house.
Vishal S Saxena - Astrologer