I'm reasonably sure that many intelligent people think they actually know that Mars exists, when what they are really doing is believing it exists, by taking 2nd and 3rd hand information and trusting it is reliable.
I'm not about to put myself on a pedestal and presume that I know what "intelligent people think", but...
On to your point regarding "belief" vs "knowledge".
If a person claims to know something, they also happen to believe it.
You cannot claim to know something while simultaneously not believing it to be true. (unless you are lying of course).
So going by the above "knowledge" is a belief and it is a subset of all the beliefs that a person holds.
What differentiates the "knowledge" belief from the other beliefs?
It is the person's epistemology.
In general that which a person classifies as "knowledge" meets their own "justification" criteria. So these are "Justified" beliefs as opposed to "unjustified" beliefs.
People's "justification" criteria differ from each other, and also differ from situation to situation. For me, mundane claims require less justification than do extraordinary claims. For example, if my wife tells me that she has baked a fresh batch of cookies, and if I can smell the cookies and see the cookies and they don't look like store bought ones then I will deem the claim justified and hold it as knowledge even though I didn't see my wife cook them.
But, if my wife told me she waved a magic wand and the cookies magically appeared, well, since magic is an extraordinary thing, never having been proven ever to exist, I will not accept this as justified knowledge. I will investigate further and insist my wife prove her extraordinary claim.
With regards to a scientific claim that Mars exists, I know that science is a methodology based on measurement and evidence, it includes the obligation to document findings and evidence and to have peers review, critique and try to disprove the claim. I can take the claim as just a topic title and choose to believe it, promote it to justified knowledge, or I can seek out the reasons to why this claim was made, find out and understand the evidence provided, seek out other scientists rebuke of the claim. These things are all publicly available if I want to put the time in.