Since annuities are insurance products, I'm going to address the insurance element of your question. But I strongly recommend that you speak to a financial consultant before making this move, to make sure an annuity is the best of the alternatives available to you.
Annuities are attractive to people who want an insurance company to take on the risk that they might live too long or outlive their assets. Annuities are particularly attractive to reasonably healthy people who think they can outlive the normal life expectancy for their age.
Here's how they work: You give the insurance company a lump sum of money, and it gives it back in the form of an income stream that you can't outlive. Payouts are usually monthly, with a minimum guaranteed period of 10 years or 20 years.
I just checked with my annuity broker. He says yes, there are life insurance companies that sell annuities paying out immediately to a 92-year-old and that they generally guarantee payments for at least five years. He says the best payout on a $500,000 deposit would be $6,715 per month for as long as your mom lives.
That works out to more than $80,000 a year. If she dies in the first five years, during the guarantee period, you will get at least $400,000 of your $500,000 investment back. Ignoring interest, it will cost you $100,000 for the peace of mind of knowing that your mother cannot outlive her $80,000 annual income stream -- even if she lives to 102 or beyond!