Stock index futures rose on Tuesday as a higher earnings outlook from software maker Adobe Systems and more mergers looked to add fuel to the December rally.

Adobe Systems Inc forecast earnings late on Monday sharply above Wall Street's projections, contrasting with a pessimistic outlook three months ago. Its shares surged almost 7 percent in premarket trade.

Alpha Natural Resources Inc made an offer for rival U.S. coal miner Massey Energy Co , the Wall Street Journal reported, citing members of Massey Energy's board. Massey shares rose 2.7 percent in premarket trade to $53.20.

The U.S. stock market continues to make new highs after taking out key technical resistance levels. The S&P 500 has rallied nearly 6 percent so far this month, defying some expectations of a pullback, and is up around 21 percent from its lows this year.

It looks as though the Santa year-end rally continues, said Peter Cardillo, chief market economist at Avalon Partners in New York. Cardillo said earnings and mergers were supporting a technically strong market.

Dutch group DSM , the world's largest vitamins maker, is buying U.S. baby food ingredients maker Martek Biosciences Corp for $1.1 billion, sending its shares up 34 percent to $31.30.

S&P 500 futures rose 4.4 points and were above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures gained 36 points, while Nasdaq 100 futures rose 5.5 points.

The S&P 500 has gained for the last 3 weeks. The index broke through the 61.8 percent Fibonacci retracement of the 2007-2009 bear market slide earlier this month. Technicians say the next stop is the 76.4 percent retracement at 1,362.

Jabil Circuit Inc posted late on Monday first-quarter results that topped analyst estimates, helped by growth across segments, and forecast a robust second quarter, sending shares of the electronics manufacturing services provider up 7 percent in premarket trading.

European stocks were up 0.6 percent in morning trade in thin volume, with record-high copper prices helping mining stocks. Investors brushed aside Moody's warning it may downgrade Portugal's debt' rating.

U.S. stocks tacked on further gains to push the S&P 500 to a two-year high on Monday, continuing a steady upward march investors say should continue in 2011.

(Editing by Padraic Cassidy)