certain : 確かな

《補語にのみ用いて》(主語となる人が)『碓信している』,信じて疑わない / (事が)『碓かな』,疑いのない(sure) / 《名詞の前にのみ用いて》ある一定の / 《名詞の前にのみ用いて》(ある程度わかっているがそれ以上ははっきりしない)『ある』…,例の… / 《名詞の前にのみ用いて》いくぶんかの,多少の,ある程度の / (…のうちの)ある人々(もの),いくつか《+『of』++『名』》
Beyond a depth of around 2,900 kilometers, a great change takes place and the mantle gives way to the core. Some seismic waves cannot pass through the core and others are bent by it. From this and other evidence, geologists conclude that the outer core is probably liquid, with a solid center. It is almost certainly made of iron, mixed with smaller amount of other elements such as nickel.